Source SummaryWavelength tunable surface plasmon resonance-enhanced optical transmission through chirped diffraction grating.Ellipsometry & Reflectometry (Harland Tompkins)

The name ellipsometry is derived from elliptically polarized light. If the material is well understood, single wavelength

ellipsometry can be used to measure layers which are a few tens of angstroms thick.

SWE and reflectometry can be used when the sample is single layer on a substrate. These two techniqes suffer when the materials is not well understood (i.e. material with unknown composition) or when multiple layers are involved.

The primary strength of spectroscopic ellipsometry is its ability to analyze multiple layers e.g. degree of crystallinity in amorphous silicon.

Maxwell equation and the resulting wave equation are analogous to a differential equation which describes the forces on a body and the resulting equation which describes the motion of the body as a function of time.

Electric field and magnetic field of light are not independent. The specification of electric field vector completely determines the magnetic field vector and therefore only electric field vector is considered when required.

The complex index of refraction is a combination of a real number and an imaginary number and is designated as N=n-jk. Where n is the index of refraction and k is the coefficient of extinction.

The extinction coefficient k is a measure of how rapidly the intensity decreases as the light passes through the material.

In an absorption medium, the decrease in intensity I per unit length z is proportional to the value of I.

The extinction coefficient is related to the absorption coefficient

k= λ4 πα

Penetration depth is the distance when the original intensity falls to 37% of initial value.

When a photon is emitted from a source, its electric field is oriented in a given direction. The electric field of the next photon will be oriented in a different direction, and in general photons are emitted with electric fields oriented in all different directions. This is called unpolarised light. In candescent light sources are

usually unpolarized whereas most lasers emit more or less polarized light.

When two linearly polarized waves with the same frequency are combined out of phase, the resultant wave is elliptically polarized.

Elliptically polarized light is generated when linearly polarized light is reflected from a surface under certain conditions. The amount of ellipticity which is induced depends on the surface (optical constants, presence of films etc.)

Soft lithographic approchaes to nanofabricationHybrid single nanowire photonic crystal and microresonator structuresTuning optical discs for plasmonic application(Burkan Kaplan et. Al.)

Plasmons are resonant interaction of light with materials of finite carrier density and mobility. Such resonant effects can be observed in metal films on dielectrics, in chemically synthesized metal nanostructures, and in lithographically defined metal nanostructures.

Resonant conditions are highly sensitive to nearby dielectric properties.

A sharper resonance improves the overall sensitivity of plasmonic devices for biomolecular sensing.

On a smooth planar interface of metal and dielectric, the surface plasmon polarization wave-vector is given by:

k SPP=ωc √ n1

2n22

n12+n2

2

In the presence of a sinusoidal surface corrugation, the coupling condition is modified by the pattern of the grating, and a plave wave approaching the surface at a particular angle of incidence can resonantly couple to the plasmonic excitation. The additional parameter introduced, namely, the grating period, relaxes the matching condition and gives greater freedom for the choice of other parameters.

For higher peak absorption, the depth and shape of the gratings need to be tuned.

Preparation of grating from CD-R: the top protective layer was peeled off by exfoliation using a tape after delineation of the surface with a sharp cutter. Different brands of CDs have different corrugation depth and grating shape.

For DVD, it has a strong protective layer, the inner surface was exposed by first mechanically cutting a notch into the side of the

disc and then applying mechanical force to peel the two sides apart.

Blue ray has nearly sinusoidal grating shape and therefore no need to modify it chemically.

Period of CD-R is 1600 nm, DVD is 740 nm and Blue-ray is 320 nm. Aluminium is used as the coating for blu-ray as its silver has enhanced absorption around 32 nm where the resonance occur for blu ray.

Untreated gratings from CD and less shap peaks due to unevenness and irregularity of the gratings periodicity and shape.

Alternative methods to modify the topography is by heat treatment e.g. 150 degree celcius for 5 min or 140 degree celcius for 30 min in an oven.

Low surface roughness is important for observing higher quality factors. The surface roughness of silver layers is increased by rapid silver evaporation. It can be reduced by reducing evaporation rate of silver.

Highly tunable surface plasmon enhanced optical transmission through periodic nanostructures (Hillier et. al.)

Optical modelling with periodic nano-structures. Construction of nanostructures via laser interference lithography Characterization of nanostructures Applications

Energy transfer from the incident photons excites a collective oscillation of free electrons, known as surface plasmon polaritons (SPP). At the metal dielectric interface, SPPs produce an evanescent wave that extend several hundred nanometer into the surrounding medium. The resonance conditions are highly sensitive to local environment, which makes them perfect for use in thin films and adsorption testing.

In the presence of a film, with larger dielectric constant than the surroundings, the increased the optical density results in a decrease in the energy associated with the SPP and corresponding red-shift in the resonance position.

The pitch value of gratings affect the location (or wavelength) of the transmission peaks while grating amplitude has a significant impact on the magnitude of the transmission enhancement.

Propagating surface plasmon resonances in two dimensional patterned gold grating templates and surface enhanced Raman

Plamonically resonant two-dimensional patterned templates can also be considered to be superior to one-dimensional counterparts as the resonant plasmonic modes in 2D structures can easily be excited using transverse magnetic (TM), transverse electric (TE), or mixed TM-TE polarized input pump. Hence an unpolarized pump laser can effectively excite the plasmonic modes and give rise to SERS

Fabrication of small periodic grating structures needs the

scattering involvement of expensive instrumentation such as e-beam lithography, focused ion bean milling, or by UV photolithography with expensive UV optics and UV lasers.

Nanoimprint lithography can also be used considered as an effective and simple tool for quick reproduction of nanopatterns. However, expensive master template are needed.

Alternatively through chemical approaches, small metal spheres can be arranged on a preformed template in a regular fashion and achieved very small gap in between them (<25nm)in order to obtain large localized fields in the nanogaps.

Shape-ControlledSynthesis andSurface PlasmonicProperties of MetallicNanostructures

The attractions of SPs lie in their potential to confine light to the metal/dielectric interface, which in turn generates intense local electromagnetic fields and greatly amplifies the weak signal typical of Raman scattering or second-harmonic generation.

Many metals (e.g., alkali metals, Mg, Al, and to some extent noble metals such as Au and Ag) can be treated as free-electron systems whose electronic and optical properties are determined by the conduction electrons alone. In the Drude–Lorentz model, such a metal is denoted as a plasma, because it contains equal numbers of positive ions (fixed in position) and conduction electrons (free and highly mobile).

Under the irradiation of an electromagnetic wave, the free electrons are driven by the electric field to coherently oscillate at a plasma frequency of ω p relative to the lattice of positive ions.

For a bulk metal with infinite sizes in all three dimensions, p can

be expressed as ω p=( N e2

ε0me )12where N is the number density of

electrons, ε 0 is the dielectric constant of a vacuum, and e and me are the charge and effective mass of an electron, respectively. Quantized plasma oscillations are called plasmons.

an electromagnetic wave impinging on a metal surface only has a certain penetration depth (<50 nm for Ag and Au), just the electrons on the surface are the most significant. Their collective oscillation are called surface plasmon polaritons.

By resonance, we mean a condition in which the frequencies and wave vectors of both incident and SP waves are approximately the same, leading to constructive interference and a stronger signal.

The gold and silver systems are unique because their densities of free electrons are in the proper range to give their nanoparticles SP peaks in the visible regime.