lasing action from random media with gain

ELSEVIER

July 1997

Optical Materials 8 (1997) 1-9 l a eFit ls

Lasing action from random media with gain

B.G. Sfez *, Z. Kotler NRC Soreq, Yavne 81800, Israel

Received 25 November 1996; revised 31 January 1997; accepted 1 May 1997

Abstract

Photon processes in scattering media with optical gain has recently attracted much interest because of the possibility of getting strong, monochromatic, laser-like light emission at low cost. We present here experimental results showing the properties of optically pumped dyed titanate solutions. In particular we introduce a mapping technique showing the existence of several physical mechanisms depending on the titanate particles density and the dye concentration. We also suggest the possibility of locking the light emission at a specific wavelength using the scatterers properties only. © 1997 Elsevier Science B.V.

1. Introduction

Recently a strong interest arose over the possibility of generating laser-like emissions from optically pumped mixed media of scatterers randomly dis- persed in laser gain media such as dye solutions. Spectral bar-coding, parts identification, tatoo re- moval or color displays are only a few of the possible applications of such a light source. Initial experiments by Lawandy's group [1,2] have shown that the presence of scatterers inside the gain medium radi- cally changed the nature of the radiation emitted by this medium: thus, strong spectral narrowing as well as giant emission enhancement (compared to luminescence) could be observed. Moreover the presence of a threshold for such an enhancement indicated the presence of a laser-like mechanism. Several t ime- domain studies of the emission [3-5], showing a pulse duration of 50 ps (instead of 2 ns for the

' Corresponding author.

luminescence), indicated that this effect was not simply due to slight modifications of the luminescence properties but involved coherence mechanisms.

Since then other groups have demonstrated that this effect is quite general and can be observed for example also in an intralipid dyed solution [6] and pigmented solutions [7]. Moreover, recently, lasing in a mixture of a semiconducting polymer and scatterers has been observed [8], indicating the possibility of electrically induced lasing action in such media.

In this paper, we investigate the origin of the possible involved mechanisms by studying the inter- play between dye concentration, scatterers density and incident pump power. In particular, our experimental results show that there might be more than one mechanism involved depending on the relative values of the different concentrations.

In Section 2 we will describe the experimental set-up and the different experimental results obtained, then we will discuss several possible mecha-

00925-3467/97/$17.00 © 1997 Elsevier Science B.V. All rights reserved. PI1 S0925-3467(97)00037-2

2 B.G. Sfez, Z. Kotler/Optical Materials 8 (1997) 1-9

nisms which might explain these results and check their compatibility with the experimental data. We will then propose two qualitative models which seem to be compatible with the data. Finally we will describe an experimental procedure which might give more physical insights and then we conclude.

2. Experiments

2.1. Experimental setup

The experimental set-up is described in Fig. 1. The laser source is a Q-switched Nd:YAG laser that produces second harmonic 7 ns duration, 532 nm wavelength pulses at a pulse repetition rate of 10 Hz. A filter (F1) was used in order to eliminate completely the 1.06 /zm wavelength radiation. The long-term source stability was checked using a pyro-

I Laser

i

E

E 4%

~ P 1

• 4%

I Neutral Density Filters

I GP1

Iris Q

HW

I GP2

ilter 532nm

Spectrometer

Sample Cell

Fig. 1. Experimental set-up. The 532 nm light from the laser is passing through a 1060 nm filter (F1), part of the light is deviated to a detector (P1). After passing a set of neutral density filters, the light intensity is modified using 2 Glan prisms (G1 and G2) and a half-wave plate HW. An iris determines the spot size. The light then impinges on the cell. An optical fiber (OF) collects light and transmits it to a spectrometer.

electric detector (PI) (Ophir). A series of neutral density filters limited the incident power to a maximum fluence of 15 m J / c m 2 per pulse in order to avoid dye bleaching. The laser power was varied using a pair of Glan polarizers (GP1 and GP2) and a half-wave plate HW. An iris allowed control of the spot size and was set in order to select only the central part of the spot. The spot diameter was fixed to 2.5 mm. Thus the incident beam could reasonably be considered as a plane wave.

The beam then impinged on a cell making an angle of approximately 10 ° with the beam direction in order to avoid self-lasing between the cell walls. A fiber spectrometer (Ocean Optics PC1000) with a ~ 2 nm resolution was used for detection. An optical fiber (OF) collected both the emission from the solution and part of the incident beam. In order to do so a piece of glass (reflectivity 4%) was inserted in the path of the excitation beam and redirected part of the beam towards the fiber. Moreover a high pass filter was inserted between the cell and the fiber. Thus back-reflected excitation light was filtered out. This set-up allowed the pump excitation and the luminescence emission to both be on the same spectrum and thus normalize the luminescence emission. The angle between the fiber axis and the excitation beam direction was about 24 ° .

2.2. Experimental conditions

The sample was a dye solution containing titanate powder. The solvent was methanol (refractive index, n D = 1.33) and the dye was Rhodamine 610 (Exci- ton), also known as Rhodamine 6B. The dye concentration was varied between 10 -5 and 10 -~ M during the experiments. The scatterers were provided by Cerac (32 nm diameter grains) and DuPont (Ti-Pure 960, 360 nm diameter grains). The scatterer concentration was varied during the experiments from 109 to 1012 par t ic les /cm 3 (for the 360 nm particles) and to 7 X 1013 part ic les /cm 3 (for the 32 nm particles).

2.3. Experimental data: changes as a function of the pump power

Figs. 2 and 3 show typical graphs obtained, respectively, at low and high pump fluence. The dashed line corresponds to a dye solution (1.4 x 10 -4 M)

B.G. Sfez, Z. Kotler / Optical Materials 8 (1997) 1 -9 3

lOO

90

8o

70

~" 50

~ 50

:J

' l ~ J , ' ~, '

o

540 560 580

"i~.. ~

i

600 620 640 660 680

Wavelength (nm)

Fig. 2. Spectra at low pump energy. The dashed line graph corresponds to pure luminescence (excitation of a 1.3 × 10 -3 M dye solution without scatterers) and the solid line graph corresponds to solution with scatterers (32 nm diameter, 3×1013 particles/cm3). The pump fluence is 0.08 rnJ /cm 2.

1 0 0 0 0

9000

8000

7000

~ " 6000 <C . ~ 5000

.~ 4000

E ILl 3000

2000

1000 -,---,--?-: 00.0 0.2 0.4 0.6 0,8 1.0 1 2 1 4 1 6 1 6 2 0

Pump energy (mJ/cm:)

Fig. 4. Peak of emission versus pump fluence. The dashed line represents emission from a solution without scatterers (1.4 × 10 -3 M) whereas the solid line represents the same solution with 3 × 10 ]3 particles per cm 3.

without scatterers whereas the solid line corresponds to the same solution with 3 × 1013 titanate particles per cm 3 (note the scale change). At low pump fluence, the scatterers do not seem to play any role. However, at strong pump fluence one can observe a very large increase in the light emission and clear spectral narrowing (from about 25 to 8 nm linewidths under these particular conditions).

In fact, it appears that beyond a certain pump threshold, the emission from the scattering solution increases linearly with the pump power. In Fig. 4 the peaks of emission of a pure dye solution and the same solution with 3 × 1013 particles/cm 3 are compared as a function of the peak power. The lumines-

cence (dashed curve) slowly increases with the pump energy and presents a saturation-type behavior. On the contrary, the emission from a scattering solution presents two different regimes. Below 0.65 mJ/cm 2, the emission is slowly increasing with the pump power (in fact this increase is only a little bit higher than that for luminescence). However above 0.65 mJ/cm 2, the emission is increasing linearly and more strongly with the pump power and at 1.5 mJ/cm 2, the emission is already more than ten times that of luminescence.

If we now compare the emission linewidths from pure dye and from a scattering solution (Fig. 5), the difference is even clearer. The luminescence linewidth (dashed line) slowly decreases as the power

300

250

2OO

~ 150 - - - - u

E 100 uJ

50

0

540

J i

560 586 600 620 640 660 680

Wavelength (nm)

Fig. 3. Spectra at strong pump energy. The experimental conditions are the same as for Fig. 2 except for the pump fluence (2.16 mJ/cm2) .

50

45

40

35

g 30

.m 25

"~ 20

. .J 15

10

g

0 0

I i i i i , i i I , I i I

1 2 3 4 5 6 7

Energy per pulse (mJ/cm 2)

I , I i 110 6 9

Fig. 5. Linewidth versus pump fluence. Same conditions as for Fig. 4.

4 B.G. Sfez, Z Kotler/Optical Materials 8 (1997) 1-9

is increased whereas, for the scattering solution, a clear threshold can be seen at roughly 0.8 mJ / cm 2. The linewidth then remains constant at 8 nm. Note that depending on the exact dye and scatterer concentrations, one observes different values for the minimum linewidth (see Section 2.4). This value can be as low as 4.8 nm for optimal conditions.

Finally, one observes a blue shift of the emission when the pump power is increased (Fig. 6), followed by a weak red-shift at higher pump powers.

2.4. Experimental data: effect of scatterer concentration

It is also possible to modify the particle concentration while keeping the pump power constant and high enough. Fig. 7 shows how the peak of emission (relative to luminescence measured at the same dye concentration but without scatterers) is modified when varying the scatterer concentration. There is only a specific range of particle concentrations where the enhancement of emission can be observed, with a maximum effect at 3 × 1013 particles/cm 3. Outside this range, the effect disappears [9].

Similarly, the linewidth of the emission presents a clear minimum for roughly the same value of the particle concentration (Fig. 7). Here too, there is a specific range of concentrations for which the linewidth is narrowed.

14

u

8

4

2

o

Ol I 10

log [TiO2] -11 (cm -3)

Fig. 7. Peak of emission enhancement versus particle concentration. The pump ftuence is fixed at I0 m J / c m 2. The particle mean diameter is 32 nm and the dye concentration is 1.5 × 10 -3 M. A maximum of the peak of emission occurs at roughly 3.2× 1013 particles/cm 3.

2.5. Experimental data: effect of dye concentration

Similar results can be found when varying the dye concentration. However, this result is less surprising since anyway, even for luminescence from a pure dye solution, there is a dye concentration optimum: for high dye concentrations there is a strong reabsorption from the dye itself and the formation of molecular dimers which reduce the dye efficiency (Fig. 10).

5 9 2

591

590

5 8 9

5 8 8

5 8 7

5 8 6

H i l i • • •

• i | • I n • •

I L r i / o 2 4 6 8 1

Pump Energy (m J)

Fig. 6. Wavelength of the peak of emission versus pump energy. A strong blue-shift can be observed from the scattering solution when pump power is increased, followed by a weak red-shift. The resolution in wavelengths is governed by the interpixel distance of the spectrometer (same conditions as Fig. 4).

3. Discussion of possible mechanisms

There are different possible physical mechanisms which may explain this emission enhancement and we will try to select among them those which are definitely not compatible with the experimental results and those which are. We can divide these mechanisms into two classes: coherent mechanisms for which the enhancement originates from a coherent behavior of the excitation and amplification mechanisms for which the enhancement originates from gain within the medium.

3.1. Super-radiance and super-luminescence

Both effects originate from a cooperative spontaneous emission of excited molecules due to coupling

B.G. Sfez, Z. Kotler / Optical Materials 8 (1997) 1-9 5

with the pump field [10,11] (super-radiance) or with the emitted field [12] (super-fluorescence). Because of this cooperative behavior, this effect grows as N 2 where N is the number of excited molecules. Thus, if one of those effects were responsible for the emission enhancement, there should be a parabolic dependence of the emission with the pump power. Fig. 4 shows a linear dependence with the pump power and, consequently, we can eliminate the possibility of a cooperative behavior as the source of the observed enhancement.

3.2. Redirected amplified spontaneous emission

A second possible explanation is the redirection of amplified spontaneous emission (ASE) towards the detector. This explanation has been proposed by Wiersma et al. [13]. ASE is known as a very strong effect, especially in dye lasers, and is usually detri- mental for optimal laser performances. ASE has several common points with lasing such as spectral narrowing of the emission, threshold behavior and strong enhancement of the emitted power (compared to luminescence) at the specific emission wavelength. However, since ASE makes use of only one component of the laser action (light amplification) and does not utilize optical feed-back, the emission characteristics depend only on the gain properties: geometric shape of the pumped region, gain optimum and so on. For example, the wavelength of emission is determined only by the gain medium properties: ASE occurs at the gain maximum, which

18

01

mm

l 1 I10

log [ T i 0 2 ] - 1 '1 (in c m -a)

Fig. 8. Emission linewidth versus particle concentration. Same conditions as for Fig. 7.

Gain region (laser beam)

"~ y ~ , ASE q i

i 1 ' ' Detector

L __ -

Scatterer

Fig. 9. ASE growth in the present experimental conditions (Fig. 1). The disk-shaped geometry of the gain region favors ASE in the direction parallel to the cell wall The presence of scatterers redirects ASE towards the detector (see text).

can be determined by the luminescence spectrum at low pump power.

Moreover, ASE develops preferentially in the longest straight path in the gain medium, since ASE growth is exponential with the distance traveled in the gain medium. For example, in the present experimental configuration described in Fig. 1, the spot diameter is 2.5 mm and the penetration depth of the pump within the medium is less than 1 mm. The gain region shape is thus a disk parallel to the front wall of the cell. ASE will propagate preferentially in the direction parallel to the wall, where the paths are longer (Fig. 9). In the experimental configuration the optical fiber mainly collects light emitted from the front wall and does not detect light emitted from the side walls. Consequently it does not detect ASE in a pure dye solution. However, the presence of the scatterers modifies this picture since ASE is now scattered in all directions. In particular, light is scattered also in the fiber detector direction and strong ASE signal can now be detected from the front wall.

Since ASE occurs at the optimum value of the gain, maximum ASE should be seen when pure dye luminescence is optimal. In order to check this possibility, we have taken the luminescence spectra of pure dye solutions for different dye concentrations, ranging from 3.1 × 10 -2 to 1.5 × 10 - 6 M (Fig. 10).

6 B.G. Sfez, Z~ Kotler/Optical Materials 8 (1997) 1-9

300

250

2OO 3

150

8 :~ ,oo E UJ

5°

o

-50 560

[ I I 31x~O~M

- -X- - 15x104 M

7.SxlO~M

. . . . . . . lx10 ~ M -

i . . . . . . . . 25xlO~M - _ _

, i 565 570 575 580 585 590 595 600 605 610

Wavelength (nm)

Fig. 10. Luminescence spectra from pure dye solutions for different dye concentrations. Luminescence from pure dye solution (without scatterers) is measured for different values of the dye concentration Note that optimal luminescence occurs for ~ 1 0 - 4

M dye concentration At 15X 10 -3 M, the luminescence is quite low.

It Can be clearly seen that the luminescence optimum occurs for a dye concentration of approximately 10 -4 M. In particular, for a concentration of 1.5 X 10 -3 M (of this pure dye solution), the luminescence signal is very weak and much red-shifted due to light reabsorption.

On the other hand we have scanned titanate concentrations and dye concentrations in order to find the strongest light emission of dye and titanate mix- tures. Figs. 11 and 12 show several emission spectra around this optimum. We have found that such an optimum occurs for a titanate concentration of 1.6 X l0 H par t ic les /cm 3 (for 360 nm diameter titanate particles) and for a dye concentration of 1.5 X 10 -3 M.

Dye concentration: 1.5 10 ̀3 M

3000 [ " " [Tie2] 3.2 10"

• ~ "~ . . . . . [Ti02] 1.6 10'* I UJ

580 582 584 586 588 590 592 594 596 598 600 602 604 606 605 610

W a v e l e n g t h ( n m )

Fig. 11. Scanning of scatterers concentration at fixed dye concentration near the emission optimum.

2500

~ " 2000

.~ 1500

E looo LU

5O0

o i

TiO~ concen~ra~on: 1.6 10" -> 2~m between particles

- - [ D y e ] 3.1 10"~M

I , / ,,, . . . . . IDYll 1.5 l o ~ [Dye} 0.78 104M

l 580 582 584 586 588 590 592 594 596 595 600 502 604 606 605 610

W a v e l e n g t h (r im)

Fig. 12. Scanning of dye concentration at fixed scatterers concentration near the emission optimum. Note that the optimum is obtained for a dye concentration of 15 X 10 -3 M, which is a much higher concentration than for pure dye solutions (see Fig. 10).

If redirected ASE was effectively responsible for the emission enhancement, the effect should be optimal when luminescence from a pure dye solution is optimal, that is for a dye concentration of ~ 10 -4 M (Fig. 10).

Since we have checked that the scattering solution emits optimally for a dye concentration of ~ 10 3 M (where the pure dye luminescence is quite low), we conclude that the proposed mechanism cannot explain the effect.

3.3. M i c r o c a v i t i e s

Microcavity mechanism seems very attractive: ba- sically, we consider that the scattering medium is made of a very large number of microcavities, each of them being determined by two or more scatterers. A photon within such a microcavity has a certain probability of remaining within the microcavity, depending on the scattering cross-section of the scatterers. For given dye and scatterer concentrations, there is an optimum cavity geometry (number of scatterers involved, distance between them and so on). Since the medium is homogeneous, all the scatterers are involved at least in one of those optimum microcavities. Thus there is a very large number of such optimum microcavities, forming a specific sub-set among all the possible closed loops of emitted photons. For this particular class of microcavities, photons experience lasing action: spectral narrowing (Fig. 3), linear dependence with the pump power

B.G. Sfez, Z Kotler / Optical Materials 8 (1997) 1-9 7

(Fig. 4) and threshold behavior (Figs. 4 and 5). Let us check now the compatibility of such a mechanism with the other experimental results.

Since the scattering process is essentially of the Rayleigh type (particles smaller than the wavelength), the scattering process is stronger at shorter wavelengths than at longer ones (A -4 dependence). The equivalent mirrors of such microcavities are thus more reflective for the shorter wavelengths. Beyond the threshold, the gain maximum is displaced towards the blue, which is compatible with the experimental results of Fig. 6.

For a given pump power and dye concentration, when the scatterer density is increased, the mean distance between scatterers is reduced. Let us consider what happens at low and high powers:

• At low power: when no scatterer is present (pure dye solution), the pump penetration in the solution is given by the dye absorption length I a-

When scatterers are introduced in the solution, this penetration depth is reduced to (l~l~c/3) 1/2, where lsc is the inverse of the product of the scattering cross section and the scatterers density [14]. Thus the pumped region is much smaller and luminescence is degraded as shown in Figs. 2 and 5 (wider luminescence linewidth, lower peak of emission for very low pump powers).

• At high power: at low scatterer density, the distance between scatterers is so large that a photon closed loop can be achieved only with a very small probability (the scattering probability from one scatterer to the other varies as d -2 where d is the distance between the scatterers). When the distance between scatterers is decreased, this probability is increased and for a certain value of the scatterers density, lasing occurs. However, simultaneously, the number of dye molecules participating to the lasing process is reduced when the distance between scatterers is reduced. When the number of molecules is not sufficient to provide enough gain, lasing ceases (in the limiting case where scatterers are stuck one to the other, there is no dye molecules at all). There is thus an optimum for the lasing action to occur, as shown in Figs. 7 and 8 for emission intensity and linewidth, respectively. The same kind of mechanism can be used in order to explain optimum for dye concentration. It must be noted that this optimum is not the one obtained as for pure dye solution (as

underlined in Section 3.2) and that the regular explanation of photon reabsorption and dimer formation is not sufficient to explain the experimental results. Finally it should be noted that a strong dependence exists between the dye and the scatterers concentrations since the former is responsible for the gain and the second for the cavity dimensions.

3.4. Amplified spontaneous emission with optical feedback

The previous mechanism involves light amplification within closed cavities. However, as it was underlined, the effective reflectivity of the scatterers decreases very quickly with the distance between them. However, as soon as there is gain, light amplification increases with the total path that the photon travels within the medium. Thus two regimes can be envisioned:

(1) For higher scatterer concentrations (the distance between scatterers is smaller than a few wavelengths), the distance between scatterers is small and the closed loop probability is high. Moreover the probability of a photon to escape from a given region of the medium is small ( 'bound photon'). Long paths within the medium can effectively be achieved through closed loops: this is the 'microcavity' regime 1.

(2) For lower scatterer concentrations, the distance between scatterers is large. The probability that a photon escapes from a given region is high ('free photon'). Photons can propagate long paths in the medium before being scattered. This regime is favor- able to ASE: amplified photons do not close loops but propagate much longer paths in the gain medium than if there was no scatterer. Thus the role of the scatterers is to provide longer paths for the photons to travel within the gain medium [16]. This is the ASE with optical feedback (OFASE) regime. Quali- tatively, the same arguments as for Section 3.3 can be applied for explaining the experimental results. Note that this mechanism is different from that of Section 3.2, since here scatterers play an active role in the amplification process. For example, shorter wavelengths will experience longer paths in the

J This regime would be close to pre-localization where coherent effects are to be expected. For an excellent review, see [15].

8 B.G. Sfez, Z. Kotler/Optical Materials 8 (1997) 1-9

medium than longer ones (since the scattering process favors shorter wavelengths) and the gain peak will be blue shifted compared to the luminescence peak.

Because of the very large number of scatterers, emission coherence properties are blurred in the case of microcavities and it is difficult to distinguish between the two effects. However, since the microcavity effect involves a much larger number of scattering events for the same photon path within the gain medium, the losses are larger than for OFASE and the emission enhancement threshold should be higher than for OFASE. Finally, it should be noted that if the scatterers have some resonance for a particular wavelength (such as a Mie resonance if the scatterer diameter is typically larger than A) and if the scatterer size distribution is very narrow, one should be able to observe enhanced emission at different locations of the luminescence spectrum (since such resonance implies increase in the scattering cross-section).

4. Mapping technique

In order to gain better understanding of the physical mechanisms involved and to build a quantitative model of the observed phenomena, we have scanned the entire parameter space: wavelength, pump power, dye concentration and fitanate concentration, which allowed us to visualize the dependence of any pa- rameters on the others. In particular we have dressed 'maps' of the emitted light as a function of both the dye and the scatterers concentration.

4.1. Description and experimental procedure

In order to do so, we adopted the following procedure: for each pump power, a full emission spectrum was taken using the fiber spectrometer. The pump power was increased in regular mannerusing fixed positions of the half-wave plate. The pump power was checked for long term stability using the photodetector P1 (Fig. 1). For each combination of dye concentration and scatterers density about 15 spectra were taken, each at different pump powers. Then the dye concentration and scatterer density

t o .

39.37 - - 45.00 33,75 -- 39.37

125 .13 *- 33.75 122 .50 -- 28,13

16,87 - 22.50

t 11 .25 -- 16.57 ~ 5,625 - 11.25

o -* 5.625

2.0 -2.5 -3,0 -3.5 .4,0 -4.5 -5.0 - 55

log(Dye)

Fig. 13. Map (dye concentration/scatterer density) of emission peak at low power.

were decreased by a fixed ratio by diluting the solution with methanol (this ratio being thus the same for the dye and the scatterers). This allowed, from a given sample combination, to derive a series of diluted samples where the ratio of the dye concentration to scatterer density was the same (diagonal line in a dye concentration/scatterer density graph). In Fig. 13 we show such a map for a low pump fluence (0.3 mJ/cm2). This map shows the peak emission intensity as a function of dye concentration and particle density. A maximum of emission can be seen for a scatterers density of roughly 1011 scatter- e rs /cm 3 and a dye concentration of 10 -2 M. At a much higher power (100 m J / cm 2) two additional peaks appear at the same dye concentration ( ~ 10 -3

M): one at low scatterers density and the other at very high scatterers density (1012 c m -3 ) (Fig. 14).

PUlI L. --2 ...... 1 2250 - 2625 ~ l~1875 - 2250

1500 *- 1875 ~ l~1125 -- 1500 ~J~750 .0 -- 1 t25 ~ . , ~ 375.0 - 7500

O 10 ~ , ~ O 375 O

' i , i , i . I

-2.0 -2.5 -3,0 35 - 40 =45 -5.0 *5.5

log(Dye)

Fig. 14. Map (dye concentration/scatterer density) of emission peak at high power.

B.G. Sfez, Z. Kotler / Optical Materials 8 (1997) 1-9 9

4.2. Results and physical insights

This mapping of the dye/scatterer concentrations shows different emission enhancement 'islands', probably originating from different physical mechanisms. On Fig. 13 (low power), only one island appears, with a relatively low enhancement factor compared to pure dye luminescence ( ~ factor 4). However, at high pump intensities, two others islands appear, one with a high enhancement factor ( ~ 20) and the other with a still high er enhancement factor ( ~ 300) (Fig. 14). These two new islands occur for the same dye concentration and probably correspond to two different 'cavity' geometries. One corresponds to a long cavity (low scatterers concentration) and the other to a small cavity (high scatterers concentration). In the case of the small cavity, although the losses are very high due to the large number of scattering events, some additional enhancement maybe due to coherent processes at the cavity length scale: the probability that the photon makes closed loops is increased. In the case of the long cavity, the effect may be due to simple redirected ASE, since the scatterer density allows only single or a very few number of scattering events.

5. Conclusion

In conclusion, we have shown the existence of local peaks in the dye concentration/scatterers concentration plan for which strong emission enhancement could be observed. Enhancements as high as 300 have been observed for optimum parameter sets. We have introduced a mapping technique which shows that several such peaks were in fact coexisting for completely different values of the dye and scatterer concentrations, inferring that different mechanisms might be involved. A precise description of the observed effects should take into account these different mechanisms. We have suggested that solutions of dye and monodispersed large scatterers (having Mie resonances), could introduce locking of the emission at specific wavelengths due to enhanced scattering cross-section of the scatterers at discrete wavelengths. This could be very useful for spectral

bar-code since the emission wavelength of the medium could be determined by the scatterers properties and not by the dye characteristics 2

Acknowledgements

We thank Mr. H. Dekoster from DuPont White Pigments (Belgium) for having kindly provided us with part of the titanate powders. We also acknowl- edge S. Feigel and M. Rosenbluh for stimulating discussions.

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[10] R.H. Dicke, Phys. Rev. 93 (1954) 99. [11] R. Bonifacio, L.A. Lugiato, Phys. Rev. A 11 (1975) 1507. [12] F. Auzel, S. Hubert, D. Meichenin, Europhys. Lett. 7 (5)

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[16] M.A. Noginov, H.J. Caulfield, N.E. Noginiva, P. Vankateswarlu, Proceedings of the IEEE Lasers and Electro- optics Society, Annual Meeting, vol. 2, 1994.

2 Dyes usually show long term degradation, which is translated in this case by a shift of the emission wavelength.

lasing action from random media with gain

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