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Optics Communications 277 (2007) 138–142

Enhanced laser performances based on energy transferin multi-dyes co-doped solid media

Yu Yang, Guannan Lin, Juan Zou, Zhiyu Wang *, Minquan Wang, Guodong Qian

Department of Materials Science and Engineering, State Key Laboratory of Silicon Materials, Zhejiang University, Hangzhou 310027, PR China

Received 4 April 2007; received in revised form 29 April 2007; accepted 30 April 2007

Abstract

Various coumarin dyes are co-doped with perylene red (P-red) and pyrromethene 567 (PM567) into vinyltriethoxysilane-derived solidmedia, respectively. Energy transfer among laser dyes has been observed, and the effect of coumarin dye concentration on the laser prop-erties has been investigated. With the presence of coumarin dye and pyrromethene 567, enhanced laser performances based on energytransfer of perylene red have been exhibited. The laser efficiency can be improved by two-fold and broad tunable range as wide as80 nm can be achieved. At the pump intensity of 1.0 J/cm2, the laser output of co-doping perylene red decreases less than 30% after30,000 pulses.� 2007 Elsevier B.V. All rights reserved.

PACS: 42.55.Mv; 78.20.�e; 78.55.�m

Keywords: Energy transfer; Laser dye; ORMOSILs

1. Introduction

Since the mid 1980s, intensive research efforts have beendevoted to the solidification of organic laser dyes, whichmay bring benefits such as non-toxic and maintenance freeas compared with liquid dye lasers [1–9]. Up to now, greatprogresses have been made in this research field and themajor obstacles which had prevented the practical use ofsolid-state dye laser (SSDL) materials i.e., the laser perfor-mances especially the photostability, have been partiallyresolved [7–13]. However, the spectrum coverage by thetunable SSDLs output, at least from the green to the red,is still a problem. With a specific laser dye, the narrow line-width laser output can only be tunable in a range of lessthan 50 nm [3–6]. The replacement of a SSDL mediumby another one emitting in different spectrum range andrealignment may be inconvenient. It should also be noted

0030-4018/$ - see front matter � 2007 Elsevier B.V. All rights reserved.

doi:10.1016/j.optcom.2007.04.055

* Corresponding author. Tel.: +86 571 87952334; fax: +86 57187951234.

E-mail address: wangzhiyu@zju.edu.cn (Z. Wang).

that there still exist some spectra regions in which theenergy conversion efficiencies were low.

In this concern, the use of dye mixtures as dopants inSSDL materials has generated much research interest inrecent years. It have been found previously that controllingthe interactions between various dyes co-doped in solidhosts, i.e., the energy transfer between dyes, improvementin laser efficiency, laser lifetime and tunable range may berealized [14,15]. Nhung et al. and Sheridan et al. hadobserved energy transfer from RB or P-orange to P-redin ORMOSILs or in organic thin films, respectively[16,17]. Ahmad et al. had obtained the normalized photo-stability as high as 350 GJ/cm2 by co-doping PM567 withC540A into PMMA, which increased the photostabilityof PM567 by five times [7]. The interaction between dyesmay also bring negative impact on the performance ofSSDL materials. Sisk et al. found that P546 would acceler-ate the photodegradation rate of P597 and P567 [18].Recently, M. Alvarez et al. realized the narrow linewidthlaser emission continuously tunable over a range of85 nm by using a dye mixture of PM567 and DCM in

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C540A+PM567+P-red

C540A+PM567

P-red

Fig. 1. Fluorescence spectra of P-red doped in VTES-derived ORMOSILsin the absence and presence of C540A and PM567 (solid:C540A + PM567 + P-red(2.5:2:1); dash: P-red; dot: C540A + PM567(2.5:2)).

Y. Yang et al. / Optics Communications 277 (2007) 138–142 139

liquid solution. But, the attempt of extending tuning rangeto the red failed when doping the dye mixture into poly-meric hosts, resulting in a tunable range of only 35 nm[19]. In our pervious studies, energy transfers from couma-rin dyes to pyrromethene and perylene family dyes andfrom PM567 to P-red have been observed [14,15]. It is sug-gested that by deliberate design of dye pair combination,the photophysical and photochemical process of dye mole-cules can be controlled and a series of efficient, stable andwide tunable SSDL materials is expected.

In this work, PM567 and various coumarin dyes includ-ing C440, C460, C503 and C540A are adopted as donors toco-dope with P-red acting as the acceptor into solid-statematrices by sol–gel process. The laser efficiency, photosta-bility and tunable properties of P-red in the presence ofvarious coumarin dyes and their dependence on the couma-rin dye concentration are investigated.

2. Experimental details

The laser dyes are purchased from Exciton and used asreceived. The laser dyes, PM567 and P-red are co-doped intoVTES-derived ORMOSILs with various concentrations ofC440, C460, C503 and C540A, respectively. The initialconcentrations of PM567 and P-red in all the samples arekept constant at 1.0 · 10�4 mol/l and 5.0 · 10�5 mol/l,respectively, whereas, the initial concentration of coumarindyes varied. The initial concentration of coumarin dyes isgiven the values 1.25 · 10�4 mol/l, 2.5 · 10�4 mol/l and3.75 · 10�4 mol/l, corresponding to the molar ratiosbetween coumarin, PM567 and P-red of 2.5:2:1, 5:2:1 and7.5:2:1, respectively. These samples are labeled as couma-rin:PM567:P-red (2.5:2:1), coumarin:PM567:P-red (5:2:1)and coumarin:PM567:P-red (7.5:2:1), respectively. Prepara-tion details of dye doped VTES-derived ORMOSILs can befound elsewhere [6]. The ORMOSIL could be machine orhand polished. The net thickness of all samples is kept nearlythe same in this work, for about 4.5 mm.

The photoluminescence (PL) spectra are taken by a Hit-achi 850 fluorescence spectrometer. The scan speed is60 nm/min and the slit width is 1.0 nm. The laser perfor-mances such as slope efficiency and tunable range of thedye doped ORMOSILs are investigated with the longitudi-nally pumped laser cavities which had been reported previ-ously [6]. The second harmonic output from a Q-switchedNd:YAG laser with pulse width of 3–5 ns (FWHM) is usedas pump source. In details, a longitudinal plano–plano con-figuration is adopted, consisting of a dichroic mirror withhigh transmission at 532 nm and high reflectance from570 to 650 nm, and an output coupler with 60–70% trans-mission at the laser wavelength. The cavity length is 3.5 cmduring the laser efficiency and lifetime measurement. Byintroducing an 1800 lines/mm holographic grating asintra-cavity dispersive element, tunable narrow linewidthlaser output can be achieved. In such cases, the cavitylength is about 12 cm. Tuning is achieved by rotating theoutput coupler, which feedback a specific wavelength for

laser oscillation. The laser emission spectra are taken bya SP 750 monochromator followed by a CCD as detector.

3. Results and discussion

In Fig. 1, the fluorescence spectra of P-red doped inVTES-derived ORMOSILs in the absence and presenceof C540A and PM567 together with the fluorescence spec-trum of C540A and PM567 co-doped in VTES-derivedORMOSILs are shown, from which energy transfer fromdonor dyes to P-red can be ascertained. Compared withthat of C540A and PM567 co-doped sample, the fluores-cence intensity of the 3-laser-dyes co-doped in the blue sidearound 547.0 nm, which can be attributed to the fluores-cence from PM567, is quenched sharply. While comparedwith those of P-red solely doped, the peak fluorescenceintensity of P-red in the presence of donor dyes is increasedby 60% and the peak wavelength blue shifts from 612.0 to605.0 nm, which are the typical indications of energy trans-fer process. Similar fluorescence enhancement and blueshift of peak wavelength have also been observed whenco-doping C440, PM567 and P-red into ORMOSILs inour previous report [14,15].

The main purpose of this work is to develop a SSDLmaterial with higher laser efficiency, photostability andextended tunable range. So, the laser slope efficiency ofP-red doped into VTES-derived ORMOSILs in the absenceand presence of PM567 and various kinds of coumarindyes and their dependence on the coumarin dye concentra-tion are investigated, and the results are presented in Table1. Higher laser efficiencies of P-red in the presence of donordyes were observed in all cases as compared with that of P-red solely doped. At optimized concentration, the slopeefficiencies of P-red in the presence of PM567 and C440or C540A are improved by one-fold, from 10.4% to

Table 1Laser performances of P-red doped in VTES-derived ORMOSILs in theabsence and presence of PM567 and coumarin dyes

Dye pair(molar ratio)

Slopeefficiency(%)

Laserthreshold(lJ)

Tunablerange (nm)

Peakconversionefficiency (%)

C440:P567:P-red (2.5:2:1)

21.0 95.3 570.7–632.0 2.4

C440:P567:P-red (5:2:1)

15.4 115.7 573.8–642.8 1.5

C440:P567:P-red (7.5:2:1)

11.8 147.6

C460:P567:P-red (2.5:2:1)

17.6 114.2 582.0–639.2 1.4

C460:P567:P-red (5:2:1)

12.5 146.8 579.8–634.6 0.8

C503:P567:P-red (2.5:2:1)

17.6 105.4 575.8–637.2 2.4

C503:P567:P-red (5:2:1)

18.8 126.4 574.3–640.4 1.9

C503:P567:P-red (7.5:2:1)

16.4 121.3 –

C540A:P567:P-red (2.5:2:1)

21.0 100.6 562.3–642.4 2.4

C540A:P567:P-red (5:2:1)

16.0 110.3 577.6–647.6 2.1

C540A:P567:P-red (7.5:2:1)

16.4 124.7 573.1–638.4 1.4

P567:P-red(2:1)

14.4 149.6 578.4–615.5 0.7

P-red 10.4 103.5 599.5–613.3 <0.2

140 Y. Yang et al. / Optics Communications 277 (2007) 138–142

21.0%, as shown in Fig. 2. The reason responsible for thelaser efficiency enhancement of P-red in the presence ofPM567 and coumarin dyes can be ascribed to the energytransfer mechanism revealed previously [14]. In short,under the excitation of 532 nm laser, the excited singlet-state PM567 can transfer its energy to ground-state P-red

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Fig. 2. Laser efficiencies of P-red doped in VTES-derived ORMOSILs inthe absence and presence of PM567 and coumarin dyes. (square:C440 + PM567 + P-red(2.5:2:1); square hollow: C460 + PM567 +P-red(2.5:2:1); up triangle: C503 + PM567 + P-red(5:2:1); down triangle:C540A + PM567 + P-red(2.5:2:1); up triangle hollow: PM567 +P-red(2:1); down triangle hollow: P-red).

due to the overlapping of the absorption and fluorescenceof these two dyes and on the other hand, the triplet-stateof PM567 and/or P-red can be quenched by coumarin dyes,thereby the excited coumarin dye molecules transfer energyto the ground-state PM567 and P-red dye molecules, result-ing in the increased fluorescence intensity and laser effi-ciency of P-red. The variation of enhancement on thelaser efficiency of P-red by various coumarin dyes can beascribed to the energy level compatibility between laserdyes, i.e., the energy level compatibility between triplet-state P-red/PM567 and ground-state coumarin dyestogether with energy level compatibility between excitedstate coumarin dyes and ground-state P-red/PM567. Itshould also be noted that beyond this coumarin concentra-tion level, the laser efficiencies of P-red decrease which maybe related to the distance between dye molecules controlledby dye concentration. On the other hand, it should also benoted that too high dopant concentration may lead to thedeterioration of the optical quality of the ORMOSILs dueto the possible hindering of dopant molecules to polymer-ization process, resulting in larger optical loss and lowertransmittance of host matrix. In other words, increasingthe coumarin concentration up to a particular value willnot in any way enhance the laser efficiency of P-red.

During the laser efficiency measurement, the laser out-put profiles of all samples are also obtained and those ofP-red doped in VTES-derived ORMOSILs in the absenceand presence of C540A and PM567 are shown in Fig. 3.Compared with that of P-red solely doped, the laser outputwavelength of P-red in the presence of donor dyes blueshifts from 615.8 to 608.4 nm. Another interesting phenom-enon which can evidence the above energy transfer mecha-nism is shown in the inset of Fig. 3. Fluorescenceattributing to C440 is observed when the C440, PM567and P-red co-doped sample is excited by 532 nm laser, indi-cating the involvement of excited singlet-state coumarin

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P-red C540A+PM567+P-red

Fig. 3. Laser output profiles of P-red doped in VTES-derived ORMOSILsin the absence and presence of C540A and PM567 (solid:C540A + PM567 + P-red(2.5:2:1); dot: P-red; inset: laser-induced fluores-cence of C440 + PM567 + P-red(2.5:2:1) excited by 532 nm laser).

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Fig. 4. Laser tunable ranges of P-red doped in VTES-derived ORMOSILsin the absence and presence of C540A and PM567. (square:C540A + PM567 + P-red(2.5:2:1); up triangle: C540A + PM567 + P-red(5:2:1); down triangle: C540A + PM567 + P-red(7.5:2:1); circle: P-red).

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Pump Pulses

C540A+PM567+P-red

P-red

Fig. 5. Laser output of P-red doped in VTES-derived ORMOSILs in theabsence and presence of C540A and PM567 as a function of pump pulses.(triangle: P-red; square: C540A + PM567 + P-red(2.5:2:1)).

Y. Yang et al. / Optics Communications 277 (2007) 138–142 141

dye molecules during the energy transfer process becausethere’s no absorption for C440 dye molecule at 532 nm.By the way, the fluorescence attributing the C540A is notachieved from the C540A, PM567 and P-red co-dopedsample under the excitation of 532 nm laser in this workdue to interference of the 532 nm pump beam which is toclose to the fluorescence of C540A peaked at about524 nm.

By introducing a holographic grating into the cavity as adispersive element, the tunable properties of P-red dopedinto VTES-derived ORMOSILs in the absence and pres-ence of donor dyes have also been studied, as shown inFig. 4 and Table 1. When solely doped, the tunable rangeof P-red is somewhat narrow, from 599.5 to 613.3 nm, withconversion efficiency lower than 0.2%. In the presence ofPM567, the tunable range of P-red is broadened in the blueside and the energy conversion efficiency increases by afactor of 3. In cases that both PM567 and coumarin dyesare adopted as donor dyes, the tunable ranges of P-redare broaden sharply, above 60 nm in most cases. Thebest result is obtained by the presence of C540A andPM567. At the optimized coumarin concentration, i.e.,1.25 · 10�4 mol/l for C540A, the tunable range from562.3 to 642.4 nm, more than 80 nm, is achieved with peakconversion efficiency as high as 2.4%. To our knowledge,this is the widest tunable range ever been reported by SSDLmaterials in this spectrum region from green to red.

From the above results, it is found that the dye paircombination labeled as C540A:PM567:P-red (2.5:2:1) ishighly efficient and widely tunable. So, the influence ofmulti-dyes co-doping on the laser lifetime is investigated.As shown in Fig. 5, the laser output of P-red in the absenceand presence of PM567 and C540A are compared at apump intensity of 1.0 J/cm2, i.e., 2.0 mJ per pulse and therepetition rate of 2 Hz. The pump intensity is relativelyhigh in order to reduce testing time. When solely doped,

the output laser energy of P-red decreases steadily duringthe pump process. After about 24,000 pulses of pumping,the output of P-red decreases to 50% of its initial value,corresponding to a normalized photostability of355.8 GJ/mol. In the presence of PM567 and C540A,nearly no reduction in the output energy of P-red isobserved during the first several thousand pulses. After30,000 pulses of pumping, the laser output of P-red is stillabove 70% of its initial value. It is obvious that the photo-stability of P-red is largely improved by the presence ofPM567 and C540A. It is suggested that the photostabilityenhancement of P-red upon the addition of C540A andPM567 may be partially attributed to the lower laser flu-ence due to the absorption of laser light by the PM567 mol-ecule, which corresponds to an optical density of about 9.4at 532 nm as taking account that the sample is about4.5 mm thick. It has also been reported previously byAhmad et al. that by co-doping PM567 with C540A intoPMMA, normalized photostability of PM567as high as350 GJ/cm2 has been obtained [7]. In our research, highestlaser efficiency and widest tunable range of P-red areachieved with the dye pair combination labeled asC540A:PM567:P-red (2.5:2:1) though the coumarin dyeconcentration is the lowest level chosen. Based on this find-ing, it is suggested that by improving the donor dye combi-nation design, the photostability of P-red may be enhancedfurther.

4. Conclusions

In conclusion, dye pair combinations consisting of cou-marin dyes, PM567 and P-red with coumarin dyes andPM567 as donor dyes and P-red as acceptor are employedas dopants to incorporate into solid-state host materials viasol–gel process. High laser efficiency, wide tunable rangefrom green to red and high photostability are achieved

142 Y. Yang et al. / Optics Communications 277 (2007) 138–142

by these multi-dyes doped SSDL materials based on energytransfer, which is suggested to be very promising to practi-cal use.

Acknowledgements

This work is supported by the National Natural ScienceFoundation of China (Nos. 50532030 and 50625206) andChina Postdoctoral Science Foundation (No. 20060390330).

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