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OpticsOptikOptikOptik 121 (2010) 11–18

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doi:10.1016/j.ijl

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Spectroscopic and lasing properties of Xanthene dyes encapsulated in silica

and polymeric matrices

Sunita Sharmaa,�, Devendra Mohana, Nageshwar Singhb,Meenakshi Sharmaa, A.K. Sharmac

aDepartment of Applied Physics, Guru Jambheshwar University of Science and Technology, Hisar 125001, Haryana, IndiabDepartment of Atomic Energy, Raja Ramanna Center of Advanced Technology, Indore, IndiacDepartment of Physics, Maharshi Dayanand University, Rohtak, Haryana, India

Received 30 November 2007; accepted 15 May 2008

Abstract

The photo-physical properties of Xanthene dyes: Basic Rhodamine Yellow (BRY), Rhodamine590perchlorate(R590p), SulforhodamineB (SRB) doped in tetramethylorthosilicate (TMOS) and poly-methylmethacrylate (PMMA)are observed. The various parameters viz. full-width at half-maxima (FWHM), peak emission wavelength, quantumyield and excited state lifetime at different concentrations ranging from �0.05 to �1mM of the dye under excitation byCopper Vapor Laser (CVL) of high repetition rate (�5.6 kHz) of are investigated. In order to identify photostability indyes, normalized photostability has been studied and found that silica gel samples containing dye are more stable thanthat of polymeric samples. This has been further understood in terms of number density of unbleached dye moleculesthat infers that photobleaching of dye molecules is not prominent at higher concentrations in glassy solid matrices.Pump intensity dependent optical gain of the samples has also been reported and efforts have been made to study theefficiency of solid-state laser samples in a cavity for the performance of the dye laser.r 2008 Elsevier GmbH. All rights reserved.

PACS: 42.60.Da; 42.70.Ce; 78.66.Jg; 78.70.En

Keywords: Silica matrices; Fluorescence; Photostability; Copper vapor laser; Solid-state dye laser

1. Introduction

Solid-state dye lasers are inherently free from defectslike flammable, toxic solvents, bulky volume and manymore problems imposed by liquid solvents [1,2]. Solid-statedye laser is expected to be compact, versatile, inexpensive,nontoxic and having many more advantages [3,4]. The

e front matter r 2008 Elsevier GmbH. All rights reserved.

eo.2008.05.005

ing author. Tel.: +01662 263176 (Off);

6240.

ess: sunphotonics@gmail.com (S. Sharma).

photo-physical properties of laser grade dyes doped indifferent solid host matrices are studied in past by variousresearch groups [5–7]. However, the aggregate formationof the dye molecules at higher concentrations is to beminimized by restricting their mobility by entrapping themin solid matrices as is observed in case of liquids.Unfortunately, problem arises because of photo degrada-tion of the dye molecules and therefore photostable dyesare proposed to be used for the better performance of thedye lasers. The molecules are required to be put into solid-state hosts of high optical quality and low thermal

ARTICLE IN PRESSS. Sharma et al. / Optik 121 (2010) 11–1812

conductivity to maintain good lasing performance [8]. Theability to dope controlled amounts of lasing species intosilica hosts using sol gel technique affords the possibility ofdeveloping a new generation of advanced tunable solid-state lasers. Recently, there has been a considerableinterest for the generation of high repetition rate solid-state laser operating at a repetition rate of few kHz with abroad band output [2,9]. The advantage is that the numberof pulses required is less to get the same output if highrepetition is used in comparison to the low repetition rate.Xanthenes are well known for their high efficiencies andhigh quantum yields [10–12]. Therefore, during the presentcourse of investigations fluorescence studies of the silicaand polymeric samples doped with these dyes of variousconcentrations are reported using high repetition rate CVL(�5.6kHz), as most of the other research groups [8,12,13]are using Nd:YAG laser of repetition rate p10Hz. Theresults are satisfactory though thermal effects may getenhanced. The future communication would consider thethermal lensing effect arising because of the use of highrepetition rate of the pump source. Further, pumpintensity dependent gain coefficient, which is a measureof optical gain is measured by using optical gain spectro-scopy technique. The details of the technique are presentedin earlier communications of Mohan et al. [14].

Fig. 1. Experimental set up in transverse configuration for

fluorescence studies.

2. Experimental

2.1. Sample preparation

The dye molecules are embedded in silica andpolymeric matrices by the low-temperature sol–gel andpolymerization techniques, respectively.

2.1.1. Silica samples

The preparation of samples by sol–gel technique ispresented by various authors [15–18]. The low-temperaturesol–gel technique is based on hydrolysis and polyconden-sation reactions of inorganic compounds. During thepresent work, the inorganic compound used is tetramethy-lorthosilicate (TMOS).

TMOS and methanol obtained from Sigma Aldrich

and dyes from Lambda Physik, USA are used as suchwithout any further purification. The chemicals aretaken in the ratio as: [TMOS+(dye +MtOH)]: [diluteHCl]H10:03

At first, the mixture of TMOS and methanolic dye wasstirred for around 30min. Then dilute HCl (catalyst) wasadded drop wise and the mixture was stirred again for3 h. Then silica gel so prepared was transferred to glasscuvette for aging at room temperature for around a week.The silica samples typically measure 15� 6� 6mm3 andvisually appear to have a good surface finish, with the endfaces seeming to be plane parallel. Many crack free

samples of lengths �10–15mm with different concentra-tions of dye were obtained by this method.

2.1.2. Polymeric samples

Methylmethacrylate (MMA) is washed three timeswith 20% sodium chloride and 5% sodium hydroxidesolution to remove foreign inclusions till the solution isclear. A few pellets of anhydrous sodium sulfate areadded to the MMA and kept for 24h before filtration. AsRhodamine dye has limited solubility in the monomerMMA [11] hence methanol is used as a solvent. Theaddition of ethanol as a plasticizer also increases the laserdamage threshold of poly-methylmethacrylate (PMMA).One gram of benzyl peroxide per 100ml of the solution isused as an initiator for polymerization. The monomer–alcohol mixture containing the dye and the initiator is putin a quartz cuvette and kept in a constant temperaturebath maintained at 50 1C for polymerization. Necessaryprecautions like proper mixing of the dye solution inPMMA, temperature control, etc. is taken for homo-geneous distribution of the dye in polymeric matrix. Thecompletely polymerized samples having dimensions15� 6� 6mm3 (blank as well as dye impregnated ofdifferent concentrations) are removed from the waterbath after around 6 days.

2.2. Measurements

The experimental set up for the measurement offluorescence is shown in Fig. 1. CVL (wavelength 510.6and 578.2 nm, repetition rate �5.6 kHz, pulse duration60 ns) is used as the excitation source for the samples.The two CVL wavelengths (578.2 and 510.6 nm) areseparated by a dichroic mirror and only 510.6 nm isallowed to pump the dye sample. The available 40mmbeam diameter is reduced as per length of the sample byusing combination of spherical lenses. The pump poweris also reduced considerably to various levels of power infew hundreds of mW through neutral density filters.A cylindrical lens of focal length 6 cm is used to focusthe pump beam on the sample. The diagnostic equip-ment in this study is a compact hand-held plug andplay spectrometer (Ocean optics, Inc., Model USB2000).

ARTICLE IN PRESSS. Sharma et al. / Optik 121 (2010) 11–18 13

The band width of the instrument is 0.3 nm. Further, theoptical gain is recorded as per the procedure alreadymentioned in our earlier communications [14] and thelasing characteristics are studied by putting the samplesinside a laser cavity. The cavity consists of an outputcoupler mirror (20% reflectivity), lasing material havingdimension: 15� 6� 6mm3, a single prism beam expan-der (magnification �8X) and a grating (2400 linesmm–1)in grazing incidence with a tuning mirror. The overallcavity length was 16 cm.

20

40

60

80

100

120

0

Tran

smitt

ance

1

2

Wavenumber (cm-1)4000350030002500200015001000500

Fig. 2. Diffused reflectance (DR) spectra of BRY-doped silica

(curve 1) and polymeric (curve 2) matrices.

3. Results and discussion

3.1. Absorption cross-section and effective

absorption coefficient

The fluorescence spectra can be understood as a resultof the contribution of cross-sections for the molecularabsorption and/or emission. This can be defined byBeer’s law as in case of absorption of pump light fromthe ground state is

IðZÞ ¼ Ið0Þ expð�N0saZÞ (1)

where I (0) is the pump intensity, I (Z) is the maximumfluorescence intensity of the dye-doped samples excitedby 100mW average power of CVL, N0 is the concentra-tion of dye molecules in ground state, Z is the thicknessof the sample and sa is the absorption cross-section.From Eq. (1):

sa ¼�N0

ZðlogðIðZÞÞ � logðIð0ÞÞÞ (2)

For the four-level dye molecule system as consideredby Alok Sharan et al. [19] after neglecting the formationof triplet states, the variation of effective absorptioncoefficient (a1) with different input intensities of laser at�10�4M concentration of dye is estimated as

a1 ¼a0IðZÞ

Ið0Þ(3)

where a0 is the absorption coefficient from the groundstate to the excited state.

3.2. Photostability

The number density of unbleached dye molecules isgiven by [10]

N2 ¼N1

1þ IðSÞ=Ið0Þ(4)

Under steady-state conditions of excitation I(0)5I(S)where

IðSÞ ¼hc

lsat(5)

Here, h is Planck’s constant, l being the wavelength ofpump laser; c is the velocity of light in vacuum and t isthe excited state relaxation lifetime. Now Eq. (4) can bewritten as

N2

N1¼

Ið0Þ

IðSÞ(6)

where N2 is the number density of unbleached dyemolecules and N1 is the number density of excited statedye molecules.

3.3. Diffused reflectance spectra

The diffused reflectance (DR) spectra of BasicRhodamine Yellow (BRY) (1mM) doped in TMOSand PMMA is shown in Fig. 2. The observed band inthe range 3255–3503 cm�1 is assigned to OH stretchingvibrations of the Si–OH group and strong Si–O bandsare associated with the peak at 928 cm�1. Some portionof the wide band is due to aromatic stretch. The peaklocated at 790 cm�1 is associated with symmetricSi–O–Si stretching. It confirms the formation of silicamatrices. A peak is observed at 1684 cm�1, whichcorresponds to b-keto ester present in the BRYstructure. The ‘‘C–O stretching vibrations’’ of estersactually consist of two asymmetrical coupled vibrations:C–C ( ¼ O)–O and OC–C, the former being moreimportant. The peak at 1105 cm�1 is due to thesevibrations. Observed peak at 1634 cm�1 is causedprimarily due to N–H bend. The peak at 2955 cm�1

corresponds to C–H stretch in alkanes. The presence ofester group in PMMA is confirmed by the band at1729 cm�1. The peak at 1442 cm�1 corresponds to –CH3

(bend). The peaks at 973, 845, 748 cm�1 give informa-tion about ¼ C–H out of plane bending in alkenespresent in PMMA. DR spectra of all Rhodamine590-perchlorate (R590p) and SulforhodamineB (SRB) arealso studied and the data confirm the successfulinclusion of the dye molecules in the host matrices.

ARTICLE IN PRESS

0

500

1000

1500

2000

2500

500Wavelength (nm)

Inte

nsity

(A.U

.)

680660640620600580560540520

Fig. 5. The fluorescence spectra of BRY doped in TMOS

(concentration 0.1mM).

520

540

560

580

600

620

640

0

Concentration (mM)

Peak

em

issi

on w

avel

engt

h (n

m)

Series1Series2Series3Series4

1.210.80.60.40.2

Fig. 6. Dependence of peak emission wavelength on dye

concentration in host matrices.

S. Sharma et al. / Optik 121 (2010) 11–1814

3.4. Concentration dependence of absorption,

fluorescence, peak emission wavelength and FWHM

The absorption spectra of various silica and polymericdoped samples are recorded on a UV–vis spectro-photometer (Varian Cary50) having band width 0.1 nmwith a percentage transmission greater than 10%throughout the range 200–1000 nm. A typical curve forBRY in silica and polymeric host is shown in Fig. 3. Thelinear variation of absorbance of the peak as a functionof concentration in silica and polymeric samples isshown in Fig. 4.

The laser-induced fluorescence spectra of dye withdifferent concentrations from 0.05 to 1mM of BRY,R590p and SRB in silica and polymeric matrix arerecorded. The result for a typical concentration of BRY(�0.1mM) is presented in Fig. 5. Similar trend influorescence spectra is observed for other concentrationsof the dye.

Fig. 6 depicts that with increase in concentration,peak emission wavelength (deduced from the fluores-cence spectra) of dye-doped sample also increases. Thevariation in peak emission wavelength ranges from 568to 577 nm in silica and from 535 to 556 nm in polymericsamples for BRY. It also varies from 560 to 583 nm for

0

0.2

0.4

0.6

0.8

1

1.2

450Wavelength (nm)

Abs

orba

nce

(A.U

.)

1.4

1.2

1

0.8

0.6

0.4

0.2

0375 425 475 525 575 625 675

Wavelength (nm)

Abs

orba

nce

(A.U

.)

700650600550500

Fig. 3. Absorption spectra of BRY in silica and PMMA

matrices.

0

1

2

3

4

5

0Concentration of dye (mM)

Abs

orba

nce

(A.U

.)

Series1Series2Series3Series4

1.210.80.60.40.2

Fig. 4. Variation of peak absorption with concentration of the

dye.

R590p and 592 to 631 nm for SRB. This is due to theoverlapping of the absorption and emission spectra, andhence the peak emission wavelengths are seen to be redshifted with increase in concentration [20]. The spectralshift is considered due to progressive increase in thepresence of aggregated dye molecules with increase indye concentration, rather than the enhancement in thelocal fields.

The full-width at half-maximum (FWHM) with thevarying concentration of dye increases as shown inFig. 7. It varies from 35 to 43 nm in silica and 37 to52 nm in polymeric samples for BRY FWHM. ForR590p its range lies between 32 and 41 nm, whereas it is34–73 nm for SRB. The increase in the peak wavelengthand FWHM with concentration of dye is in conformitywith the results of RhodamineB solution reported byBindhu et al. [21].

3.5. Concentration dependence of absorption cross-

section

The variation of absorption cross-section with con-centration of dye shows decreasing trend with increase inconcentration (Fig. 8). It is due to the aggregation of dyemolecules at high concentrations, resulting in decreasein surface area and hence the absorption cross-section.The mutual interaction of dye molecules at highconcentration is responsible for the concentration-dependent absorption cross-section changes [22]. Butits inverse effect is observed in BRY-doped polymericsamples.

ARTICLE IN PRESS

30

35

40

45

50

55

0Concentration of dye (mM)

FWH

M (n

m) Series1

Series2

Series3

Series4

25

35

45

55

65

75

85

0

Concantration of dye (mM)

FWH

M (n

m) Series1

Series2

Series3

Series4

0.120.10.080.060.040.02

1.210.80.60.40.2

Fig. 7. Variation of FWHM with dye concentration at

510.6 nm

0.08

0.1

0.12

0.14

0

Concentration of dye (mM)

Abs

orpt

ion

cros

s-se

ctio

n (A

.U.)

Series1

Series2

Series3

Series4

1.210.80.60.40.2

Fig. 8. Variation of absorption cross-section with various

concentrations of dye.

0

1000

2000

3000

4000

5000

0Pump intensity ( A.U.)

Effe

ctiv

e ab

sorp

tion

co-e

ffici

ent (

A.U

.)

Series1

Series2

Series3

Series4

0.060.050.040.030.020.01

Fig. 9. Intensity dependence of effective absorption coefficient

of BRY at 0.1mM in silica host.

0

0.2

0.4

0.6

0.8

1.0

1.2

0 50 100 150 200 250 300 350 400 450 500 550 600 650Power of Laser (in mW)

Nor

mal

ised

Max

imum

Inte

nsity

(in

A.U

.)

y = -5.80E-7 x2 + 0.00205 x1 - 0.0191, R:0.0230, max dev:0.0347

Fig. 10. Normalized maximum intensity as a function of

pump power.

1.0165

1.017

1.0175

1.018

1.0185

1.019

1.0195

1.02

1.0205

0Concentration of dye (mM)

N2/

N1

Series1

Series2

Series3

Series4

1.210.80.60.40.2

Fig. 11. Variation of N2/N1 (N2 is the number density of

unbleached dye molecules/N1 is the number density of excited

state dye molecules) with concentration of dye.

S. Sharma et al. / Optik 121 (2010) 11–18 15

3.6. Pump intensity dependence of effective

absorption coefficient

A decreasing trend in the effective absorptioncoefficient is observed with variation in pump intensityat �10�4M concentration of dye (Fig. 9).

Fig. 10 shows the normalized maximum intensityversus pump power for �0.1mM concentration ofBRY. The curve is a slightly quadratic fit rather thanthe linear. However, at the lower pump powers the curveis considered to be linear and therefore the thresholdpump power is estimated to be �10mW.

3.7. Photostability

Photostability of a dye can be understood in terms ofthe ratio of number density of unbleached dye molecules(N2) and number density of excited state dye molecules

(N1). N2/N1 increases with concentration as studied bynumerous researchers [1,4,13,15] in different classes ofdyes under various experimental conditions. During thepresent course of investigations, the authors have madeefforts to study the photostability at various concentra-tions of dye ranging from 0.05 to 1mM in silica andpolymeric hosts (Fig. 11). The mobility and theconcentration of dye molecules are two importantfactors that decide the probability of photo degradationof the dye molecules. The mobility of the molecules isminimized by entrapping them into host matrices.As the concentration of dye molecules in silica host

ARTICLE IN PRESSS. Sharma et al. / Optik 121 (2010) 11–1816

increases, the number density of unbleached dyemolecules also increases and hence photobleaching ofdye molecules is not prominent at higher concentrations.However, the formation of aggregate at higher concen-trations may, in general, affect the degradation of thedye molecules.

The studies related to photostability, an importantparameter that is defined as the number of laser pulsesrequired to reduce the laser output intensity to 50% ofits initial value, with high repetition rate presentedduring the present course of investigations will be ofmuch interest in search of active materials for thedevelopment of solid-state dye laser systems [23].

Photostability that determines the performance of thefluorescence is determined by irradiating the optimumconcentrations of the dye-doped samples by CVL. Thefluorescence intensity as a function of the duration ofirradiation is shown in Fig. 12. The curve shows that therelative decrease of the fluorescence intensity withexposure time in BRY-doped silica samples. However,after certain time of exposure �50min, interestinglythere is subsequent increase in fluorescence, which is

2109

2113

2117

2121

2125

0Time (min.)

Inte

nsity

(A.U

.)

8070605040302010

Fig. 12. Variation of peak fluorescence intensity with irradia-

tion time of the samples.

Table 1. Various photo-physical parameters of dyes in silica and p

Dye Concentration

(mM)

Molar a

(M�1m�

BRY+TMOS 0.05 1,082,71

0.1 559,51

0.5 148,07

1 65,27

BRY+PMMA 0.05 263,22

0.1 113,40

0.5 36,35

1 161,93

R590p+TMOS 0.05 183,00

0.1 195,63

0.5 256,00

1 530,00

SRB+TMOS 0.05 6,300,00

0.1 4,260,00

0.5 238,00

1 716

termed as ‘fluorescence recovery after photobleaching ofdye molecules’. This is understood to be due topenetration of pump radiation deeper inside the sample,for finding more available dye molecules, as the surfacemolecules get bleached.

Further, the normalized photostability, the accumu-lated pump energy absorbed by the system per mole ofthe dye molecules before the output laser intensity fallsto one half of its initial value, is determined by [1]

EPN1=2

106pr2lc

where EP is the pulse energy, N1/2 is the number ofpulses to half input power, r is the radius of pump beamon the surface of the silica sample, l is the samplethickness and c is the dye concentration. From theresults it is found that the silica samples are 10 timesmore photostable than polymeric samples for BRY.

3.8. Photo-physical parameters

Various photo-phyical parameters viz: molar absorp-tivity, Quantum yield and fluorescence lifetime of dye-doped samples at different concentrations have beencalculated.

Molar absorptivity is calculated from Beer’s Law:log 1/T ¼ ecx, where e is molar absorptivity, c is theconcentration, x is the optical path and T is thetransmittance. The values of e is given in Table 1.

Further, the quantum yield is obtained by using therelationship Z ¼ A0=h, where h is the height of absorp-tion peak, A0 is the area under the emission curve andthe wavelength axis of the emission spectrum and Z is

olymeric hosts

bsorptivity1)

Quantum

yield

Fluorescence

lifetime (ns)

2 0.71 7.2

9 0.69 6.7

3 0.60 5.8

5 0.51 4.9

1 0.79 7.0

0 0.72 6.5

6 0.69 6.1

8 0.61 4.7

0 0.70 7.1

4 0.67 6.8

0 0.61 5.6

0 0.53 5.2

0 0.80 6.9

0 0.76 6.8

0 0.71 5.9

6 0.66 5.3

ARTICLE IN PRESS

Table 2. Variation of peak emission wavelength and FWHM

with dye concentration

Dye Concentration

(mM)

Peak

wavelength

(nm)

FWHM

(nm)

BRY+TMOS 0.05 568 35

0.1 571 36

0.5 600 41

1 577 43

BRY+PMMA 0.05 535 45

0.1 556 52

0.5 561 37

1 553 48

R590p+TMOS 0.05 560 37

0.1 583 41

0.5 570 32

1 564 38

SRB+TMOS 0.05 598 36

0.1 592 34

0.5 631 73

1 611 62

0

0.1

0.2

0.3

0.4

0.5

0Normalised pump power

Inte

nsit

y ga

in c

oeff

icie

nt (

cm-1

)

Series1

Series2

Series3

Series4

1.210.80.60.40.2

Fig. 13. Variation of intensity gain coefficient with concentra-

tion. Note: Series1 ¼ BRY in TMOS; Series2 ¼ BRY in

PMMA; Series3 ¼ R590p in TMOS; Series4 ¼ SRB in TMOS.

S. Sharma et al. / Optik 121 (2010) 11–18 17

the quantum yield. The fluorescence lifetime t iscalculated by using the following expression by usingStrickler–Berg Eq.:

1

t¼ 2:88� 10�9n2Z�1 �

RF ðvÞdvR

v�3F ðvÞdv�

Z�ðvÞ

vdv

where the values of e(v) molar absorptivity and quantumyield are known, n is the refractive index (1.5 for silicaand 1.49 for polymeric). F(v) is the fluorescence intensityand v is the wave number. Table 2 represents the variousphoto-physical parameters calculated. The molar ab-sorptivity decreases with increase in concentration inboth host materials. A comparative study between twodifferent hosts shows that it is more in the dye-dopedsilica matrices rather than in polymeric samples.

3.9. Optical gain and the lasing characteristics

Fig. 13 shows the variation of intensity gaincoefficient with pump intensity. The gain decreases athigher pump intensity in case of BRY-doped samples.However, it becomes saturated in case of R590p andSRB. The decrease in the values in case of BRY ismainly attributed to the excited state absorption fromfirst excited singlet state to higher states, whereas in caseof R590P and SRB, the phenomena of excited stateabsorption is expected at slightly more higher pumpintensity. Though the intensity of the laser emissiondecreases in all the cases in comparison to the dyes inliquid environment as reported in the literature

[14,21,24,25], the lasing spectral range becomes nar-rower from 10 to18 nm and 12 to 27 nm in BRY in silicaand polymeric samples and 7–16 nm in R590p and9–48 nm in SRB, when the samples are put in the cavity.

4. Conclusion

Samples doped with various concentrations of dyesranging from 0.05 to 1mM are prepared by exploitinglow-temperature sol–gel and polymerization technique.Laser-induced spectroscopic measurements are madeusing 510.6 nm wavelength of CVL. It is observed thatthe variation in peak emission wavelength varies from568 to 577 nm in silica and from 535 to 556 nm inpolymeric samples for BRY. It also varies from 560 to583 nm for R590p and 592 to 631 nm for SRB. FWHMvaries from 35 to 43 nm in silica and 37 to 52 nm inpolymeric samples for BRY FWHM. For R590p itsrange lies between 32 and 41 nm, whereas it is 34–73 nmfor SRB. There is an abrupt change in spectroscopicproperties of the sol–gel-derived Rh6G-doped glassysample of �0.05mM concentration as noticed fromFWHM, peak wavelength of the fluorescence and alsofrom the ratio of number density of unbleached dyemolecules to the number density of excited state dyemolecules (N2/N1). Therefore, the optimum concentra-tions near to 0.05mM are suitable to act as gain mediumfor the development of solid-state dye lasers. Futurestudies will be emphasized for more significant improve-ments for the wide range wavelength tuning of solid-state amplifiers and lasers by using different repetitionrate of the pumping source.

Acknowledgements

The authors are grateful to Department of Scienceand Technology, New Delhi, for providing financialassistance and Mr. Khatak from Raja Rammana Centrefor Advanced Technology, Indore, for providing help ingetting some of the results.

ARTICLE IN PRESSS. Sharma et al. / Optik 121 (2010) 11–1818

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