1

CHAPTER – I

INTRODUCTION TO CRYSTAL GROWTH BY

UNIDIRECTIONAL SR TECHNIQUE AND NONLINEAR OPTICS

1.1 INTRODUCTION

Crystals have interested man because of their beauty and rarity. The

enchanting colours, the smooth surfaces with scintillating reflections of light,

the definite and varied shapes with sharp edges, the deep transparency of some

perfect crystals, all together aroused the aesthetic sense of early man who used

them as ornaments. The fantasy of their external beauty was understood more

thoroughly through the natural laws of physics, mathematics and chemistry.

The contents of the crystals and their insides were explored, analyzed and

understood by modern methods of diffraction as well as with the help of

spectroscopic and electron microscopic techniques. The external shapes, planes

and colours were correlated with the internal atomic content and their

arrangements in unequivocal terms. Thus grew a science, the study of “crystal

growth and characterization”.

Crystal growth is a multidisciplinary field covering physics, chemistry,

electrical engineering, metallurgy, crystallography, mineralogy etc. In the past

few decades, there has been a growing interest in crystal growth process,

particularly in view of the increasing demand for materials for technological

applications [1-3]. The strong influence of single crystals in the present day

technology is evident from the recent advancements in the fields of

semiconductors, polarizers, transducers, infrared detectors, ultrasonic

amplifiers, ferrites, magnetic garnets, solid state lasers, nonlinear optic,

piezoelectric, acousto-optic, photosensitive materials and crystalline thin films

2

for microelectronics and computer industries. Hence, in order to achieve high

performance from the device, good quality single crystals are needed.

This chapter deals with the various methods of growing single crystals

and in particular, the solution growth method, and the development of the

nonlinear optical crystals along with the theory of nonlinear optics.

1.2 METHODS OF CRYSTAL GROWTH

The phenomenon of crystal growth is widely observed in nature and it is

found to occur in different ways, depending upon the material involved. Over

the period of time, a better understanding of the process has led to the

development of several techniques for the growth of single crystals. The

methods of growing crystals are very wide and mainly dictated by the

characteristics of the material and its size. The methods of growing single

crystals may be classified according to their phase transformations.

Growth from Solid → Solid−solid phase transformation

Growth from liquid → Liquid−solid phase transformation

Growth from vapour → Vapour−solid phase transformation

The above methods have been discussed in detail by several authors [2],

[4, 5]. The different techniques of each category are found in reviews and

books by Faktor and Garrett [6] on vapour growth, Brice [7] on melt, Henisch

[8] on gel growth, Buckley [9] on solution growth and Elwell and Scheel [10]

on high temperature solution growth.

For a successful crystal growth experiment, it is necessary to know the

following information.

(i) Proper examination of the physical and chemical properties of the

material under consideration. This step is essential to decide the most

suitable technique.

3

(ii) To identify the process parameters that are likely to influence the growth.

(iii) To identify the constraints that might be faced in the optimization of the

processing parameters.

An efficient process is the one, which produces crystals adequate for

their use at minimum cost. The growth method is essential because it suggests

the possible impurity and other defect concentrations. Choosing the best

method to grow a given material depends on the material’s characteristics.

1.2.1 Low temperature solution growth

The NLO materials chosen for the present study have been grown from

low temperature solution growth technique. Solution growth is particularly

suited to those materials, which suffer from decomposition at high

temperatures and undergo phase transformations below the melting point. The

method of crystal growth from low temperature aqueous solutions is extremely

popular in the production of many technologically important crystals. The

growth of crystals by low temperature solution growth involves weeks, months

and sometimes years.

Among the various methods of growing single crystals, solution growth

at low temperatures occupies a prominent place owing to its versatility and

simplicity. Materials having moderate to high solubility in temperature range,

ambient to 100 oC at atmospheric pressure can be grown by low temperature

solution growth method. This method is the most widely used method for the

growth of single crystals, when the starting materials are unstable at high

temperature [11]. This method is widely used to grow bulk crystals, which have

high solubility and have variation in solubility with temperature. Growth of

crystals from solution at room temperature has many advantages over other

growth methods though the rate of crystallization is slow. Since growth is

carried out at room temperature, the structural imperfections in the solution

4

grown crystals are relatively low [12]. After undergoing so many modification

and refinements, the process of solution growth now yields good quality

crystals for a variety of applications.

Low temperature solution growth can be subdivided into the following

methods:

(i) Slow cooling method

(ii) Slow evaporation method and

(iii) Temperature gradient method

1.2.1a Slow cooling method

Slow cooling is the easiest method to grow bulk single crystals from

solution. However, the major disadvantage of slow cooling method is the need

to use a range of temperatures. The possible range of temperature is usually

narrow and hence much of the solute remains in the solution at the end of the

growth run. The use of wide range of temperatures may not be desirable

because the properties of the grown crystals may vary with temperature.

Temperature stability may be increased by keeping the solution in large water

bath or by using a vacuum jacket. This technique needs only a vessel for the

solution in which the crystals grow. The height, radius and volume of the

vessel are so chosen as to achieve the required thermal stability. Achieving the

desired rate of cooling is a major technological hurdle. This method also has

the difficulty of requiring a programmable temperature controller. In spite of

these disadvantages, the method is widely used with great success [7]. The

temperature at which crystallization begins is usually within the range 40-70 oC

and the lower limit of cooling is the room temperature.

Hiroaki Yuan et al [13] have grown organic nonlinear optical

4-dimethylamino-4-methyl-4-stibazolium tosylate (DAST) crystals by slow

cooling method. KDP crystals were grown by Rajesh et al [14] from aqueous

5

solutions with organic additives by slow cooling method for obtaining better

nonlinear properties. Nixon et al [15] have reported the growth of 4-nitro-4-

methoxy benzylidene aniline (NMOBA) by employing restricted evaporation

and slow cooling methods. In the case of KDP, the addition of 5 M % of KCl

resulted in the rapid growth of the crystals and also suppressed the

incorporation of metal ion impurities into the crystal lattice Li et al [16].

A new chelating agent diethylenetriaminepentaacetic acid (DTPA) was

employed by Haja Hameed et al [17] to investigate its effect on the metastable

zone width, crystal growth and characterization of 4-dimethylamino-N-methyl-

4-stilbazolium tosylate (DAST) single crystals. Bulk crystals of two new

organic nonlinear optical (NLO) materials of hydroxyethylammonium-L-

tartarate monohydrate and hydroxyethyl ammonium-D-tartrate monohydrate

have been successfully grown by slow-cooling method. Its structural,

spectroscopic, nonlinear and thermal properties were also discussed [18].

Bhaskaran et al [19] have synthesized a new nonlinear optical material,

namely, tetrakis thiourea nickel chloride (TTNC) and single crystals were

grown from mixed solvent of water and isopropanol by both slow evaporation

and slow cooling methods. The growth habits and transparency of KDP crystals

doped with different concentrations of sulphate were studied by Jianqin Zhang

et al [20], the crystals were grown by both the traditional temperature lowering

method and the rapid growth method. Single crystals of triglycine sulfate

(TGS) with L-glutamine and L-methionine were grown in aqueous solution by

a slow cooling method by Bharthasarathi et al [21]. Ravikumar et al [22] have

grown a new nonlinear optical active inorganic crystal of cadmium iodate

(CDI) by slow cooling method. The effects of the addition of L- lysine

monohydrochloride dehydrate (L-MHCl dehydrate) on the growth and various

properties of ammonium dihydrogen orthophosphate (ADP) single crystal

grown by slow cooling method have been studied [23]. Single crystals of

6

Cesium hydrogen L-maleate monohydrate, have been grown by the

conventional slow cooling technique from aqueous solution, the grown crystals

display both platelet and prismatic morphologies depending on the imposed

supersaturation [24].

1.2.1b Slow evaporation method

This technique is similar to the slow cooling method in terms of

apparatus requirements. In this method, the saturated solution is kept at a

particular temperature and provision is made for evaporation. The Manson Jar

Crystallizer used for the solution growth technique is shown in Figure 1.1. If

the solvent is non-toxic like water, it is permissible to allow the evaporation

into the atmosphere. Typical growth conditions involve a temperature

stabilization of about 0.05 oC and rate of evaporation of a few mm3/h. The

evaporation technique has an advantage viz. the crystals grow at a fixed

temperature. But inadequacies of the temperature control system still have an

effect on the growth rate. This method can effectively be used for materials

having moderate temperature coefficient of solubility. Evaporation of solvent

from the surface of the solution produces highly local supersaturation and

formation of unwanted nuclei. Small crystals are also formed on the walls of

the vessel near the surface of the liquid from the material left after evaporation.

These tiny crystals fall into the solution and hinder the growth of the crystal.

Another disadvantage lies in controlling the rate of evaporation. A variable rate

of evaporation may affect the quality of the crystal. In spite of these

disadvantages, this is a simple and convenient method of growing single

crystals of large size.

Single crystals of pyridinium perchlorate (PyClO4) were grown by slow

evaporation of the water-ethanol (1:1) solution at constant room temperature

[25]. The habits of NMBA (4-nitro-4-methyl benzylidene aniline) crystals

grown with 10 different organic solvents were studied by Srinivasan et al [26]

7

by restricted evaporation method. The growth of crystals of pure and doped

(Sr2+ and Mn2+) ammonium tetrachlorozincate (AZC) in different

concentrations by slow evaporation technique was reported by Gaffer et al [27].

The morphology, optical absorption and dc conductivity studies indicated the

influence of dopants in the crystal lattice.

Dhanuskodi et al [28] have grown 1-Ethyl-2, 6-dimethyl-4-hydroxy

pyridinium chloride dihydrate and bromide dihydrate salts by the slow

evaporation of aqueous solution at 30 °C. Single crystals of 2-amino-5-

chlorobenzophenone (2A-5CB) were grown by employing slow evaporation

technique using acetone as solvent [29,30].

Figure 1.1 Manson Jar Crystallizer

Liu et al [31] have grown L-arginine trifluoroacetate (LATF) from

aqueous solution and studied the influence of pH value on the crystal growth

using the micro crystallization method. Single crystals of cadmium thiourea

sulfate (CTS) and magnesium cadmium thiourea sulfate (MCTS) have been

successfully grown from aqueous solution by slow evaporation technique using

predetermined solubility data. The basic growth parameters of the crystal

8

nuclei of the grown crystals of CTS and MCTS were evaluated based on the

classical theory of homogeneous nucleation [32]. Cadmium mercury

tetrathiocyanate single crystals were grown from acetone-water (4:1) mixed

solvent by slow evaporation solution technique [33]. An organometallic

material of mercury chloride thiocyanate (MCCTC) was synthesized in water-

methanol mixed solvent [34]. The growth of a new inorganic mixed borate of

barium strontium borate (BSB) has been reported by solution growth technique

using slow solvent evaporation method [35]. Allylthiourea cadmium bromide

(ATCB), a promising organometallic second order nonlinear optical material

was grown by isothermal solvent evaporation as well as by conventional

temperature lowering methods. The growth mechanism and surface features of

the as grown single crystals were analyzed by chemical etching analysis [36].

Single crystals of L-phenylalanine L-phenylalaninium perchlorate (LPAPCl), a

semiorganic nonlinear (NLO) material have been successfully grown upto a

size of 14 X 5 X 3 mm3. Nonlinear optical study reveals that the SHG

efficiency of LPAPCl is nearly 1.4 times that of KDP. The laser damage

density is found to be 7.4GW/cm2 [37]. Metal complexes of thiourea with

group II transition metals (Zn, Cd) as central atom and period III elements (S,

Cl) were synthesized by chemical reaction method and single crystals were

grown from aqueous solution by slow evaporation method [38]. The

investigation by Parikh et al [39] indicates that when KDP is doped with amino

acid L-alanine the SHG efficiency increases; where as the thermal stability of

the sample decrease. Bis thiourea zinc bromide (BTZB) a semiorganic NLO

material, has been synthesized and single crystals of size in the range of

5x4x3mm3 have been grown from aqueous solution by slow evaporation

method and the z-scan studies it is found that the material has large nonlinear

response [40]. Recently semiorganic compound, L-proline strontium chloride

monohydrate (L-PSCM) was grown from its aqueous solution at room

temperature [41].

9

1.2.1c Temperature gradient method

This method involves the transport of the materials from hot region

containing the source material to be grown to a cooler region where the

solution is supersaturated and the crystal grows.

The main advantages of the method are that:

i) Crystal grows at fixed temperature

ii) This method is insensitive to changes in temperature, provided

both the source and the growing crystal undergo the same change

iii) Economy of solvent and solute

On the other hand, changes in the small temperature difference between

the source and the crystal zones have a large effect on the growth rate.

1.3 CRITERIA FOR OPTIMIZING SOLUTION GROWTH PARAMETERS

The growth of good quality single crystals by slow evaporation and slow

cooling techniques require the optimized conditions and the same may be

achieved with the help of the following norms: (i) Material purification

(ii) Solvent selection (iii) Solubility (iv) Solution preparation (v) Seed

preparation (vi) Agitation (vii) Crystal habit and (viii) Cooling rate.

1.3.1 Material purification

An essential prerequisite for success in crystal growth is the availability

of material of the highest purity attainable. Solute and solvents of high purity

are required, since impurity may be incorporated into the crystal lattice

resulting in the formation of flaws and defects. Sometimes impurities may slow

down the crystallization process by being adsorbed on the growing face of the

crystal which changes the crystal habit. A careful repetitive use of standard

10

purification methods of recrystallization followed by filtration of the solution

would increase the level of purity.

1.3.2 Solvent selection

A solution is homogeneous mixture of a solute in a solvent. Solute is the

component, which is present in a smaller quantity and the one which gets

dissolved in the solvent. For a given solute, there may be different solvents.

The ideal solvent should yield a prismatic habit in the crystal and should have

the following characteristics [42].

(i) high solubility for the given solute

(ii) high positive temperature coefficient of solubility

(iii) low viscosity

(iv) low volatility

(v) less corrosion and non-toxicity

(vi) density less than that of the bulk solute

(vii) cost advantage

Solvent commonly used include water, both light (H2O) and heavy

(D2O), ethanol, methanol, acetone, carbon tetrachloride, hexane, xylene and

many others. Solvents having all the above characteristics together, however,

do not exist. Almost 90% of the crystals produced from low temperature

solutions are grown by using water as a solvent. Probably no other solvent is as

generally useful for growing crystals as is water. Some properties that account

for this are its high solvent action, which is related to its high dielectric

constant, its stability, its low viscosity, its low toxicity and its availability.

1.3.3 Solubility

Solubility is an important parameter, which dictates the growth

procedure. If the solubility is too high, it is difficult to grow bulk single crystals

and too low solubility restricts the size and growth rate of the crystals. Neither

a flat nor a steep solubility curve will enable the growth of bulk crystals from

11

solution. If the solubility gradient is very small, slow evaporation of the solvent

is the other option for crystal growth to maintain the supersaturation in the

solution. The solubility characteristics of the solute in a given solvent have a

considerable influence on the choice of a method of crystallization.

Low temperature solution growth is mainly a diffusion-controlled

process; the medium must be less viscous to enable faster transfer of the

growth units from the bulk solution by diffusion. Hence a solvent with less

viscosity is preferable. Supersaturation is an important parameter for the

solution growth process. The solubility data at various temperatures are

essential to determine the level of supersaturation. Hence, the solubility of the

solute in the chosen solvent must be determined before starting the growth

process [43].

The solubility of the solute can be determined by dissolving the solute in

the solvent maintained at a constant temperature with continuous stirring. On

reaching saturation, equilibrium concentration of the solute can be determined

gravimetrically. A sample of the clear supernatant liquid is withdrawn by

means of a warmed pipette and a weighed quantity of the sample is analyzed.

By repeating the above procedure for different temperatures, the solubility

curve can be plotted. Solubility of most substances increases with temperature

(the temperature coefficient of solubility is positive).

1.3.4 Solution preparation and crystal growth

For solution preparation, it is essential to have the solubility data of the

material at different temperatures. Sintered glass filters of different pore size

are used for solution filtration. The clear solution, saturated at the desired

temperature is taken in a growth vessel. For growth by slow cooling, the vessel

is sealed to prevent the solvent evaporation. Solvent evaporation at constant

temperature can be achieved by providing a controlled vapour leak. A small

crystal suspended in the solution is used to test the saturation. By varying the

12

temperature, a situation where neither the occurrence of growth nor dissolution

is established. The test seed is replaced with a good quality seed. All unwanted

nuclei and the surface damage on the seed are removed by dissolving it at a

temperature above the saturation point. The temperature is then lowered to the

equilibrium temperature and the growth commences. Solvent evaporation can

also be helpful in initiating the growth. The quality of the grown crystal

depends on the (a) nature of seed (b) cooling rate employed and (c) agitation of

the solution.

1.3.5 Seed preparation

Seed crystals are prepared by self-nucleation under slow evaporation

from a saturated solution (Figure 1.2). Seeds of good visual quality, free from

any inclusion and imperfections are chosen for growth. Since strain free reface

of the seed crystal results in low dislocation content, a few layers of the seed

crystal are dissolved before initiating the growth. Defects present in an

imperfect seed propagate into the bulk of the crystal, which decreases its

quality. Hence, seed crystals are prepared with care. The quality of the bulk

crystal is usually slightly better than that of the seed.

Figure 1.2 Apparatus for the preparation of seed crystals

13

1.3.6 Agitation

To have a regular and even growth, the level of supersaturation has to be

maintained equally around the surface of the growing crystal. An uneven

growth leads to localized stresses at the surface generating imperfection in the

bulk crystals. Moreover, the concentration gradient that exist in the growth

vessels at different faces of the crystal cause fluctuations in supersaturation,

seriously affecting the growth rate of individual faces. The gradient at the

bottom of the growth vessel exceeds the metastable zone width, resulting in

spurious nucleation. The degree of formation of concentration gradients around

the crystal depends on the efficiency of agitation of the solution. This is

achieved by agitating the saturated solution in either direction at an optimized

speed using a stirrer motor.

1.3.7 Crystal habit

The growth of a crystal at approximately equivalent rates along all the

directions is a prerequisite for its accurate characterization. This will result in a

large bulk crystal from which samples of any desired orientation can be cut.

Further, such large crystals should also be devoid of dislocation and other

defects. These imperfections become isolated into defective regions surrounded

by large volumes of high perfection, when the crystal grows with a bulk habit.

In the crystals which grow as needles or plates, the growth dislocations

propagate along the principal growth directions and the crystals remain

imperfect [42]. Needle like crystals have very limited applications and plate

like crystals need to be favourably oriented.

Changes of habit in such crystals which naturally grow as needles or

plates can be achieved by any one of the following ways:

(i) Changing the temperature of growth

(ii) Changing the pH of the solution

14

(iii) Adding a habit modifying agent and

(iv) Changing the solvent

Achievement in this area is of great industrial importance where such

morphological changes are induced during crystallization to yield crystals with

better perfection and packing characteristics.

1.3.8 Cooling rate

Supersaturation, the driving force which governs the growth of a crystal,

is achieved by lowering the temperature of a solution. Temperature and

supersaturation have to be precisely controlled for desirable results. The growth

rate is maintained linear in order to grow large crystals. This requires an

increase in the supersaturation level and linear cooling will not provide this.

Hence, after the initial growth, the rate of temperature lowering is increased.

Operation within the metastable limit occurs without any spurious nucleation in

the solution. A large cooling rate changes the solubility beyond the metastable

limit. Further, fluctuations in supersaturation may encourage solution

inclusions of flaw in growing crystals. Hence, a balance between the

temperature lowering rate and the growth rate has to be maintained.

1.3.9 Factors that influence the perfection of the crystal

Using the techniques described in the above sections, large size, well-

faceted, optically clear crystals can be produced. There are four basic factors

that determine the perfection of the grown crystal.

The perfection of the final crystal is based on:

(i) The purity of the starting materials

(ii) The quality of the seed crystal

(iii) Cooling rate employed and

(iv) The efficiency of agitation

15

Hence, high quality single crystals can be grown from quality seeds in

an efficiently stirred solution.

1.3.10 Need for growing bulk size single crystal

The molecular structure and bonding interactions of the crystals chosen

for the present study could be perceived even with the help of tiny crystals

from X-ray measurements. But, for the effective use of these crystals in the

field of opto-electronics as an electro-optic modulator, frequency doubler and

for optical parametric amplification, bulk crystal with good optical quality are

needed. The dimensions of the crystals for such device fabrications range from

centimeters to inches. Though there are different methods based on different

principles for growing good crystal, the following considerations seem to be

common to all of them.

(i) Temperature fluctuation should be avoided

(ii) Growth should proceed at a constant rate

(iii) Rate of growth should be as low as possible

(v) Large temperature gradient should be avoided

(vi) Starting material should be of high purity

1.4 UNIDIRECTIONAL SANKARANARAYANAN-RAMASAMY (SR) TECHNIQUE

1.4.1 Limitations of conventional slow evaporation method

In the conventional slow solvent evaporation growth, all crystals

bounded by planar habit faces contain separate regions common to each facet

having their own sharply defined growth direction known as growth sectors.

The boundaries between these growth sectors are more strained than the

extended growth sectors due to mismatch of the lattices on either side of the

boundary as a result of preferential incorporation of impurities into the lateral

16

section [44]. Further, in solution growth method, many of the commonly

observed characteristic growth-induced defect structure comprising growth

sectors and boundaries, growth banding, solvent inclusions, dislocations, twins

and stacking faults can be attributed to impurities [45]. Beside the relatively

low growth efficiency, the common problem the above method is that it is very

difficult to control the growth of crystal along a given direction. In other words,

the grown crystals must be cut and polished to obtain the specific crystal faces

before they are applied as a linear or nonlinear optical device. Many methods

have been developed for cutting cylindrical crystals. Unfortunately, the cutting

and polishing in a desired direction as phase matching angle are difficult for

organic NLO crystals because of their poor chemical stability and brittleness.

Cutting and machine working of single crystals result in the appearance of

structural defects and, what is most important, in these processes the expensive

material ends up as scraps. To minimize the above problems, crystals must be

grown with specific orientation in a growth vessel at room temperature. Hence,

there is a need to investigate a possible single crystal growth technique to grow

high quality crystal with a reasonable yield. Further, growth of crystal with

specific orientation has tremendous value in terms of its significance towards

device application.

1.4.2 Unidirectional growth of crystals: Common issues

High quality phase matched second harmonic generation (SHG) single

crystal is the current interest in the field of nonlinear optical materials [46].

Various attempts to grow bulk single crystals for optoelectronic applications

have been reported in the literature using solution method, vapour method,

the Bridgman–Stockbarger method [47], Czochralski method and recently

micro-tube Czochralski method [48]. In particular, novel growth methods from

melt such as seed-oriented under cooled melt growth method [49], the indirect

laser heated pedestal growth method [50], and horizontal Bridgman–

17

Stockbarger method [51] with specially designed glass cell as crucible are the

specialized growth methods to grow oriented crystal towards a phase-matched

direction. All the above mentioned crystal growth methods involve growth at

elevated temperature and there by lead to thermally induced growth defects,

and in addition these methods employ complicated equipment and multi-step

processes. These problems limit the growth of large size crystals and the

growth of crystals in unidirection is a challenging task. In this connection, a

novel unidirectional crystal growth method was reported by Sankaranaryanan

and Ramasamy [52]. It offers a solution growth method at room temperature

involving less sophisticated equipment to grow unidirectional single crystal

with cylindrical morphology, 100% solute crystal conversion efficiency and

ease in scaling up of crystal diameter. In contrast to the conventional slow

solvent evaporation technique, the crystal is restricted to grow with a specific

direction and inside a growth ampoule. This yielded a crystal with cylindrical

morphology in contrast to the crystal with planar habit face. The solution

growth method is the basic method which is economically more viable and

simple and there are no problems with thermal decomposition.

1.4.3 Fundamentals of Sankaranarayanan and Ramasamy (SR) tehcnique

Crystallization from the solution is an important process and is a two-

step process i.e the nucleation and crystal growth. The driving force for

crystallization is the degree of supersaturation which has been commonly

expressed as the difference in concentration between the supersaturated and

saturated solutions. Compared to other crystal growth techniques, it has been

widely used to grow several types of crystals at ambient temperature.

Probably no other solvent is as generally used for growing crystals as water. If

the solvent is volatile, precautions must be taken to prevent volatilization which

promotes spurious nucleation due to temperature and concentration changes.

18

In the case of organic materials, significant efforts have been made to

grow large sized crystals due to their potential application in optoelectronics

and nonlinear optical applications. However, their low melting point, weaker

mechanical properties, low thermal conductivity and the ease of supercooling

of organic materials lead to growth related problems while growing crystals

from supersaturated solutions or from melt. Extremely low growth rate and

thermal gradients are usually required for growing single crystals from melt.

From this point of view, a novel crystal growth method has been

proposed to grow organic single crystal with specific orientation in an ampoule

at room temperature [52].

1.4.4 Experimental set-up

The schematic diagram of the experimental set up is shown in

Figure.1.3. It consists of a growth ampoule made out of glass with seed

mounting pad. An outer glass shield tube protects and holds the inner growth

ampoule. A ring heater positioned at the top of the growth ampoule is

connected to the temperature controller and it provides the necessary

temperature for solvent evaporation. The temperature around the growth

ampoule is to be set based on the solvent used and is controlled with the aid of

the temperature controller. Depending on the growth rate of the crystal, the ring

heater is moved downwards using a translation mechanism. In a typical

procedure, the required solution of optimized saturation is prepared in a solvent

and then it is transferred to the growth vessel and the entire experimental set-up

is placed in a dust free hood [52].

19

Figure 1.3 Schematic diagram of the SR experimental set-up

1.4.5 Growth Procedure

Before conducting the actual crystal growth experiments, solubility

experiment is carried out in different solvents at room temperature and the

suitable solvent is identified. According to the solubility data, saturated

solution is prepared and transferred to conventional growth vessel (beaker) for

preparing seed crystal by the slow solvent evaporation technique. Based on the

quality of the grown crystals, a suitable seed crystal having a reasonable size is

selected for single crystal growth with specific orientation. Based on the

observations related to induction period, growth rate and the solvent properties,

a particular solvent is selected for conducting the single crystal growth

experiment with specific orientation. Prior to growth, care has been taken to

avoid to any contamination from the growth vessel which can lead to spurious

nucleation. Also, special care is to be taken for the preparation of the solution.

The seed crystal is chemically polished and a specific orientation is

selected to impose the orientation in the growing crystal. The glass ampoule for

20

conducting the single crystal growth is carefully mounted along with the seed

of a particular plane to face towards the solution poured into the vessel. The

entire set-up is porously sealed and placed in a dust-free hood.

Once the system attains equilibrium, the growth is initiated with a

suitable temperature provided by the ring heater at the top of the supersaturated

solution. The effective zone width of the solution and the maximum

temperature of the ring heater determine the effective evaporation rate of the

supersaturated solution for a given diameter of the ampoule. Due to the

transparent nature of the solution and the experimental set-up, real time close-

up observation will help to find out the solid-liquid interface. Under optimized

condition highly transparent crystal growth is seen. The shape of the solid-

liquid interface is monitored during the growth process and it was observed that

unlike in melt growth technique any change in the growth parameters of this

technique such as effective zone width temperature and the lowering rate of the

ring heater did not have any visible effect in the shape of the solid-liquid

interface. However, in few cases (as reported for benzophenone crystal); rapid

growth rate is also observed as evidenced from the less transparent region of

the ingot than the other regions of the ingot grown under optimized conditions

[52]. This fact is proved again by the existence of another less transparent

region of the ingot where fast growth was deliberately introduced. Depending

on the values of these growth parameters, the solvent evaporation rate can be

controlled more effectively since controlling of solvent evaporation results in

the controlling of degree of supersaturation in the solution.

Impurities affect the growth of different faces of the crystal to different

extents. The presence of impurities may accelerate, retard or even stop the

further outgrowth of individual crystal faces, sometimes producing a habit

change. As it it now possible to achieve growth on any desired face, the

influence of specific impurities on different faces can be found out. As

21

impurity concentration varies in different faces it is possible to achieve

minimum impurity concentration in the grown crystals by choosing the

appropriate growth faces.

Microbial growth has been causing serious concern in solution growth

experiments largely due to aging of the solution. However, as fresh solution

can be constantly fed during the crystal growth, the problems associated with

microbial growth can be avoided in this method. Thus the present method

encourages the growth of amino acid crystals, which usually suffer by

microbial growth. Another added advantage is; when a suspension thread is

used in crystal growth, the region close to the thread often affects its quality.

But this situation is avoided by SR method. Sankaranarayanan and Ramasamy

method offers an elegant way to evaluate the growth rate of different faces of a

crystal by choosing the appropriate seed face for conducting growth.

1.4.6 Modified SR methods

The Sankaranarayanan and Ramasamy method, which was originally

designed to grow large size organic crystals, is now turning out to be the best

method not only for the organic crystals but also for the inorganic and

semiorganic crystals. The method has been modified by few researchers

according to the choice of the materials and other requirements.

Balamurugan et al [44] have successfully grown potassium acid

phthalate (KAP) crystal of length 140 mm and diameter 20 mm, from aqueous

solution using modified SR method setup. The modifications were mainly

focused for growth from aqueous solution. The SR method setup has been

modified in some aspects in order to grow KAP crystals. In the original form of

the SR method setup [52], depending on the growth rate of the crystal, the ring

heater has to be moved downwards using a translation mechanism. But, it is

difficult to translate the heater at the rate of crystal growth. Also the top of the

growth container (ampoule) was of the same diameter as the middle of the

22

ampoule. In the modified assembly, the top of the ampoule has a bigger

diameter compared to the middle, so that from the surface of the solution more

and more evaporation will take place and also the ring heater is not translated

but fixed on the top of the ampoule. The crystallizer is kept in a water bath to

avoid the temperature fluctuation of the daily variation. The modified SR

method setup is shown in Figure 1.4. The ring heater is connected to

temperature controller and it provides the necessary temperature. The mercury

thermometer shows the temperature near the seed. The top cover is preventing

the evaporation of water from bath and it allows the evaporation of the solvent.

An advantage of this method is that it can be used to grow single crystals of

substances that have a positive, zero, or negative coefficient of solubility.

Additionally, it is suitable for crystal growth of solids that have a narrow

temperature range for thermal stability since this process is carried out

isothermally.

Figure 1.4 Modified SR method experimental setup

23

Another modified SR method procedure has been reported by Bairava

Ganesh et al [53]. An efficient semiorganic nonlinear optical crystal

L-Glutamic acid hydrochloride has been grown by using the novel uniaxial

crystal growth method of Sankaranarayanan and Ramasamy with a slight

modification in the experimental setup. This method allows the crystals to grow

in one specified axis with well developed facet. In this method, the ring heater

is replaced with a furnace-like arrangement. It can be compared with a single

zone Bridgman furnace. The heater assembly consists of a 6 cm diameter, 30

cm long cylindrical glass tube with the heating element wound over it. The

assembly is designed in order to obtain a temperature profile with maximum

temperature at the top. The temperature reduces as we move from top to bottom

of the furnace creating a gradient along the axis. The gradient is adjusted

according to the requirement by varying the spacing between each winding.

The growth ampoule with a seed fitted at the bottom is filled with the saturated

solution of L-Glutamic acid hydrochloride. The ampoule is placed along the

axis of the growth assembly. The temperature gradient creates a concentration

gradient along the growth ampoule, with a maximum supersaturation at the

bottom of the ampoule and a minimum at the top of the tube, thereby avoiding

any possibility of a spurious nucleation along the length of the ampoule. The

temperature of the furnace is controlled with an indigenously developed

controller with an accuracy of ± 0.1 ˚C. The excess the solute generated by

evaporation of the solution is driven down the ampoule by the temperature

gradient of the furnace setup. Thus the growth commences upon the seed fixed

at the bottom of the ampoule with desired orientation. After one week,

cylindrical shaped crystal with good optical quality has been obtained. With the

modified apparatus, L-Glutamic acid hydrochloride crystals of length 60 mm

and 10 mm diameter was successfully harvested [53].

24

Justin Raj et al [54] have employed an assembly of alternating 40 W

filament lamps to replace the ring heater and also the bottom seed fitted thistle

funnel replaced the ampoule for growing KDP crystal. The schematic

arrangement is shown in Figure. 1.5. The thistle funnel was placed along the

axis of the growth assembly maintaining steady temperature around it. The

temperature gradient (0.5 K per cm) has been adjusted according to the

requirement by varying the spacing between the lamps placed alternately. The

seed-fitted funnel was filled with saturated solution of the KDP salt. The

temperature near funnel was maintained at 313 K using a temperature

controller setup for the evaporation of saturated solution at the top of the

funnel. The seed fused portion of the funnel is immersed in water coloumn to

maintain the required temperature gradient. The outer surface of the growth

assembly was maintained at room temperature. The temperature gradient

makes the concentration gradient maximum at the bottom and minimum at the

top of the funnel for avoiding the spurious nucleation along the axial dimension

of the funnel. The growth rate of the crystal was found to be around 5 mm per

day. KDP crystal of 5 mm diameter and 60 mm length has been grown

successfully within a period of 30 days. The grown crystal exhibited the

cylindrical morphology same as that of the growth vessel.

25

Figure 1.5 Crystal growth assembly

Dinakaran and Jerome Das [55] reported on optically transparent bulk

single crystal of lithium para-nitrophenolate trihydrate (NPLi) along (1 1 0)

plane using the uniaxial crystal growth method of Sankaranarayanan–

Ramasamy with a slight modification in the growth assembly. The schematic

representation of the apparatus is shown in Figure 1.6. It consists of heating

coil, thermometer, inner container, temperature controller, growth vessel and

water bath. A ring heater fixed at the top of cylindrical glass tube of diameter 6

cm and height 40 cm was used as inner container and a short cylindrical

constant temperature water bath acted as the outer container. The assembly was

designed in such a way to obtain a maximum temperature profile at the top.

The ring heater connected to microprocessor controlled thermocouple provides

a constant temperature of 318 K at the top of ampoule. A seed was fixed at the

bottom of the ampoule and filled with the saturated solution of NPLi which

was mounted along the axis of the inner cylinder. The ampoule was designed

with an inner L-bend, which controls spontaneous nucleation on the top wall of

the ampoule and prevents poly crystallization. The water level inside the water

bath was increased with respect to the growth in the ampoule. The temperature

gradient creates a concentration gradient along the growth ampoule, having a

26

maximum super saturation at the bottom of the ampoule and a minimum at the

top of, thereby avoiding any possibility of a spurious nucleation along the

length of its ampoule. The excess solute generated by evaporation of the

solution is driven down the ampoule by the temperature gradient of the setup.

With this modified apparatus, NPLi crystal of 80 mm length and 12 mm

diameter has been grown successfully within a period of 12 days.

Recently, Rajesh et al [56] used another modified SR method setup to

segregate the impurities present in the solution. Many identical slots were made

in the ampoule with equal distance above the seed mounting pad. A crystal

when it is growing segregates the impurities and all the segregated impurities

are staying near to the growing crystal and there is no way to go away from the

region in the normal SR method experimental assembly. When the

concentration of the segregated impurities increases, the quality of the crystal

will become bad. It is well known that the matter diffuses from regions of high

concentration to regions of low concentration.

Figure 1.6 Modified SR Crystal growth setup

The slots made in the ampoule allow diffusion of impurities from the

high concentration to the low concentration medium, i.e the impurities present

near the crystal diffuse to the outer ampoule and several slots were made to

continue this process throughout the crystal growth process. The slots are in

27

rectangular shape and 6 mm in length and 2 mm in breadth. The diagram of the

ampoule with slots is shown in Figure. 1.7. It is expected that the impurities

segregated by the growing crystal diffuse away from the crystal vicinity to the

outer ampoule thus avoiding the defects in the crystal and other deleterious

effects of the impurities. The ampoule is placed into another big ampoule.

Saturated solution of ADP (900 ml) was used for growth. The solution was

prepared at 33 оC and it was overheated to 35 оC for few hours and again

reduced to 33 оC. Filtered solution was carefully transferred into the growth

vessel. Both the inner and outer ampoules are now filled with the solution. It is

arranged such that the inner ampoule is just taller than the outer ampoule

because, the inner ampoule is covered with porously sealed cover and allows

controlled evaporation and the outer ampoule is covered fully and no

evaporation is allowed. The ring heaters placed on the top and bottom of the

outer ampoule provide the necessary temperatures. The growth rate was

approximately 1.5 mm/day for the given ampoule of diameter 45 mm. After 40

days of the growth a good quality crystal of size 40 mm in diameter and 45 mm

in length was harvested.

28

Figure 1.7 Slotted ampoule diagram of modified SR method

1.4.7 A review on SR method grown crystals

The SR method, introduced in the year 2005 was originally designed to

grow large size organic crystals. As the growth process in organic crystals by

other conventional methods present various growth related problems, an

attempt was initially made to grow organic nonlinear optical crystal of

benzophenone and Sankaranarayanan and Ramasamy were successful in

growing unidirectional crystal of 60 mm diameter. Table 1.1 presents the list of

various crystals grown by SR method. A brief review of the work done by the

crystal growers on SR method is also highlighted.

29

Table.1.1 List of crystals grown by SR method

Harvested size Crystal Diameter

(mm) Length (mm)

Reference

Benzophenone 20 50 Sankaranarayanan and Ramasamy (2005) [52]

ADP 20 60 Sethuraman et al (2006) [57]

BMZ 15 40 Vijayan et al (2007) [58]

L-Glutamic hydrochloric acid

10 50 Ganesh et al (2007) [53]

KAP 20 140 Balamurugan et al (2007) [44]

NPLi 12 80 Dinakaran et al (2008) [55]

HA 10 23 Vijayan et al (2008) [60]

TGS 18 150 Senthil Pandian et al (2008) [59]

KDP 10 110 Balamurugan et al (2009) [61]

L-LMHCl 17 80 Senthil et al (2009) [63]

Ammonium chloride doped ADP

10 100 Rajesh and Ramasamy (2009) [62]

SA 35 150 Senthil Pandian et al (2010) [64]

LAM 18 52 Mohd. Shakir et al (2010) [65]

BTCZC 12 95 Uthrakumar et al (2011) [66]

BTCA 15 45 Ganesh et al (2011) [67]

30

Uniaxial benzophenone crystals along (1 1 0), (0 1 0) and (1 0 0)

orientation were grown by uniaxially solution-crystallization method of

Sankaranarayanan–Ramasamy (SR). A transparent uniaxial benzophenone

crystal having dimension of 500 mm length and 55 mm diameter was grown at

room temperature [68]. In contrast to the conventional solution growth method,

the growth rate along each direction was measured at ease during the respective

growth experiment by monitoring the elevation of the solid–liquid interface.

The scaling up was found to be less complicated compared to hitherto known

crystal growth methods.

A transparent uniaxial benzophenone crystal was grown at room

temperature by Arivanandhan et al [69]. In contrast to the conventional solution

growth method, the growth rate along each direction was measured at ease

during the respective growth experimental by monitoring the elevation of the

solid-liquid interface and found to be 2,4 and 6 mm /day along the (1 1 0),

(0 1 0) and (1 0 0) directions, respectively, for a chosen supersaturation.

(1 0 0) oriented benzophenone single crystal with large size (500 mm length,

55 mm diameter) have been grown by uniaxially solution-crystallization

method. A constant growth rate 100mm/day along (1 0 0) direction was

achieved at room temperature with 100% solute-crystal conversion efficiency.

X-ray topographic studies illustrate the absence of growth sectors and solvent

inclusion which are the prime inherent problem in the conventional solution

grown crystals.

Arivanandhan et al [70] have grown unidirectional benzophenone single

crystals grown by vertical Bridgman (VB), microtube-Czochralski (mT-CZ),

uniaxially solution crystallization method of Sankaranarayanan–Ramasamy

(SR) and made a systematic investigation on the growth parameters and

compared the three methods. The crystals grown by the three methods were

characterized using X-ray diffraction (XRD), high resolution XRD (HRXRD),

31

laser damage threshold (LDT) studies and the results were compared. The

structural perfection and LDT of the benzophenone grown by VB, mT-CZ and

SR methods were compared. HRXRD studies reveal that the SR grown sample

has relatively high crystalline perfection than the samples grown by other

methods. The VB grown crystal was found to have low crystalline perfection

due to the difference in thermal expansion of the growing crystal and the

ampoule which may lead to the occurrence of plastic deformation in the grown

crystal during the post-growth annealing process. The SR grown sample has

high LDT than the crystals grown by other methods, probably due to low

dislocation density in the SR grown ingots.

Good quality single crystal of hippuric acid (HA) has been grown by

Vijayan et al [60] using SR method. The crystalline perfection has been

evaluated from the HRXRD analysis and it is found to be reasonably good. The

grown crystal is found to have 1.54 times relative SHG efficiency than that of

KDP.

Unidirectional (0 0 1) bulk ferroelectric Tri glycine sulphate (TGS)

single crystal of diameter 18 mm and length 150 mm was successfully grown

by SR method. The value of Meyer's index for conventional and SR grown

TGS crystal was estimated as 2.8 and 1.8 respectively. The TGS crystals grown

by SR method are found to have higher hardness than the conventional method

grown crystals [59].

Balamurugan et al [61] successfully employed SR method in growing

inorganic NLO crystal of KDP directed along (0 0 1) with dimensions of

10 mm diameter and 110 mm length. The HRXRD analysis indicates that the

crystalline perfection is excellent without having any very low angle internal

structural grain boundaries. The better laser damage threshold value indicates

that the direction controlled KDP crystal has high damage resistance and hence

the grown crystals are useful in high powder frequency conversion application.

32

The SR method grown KDP has higher transmittance and higher hardness

value compared to conventional method grown crystals. The improved

transparency and size make this crystal suitable for SHG and optoelectronic

device fabrication.

Rajesh and Ramasamy [62] investigated the growth of (0 0 1) directed

ammonium dihydrogen phosphate (ADP) single crystal with the addition of

1 mol% of ammonium chloride in the mother solution by Sankaranarayanan-

Ramasamy method. The presence of ammonium chloride in the growth

medium suppressed the metal ion impurities and improved the quality of the

crystal. The grown crystal is found mechanically harder and thermally more

stable than the pure ADP. Higher decomposition temperature as well as

development of cracks at higher loads seems to be in conformity with each

other. Ammonium chloride doped crystals show higher intensity of green

signals than pure ADP crystals because of its better optical quality.

Senthil et al [63] have developed large diameter semi-organic

L-lysinemonohydrochloridedihydrate (L-LMHCl) single crystal using SR

method. The width of metastable zone was determined which was helpful for

the growth of larger diameter L-LMHCl dihydrate single crystal. The gravity

driven concentration gradient effect in the SR method was analyzed at different

heights. The faces which have higher attachment energy grow faster. The

results from various characterization studies demonstrate the suitability of this

method to obtain nonlinear element right during crystal growth, thus,

decreasing material consumption when making products for nonlinear optical

applications. Similarly, another unidirectional bulk semi-organic NLO single

crystal of L-LMHCl (18mm diameter and 70mm length) with the growth rate

of 5 mm/day was reported by Ramesh Babu et al [71] employing SR method.

The microbial growth was avoided and the growth conditions were optimized.

33

The SR method grown L-LMHCl dihyrate has transmittance of 96% and

moderately good hardness in the (1 0 0) plane.

Recently, Senthil Pandian et al [64] have grown a uniaxial sulphamic

acid single crystal having dimensions of 35 mm diameter and 150 mm length

by SR method. Etching behaviour of the (1 0 0) plane of conventional and the

Sankaranarayanan–Ramasamy method grown sulphamic acid crystals was

investigated with different etchants. Vickers microhardness test on the (1 0 0)

plane confirmed the mechanical stability of the conventional and the

Sankaranarayanan–Ramasamy method grown sulphamic acid crystals. High

resolution X-ray diffraction results show that the crystalline perfection of

sulphamic acid single crystals grown by the Sankaranarayanan–Ramasamy

method is extremely good compared to the conventional slow evaporation

method grown sulphamic acid crystal.

Bulk single crystal of semi-organic bis (thiourea) cadmium zinc chloride

was grown aqueous solution by a modified SR method and its high resolution

XRD measurements substantiate the excellent quality of the crystal free from

major defects like structural grain boundaries and inclusions [72].

L-asparagine monohydrate (LAM), an organic compound from the

amino acid family has been grown by slow evaporation solution technique as

well as SR method. The crystal perfection was assessed by high resolution

XRD and etching studies and it was found that the quality of the SR crystal is

better than those grown by slow evaporation solution technique [65].

Ganesh et al [67] developed (1 1 1) oriented bis thiourea cadmium

acetate (BTCA) crystal of diameter 15mm and length 45mm by a unidirectional

SR method and compared the high resolution X-ray diffraction (HRXRD),

chemical etching, Vickers microhardness, UV-Vis, dielectric studies and

differential scanning calorimetry of the sample with those grown by

conventional slow evaporation method.

34

1.5 NONLINEAR OPTICS

Almost all real systems are nonlinear. Nonlinear optical (NLO)

materials play a major role in nonlinear optics and in particular, they have a

great impact on information technology and industrial applications. In the last

decade, however, this effort has also brought its fruits in applied aspects of

nonlinear optics. This can be essentially traced to the improvement of the

performances of the NLO materials. The understanding of the nonlinear

polarization mechanisms and their relation to the structural characteristics of

the materials has been considerably improved. The new development of

techniques for the fabrication and growth of artificial materials has

dramatically contributed to this evolution. The aim is to develop materials

presenting large nonlinearities and satisfying at the same time all the

technological requirements for applications such as wide transparency range,

fast response and high damage threshold.

1.5.1 Nonlinear optical phenomenon

Nonlinear optics (NLO) deals mainly with various new optical effects

and novel phenomena arising from the interactions of intense coherent optical

radiation with matter. One of the most intensively studied nonlinear optical

phenomena and specifically the NLO property studied in the present thesis, is

second harmonic generation. Nonlinear optics (NLO) is the study of the

interaction of intense electromagnetic field with materials to produce modified

fields that are different from the input field in phase, frequency or

amplitude [73].

Second harmonic generation (SHG) is a nonlinear optical process that

results in the conversion of an input optical wave into an output wave of twice

the input frequency. The process occurs within a nonlinear medium, usually a

crystal. Such frequency doubling processes are commonly used to produce

green light (532 nm) from, for example, a Nd:YAG (yttrium-aluminium-

35

garnet) laser operating at 1064 nm. The light propagated through a crystalline

solid, which lacks a center of symmetry, generates light at second and higher

harmonics of the applied frequency. This important nonlinear property of non-

centrosymmetric crystals is called second harmonic generation (SHG) and this

phenomenon and the materials in which it occurs are the subject of intense

study.

Nonlinear optics is completely, a new effect in which light of one

wavelength is transformed to light of another wavelength. The creation of light

of new wavelength can be best understood, as we think about the electrons in

nonlinear crystal. Electrons in a nonlinear crystal are bound in potential well,

which acts like a spring, holding the electrons to lattice the points in the crystal

(Figure 1.8). If an external force pulls an electron away from its equilibrium

position the spring pulls it back with a force proportional to the displacement.

Figure 1.8 Electrons in a nonlinear crystal are bound in a potential well, holding the electrons to lattice points in a crystal

36

The spring’s restoring force increases linearly with the electron

displacement from its equilibrium position. The electric field in a light wave

passing through the crystal exerts a force on the electrons and pulls them away

from their equilibrium position. In an ordinary optical material ie., linear

optical material the electrons oscillate about their equilibrium position at the

frequency of this electronic field.

The nonlinear material is different from the linear material in several

aspects. We can think of a nonlinear material as the one whose electrons are

bound by very short springs. If the light passing through the material is intense

enough, its electric field can pull the electrons so far that they reach the end of

their springs. The restoring force is no longer proportional to the displacement

and then it becomes nonlinear. The electrons are jerked back roughly rather

than pulled back smoothly and they oscillate at frequencies other than the

driving frequency of the light wave. These electrons radiate at the new

frequencies, generating the new wavelength of light. The exact values of the

new wavelengths are determined by conservation of energy. The energy of the

new photons generated by the nonlinear interaction must be equal to integral

multiple of the energy of the photon used. Figure 1.9 shows the photons

involved in the second harmonic generation process.

Figure 1.9 Two photons are welded together to produce a single photon with the energy of both original photons

IR jjjNNNNN SR

IR

Two photons in one photon out

NON

LINEAR MEDIUM

37

When the electromagnetic field of a laser beam is illuminated on an

atom or a molecule, it induces electric polarization, which gives rise to many of

the unusual and interesting properties that are optically nonlinear. In a

dielectric material, the influence of an electric field causes distortion in the

spatial distribution between the electrons and the nucleus. These distortions

cause electric dipoles, which in-turn manifest as polarization [74]. At very low

fields, the induced polarization is directly proportional to the electric field.

However, at intense electric fields, polarization becomes independent of the

field and the susceptibility becomes field dependent. The induced polarization

is capable of multiplying the fundamental frequency to second, third order and

even higher harmonics. The re-radiation from the oscillating dipoles differs in

amplitude with respect to the incident sinusoidal electric field. As a

consequence, the distorted reradiated waves contain different frequencies from

that of the incident wave.

1.5.2 Theoretical explanation of nonlinear optics

The explanation of nonlinear effects lies in the way in which a beam of

light propagates through a solid. The nuclei and associated electrons of the

atoms in the solid form electric dipoles. The electromagnetic radiation interacts

with these dipoles causing them to oscillate, which by the classical laws of

electromagnetism, results in the dipoles themselves acting as sources of

electromagnetic radiation.

If the amplitude of vibration is small, the dipoles emit radiation of the

same frequency as the incident radiation. As the intensity of the incident

radiation increases, the relationship between irradiance and amplitude of

vibration becomes nonlinear resulting in the generation of harmonics in the

frequency of radiation emitted by the oscillating dipoles. Thus frequency

doubling or second harmonic generation (SHG) and indeed higher order

frequency effects occur as the incident intensity is increased.

38

In a nonlinear medium the induced polarization is a nonlinear function

of the applied field. A medium exhibiting SHG is a crystal composed of

molecules with asymmetric charge distributions arranged in the crystal in such

a way that a polar orientation is maintained throughout the crystal.

At very low fields, the induced polarization is directly proportional to

the electric field.

P�

= εoχ E�

(1.1)

Where χ is the linear susceptibility of the material, E�

is the electric field

vector, εo is the permittivity of free space.

At high fields, polarization becomes independent of the field and the

susceptibility becomes field dependent. Therefore, this nonlinear response is

expressed by writing the induced polarization as a power series in the field.

P�

= εo χ (1) E�

+ χ (2) E�

. E�

+ χ (3)E�

. E�

. E�

+... (1.2)

In nonlinear terms, product of two or more oscillating fields gives

oscillation at combination of frequencies and therefore the above equation can

be expressed in terms of frequency as:

P�

(-ωo) = εo χ (1) (-ωo; ω1). E�

(ωo) + χ (2) (-ωo; ω1, ω2). E�

ω1. E�

ω2 +

χ (3) (-ωo; ω1, ω2, ω3). E�

ω1. E�

ω2. E�

ω3 +…. (1.3)

Where, χ(2), χ (3) …. are the nonlinear susceptibilities of the medium. χ(1)

is the linear term responsible for material’s linear optical properties like

refractive index, dispersion, birefringence and absorption. χ(2) is the quadratic

term which describes second harmonic generation in noncentrosymmetric

materials. χ (3) is the cubic term responsible for third harmonic generation,

stimulated Raman scattering, phase conjugation and optical instability. Hence

the induced polarization is capable of multiplying the fundamental frequency to

second, third and even higher harmonics. The coefficients of χ (1), χ (2) and χ (3)

39

give rise to certain optical effects. These are listed in Table 1.2. If the molecule

or crystal is centrosymmetric then χ(2) = 0. If a field +E�

is applied to the

molecule (or medium), equation 1.3 predicts that the polarization induced by

the first nonlinear term is predicted to be +2E�

, yet if the medium is

centrosymmetric the polarization should be –2E�

.

This contradiction can only be resolved if χ(2) = 0 in centrosymmetric

media. If the same argument is used for the next higher order term, +E�

produces polarization +3E�

and –E�

produces – 3E�

, so that χ(3) is the first non-

zero nonlinear term in centrosymmetric media. In second harmonic generation,

the two input wavelengths are the same 2ω1 = ω2 or (λ1= 2 λ2).

During this process, a polarized wave at the second harmonic frequency

2ω1 is produced. The refractive index, n1 is defined by the phase velocity and

wavelength of the medium. The energy of the polarized wave is transferred to

the electromagnetic wave at a frequency ω2.

Table 1.2 Optical effects of nonlinear materials

Order Susceptibility Optical effects Applications

1 χ (1) Refraction Optical fibers

2 χ (2) SHG (ω+ω = 2ω)

Frequency mixing (ω1±ω2=ω3)

Pockels effect (ω+o =ω)

Frequency doubling

Optical parametric oscillators

Electrooptical modulators

3 χ (3) 4 wave mixing Phase gratings

Kerr effect Optical amplitude

Raman Coherent spectroscopy Real time holography

Ultra high speed optical gates

Amplifiers, choppers etc.

40

The phase velocity and wavelength of this electromagnetic wave are

determined by n2, the refractive index of the doubled frequency. To obtain high

conversion efficiency, the phase vectors of input beams and generated beams

are to be matched.

( ) 02

12

=−

=∆nn

kλ

π (1.4)

Where, k∆ represents the phase–mismatch. The phase–matching can be

obtained by angle tilting, temperature tuning or other methods. Hence, to select

a nonlinear optical crystal, for a frequency conversion process, the necessary

criterion is to obtain high conversion efficiency.

The conversion efficiency η is given by

2

2

.

sin

∆∆

=Lk

kLdLP eff�

η (1.5)

Where, deff is the effective nonlinear coefficient, L is the crystal length,

P�

is the input power density and k∆ is the phase – mismatching.

In general, higher power density, longer crystal, large nonlinear

coefficients and smaller phase mismatching will result in higher conversion

efficiency. Also, the input power density has to be lower than the damage

threshold of the crystal. Table 1.3 lists the laser and crystal parameters for

selecting a NLO crystal for a particular application.

41

Table 1.3 Parameters for selecting a NLO crystal

Laser parameters Crystal parameters

NLO process Type of phase matching

Power, Repetition rate Damage threshold

Divergence Acceptance angle

Band width Spectral acceptance

Beam size Crystal size, Walk off angle

Pulse width Group velocity mismatching

Environment Moisture, temperature acceptance

1.6 CRITERIA FOR SELECTING USEFUL NONLINEAR OPTICA L MATERIALS

The “ideal” nonlinear crystal does not exist. The applicability of a

particular crystal depends on the nonlinear process used, the desired device

characteristics and the pump laser. Special material properties that are important

in one application may not be significant in another. For instance, efficient

doubling of very high power lasers having poor beam quality requires a material

with large angular bandwidth. A crystal, which has a smaller nonlinearity but

allows noncritical phase matching, will perform better than one which is more

nonlinear, but is critically phase matched. On the other hand, for the doubling of

femtosecond optical pulses, the preferred material will be one with a large

nonlinearity so that a very thin crystal can be used to avoid dispersive broadening

of the second harmonic output pulses [75].

For a material that has favourable features such as large nonlinearity, high

damage threshold, favourable crystal growth habits etc., an application can

invariably be found that uses the crystal efficiently. From a material point of

42

view, only general criteria can be established to gauge the usefulness of a

nonlinear crystal. For specialized applications where device performance

requirements are well established, quantitative criteria for the selection of

suitable nonlinear crystals can be obtained which are often invaluable in aiding

system design.

Nonlinear frequency converters are most commonly used with an

efficient nontunable laser source. Obviously, the nonlinear crystal should have

good transparency at the pump laser wavelength. Specific applications of

nonlinear crystals currently of interest can be divided into the following

efficient harmonic generation and up-conversion, optical parametric oscillator,

frequency conversion of ultrashort pulses, frequency conversion of high

average power sources, frequency conversion of low average power sources,

and laser fusion.

The resulting urge of interest in the development of other materials with

superior optical quality and improved nonlinear properties soon led to the

discovery of a number of early materials, including ammonium dihydrogen

phosphate (NH4H2PO4), potassium dihydrogen phosphate (KH2PO4), lithium

niobate (LiNbO3), barium sodium niobate (NaNbO3), lithium iodate (LiIO3),

lithium formate monohydrate, potassium niobate (KNbO3) and barium titanate

(BaTiO3), potassium pentaborate (KB5O8.4H2O) and ammonium pentaborate

(NH4B5.4H2O), urea, potassium titanyl phosphate, beta barium borate and

lithium borate [76]. Crystal growth and further characterization of these

materials has been identified as high-priority research areas by a high level

expert committee in the workshop on nonlinear optical materials [77]. These

crystals played an important role in the establishment of nonlinear optics as a

major area of laser science. Subsequently, intensive efforts were expanded and

are continued till today in search for new and better nonlinear materials.

43

Kurtz and Perry powder SHG method was introduced at the end of

1960’s. In this method, a powdered sample is irradiated with laser and scattered

light is collected and analyzed for its harmonic content with the use of suitable

filters. For the first time, rapid and qualitative screening for second order NLO

effect was possible. The stage was set for a rapid introduction of new materials,

both inorganic and organic.

1.7 TRENDS IN ORGANIC NLO MATERIAL DEVELOPMENT

For the past two decades, the search for new NLO materials has

concentrated primarily on organic compounds owing to their large nonlinearity.

The NLO properties of large organic molecules and polymers have been the

subject of extensive theoretical and experimental investigations during the past

two decades and they have been investigated widely due to their high nonlinear

optical properties, rapid response in electro-optic effect and large second or

third-order hyperpolarizabilities compared to inorganic NLO materials.

The low temperature solution growth technique is widely used for the

growth of organic compounds to get quality single crystals. Vijayan et al [78]

have grown p-hydroxy acetophenone (C8H8O2), one of the potential organic

NLO materials. Nagaraja et al [79] showed that benzoyl glycine (BG), an

organic nonlinear crystal grown by slow evaporation from DMF solution has

the advantages of both the organic and inorganic NLO materials. Owing to

high nonlinear efficiency, high melting point, good chemical stability, less

sublimation problems and improved hardness and cleavage properties (unlike

other organic materials), benzoyl glycine is found to be a promising material

for NLO applications. Lakshmana Perumal et al [80] further extended the effort

in synthesizing 4-methoxy benzaldehyde-N-methyl-4-stilbazolium tosylate

(MBST), which is a derivative of stilbazolium tosylate. The Kurtz powder SHG

measurements on MBST showed that the peak intensity is 17 times more than

that of urea.

44

Urea has been used in an optical parametric oscillator to generate

tunable radiation throughout the visible region but intrinsic absorption and

phase matchability considerations make it unsuitable for wavelengths longer

than 1000 nm (Rosker and Tang) [81]. The efforts made to resolve the

problems associated with urea have not been successful. The newly grown

binary UNBA crystal by Rai et al [82] is thermally and mechanically harder

than the crystal of the parent components. It is quite transparent almost in the

entire the UV region and hence it can be used for producing green/blue laser

light. Lin et al [83] have synthesized two component urea-mNBA systems and

urea-L-malic acid systems with different urea compositions. Jun Shen et al [84]

have grown single crystals of L-tartaric acid-nicotinamide and D-tartaric acid-

nicotinamide by the temperature lowering method from aqueous solution.

Single crystal of 3-methyl 4-nitropyridine 1-oxide (POM) was grown by

Boomadevi [85]. Manivannan and Dhanushkodi [86] have grown 3-[(1E)-N-

ethylethanimidoyl]-4 hydroxy-6-methyl-2H-pyran 2-one, by slow evaporation

technique and found that the SHG efficiency is close to that of urea.

A new ligand N-(3-fluorophenyl) naphthaldimine has been synthesized

by Unver et al [87]. The electric dipole moment (µ) and the first

hyperpolarizability (β) values of the N-(3-fluorophenyl) naphthaldimine have

been computed and the results reveal that the synthesized molecule might have

microscopic nonlinear optical (NLO) behaviour with non-zero values.

L-arginine acetate (LAA) is an organic nonlinear optical material and has a

wide optical transmission window between 220 and 1500 nm. Its laser damage

threshold and SHG efficiency are comparable with that of KDP. The N-(3-

nitrophenyl) phthalimide (N3NP) is a phase-matchable NLO crystal and can be

used as an efficient frequency doubler and optical parametric oscillator due to

its high SHG conversion efficiency, which was grown by slow evaporation

technique using DMF solvent [88]. Shaokang Gao et al [89] have synthesized

45

the N-(4-nitrophenyl)-N-methyl-2-aminoacetonitrile (NPAN) material and

single crystals of dimensions 36 x 8 x 8 mm3 were harvested. Second-harmonic

generation (SHG) in the NPAN crystal was observed using Nd:YAG laser with

a fundamental wavelength of 1064 nm. An organic NLO material, 4-OCH3-4′-

nitrochalcone (MNC), has been synthesized and grown by Patil et al [90] which

has NLO efficiency 5 times more than that of KDP.

A new organic crystal of semicarbazone of 2–amino–5–chloro–

benzophenone (S2A5CB) has been grown and characterised by proton nuclear

magnetic resonance by Sethuraman et al [57] and its second harmonic

generation property was confirmed by Kurtz powder method. Vibrational

spectral analysis of the non-linear optical material, L-prolinium tartrate (LPT)

was carried out using NIR-FT-Raman and FT-IR spectroscopy by

Padmaja et al [91].

Jagannathan et al [92] have synthesized the organic material

4-Ethoxybenzaldehyde-N-methyl 4-Stilbazolium Tosylate (EBST), a derivative

in Stilbazolium Tosylate family. Its NLO efficiency is 11 times greater than

that of urea. Studies on the nucleation kinetics of Sulphanilic acid (SAA) single

crystals were reported by Mythili et al [93]. The laser damage threshold values

of the SAA crystals are found to be 7.6 and 6.6 GW/cm2 for single and multiple

shots, respectively. Single crystals of pure and Cu2+ and Mg2+ doped L-arginine

acetate (LAA) were grown by Gulam Mohamed et al [94] using slow

evaporation method. It is observed that both Cu2+ and Mg2+ dopants have

increased the percentage of transmission in LAA. Ravindra et al [95] reported

that the solution grown NLO crystals of p-chloro dibenzylideneacetone

(CDBA) are thermally stable up to 250 ºC. Ammonium malate (AM), an

organic nonlinear optical material has been synthesized from aqueous solution.

The structural perfection of the grown crystals has been analyzed by high

resolution XRD rocking curve measurement [96]. Single crystals of L-lysine

46

acetate, an organic nonlinear optical material were grown by Sun et al [97]

using the controlled evaporation of its aqueous solution. Good quality

benzophenone (BP) crystals were grown by solution technique using CHCl3 as

solvent by adopting slow evaporation method at room temperature was

reported by Madhurambal et al [98]. The growth of a organic nonlinear optical

(NLO) crystal of 2-aminopyridinium maleate (2APM) in larger size has been

reported by [52] by slow evaporation method. A new semi-organic nonlinear

optical crystal, L-Phenylalanine L-Phenylalaninium perchlorate (LPPAPC) has

been grown through synthesis between L-Phenylalanine and perchloric acid.

Bulk crystals of dimension 5.5 x 0.4 x 0.3 cm3 were obtained by submerged

seed solution method [99]. A new nonlinear optical single crystal L- alaninium

fumarate (LAF) belonging to the amino acid group was grown by slow

evaporation solution growth technique [100]. The organic nonlinear optical

crystal of amino-carboxyl acid family, L-lysinium trifluoroacetate (LLTF) was

successfully grown from its aqueous solution by the temperature-lowering

technique. Its growth morphology was investigated by X-ray single diffraction

data and the growth habits were studied using micro-crystallization method

[101]. Urea ninhydrin monohydrate (UNM) was synthesized and grown from

aqueous solution employing the slow evaporation method [102]. Recently,

Jerald Vijay et al [103] have investigated the rapid growth of DAST by

adopting the slope nucleation method and by rapidly evaporating the solvent.

Thin plates of organic NLO crystal of DAST are grown within a period of 72 h

by carefully optimizing the growth conditions. Picric acid and its complexes

with amino acids viz., L-prolinium picrate, L-valinium picrate and

L-asparaganium picrate show very high SHG efficiency [104]. A novel

nonlinear optical crystal of tris (glysine) calcium (II) dichloride (TGCC) of

dimensions 34 x 23 x 5 mm3 was grown by slow evaporation technique and the

second harmonic conversion property of TGCC was identified by the Kurtz and

Perry technique [105]. Recently Gupta et al [106] studied the synthesis and

47

growth of a new nonlinear optical material, L-proline strontium chloride

monohydrate (L-PSCM) of amino acid family and found that SHG efficiency

of the crystal is nearly one tenth of the KDP. L-Arginine meleatate dihydrate

(LAMD, C6H14N4O2.C4H4O4.2H2O), a complex of strong amino acid has been

successfully grown by SR method by Urit Charoen-In et al [72]. SHG

measurements indicate that the SHG efficiency of LAMD crystal is roughly 4

times that of KDP. A recent study by Amalanathan et al [107] throws a new

interest in the possible application of amino acid crystals in terahertz (THz)

technology. The molecular structure, vibrational spectra and NLO properties of

L-valine hydrobromide are reported. The charge transfer interaction, calculated

first order hyperpolarizability and the HOMO-LUMO energy gap explain the

NLO activity of the molecule. The calculated first order hyperpolarizability is

found to be 6.617e-30e.s.u., which is 25 times that of urea. Vibrational and NBO

analysis confirms the N-H…..Br hydrogen bonding. The increase of β value is

additive property to exhibit nonlinear optical activity. Hence the L-valine

hydrobromide crystal can be a better entrant for the THz application [107].

1.8 IMPORTANCE OF L-TARTARIC, L-ALANINE AND L-PHENYLALANINE BASED NLO CRYSTALS

On the search towards new NLO materials with better mechanical

properties, many researchers have focused on the small organic molecules

having a large dipole moment and a chiral structure. These molecules are

usually linked through the hydrogen bond. The tartaric acid molecules are

bonded into a layer by O…..O-type hydrogen bonds to generate a two

dimensional frame work. Tartaric acid forms a broad family of hydrogen-

bonded crystals. The tartrates structure can be used to discuss a design strategy

for the engineering of crystals with predesigned architecture. The salts of

tartaric acid were intensively studied by means of structural, spectroscopic,

optical, dielectric methods. Some tartrates exhibit structural phase transisions

and in many cases vibrational spectroscopy was effectively used to study them

48

[108]. The influence of the strong and very strong hydrogen bonds on the

nonlinear optical properties of the crystal was already considered.

The multidirectional hydrogen-bonded tartrate anions provide a

conformational rigid environment for the incorporation of cations to form

acentric crystalline salts of SHG materials. Few examples for this category are;

L-lysine-L-tartaric acid, L-tartaric acid-nicotinamide and L-alanine tartrate

complex [108,109].

Because of the ability of enhancing the macroscopic nonlinearity in a

synergistic mode and initiating multidirectional hydrogen bonds, tartaric acid

was chosen to synthesize nonlinear materials. Hence, in the present thesis the

growth of selected salts of tartaric acid are attempted with success and the main

emphasis is to grow L-tartaric acid (LTA), L-tartaric acid-nicotinamide (LTN),

L-alaninium tartrate (LAT), L-phenylalanine hydrochloride (LPHCl) and

L-phenylalaninium maleate (LPM) single crystals by unidirectional SR

method.

L-tartaric acid-nicotinamide (LTN) is yet another interesting category of

new organic NLO material, which crystallizes in the monoclinic crystal system

with space group P21. The lattice parameter values are a =7.650 Ǻ, b =15.499Ǻ

and c = 10.506 Ǻ. The transmission range of LTN is found to be better than

KDP [109].

L-alanine is an efficient organic NLO compound under the amino acid

category. Single crystals of L-alanine was grown and characterized by Razzetti

[110]. Vijayan et al [111] have reported the bulk growth and characterization

studies of L-alanine single crystal. Rajan Babu et al [112, 113] reported the

growth of single crystals of L-alanine derivatives such as

L-alanine tetrafluoroborate (L-A1FB) and studied their fundamental growth

properties. The linear optical properties showed that L-alanine family crystals

have lower cut-off wavelength in the UV-region. Among all L-alanine

49

derivatives, L-A1Ac possesses high transmittance of 80 %. The powder SHG

test confirms that the nonlinear optical property of the grown crystals of

L-alanine derivatives is comparable with other semiorganic crystals.

Dhanuskodi and Vasantha [114] have reported the structural, thermal and

optical characterization of L-alaninium oxalate (LAO). LAO has its

transparency window from 230 nm onwards, suggesting the suitability of LAO

for SHG of the 1064 nm radiation and for other applications in the blue violet

region. Solution grown single crystals of LAO have produced SHG efficiency

of 1.2 times that of KDP [115]. Photoacoustic studies and thermal properties of

the NLO compound, L-alaninium maleate were reported by Martin Britto Dhas

et al [116].

The crystal structures of amino acids and their complexes have provided

a wealth of interesting information to the patterns of their aggregation and the

effect of other molecules and ions on their interactions and molecular

properties. Among them, L-phenylalanine is an essential protein amino acid,

which is used by the body to build neurotransmitters. The phenylalanine

molecules are related by a non-crystallographic pseudo two fold symmetry.

There have been several spectroscopic studies on the behaviour of many amino

acids and peptides including phenylalanine and on complexes involving amino

acids, organic molecules and metal ions [117].

1.9 SCOPE OF THE THESIS

The ever increasing demand for highly efficient nonlinear optical (NLO)

crystals for visible and ultraviolet regions is extremely important for laser and

material processing. In this context, the design and growth of single crystals

suitable for such requirements, assumes centre stage.

Organic NLO crystals formed with L-tartaric acid, L-alanine and

L-pheynyl have been identified as potential candidates for replacing KDP in

nonlinear optical applications. Hence attempts are made to grow L-tartaric acid

50

(LTA), L-tartaric acid-nicotinamide (LTN), L-alanine tartrate (LAT),

L-phenylalanine hydrochloride (LPHCl) and L-phenylalaninium maleate

(LPM) single crystals. Keeping in view, the importance attached to the

materials chosen for the present thesis, the growth of these crystals has been

carried out with the focus on unidirectional method along with the conventional

low temperature method. The grown crystals are characterized to identify the

physicochemical properties for possible exploitation of the developed materials

for future applications.

The present investigation is aimed at

(i) Synthesizing the chosen materials for the growth of single crystals

(ii) Determining the solubilities of the materials

(iii) Growing bulk size single crystals by unidirectional and conventional low temperature techniques

(iv) Identifying the crystal structure by single crystal X-ray diffraction analysis

(v) Estimating the crystalline quality by high resolution X-ray diffractrometry (HRXRD) study

(vi) Characterizing the grown crystals by Kurtz powder NLO test

(vii) Laser damage threshold measurement

(viii) FT-IR and Optical transmission/absorption studies

(ix) NMR spectral studies

(x) Studying of thermal behaviour of the grown crystals

(xi) Determining microhardness values

(xii) Measuring the dielectric constant and dielectric loss of the grown crystals

(xiii) Carrying out ac/dc conductivity study of the grown crystals and

(xiv) Carrying out photoconductivity study of the grown crystals.

51

REFERENCES

1. Laudise R.A. (1975), ‘Crystal growth and characterization’, Ed. Ueda R. and Millin J.B., North-Holland Publishing Co.

2. Brice J. C. (1986), ‘Crystal Growth Process’, John Wiley and Sons, New York.

3. Nalwa H.S. and Miyata S. (1996), ‘Nonlinear Optics of Organic Molecules and Polymers’, CRC Press Inc., New York.

4. Pamplin B.R. (1980), ‘Crystal Growth’, Pergman Press, London.

5. Chernov A.A. (1984), ‘Modern Crystallography III-Crystal Growth’, Springer-Verlag, Solid State Series, Vol. 36, Berlin.

6. Faktor M.M. and Garrett I. (1974), ‘Growth of Crystals from Vapour’, Chapmann and Hall, London.

7. Brice J.C. (1973), ‘The growth of crystals from liquids’, North Holland publishing company, Amsterdam.

8. Henisch H.K. (1988), ‘Crystals in gels and Liesegang rings’, Cambridge Univ. Press. USA

9. Buckley H.E. (1951), 'Crystal Growth', John Wiley and Sons, New York.

10. Elwell D. and Scheel H.J. (1975), ‘Crystal Growth for High Temperature Solutions’, Academic Press Inc., London.

11. Pamplin B.R. (1979), ‘Crystal Growth’, Pergamon Press, Oxford.

12. Brice J. C. (1972), ‘The growth of crystals from liquids’, Wiley, New York.

13. Hiroaki Yuan., Takashashi Y., Junko Yabuzaki, Yusuke Mori and Takatomo Sasaki, J. Cryst. Growth. 198 (1999) 568-571.

14. Rajesh N.P., Kannan V., Santhanaraghavan P., Ramasamy P. and Lan C.W. Materials letters. 52 (2002) 326-328.

52

15. Nixon A., Haja Hameed A.S., Thenappan L., Noel M. and Ravi G., Mater. Chem. Phys. 88 (2004) 90-96.

16. Li G., Liping X., Su G., Zhuang X., Li Z., He Y., J. Cryst. Growth. 274 (2005) 555 –562.

17. Haja Hameed A.S., Rohani S., Yu W.C., Tai C.Y. and Lan C.W., Mater. Chem. Phys. 102 (2007) 60-66.

18. Yun Zhang., Yonggang Wang., Yunxia Che and Jimin Zheng., J. Cryst. Growth. 299 (2007) 120-124.

19. Bhaskaran A., Ragavan C.M., Sankar R., Mohankumar R. and Jayavel R., Cryst. Res. Technol. 42 (2007) 477-482.

20. Jianqin Zhang., Shenglai Wang., ChangShui Fang., Xun Sun., Qingtian Gu., Yiping Li., Bo Wang., Bing Liu and Xiaoming Mu., Materials Letters. 61(2007) 2703-2706.

21. Bharthasarathi T., Siva Shankar V., Jayavel R., Murugakoothan P., J. Cryst. Growth. 311 (2009) 1147-1151.

22. Ravi Kumar S.M.., Melikechi N., SelvaKumar S., Sagayaraj P., J. Cryst. Growth, 311 (2009) 337-341.

23. Rajesh P., Ramasamy P., Mahadevan C.K., J. Cryst. Growth. 311 (2009) 1156-1160.

24. Babu Reddy J.N., Vanishri S., Kamath G., Elizabeth S., Bhat H.L., Isakov D., Belsley E., Gomes D.M., Aroso T.L., J. Cryst. Growth, 311 (2009) 4044-4049.

25. Czarnecki P., Wasicki j., Pajak Z., Goc R., Mahaszyfiska H and Habryto S., Journal of Molecular Structure. 404 (1997) 175-180.

26. Srinivasan K., Sankaranarayanan K., Tangavelu S and Ramasamy P., J. Cryst. Growth. 212 (2000) 246-254.

27. Gaffar M.A., Abu El-Fadl A.., Bin Anooz S., Cryst. Res. Technol. 40 (2005) 204-211.

28. Dhanuskodi S., Manivannan S and Kirschbaum K., Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy. 64 (2006) 504-511.

53

29. Ramesh Babu R., Vijayan N., Gunasekaran M., Gopalakrishnan R. and Ramasamy P., J. Cryst. Growth. 265 (2004) 290-295.

30. Gulam Mohamed M., Rajarajan K., Mani G., Vimalan M., Prabha K., Madhavan J., Sagayaraj P., J.Cryst. Growth, 300 (2007) 409- 414.

31. Liu X.J., Wang Z.Y., Xu D., Wang X.Q., Song Y.Y., Yu W.T and Guo W.F., J. Alloy. Compoun., 441 (2007) 323-326.

32. Daniel M., Jeyarani Malliga M., Sankar R., Jayaraman D., Mater. Chem. Phys. 114 (2009) 18-22.

33. Raghavan C.M., Pradeepkumar R., Bhagavannarayana G., Jayavel R., J. Cryst. Growth. 311 (2009) 3174-3178.

34. Ravi Kumar S.M., Melikechi N., SelvaKumar S., Sagayaraj P., J. cryst. Growth. 311 (2009) 2454-2458.

35. Prabha K., Rajesh Kumar., Shi Feng Du., Vlmalan M., Dayalan A and Sagayaraj P., Mater. Chem. Phys. 121 (2010) 22-27.

36. Perumal R and Moorthy Babu S, J. Cryst. Growth., 310 (2008) 2050-2057.

37. Cyrac Peter .A, Vimalan. M, Sagayaraj .P,Madhavan.J, Physica B. 405 (2010) 65–71

38. Sabari Girisun and Dhanuskodi, Materials Research Bulletin.45 (2010) 88–91.

39. Parikh K.D, Dave D.J, Parekh B.B, Joshi M.J., Cryst. Res. Technol. 45 (2010) 603-610.

40. Sabari Grisun .T.C, Dhanuskodi .S, Mangalaraj .D, Philip .J., Synthesis, growth and characterization of bisthiourea zinc bromide for optical limiting applications, “Current Applied Physics” 11 (2011) l-6.

41. Manoj K Gupta, Nidhi Sinha, Binay Kumar Physica B. 406 (2011) 63-67.

42. Pamplin B.R. (1979), ‘Crystal Growth’, Pergamon Press, Oxford.

43. Christian R. (1990), ‘Solvents and solvent effects in organic chemistry’, VCH, New York.

54

44. Balamurugan N., Lenin M., Ramasamy P., Materials Letters. 61 (2007) 1896.

45. H.G. Gallagher, R.M. Vrcelj, J.N. Sherwood, J. Cryst. Growth. 250 (2003) 486.

46. Sankaranarayanan K. J. Cryst. Growth. 284 (2005) 203-208.

47. Arivanandhan M., Sankaranarayanan K., Ramamoorthy K., Sanjeeviraja C and Ramasamy P, Cryst. Res. Technol. 39 (2004) 692.

48. Sankaranarayanan K and Ramasamy P, J. Cryst. Growth. 193 (1998) 252.

49. Wang .W, Huang.W, Ma.Y, Zhao.J, J. Cryst. Growth. 270 (2004) 469.

50. Yokoo A., Tomaru S., Yokohana I., Kobayashi H., Itoh H and Kaino T, J. Cryst. Growth. 156 (1995) 279.

51. Taima T., Komatsu K., Onodera S and Kaino T, J. Cryst. Growth. 229 (2001) 558.

52. Sankaranarayanan K and Ramasamy P, J. Cryst. Growth. 280 (2005) 467-473.

53. Bairava Ganesh R., Kannan V., Sathyalakshmi R., Ramasamy P. Materials Letters. 61 (2007) 706.

54. Justin Raj C, Krishnan S, Dinakaran S, Uthrakumar and Jerome Das S, Cryst. Res. Technol., 43 (2007) 245-247.

55. Dinakaran S and Jerome Das S. J. Cryst. Growth. 310 (2008) 410.

56. Rajesh.P, Ramasamy.P, Bhagavannarayana.G and Binay Kumar Curr. Appl. Phys., 10 (2010) 1221-1226.

57. Sethuraman K, Ramesh Babu R, Gopalakrishnan R and Ramasamy R, J. Cryst. Growth, 294 (2006) 349.

58. Vijayan N, Nagarajan K, Alex M.Z. Slawin, Shashidharan Nair C.K and Bhagavannarayana G, Crystal growth and design. 7 (2007) 445-448.

59. Senthil Pandian M., Balamurugan N., Ganesh V., Raja Shekar P.V., Kishan Rao K., Ramasamy P. Materials Letters. 62 (2008) 3830.

55

60. Vijayan N, Bhagavannarayana G, Alex M.Z. Slawin, Materials Letters. 62 (2008) 2480–2482.

61. Balamurugan S., Ramasamy P., Sharma S.K., Yutthapong Inkong and Prapun Manyun, Mater. Chem. Phys. 117 (2009) 465-470.

62. P. Rajesh., P. Ramasamy., Materials Letters., 63 (2009) 1260-2262.

63. Senthil A, Ramesh Babu R, Balamurugan N and Ramasamy P, J. cryst. Growth. 311 (2009) 544-547.

64. Senthil Pandian M, Urit Charoen In, Ramasamy P, Prapun Manyum., Lenin M and Balamurugan N, J. Cryst. Growth. 312 (2010) 397–401

65. Mohd. Shakir, Riscob B, Maurya K.K, Ganesh V, Wahab M.A, Bhagaavannarayana G, J. Cryst. Growth. 312 (2010) 3171-3177.

66. Uthrakumar R, Vesta R, Bhagavannarayana G, Robert R, Jerome Das S, Journal of Alloys and compounds. 509 (2011) 2343-2347.

67. Ganesh V, Ch. Snehalatha Reddy, Mohd.Shakir, Wahab M.A, Bhagavannaraya G, Krishan Rao K, Physica B. 406 (2011) 259-264.

68. Ramasamy P. J. Cryst. Growth. 310 (2008) 1501.

69. Arivananthan.M, Sankaranarayan K and Ramasamy P, J. Cryst. Growth. 310 (2008) 1493-1496.

70. Arivananthan M, Xinming Huang, Satoshi Uda, Bhagavannarayana G, Vijayan N, Sankaranarayanan K and Ramasamy P, J. Cryst. Growth. 310 (2008) 4587-4592.

71. Ramesh Babu R, Sethuraman K, Gopalakrishan R and Ramasamy P, J. Cryst. Growth. 297 (2006) 356.

72. Urit Charoen-In., Ramasamy P, Manyum P, J. Cryst. Growth., 318 (2011) 745-750.

73. Firdous Anwar (1988), ‘The assessment of urea related compounds as nonlinear optical materials and the single crystal growth of urea and NN’ dimethyl urea’, Ph.D thesis, Dept. of Pure and Applied Chemistry, University of Strathclyde, U.K.

56

74. Narasimhamurthy T.S. (1981), ‘Photoelastic and electro-optic properties of crystals’, Plenum press, New York.

75. Cheng L.K., Bosenberg W. and Tang C.L., Prog. Crystal Growth and Charact. 20 (1990) 9-57.

76. Dmitriev V.G., Gurzadyan C.G., and Nicogosyam D.N., (1999) Hand book of nonlinear optical crystal, Springer Verlag, NY.

77. Betterman B.W and Cole H., (1964), ‘Dynamical Diffraction of X Rays by Perfect Crystals’, Rev. Mod. Phys. Vol. 36, pp. 681-717.

78. Vijayan N., Ramesh Babu R., Gunasekaran M., Gopalakrishnan K., Kumaresan R., Ramasamy P., Lan C.W., J. Cryst. Growth. 249 (2003) 309-315.

79. Nagaraja H.S., Upadhyaya V., Mohan Rao P., Sreeramana Aithal P. and Bhat A.P., J. Cryst. Growth. 193 (1998) 674-678.

80. Lakshmana Perumal C.K., Arul Chakkaravarthi A., Rajesh N.P., Santhana Raghavan P., Huang Y.C., Ichimura M. and Ramasamy P., J. Cryst. Growth. 240 (2002) 212–217.

81. Rosker M.J., and Tang C.L., J. Opt. Soc. Am. B. 2 (1985) 691–696.

82. Rai R.N., Ramasamy P. and Lan C.W., J. Cryst. Growth. 235 (2002) 499–504.

83. Lin Y.Y., Rajesh N.P., Santhana Raghavan P., Ramasamy P and Huang Y.C., Materials Letters. 56 (2002) 1074-1077.

84. Jun Shen, Jimin Zheng, Yunxia Che and Bin Xi., J. Cryst. Growth. 257 (2003) 136–140.

85. Boomadevi S., Mittal H.P. and Dhanasekaran R., J. Cryst. Growth. 261 (2004) 55−62.

86. Dhanuskodi S. and Manivannan S, J. Cryst. Growth. 262 (2004) 395–398.

87. Unver H., Karakas A., Elmali A. and Durlu T. N., J. Mol. Struct. 737 (2005) 131-137.

88. Manivannan S., and Dhanuskodi., J. Cryst. Growth. 262 (2004) 473–478

89. Shaokang Gao., Weijun Chen., Guimei Wang and Jianzhong Chen., J. Cryst. Growth. 297 (2006) 361-365.

57

90. Patil P. S., Dharmaprakash S.M., Hoong-Kun Fun and Karthikeyan M.S., J. Cryst. Growth. 297 (2006) 111-116.

91. Padmaja L., Vijayakumar T., Hubert Joe I., Reghunadhan Nair C. P., Jayakumar V. S., J. Raman Spectroscopy. 37 (2006) 1427-1441.

92. Jagannathan K., Kauulainathan S., Gnanasekaran T., Vijayan N. and Bhagavannarayana G., Cryst. Res. Technol. 42 (2007) 483-487.

93. Mythili P., Kanagasekaran T. and Gopalakrishnan R., Cryst. Res. Technol. 42 (2007) 791-799.

94. Gulam Mohamed M., Rajarajan K., Mani G., Vimalan M., Prabha K., Madhavan J., Sagayaraj P., J. Cryst. Growth. 300 (2007) 409- 414.

95. Ravindra H.J., John Kiran A., Satheesha Rai Nooji., Dharmaprakash S.M., Chandrasekharan K., Balakrishna Kalluraya, Fabian Rotermund,, J. Cryst. Growth. 310 (2008) 2543-2549.

96. Anandha Babu and Ramasamy., J. Cryst. Growth, 311 (2009) 1185-1189.

97. Sun Z.H., Zhang G.H., Wang X.Q, Cheng X.F., Liu X.J., Zhu L.Y., Fan H.L., Yu G., Xu D., J. Cryst. Growth. 310 (2008) 2842-2847.

98. Madhurambal G., Ramasamy P., Anbu Srinivasan P., Suganthi M., Vasudevan G and Mojumdar S.C., J. therm. Anal. calorimetry. 94 (2008) 45-51.

99. Siva Shankar V., Siddheswaran R., Sankar R., Jayavel R., Murugakoothan P., Mater. Letter. 63 (2009) 363-365.

100. Ramachandra Raja C., Antony Joseph A., Mater. letter. 63 (2009) 2507-2509.

101. Sun Z.H., Zhang G.H., Wang X.Q, Yu G., Zhu L.Y., Fan H.L., Xu D., J. Cryst. Growth. 311 (2009)) 3455-3460.

102. Uma Devi T., Lawerence N., Ramesh Babu R., Selvanayagam S., Stoeckli- Evans H., Ramamurthi K., J. Cryst. Growth. 311 (2009) 3485-3490.

103. Jerald Vijay R, Melikechi N, Rajesh Kumar T, Joe G.M. Jesudurai, Sagayaraj P., J. Cryst. Growth. 312 (2010) 420–425.

58

104. Natarajan S, Umamaheshwaran M, Kalyana Sundar J, Suresh J and Martin Britto Dhas S.A., Spectrochimica Acta Part A: Mol. Biomol. Spect. 77 (2010) 160-163.

105. Dhanaraj P.V, Rajesh N.P., Physica B. 406 (2011) 12-18.

106. Gupta, M.K., Sinha, N., Kumar, B., Physica B: 406 (2011) 63-67.

107. Amalanathan M, Hubert I, Rastogi V.K, J. Mol. Struct. 985 (2011) 48-56.

108. Debrus S., Marchewka M.K., Baran J., Drozd M., Czopnik R., Pietraszko A., Ratajczak H., J. Solid State Chem.. 178 (2005) 2880

109. Haja Hameed A.S., and Lan C.W., J. Cryst. Growth. 270 (2004) 475-480.

110. Razzetti C., Ardoino M., Zanotti L., Zha M. and Paorici C., Cryst. Res. Technol. 37 (2002) 456-465.

111. Vijayan N., Rajasekaran S., Bhagavannarayana G., Ramesh Babu R., Gopalakrishnan K., Palanichamy M., Ramasamy P., Cryst. Growth. Des., 6 (2006) 2441-2445.

112. Rajan Babu D., Jayaraman D., Mohan Kumar R. and Jayavel R., J. Cryst. Growth. 245 (2002) 121–125.

113. Rajan Babu D., Jayaraman D., Mohan Kumar R., Ravi G. and Jayavel R., J. Cryst. Growth. 250 (2003) 157–161.

114. Dhanuskodi S. and Vasantha K., Cryst. Res. Technol. 39 (2004) 259–265.

115. Vimalan M., Ramanand A. and Sagayaraj P., Cryst. Res. Technol. 42 (2007) 1091-1096.

116. Martin Britto Dhas S. A. and Ramachandran E., Raji P., Ramachandran K. and Natarajan S., Cryst. Res. Technol. 42 (2007) 601-606.

117. Cyrac Peter A, Vimalan M, Sagayaraj P and Madhavan J, Physica B, 405 (2010) 65-71.