study of emission characteristics in laser dye mixtures encapsulated in silica gel matrices

Available online at www.sciencedirect.com

www.elsevier.com/locate/optmat

Optical Materials 30 (2008) 1273–1283

Study of emission characteristics in laser dye mixtures encapsulatedin silica gel matrices

Meenakshi Sharma a,*, D. Mohan a, R.D. Singh b, Nageshwar Singh c

a Department of Applied Physics, G. J. University of Science and Technology, Hisar 125 001, Indiab Department of Physics, Jamia Milia Islamia, New Delhi, India

c Laser Systems Engineering Division, RRCAT, Indore, India

Received 27 April 2007; received in revised form 11 June 2007; accepted 11 June 2007Available online 20 July 2007

Abstract

The fluorescence emission characteristics of two different laser grade dye combinations: Rhodamine 6G (R6G) co-doped with diso-dium fluorescien (FDS) and sulforhodamine B (SFB) co-doped with FDS in tetra-ethyl-ortho-silicate (TEOS) using sol–gel technique arestudied upon excitation under copper vapor laser (CVL). Energy transfer from FDS (donor) to R6G and SFB (acceptors) respectively,are observed by varying acceptor concentration in the range from �0.3 mM to �5 mM. The related parameters such as peak emissionwavelength, FWHM, lifetime of excited states, quantum yield and energy transfer rate constants of molecules in combination in silica gelmatrices are determined. Dye combination shows the improved fluorescence photo-stability in silica doped samples.� 2007 Elsevier B.V. All rights reserved.

Keywords: Dye laser; Energy transfer; Photo-stability; FWHM; Silica gel matrices; Copper vapor laser

1. Introduction

During the past few years, photo-physical properties oflaser grade dyes in various solid matrices are attractinggreat attention because of their numerous optical applica-tions such as guest dopants in organic LEDs, for energytransfer experiments, development of sensors and tunablesolid state lasers [1–3]. It is reported in the literature thatthe type of host determines the characteristic of laser per-formance [4–6]. Incorporating laser dye molecules in solidmatrices eliminates many of the problems, which are facedin designing liquid dye lasers [7–9]. There are many impor-tant factors that prove to be preferable for sol–gel tech-nique; such as the technique is low cost and capable ofproducing uniform glasses of high purity at low processingtemperature which allows the inclusion of fluorescent dyes

0925-3467/$ - see front matter � 2007 Elsevier B.V. All rights reserved.

doi:10.1016/j.optmat.2007.06.006

* Corresponding author. Tel.: +91 01662 263176; fax: +91 01662276240.

E-mail addresses: [email protected], [email protected] (M. Sharma).

and other optically active molecules, that could not with-stand the higher processing temperature of standard glassproduction; porosity is also controlled in a desired wayby using this technique [10]. Therefore, doping the mostphoto-stable fluorescent molecules in glass matrices formsan important area of research; it offers an environment tostudy the photo-physics of dye molecules as well as theirinteraction with the matrix [11,12]. Aggregation effectsare seen to reduce during gelation process and the photo-stability of the dyes is found to enhance when encapsulatedin a sol–gel derived matrix [13].

Various dye combinations are being used in solutions inorder to produce tunability in dye lasers [14–17]. In suchlasers, the acceptor dye is indirectly pumped by pumpingsource via energy transfer from the excited donor dye mol-ecules. As there are few publications related to the encap-sulation of dye combinations into solid matrices [18–20],therefore, during the present course of investigations, rho-damine 6G (R6G) co-doped with disodium fluorescien(FDS) and sulforhodamine B (SFB) co-doped with FDSin tetra-ethyl-ortho-silicate (TEOS) using sol–gel technique

mailto:[email protected]

mailto:minkawski@yahoo. com

mailto:minkawski@yahoo. com

Fig. 1. Chemical structures of various dyes (a) sulforhodamine B, (b)disodium fluorescein and (c) rhodamine 6G.

1274 M. Sharma et al. / Optical Materials 30 (2008) 1273–1283

are studied upon excitation under copper vapor laser(CVL). FDS at various concentrations ranging from�0.3 mM to �5 mM are also encapsulated in silica matri-ces for producing reference samples. The chemical struc-tures of dyes are shown in Fig. 1. There are very fewstudies seen in the literature [21–24] in bi-mixtures of dyesin sol–gel silica derived samples under copper vapor laserof high repetition rate. The present communication dis-cusses the results of various photo-physical parameterssuch as peak emission wavelength, full width at half max-ima (FWHM), excited state lifetime, quantum yield andenergy transfer rate constants estimated through steadystate emission measurements in the samples upon excita-tion with copper vapor laser (510 nm) of 6 kHz repetitionrate. Fig. 2 shows the typical absorption and fluorescencespectra of FDS, SFB and R6G in ethanolic solutions.The peak fluorescence of FDS lies at �505 nm (with a widespectral region of 430–750 nm) and R6G and SFB havingabsorption at �520 nm and �565 nm respectively, therebygiving best opportunity of spectral overlapping, appropri-ate for energy transfer from FDS as donor molecules toR6G and SFB as acceptor molecules. Further, photo-sta-bility, in both the combinations, is also estimated to helpin obtaining efficient energy transfer dye laser.

The important mechanisms involved in energy transferare [25]: Radiative energy transfer, where one type of dye

Fig. 2. (a) Absorption spectra of FDS, (b) fluorescence spectra of FDS

molecules act as donor that is optically excited by absorb-ing pump radiation and transferring the energy to anotherdye molecules (acceptor); thus pumped indirectly and acts

, (c) absorption spectra of R6G and (d) absorption spectra of SFB.

M. Sharma et al. / Optical Materials 30 (2008) 1273–1283 1275

as a laser source; Resonance energy transfer due to longrange dipole–dipole interaction [26–29]. The energy trans-fer reduces the emission intensity of excited donor (D*)and allows emission from excited acceptor (A*), which isrepresented as

Dþ hm! D�

D � þA! DþA�

A� ! Aþ Fluorescence emission

2. Experimental

2.1. Sample preparation

The laser grade organic dyes FDS, R6G and SFB areobtained from Ms Exciton Inc. Ohio, USA, and used with-out further purification. The precursor, TEOS is procuredfrom Sigma chemicals and found to be transparent andnon-fluorescent in the range of excitation and fluorescenceemission. The various concentrations of the dyes alone andin bi-mixture combinations are incorporated in TEOS byusing sol–gel method [30].

Silica glass samples are prepared by hydrolysis of TEOSwith water in a common solvent ethanol and followed bysubsequent ploy-condensation. The dye is first dissolvedin ethanol (5 ml), and then mixed with TEOS (5 ml),HCL (0.3 ml) and formamide (1 ml). An acid catalystHCL and formamide as a drying control chemical additive(DCCA) is used to prepare monolithic silica glass samples.The chemicals are taken in the ratio as, [TEOS + EtOH +dye alone or dye bi-mixture + HCL]: [H2O]:[Formamide]::10:03:01.

The solution is then stirred for approximately 3 h forhomogenous mixing and then kept for drying and gelling.Finally, the solution is poured into polythine containerswith pinholes in their lids and kept in oven at 40–45 �Cfor drying and aging for around two weeks. During drying

CVL

λ = 510 nm

Cylindrical lens

Sample

Fig. 3. Optical set-

shrinkage of 60–80% occurs. The samples of 10 mm ·4 mm · 4 mm are obtained that can be cut and polishedinto blocks of convenient size for the laser experiments.Bi-mixture samples: FDS/R6G and FDS/SFB in whichFDS (donor) concentration is fixed at �5 mM and R6G(acceptor) and SFB (acceptor) concentration varying inrange from �0.3 mM to �5 mM are incorporated in silicamatrices. Few samples of FDS/R6G and FDS/SFB inwhich FDS (donor) of �0.3 mM and �0.5 mM concentra-tion: R6G (acceptor) and SFB (acceptor) at fixed concen-tration of �1.5 mM are also prepared.

2.2. Optical set-up

The experimental arrangement for fluorescence mea-surements is shown in Fig. 3. Copper vapor laser(k = 510 nm, 60 ns pulse duration, 6 kHz repetition rate,20–30 W average power, 47 mm beam diameter and energy34–50 lJ) is used as excitation source. The available 47 mmbeam diameter is reduced with the help of combination ofspherical lenses, so as to fall on the sample as per its length.The pump power is also considerably reduced to 215 mWusing neutral density filters. The pump beam is line focusedon to the sample, along the length, through cylindrical lensof 6 cm focal length. The diagnostic equipment used in thisstudy is a compact hand held plug and play spectrometer(Ocean Optics Inc., USB 2000). The instrumental band-width for this set-up is 0.3 nm.

3. Results and discussion

3.1. Photo-physical parameters

The absorption spectra of various silica-doped samplesare recorded on UV–Vis spectrophotometer (Varian Cary50). The spectrophotometer bandwidth is of 0.1 nm witha percentage transmission greater than 10% throughoutthe range 200–1000 nm. Fig. 4a, b and c shows the linear

Mirror

Spectromet

PC

Fluorescence

up using CVL.


variation of absorbance of the peak as a function of con-centration in silica gel samples of FDS, FDS/R6G andFDS/SFB. Molar absorpity is calculated from the slopesof these graphs, which is according to the Beer’s lawequation

Log1

T¼ Log

I0

I¼ ecx ¼ A

where A is the absorbance; e is molar absorpity; I0 is initialintensity of light; I is the intensity after absorption; c is theconcentration; x is the optical path and T is the transmit-tance. The value of molar absorpity is given in Table 1.

Further, the quantum yield is obtained by using therelationship

g1

g2

¼ A1h2

A2h1

where h1 and h2 are the heights of absorption peak; A1 andA2 are the areas under the curve and g1 and g2 are thequantum yields that refer to two dyes, respectively. PM

0

0.5

1

1.5

2

0 0.5 1 0.5 2

0 0.5 1 0.5 2

Corr.=0.9984

0

0.2

0.4

0.6

0.8 Corr.=0.9574

0

0.5

1

1.5

0 2 4

Concentration (mM)

Concentration (mM)

Concentration (mM)

Ab

sorb

ance

(ar

b. U

nit

s)A

bso

rban

ce (

arb

. Un

its)

Ab

sorb

ance

(ar

b. U

nit

s)

Corr.=0.9702

6

c

b

Fig. 4. Variation of absorbance of the peak versus concentration: (a)FDS, (b) FDS/R6G and (c) FDS/SFB.

567 in ethanol is taken as standard of reference for calcu-lating absolute quantum yield as reported by Yariv et al.[31]. The fluorescence lifetime (s) is calculated by usingthe following expression

1

s¼ 2:88� 10�9n2g�1 �

RF ðvÞdv

Rv3F ðvÞdv

�R

eðvÞvdv

where the values of molar absorpity and quantum yield areknown, n is the refractive index (�1.5); F(m) is the fluores-cence intensity and m is the wave number (m = k�1). Table 1presents the various photo-physical parameters calculatedfor bi-mixtures: FDS/R6G and FDS/SFB in which FDS(donor) concentration is fixed at �5 mM and R6G (accep-tor) and SFB (acceptor) concentration is varied in therange from �0.3 mM to �5 mM. It is observed that donorfluorescence lifetime decreases with increasing acceptorconcentrations leading to high transfer rate. The behaviorof life time of the donor at low concentration (<0.3 mM)is understood due to the effect of radiation trapping thatcauses the increase in lifetime of the excited state. Around�0.3 mM concentration, the change in life time value isvery small indicating the dominance of radiative type en-ergy transfer process in dye mixture. This shows that atlower concentration Forster type energy transfer is smalleras compared to radiative type energy transfer. There isgradual decrease in the lifetime value with increasing accep-tor concentration that is attributed due to the concentra-tion quenching or decrease of intermolecular distance,which reflects the increase in intermolecular interactions.The fluorescence quantum yield decreases with increasingconcentration thereby causes the energy trap that ulti-mately decreases the lifetime. In the previous study onthe energy transfer studies through time resolved spectraof Coumarin–Rhodamine combination [32], the behaviorof lifetime of the dye molecules is reported in a similarmanner.

3.2. Concentration dependence of peak wavelength andFWHM

The fluorescence spectra of the dye mixture (FDS/R6G)in which the FDS concentration fixed at �5 mM and R6Gconcentration varies in the range from �0.3 mM to�5 mM is shown in Fig. 5. The fluorescence intensity ofR6G increases with increase of its concentration at fixeddonor concentration because of reduction of intermolecu-lar distance between the donor and acceptor molecules.Similar phenomenon is also observed in the case of dyemixture (FDS/SFB). The variation in peak emission wave-length as a function of concentration of the dyes underinvestigation is given in Figs. 6a, b and c. Further, the var-iation of full-width at half-maxima (FWHM) of emissionas a function of concentration in different dye/host combi-nations when excited by CVL laser is presented in Figs. 7a,b and c. It implies that the FWHM increases with increasein concentration due to inner filter effects and hence shifts

Table 1Various photo-physical parameters of dye mixtures in silica glass

Dye in TEOS Concentrationa (mM) Molar absorpity (e) (M�1 cm�1) Quantum yieldb (g) Donor fluorescence lifetimec (s) (ns) 1/s (109 s�1)

FDS/R6G 5:0 2666 0.81 6.8 0.145:0.3 1333 0.75 6.0 0.165:0.5 1000 0.74 4.5 0.225:1 800 0.67 4.3 0.235:1.5 600 0.56 4.2 0.235:5 240 0.43 3.2 0.31

FDS/SFB 5:0 2666 0.95 6.8 0.145:0.3 1000 0.88 6.2 0.165:0.5 820 0.78 4.6 0.215:1 600 0.75 4.3 0.235:1.5 533 0.63 3.2 0.315:5 500 0.54 3.1 0.32

a Uncertainty in dye concentration = ±0.1 mM.b Quantum yield = ±0.05.c Donor fluorescence lifetime = ±0.05 ns.

200

250

300

350

400

450

500

550

600

650

500 550 600 650 700Wavelength (nm)

Flu

ore

scen

ce (

arb

. Un

its)

1

2

3 45

13

Fig. 5. Fluorescence spectra of FDS/R6G: (1) 5/0 mM, (2) 5/5 mM, (3) 5/1.5 mM, (4) 5/1 mM and (5) 5/0.5 mM.


to higher energies. Table 2 presents the comparative studyof peak emission wavelength and FWHM in the silica sam-ples. FDS silica samples shows the variation of �544–�552 nm in peak wavelength and �34–�69 nm in FWHMat concentration ranging from �0.3 mM to �5 mM; FDS/R6G silica samples shows the variation of �584–�626 nmin peak wavelength and �42–�64 nm in FWHM of accep-tor concentration varies from �0.3 mM to �5 mM withdonor concentration fixed at �5 mM; FDS/SFB silica sam-ples shows the variation of �559–�663 nm in peak wave-length and �34–�92 nm in FWHM of acceptorconcentration varies from �0.3 mM to �5 mM with fixeddonor concentration at �5 mM. This comparison revealedthe fact that the energy transfer process takes place satis-factory well in dye mixtures and the fluorescence emissionis from R6G and SFB, though FDS molecules are pumpedin majority by the CVL. Such behavior is observed insolutions of the dye mixtures by various research groups[33–36].

It is seen that in all the cases (including FDS samples)peak emission wavelength is red shifted with increase inacceptor dye concentration, as with the increase in concen-tration there is increase in the absorption peak, thereby redshifting the lasing wavelength through self absorption atshorter wavelengths. Similar explanation is also suggestedby Nhung et al. [23] in case of other mixtures. Fig. 6 alsosuggests that the shift in peak emission wavelength of dyemixture samples is more than FDS samples. Fig. 8a andb shows the fluorescence spectra of R6G and SFB withor without FDS excited under 510 nm of CVL. It showsthat the fluorescence is enhanced with addition of FDS,which is illustrated as: the absorption peaks of R6G andSFB dye molecules are closer to the pump radiation wave-length, because of which there is energy absorption due todirect pumping as well along with indirect pumping fromFDS dye molecules as energy transfer process. This is evi-dent from the results that acceptor molecules in dye mix-tures under reference are excited both by direct pumping

542

544

546

548

550

552

554

0 1 2 3 4 5 6Concentration (mM)



Pea

k w

avel

eng

th (

nm

)P

eak

wav

elen

gth

(n

m)

Pea

k w

avel

eng

th (

nm

)

570580590600610620630640

540560580600620640660680

c

b

Fig. 6. Dependence of peak emission wavelength versus concentration: (a)FDS concentration from �0.3 mM to �5 mM, (b) FDS/R6G concentra-tion, FDS (donor) fixed at �5 mM and R6G (acceptor) concentrationvarying from �0.3 mM to �5 mM and (c) FDS/SFB concentration, FDS(donor) fixed at �5 mM and SFB (acceptor) concentration varying from�0.3 mM to �5 mM.

0

20

40

60

80

FW

HM

(n

m)

0

20

40

60

80

FW

HM

(n

m)

FW

HM

(n

m)

0

20

40

60

80

100



0 1 2 3 4 5 6

Concentration (mM)

b

c

Fig. 7. Dependence of FWHM of emission versus concentration (a) FDSconcentration from �0.3 mM to �5 mM, (b) FDS/R6G concentration,FDS (donor) fixed at �5 mM and R6G (acceptor) concentration varyingfrom �0.3 mM to �5 mM and (c) FDS/SFB concentration, FDS (donor)fixed at �5 mM and SFB (acceptor) concentration varying from �0.3 mMto �5 mM.


at 510 nm and by re-absorption of energy by the excitationtransfer from donor molecules that absorbed the pumpphotons initially. Furthermore, Table 2 presents the resultsof tuning range and it can be deduced that the FWHM val-ues of the dye combination samples are larger and redshifted as compared to FDS dye samples alone at relativelysame concentration. This is due to the fact that at low con-centrations of the acceptor molecules, the intermoleculardistance between the donor and acceptor molecules is largeand therefore emission from the donor molecules takesplace faster than the energy transfer rate to the acceptor.At higher acceptor concentrations, there is a competitionbetween the energy transfer to acceptor and the emissionof the donor. Moreover, it is also observed that at lowerconcentration of the donor as compared to the acceptor,the fluorescence spectra is very similar in shape, but themain contribution is because of the acceptor molecules.The comparison of FWHM of FDS, FDS/R6G andFDS/SFB is almost comparable for the �1 mM fixed

concentration of the acceptor while taking �0.3 mM and�0.5 mM concentrations of the donor. This type of combi-nation where the donor concentration is less than theacceptor concentration is not of use for energy transfer.

3.3. Energy transfer rate constants

It is reported by various researchers working in pulsedlaser pumped liquid dye laser systems [37–39] that radiativeand non-radiative energy transfers are the two main typesof energy transfer processes. In the first case, the donormolecules transfer their energy to acceptor molecules afterexcitation by pump radiation and in later due to interactionbetween donor and acceptor molecules that is also knownas the long range dipole–dipole energy transfer process[40]. The rate constants for energy transfer for dye mixturesare estimated by stern–volmer plots represented by the fol-lowing equations [41]

ID

IDA

¼ 1þ KsD½A�

and

Table 2Variation of emission peak wavelength & FWHM versus concentration

Dye inTEOS

Concentrationa

(mM)Peak wavelength(nm)

FWHM(nm)

FDS 0.3 544 340.5 545 351 547 401.5 551 635 552 69

FDS/R6G 5:0.3 584 425:0.5 589 445:1 605 485:1.5 618 585:5 626 64

FDS/SFB 5:0.3 559 345:0.5 578 555:1 607 585:1.5 633 795:5 663 92

R6G 0.5 589 40SFB 1 578 49FDS/R6G 0.3:1 600 45

0.5:1 605 48FDS/SFB 0.3:1 602 55

0.5:1 607 58

a Uncertainty in dye concentration = ±0.1 mM.


QD

QDA

¼ 1þ KnrsD½A�

where ID and IDA are fluorescence intensities of donor inthe absence and presence of acceptor [A], respectively.QD and QDA are corresponding quantum yields; sD is thefluorescence lifetime of donor without acceptor; total rateconstant K (sum of radiative and non-radiative transferrate constants) and non-radiative transfer rate constantsKnr are calculated from the slopes of ID/IDA versus [A]curve and QD/QDA versus [A] curve, respectively, whichare straight lines as shown in Fig. 9a and b. The criticaltransfer radius is given by [25]

R0 ¼7:35

ð½A�1=2Þ�1=3

where [A]1/2 is half quenching concentration. Table 3 showsthe values of rate constants calculated for different dyecombinations in solid silica in which donor concentrationis fixed at �5 mM and acceptor concentration varies inthe range from �0.3 mM to �5 mM. The value of R0 isfound to be 58.33 A and 62.84 A for FDS/R6G andFDS/SFB combinations, respectively, which is in the rangeof 30–100 A as the Forster critical radii for aromatic mol-ecules [42]. The slope of ID/IDA is evaluated to be0.109 · 104 L M�1 and 0.208 · 104 L M�1 for FDS/R6Gand FDS/SFB, respectively; slope of QD/QDA is 0.36 ·104 L M�1 and 0.28 · 104 L M�1 for FDS/R6G andFDS/SFB silica sol–gel samples, respectively. From theseslopes, the values of Kr and Knr calculated are0.38 · 1012 L M�1 s�1 and 0.08 · 1012 L M�1 s�1 for FDS/R6G; 0.29 · 1012 L M�1 s�1 and 0.10 · 1012 L M�1 s�1 for

FDS/SFB silica glass samples, respectively. Therefore, theratio of radiative to non-radiative transfer rate constantsis evaluated as 4.7 and 2.9 in FDS/R6G and FDS/SFBsamples, respectively. These factors indicate that the radia-tive energy transfer is comparatively much effective thannon-radiative energy transfer process.

Further, the total transfer efficiency (F) and the non-radiative transfer efficiency (Fnr) at different concentrationsare determined by using the following equations [43]

F ¼ 1� IDA

ID

F nr ¼ 1� sDA

sD

where sDA is the fluorescence lifetime of donor in the pres-ence of an acceptor. Fig. 10a and b shows the dependenceof total transfer efficiency and non-radiative transferefficiency on acceptor concentration. The total transfer effi-ciency F can be written as sum of two parts (F = Fnr + Fr);where Fr is radiative transfer efficiency. Fig. 11 shows thevariation of Fr/Fnr with acceptor concentration. Thebehavior of dye molecules in silica matrices is not expectedto change much as the environment offer by the silica poresin solid matrices has same chemical similarity offered by theethanol in liquid dyes. The silanol group (Si–OH) of silicapore provides active citation for hydrogen bonding withother molecules such as dye molecules, solvent moleculesor water [44]. Further on increasing the dye concentration,the interaction between neighboring molecules that are ineach others vicinity increases, thereby lowering the excitedstate energy and hence reduction of the fluorescence life-times. This is true also because of high probability of radi-ative energy transfer between dye molecules. Furthermore,there are meager chances of dimmerization in silica hostand the results show that there is interaction between closepairs of dye molecules and hence rapid energy transfertakes place.

3.4. Photo-stability

Photo-stability that determines the performance of fluo-rescence emission is studied by irradiating the samples ofoptimum concentrations under excitation by CVL. Thefluorescence intensity as a function of the duration of irra-diation is shown in Fig. 12. The curve shows the relativedecrease of the fluorescence of R6G and SFB dyes as com-pared to that of co-doped with FDS. This is the expectedresult as observed from the fluorescence spectra of FDS/R6G and FDS/SFB combination discussed above. How-ever, after certain time of exposure �80 min, interestingly,there is subsequent increase in fluorescence in all the cases,which is termed as ‘fluorescence recovery after photo-bleaching’ (FRAP) of dye molecules [45]. This is under-stood to be due to the penetration of pump radiation dee-per inside the sample, for finding more available dyemolecules, as the surface molecules get bleached. More-

0

500

1000

1500

2000

2500

3000

3500

4000

4500

450 500 550 600 650 700 750

wavelength (nm)

wavelength (nm)

1

2

0

500

1000

1500

2000

2500

3000

3500

4000

4500

500 550 600 650 700

flu

ore

scen

ce (

arb

. Un

its)

flu

ore

scen

ce (

arb

. Un

its)

1

2

b

Fig. 8. (a) Fluorescence spectra of (1) FDS/R6G with concentration (0.5/5 mM) and (2) R6G (0.5 mM) doped silica glass and (b) Fluorescence spectra of(1) FDS/SFB with concentration (0.5/5 mM) and (2) SFB (0.5 mM) doped silica glass.

0

2

4

6

0 20 40 60

I D / I

DA

I D / I

DA

Corr.=0.9961

0.5

1

1.5

2

2.5

3

0 10 20 30 40 50

Acceptor concentration (10-4 M/L)Acceptor concentration (10-4 M/L)

Acceptor concentration (10-4 M/L)Acceptor concentration (10-4 M/L)

QD

/ Q

DA

QD

/ Q

DA

Corr.=0.9610

0

1

2

3

4

5

6

0 10 20 30 40 50 60

Corr.=0.9927

0

0.5

1

1.5

2

2.5

3

0 10 20 30 40 50

Corr.=0.9464

Fig. 9. ID/IDA versus acceptor concentration for dye mixtures (a) FDS/R6G, (b) FDS/ SFB; QD/QDA versus acceptor concentration for dye mixtures, (c)FDS/R6G and (d) FDS/SFB.


Table 3Energy transfer rate constants for dye mixture in silica glass at fixed donor concentration of �5 mM and varying acceptor concentration in the range from�0.3 mM to �5 mM

Dye mixture [A]1/2 R0 Slope (ID/IDA) L M�1 Slope (QD/QDA) L M�1 K L M�1 s�1 Knr L M�1 s�1 Kr L M�1 s�1

FDS/R6G 2 · 10�3 M 58.33 A 0.109 · 104 0.36 · 104 0.46 · 1012 0.08 · 1012 0.38 · 1012

FDS/SFB 1.6 · 10�3 M 62.84 A 0.208 · 104 0.28 · 104 0.39 · 1012 0.10 · 1012 0.29 · 1012

0

0.5

1

1.5

2

6040200Acceptor concentration (10-4 M/L)

6040200Acceptor concentration (10-4 M/L)

Fr/F

nr

Fr/F

nr

0

0.2

0.4

0.6

0.8

1b

Fig. 11. Dependence of acceptor concentration on Fr/Fnr for dye mixtures(a) FDS/R6G and (b) FDS/SFB.

0

0.2

0.4

0.6

0.8

0 20 40 60Acceptor concentration (10-4 M/L)

0 20 40 60Acceptor concentration (10-4 M/L)

Tra

nsf

er E

ffic

ien

cy

0

0.2

0.4

0.6

0.8

Tra

nsf

er E

ffic

ien

cy

b

Fig. 10. Dependence of acceptor concentration on total (h) and non-radiative energy transfer efficiency (D) for dye mixtures (a) FDS/R6G and(b) FDS/SFB.

3000

4000

5000

6000

7000

8000

nsi

ty (

arb

. Un

its)


over, FDS/SFB silica sample shows the maximum fluores-cence photo-stability in silica host matrices.

Further, normalized photo-stability is determined in allthe cases under CVL excitation. The normalized photo-sta-bility is defined as the accumulated pump energy absorbedby the system per mole of the dye molecules before the out-put laser intensity falls to one half of its initial value and isgiven as [46]

EpN 1=2

p� r2 � l� c� 106

0

1000

2000

0 50 100 150time (min)

inte

Fig. 12. Exposure time dependence of fluorescence intensity of: (j) FDS(5 mM)/R6G (1 mM), (h) R6G (1 mM), (–) FDS (5 mM)/SFB (1 mM)and (·) SFB (1 mM) doped silica samples.

where Ep is the pulse energy, N1/2 is the number of pulsesto half output power, r is the radius of pump beam on thesurface of silica sample, l is the sample thickness and c isthe concentration of the dye. Fig. 13a and b shows the out-put energy of the FDS/R6G, R6G, FDS/SFB and SFB as afunction of the CVL pulses. The normalized photo-stabilityfor the solid samples: FDS/R6G as 45 ± 5 GJ/M; FDS/SFB is calculated as 55 ± 5 GJ/M; R6G as 24 ± 5 GJ/Mand SFB as 30 ± 5 GJ/M. The photo-stability values ofbi-mixture dyes (FDS/R6G and FDS/SFB) are more thanthat of R6G and SFB alone. This indicates the better sta-bility of mixed dyes doped in TEOS when excited underCVL pulses. Further, in the R6G and SFB dyes alone the

energy starts decreases after �5400000 laser pulses andreaches at its half value around �70 00000 and�80 00000, respectively. However, in the presence ofFDS, the energy begin to decrease after �7200000 laserpulses and approaches to half value around �115 00000

FDS/R6G (5/1 mM)

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

0 2000000 4000000 6000000 8000000 10000000 12000000

No. of CVL pulses

No

rmal

ized

ou

tpu

t en

erg

yR6G (1mM)

00.10.20.30.40.50.60.70.80.9

1

0 2000000 4000000 6000000 8000000

No. of CVL pulses

No

rmal

ized

ou

tpu

t en

erg

y

FDS/SFB (5/1 mM)

0

0.2

0.4

0.6

0.8

1

0 2000000 4000000 6000000 8000000 10000000 12000000 14000000

No. of CVL pulses

No

rmal

ized

ou

tpu

t en

erg

y

SFB (1mM)

0

0.2

0.4

0.6

0.8

1

0 2000000 4000000 6000000 8000000

No. of CVL pulsesN

orm

aliz

ed o

utp

ut

ener

gy

Fig. 13. The output energy as a function of the number of CVL pulses (a) FDS (5 mM)/R6G (1 mM) and (b) FDS (5 mM)/SFB (1 mM).


laser pulses. It is evident that the addition of FDS leads toat least 2-fold increase in photo-stability of R6G and SFB.The measurements of photo-stability is useful to find themaximum stability of the dye molecules for the choice ofactive media for solid state dye laser systems which pos-sesses both high laser efficiency and photo-stability.

4. Conclusion

Laser grade dye FDS and combination of R6G and SFBdye molecules co-doped with FDS, in silica gel matrices

may prove to be the promising materials as solid stateactive media for tunable lasers. Energy transfer rate param-eters are discussed in these bi-mixtures by irradiating themunder 510 nm of CVL. Studies refer that the emission canbe tuned in the range from �584 nm to �626 and�559 nm to �663 nm in case of FDS/R6G and FDS/SFB combinations, respectively. The results also show thatthe FDS/SFB samples are more photo-stable than FDS/R6G samples in silica gel matrices. Further the additionof FDS dye molecules leads 2-fold increase in photo-stabil-ity of R6G and SFB.


Acknowledgements

The financial support in the form of major researchgrant from Department of Science and Technology, NewDelhi, and University Grant Commission, New Delhi, isgratefully acknowledged. One of the authors is thankfulto Mr. H. S. Vora of RRCAT, Indore, in getting someexperimental results.

References

[1] G. Hungerford, K. Suhling, J.A. Ferreira, J. Photochem. Photobiol.A 129 (1999) 71.

[2] R.T. Nedumpara, B. Paul, A. Santhi, P. Radhakrisnan, V.P.N.Nampoori, Spetrochem. A 60 (2004) 435.

[3] S. Singh, V.R. Kanetkar, G. Sridhar, V. Muthuswamy, K. Raja, J.Lumin. 101 (2003) 285.

[4] J. Livage, T. Coradin, C. Roux, J. Phys.: Condense. Matter 13 (2001)673.

[5] A.M. Weiss, E. Yariv, R. Reisfeld, Opt. Mater. 24 (2003) 31.[6] R. Reisfeld, Opt. Mater. 16 (2001) 1.[7] H.R. Aldag, S.M. Doltov, M.F. Koldunov, Y.V. Kravchenko, A.A.

Manenkov, D.P. Pacheco, A.V. Reznichenko, G.P. Roskova, in:SPIE, vol. 3924, 2000, p. 133.

[8] A. Venkateswara Rao, P.B. Wagh, D. Haranath, P.P. Risbud, S.D.Kumbhare, Ceram. Int. 25 (1999) 505.

[9] D. Lo, S.K. Lam, C. Ye, K.S. Lam, Opt. Commun. 156 (1998) 316.[10] R. Reisfeld, J. Non-Cryst. Solids 121 (1990) 254.[11] B. Dunn, J.I. Zink, J. Mater. Chem. 1 (1991) 903.[12] S. Yang, F. Meng, H. Tian, K. Chen, Euro. Poly. J. 38 (2002)

911.[13] P. Innocenzi, H. Kozaka, T. Yoko, Non-Cryst. Solids 201 (1996) 26.[14] R.D. Singh, A.K. Sharma, N.V. Unnikrishnan, D. Mohan, J. Mod.

Opt. 37 (1990) 37.[15] D. Mohan, S. Sanghi, R.D. Singh, J. Photochem. Photobio. A 68

(1992) 77.[16] E. Weiss, S. Speiser, Chem. Phys. Lett. 40 (1976) 220.[17] S. Nad, H. Pal, J. Phys. Chem. A 104 (2000) 673.[18] V.A. Gaisenok, S.V. Sergeev, J. Spectro. 54 (1991) 450.

[19] A.P. Klishchenko, I.N. Kozlov, A.I. Kozlov, J. Appl. Spectro. 72(2005) 64.

[20] T.W. Lee, O.O. Park, H.N. Cho, Y.C. Kim, Curr. Appl. Phys. 1(2001) 363.

[21] B.J. Scott, M.H. Bartt, G. Wirnsberger, G.D. Stucky, J. Phys. Chem.A 107 (2003) 5499.

[22] Y. Yang, G. Qian, D. Su, Z. Wang, M. Wang, Chem. Phys. Lett. 402(2005) 389.

[23] T.H. Nhung, M. Canva, F. Chaput, H. Goudket, G. Roger, A. Brun,D.D. Manh, N.D. Hung, J.P. Boilot, Opt. Commun. 232 (2004) 343.

[24] M. Fukuda, K. Mito, Jpn. J. Appl. Phys. 39 (2000) 3470.[25] R. Katraro, A. Ron, S. Speiser, Chem. Phys. Lett. 52 (1977) 16.[26] J.B. Birks, S. Georghious, J. Phys. B 1 (1968) 958.[27] J.B. Birks, M.S.S.C.P. Leitte, J. Phys. B 3 (1970) 513.[28] N.J. Turro, Mod. Mol. Photochem, Benjamin, New York, 1978.[29] R.B. Banger, T.S. Varadarajan, J. Lumin. 55 (1993) 49.[30] K.C. Chen, T. Tsuchiya, J.D. Mackenzie, J. Non-Cryst. Solids 81

(1986) 227.[31] E. Yariv, S. Schultheiss, T. Saraidarov, R. Reisfeld, Opt. Mater. 16

(2001) 29.[32] R.D. Singh, D. Mohan, A.K. Sharma, N.V. Unnikrishanan, Pram. J.

Phys. 34 (1990) 77.[33] M.G. Reva, A.L. Akimov, N.V. Korolkova, N.V. Kurokhtin, B.M.

Uzhinov, Sov. J. Quant. Elec. 12 (1985) 2494.[34] D. Mohan, S. Sanghi, J. Photochem. Photobio. A 116 (1998) 75.[35] R.J. Bennet, J. Chem. Phys. 21 (1964) 3037.[36] D.L. Dexter, J. Chem. Phys. 21 (1953) 836.[37] D. Avnie, Acc. Chem. Res. 28 (1995) 328.[38] R. Ghazy, S.A. Zim, M. Shaheen, F.E. Mekawey, Opt. Laser

Technol. 34 (2002) 99.[39] M. Ali, S.A. Ahmed, K. Mitwally, Appl. Opt. 28 (1989) 3709.[40] A. Imhof, M. Megens, J.J. Engelberts, D.T.N. de Lang, R. Sprik,

W.L. Vos, J. Phys. Chem. B 103 (1999) 1408.[41] N. Tanaka, W.N. Sisk, J. Photochem. Photobio. A 172 (2005) 109.[42] A.K. Sheridan, A.R. Buckely, A.M. Fox, A. Bacher, D.D.C. Bradely,

J Appl. Phys. 92 (2002) 6367.[43] L. Taneja, A.K. Sharma, R.D. Singh, Opt. Commun. 111 (1994) 463.[44] M.D. Rahn, T.A. King, Appl. Opt. 34 (1995) 8260.[45] E.V. Keuren, W. Schrof, Macromol. 36 (2003) 5002.[46] Y. Yang, M. Wang, G. Qian, Z. Wang, X. Fan, Opt. Mater. 24 (2004)

621.

study of emission characteristics in laser dye mixtures encapsulated in silica gel matrices

Documents