- Email: [email protected]
Studies on laser action from polymeric matrices based on trimethylsilyl methacrylate doped with pyrromethene 567 dye
15 January 2002
Optics Communications 201 (2002) 437–445 www.elsevier.com/locate/optcom
Studies on laser action from polymeric matrices based on trimethylsilyl methacrylate doped with pyrromethene 567 dye A. Costela a, I. Garcıa-Moreno a,*, M.L. Carrascoso b, R. Sastre b b
a Instituto de Quımica-Fısica ‘‘Rocasolano’’, CSIC, Serrano 119, 28006 Madrid, Spain Instituto de Ciencia y Technologyıa de Polımeros, CSIC, Juan de la Cierva 3, 28006 Madrid, Spain
Received 12 September 2001; received in revised form 8 November 2001; accepted 27 November 2001
Abstract The lasing properties of pyrromethene 567 (PM567) dissolved in solid poly-trimethylsilyl-methacrylate (TMSMA) cross-linked with ethylene glycol dimethacrylate (EGDMA) and copolymerized with methyl methacrylate (MMA) have been investigated. The vol/vol proportion of the different comonomers in each copolymer formulation was systematically varied, and the effect of each composition on the laser action of PM567 was evaluated. The laser samples were transversely pumped at 534 nm with 5.5 mJ/pulse from a frequency doubled Q-switched Nd:KGW laser. Lasing efficiencies of up to 14% and good stability with no sign of degradation after 10,000 pump pulses at 1 Hz in the copolymer P(TMSMA:MMA 50:50) were demonstrated. Pumping this sample at 10 Hz, the laser emission of PM567 remained at 45% of its initial value after 40,000 pulses. Ó 2002 Elsevier Science B.V. All rights reserved.
1. Introduction Over the last decade there has been a considerable interest in the development of solid-state dye lasers, where an organic dye is incorporated into an adequate solid matrix [1], as an attractive alternative to conventional liquid-state dye lasers. A solid dye laser, retaining the versatility of liquid dye lasers, allows the design of compact and selfcontained laser systems, which facilitates their utilization in industrial or medical environments.
* Corresponding author. Tel.: +34-91-561-9400; fax: +34-91564-2431. E-mail addresses: [email protected] (I. Garcıa-Moreno), [email protected] (R. Sastre).
The laser performance of organic dyes is ultimately limited by triplet–triplet absorption over their lasing spectral region. Over the late 1980s and early 1990s, Boyer and co-workers [2–5] synthesized a number of organic dyes belonging to a new class of laser dyes, namely, the pyrromethene– BF2 ðPM–BF2 Þ complexes, which exhibit reduced triplet–triplet absorption and high fluorescence quantum yields. These new dyes present laser emission over the spectral region from green/yellow to red, competing with the well-known rhodamine dyes. In particular, some of these new dyes outperform both in efficiency and tunability the widely employed rhodamine 6G and rhodamine B laser dyes in liquid solution [6]. On the other hand, the presence of aromatic amine groups in their structure renders these pyrromethene dyes
0030-4018/02/$ - see front matter Ó 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 3 0 - 4 0 1 8 ( 0 1 ) 0 1 7 2 3 - 0
438
A. Costela et al. / Optics Communications 201 (2002) 437–445
vulnerable to photochemical reactions with oxygen which makes them relatively unstable in air-saturated solutions [7]. Laser action from solid solutions of PM–BF2 complexes in materials such as sol–gel [7–11], epoxy resins [12–14], conventional polymers [7,9,13,15– 25], and glass–polymer composites [7,9,11] has been reported. Polymers, in particular, are very attractive materials to be used as solid hosts for lasing dyes, as they offer a number of advantages which include high optical homogeneity, good chemical compatibility with organic dyes and control over relevant properties such as free volume, chemical composition, molecular weight and viscoelasticity. This attractiveness prompted us to perform a systematic study on the effect on the laser performance of dye pyrromethene 567 (PM567) of the composition of the polymeric matrix, and good efficiency with reasonable photostability was demonstrated for some of the formulations prepared [24,25]. Thermal degradation under laser irradiation of the studied polymeric materials seemed to play an important role in their behaviour, impairing their lasing photostability. One way to improve the thermal resistance of the host material without losing the benefits provided by polymers is using inorganic–organic copolymers, composed of inorganic oxidic structures substituted or cross-linked by organic groups [26–28]. A previous step to develop these hybrid materials would be the synthesis of organic polymers containing silyl groups as pendant substituents of the main chain which can ulteriorly be involved in the hydrolysis and condensation steps in order to obtain cross-linked oxidic structures. To this aim, we have proceeded to explore the possibility of obtaining new solid materials based on trimethylsilyl-methacrylate (TMSMA) monomer. The plasticity and free volume of the matrix were modified by copolymerization with monomers such as methyl methacrylate (MMA) and ethylene glycol dimethacrylate (EGDMA), respectively. The vol/vol proportion of both monomers in each copolymer formulation was systematically varied, and the effect of each composition on the lasing properties of PM567 was evaluated and compared with the efficiency reached in liquid solution of tetramethylsilane (TMS), which is the solvent that mimics the
new polymer matrix. The preparation of these new materials is described in detail and their laser parameters under transversal pumping at 534 nm are characterized. Comparison with previous results obtained with PM567 incorporated into polymeric matrices shows significant increases in photostability for some of the new materials.
2. Experimental The laser system experimental setup was the same as previously described [24]. Solid laser samples in cylindrical shape, forming rods of 10 mm diameter and 10 mm length, were prepared with a PM567 concentration of 1:5 103 M so that the optical density of the samples at the 534 nm pump wavelength was about 18 for a 1 cm optical path. The ends of the laser rods were polished by hand to obtain reasonably flat surfaces. No attempt to produce laser grade flat surfaces was made. The air-equilibrated samples were transversely pumped at 534 nm with 5.5 mJ, 6 ns FWHM pulses from a frequency doubled Q-switched Nd:KGW (Potassium Gadolinium Tungstate) laser (Monocrom STR-2+) at a repetition rate of 1 Hz. A cut parallel to the axis of the laser rod defined a lateral flat surface onto which the exciting pulses were line-focused by using a combination of one spherical ðf ¼ 50 cmÞ and two cylindrical quartz lenses (f ¼ 15 cm and þ15 cm, respectively) perpendicularly arranged. The first cylindrical lens widened the spherical cross-section of the pump beam to illuminate the complete 1 cm length of the dye sample; then, the second cylindrical lens focused the pump pulses onto the input surface of the solid sample to form a line of 0:3 10 mm, so that the 2 pump fluence was 180 mJ=cm . The oscillator cavity consisted of a 90% reflectivity flat aluminium mirror and the end face of the solid sample as the output coupler, with a cavity length of 2 cm. The cavity was not optimized and did not have tuning elements to select the wavelength. The dye and pump laser pulses were characterized with the following instruments: GenTec ED-100A and ED-200 pyroelectric energy meters, ITL TF 1850 fast risetime photodiode, Tektronix 7934 storage oscilloscope and Tektronix 2430
A. Costela et al. / Optics Communications 201 (2002) 437–445
digital-storage oscilloscope, CVI CM110 monochromator, and EMI 9783B photomultiplier. In the photodegradation studies dye and pump laser signals were sampled with boxcars (Stanford Research, Model 250). All the integrated signals were digitized and processed using a PC computer via a Computerboard DASH-8 interface [29]. The estimated error of the energy measurements was 10%. Absorption and fluorescence spectra of the laser materials were obtained from thin discs cut out from the same samples used in the lasing experiments and measured, respectively, on a Shimadzu UV-265 FS spectrophotometer and on a PerkinElmer LS-50B luminescence spectrometer. 2.1. Materials 1,3,5,7,8-Pentamethyl-2,6-diethylpyrromethenedifluoroborate (PM567, laser grade from Exciton) was used as received. The purity of the dye was found to be > 99%, as determined by spectroscopic and chromatographic methods. The dye was incorporated into the different matrices following the methods described below. Tetramethylsilane (TMS, Aldrich), and methyl methacrylate (MMA, Merck) were washed three times with 10% vol. aqueous sodium hydroxide to remove the inhibitor and then twice with distilled water. The monomer, dried over anhydrous MgSO4 , was then distilled under reduced pressure before use. These experimental procedures allow the removal of impurities, especially stabilizers, which occur in commercial monomers. Ethylene glycol dimethacrylate (EGDMA, Merck), tetramethylsilane (TMS, Aldrich) and trimethylsilyl methacrylate (TMSMA, Aldrich) were used as received. A final degree of purity higher than 99.5% was obtained, as checked by spectroscopic and chromatograpic methods. 2.2. Preparation of polymer dye samples Freshly purified monomers were used to prepare the samples. The adequate amount of PM567 dye was dissolved in mixtures with different vol/vol proportions of TMSMA and the corresponding comonomers, and the resulting mixtures were placed in an ultrasonic bath until complete disso-
439
lution of the dye. After further addition of 0.015 g/l of 2,20 -azobis(isobutyronitrile) (AIBN), used as free radical initiator, the mixtures were again sonicated. AIBN is the thermal polymerization initiator of choice, since it leaves UV-transparent end-groups on the copolymer. The resulting solutions were filtered into appropriate cylindrical polypropylene moulds using a 0:45 lm pore size filter followed by a 0:2 lm pore size filter (Whatman Lab., PTFE disposable filters). After careful deaeration by bubbling dry argon during 10 min, the moulds were sealed. An inert atmosphere avoids the well-known oxygen inhibition of radical polymerization. Polymerization was performed in the dark at 40 °C over a period of two days and then at 45 °C for about one day. The temperature was then raised to 60 °C and increased slowly up to 80 °C over a period of 7 days, in order to decompose the residual AIBN. Finally, the temperature was reduced in steps of 5 °C per day until room temperature was reached, and only then the samples were unmoulded. This procedure was essential in order to reduce the buildup of stresses in the polymer samples due to thermal shock.
3. Results and discussion Laser emission with an efficiency of 49% (defined as the ratio between the energy of the dye laser output and the energy of the pump laser incident on the sample surface) was obtained from a 1:5 103 M air-equilibrated solution of PM567 dye in TMS. The spectral profiles of the stimulated emission together with the absorption and fluorescence spectra of PM567 in TMS are shown in Fig. 1. Laser emission peaked at 560 nm, with 6 nm bandwidth, much narrower than the fluorescence spectral width. The beam divergence of the laser emission was 5 mrad, the pulse duration was 5 ns FWHM, and the minimum pump energy producing laser emission was of the order of 50 lJ. In previous studies of PM567, we have properly characterized the lasing properties of this dye using a variety of solvents, which can be roughly classified as hydrogen-bond donor (HBD) solvents, dipolar non-HBD solvents and apolar
440
A. Costela et al. / Optics Communications 201 (2002) 437–445
Fig. 1. UV/Vis absorption, normalized fluorescence ðkexc ¼ 515 nmÞ and stimulated emission ðkexc ¼ 534 nmÞ spectra of PM567 dissolved in TMS.
non-HBD solvents [30,31]. From these results, it was inferred that very polar solvents, such as 2,2,2trifluoroethanol and methanol, are recommended as the best liquid media for the laser operation of PM567, with efficiencies as high as 56% and 52%, respectively, being observed in these solvents. This behaviour was related to the electrostatic stabilization of the chromophoric positive charge by the dielectric constant of the solvent, which decreases the non-radiative deactivation processes of the dye [32]. However, when a solution of PM567 in the non-polar HBD solvent TMS was pumped under the same experimental conditions, the high lasing efficiency reported above was observed. This effect could be a consequence of the high chemical stability of the TMS molecule which prevents possible interactions and/or dye/solvent aggregations. A full understanding of this behaviour requires a deeper analysis of the radiative and non-radiative deactivation mechanisms as well as dye-solvent interactions in both the ground and excited states of the chromophore. Nevertheless, we have previously observed that fluorescence quantum yield and lasing efficiency do not linearly correlate with any solvent parameters [30]. To mimic the TMS solvent, we selected first the homopolymer TMSMA as the host matrix for the PM567 dye. When the polymerization was carried out, a very soft material resulted, with a glass transition temperature (Tg ) lower than the room
temperature, unfit for subsequent mechanization and proper polishing. To overcome this problem, we developed solid polymeric matrices of TMSMA with different amounts of the cross-linking monomer EGDMA, up to 15%, which allowed us to obtain materials with the mechanical, optical and solvatation properties appropriate to sustain laser action in the solid state. The lasing efficiencies of PM567 in the different cross-linked polymeric compositions under study are tabulated in Table 1. For comparison, the laser parameters of PM567 doped in PMMA homopolymer pumped under identical experimental conditions are also included in Table 1. The dependence of the laser output on the number of pump pulses in the same position of the sample was studied for the different materials at a repetition rate of 1 Hz. In the last column of Table 1 are listed the lifetimes of the solid samples defined as the intensity of the laser output after 10,000 pump pulses referred to the initial intensity of the laser emission [In ð%Þ ¼ ðIn =I0 Þ 100, where I0 is the initial lasing intensity]. When comparing the efficiencies of the solid samples with those of the liquid solutions, it has to be remarked that improvements in the lasing effiTable 1 Laser performancea of PM567 dissolved in MSMA with different monomersb added in various proportions vol/vol Material
kmax (nm)
Eff (%)
I10;000 (%)
TMS PMMA P(TMSMA:EGDMA 99:1) P(TMSMA:EGDMA 95:5) P(TMSMA:EGDMA 90:10) P(TMSMA:EGDMA 85:15) P(TMSMA:MMA 70:30) P(TMSMA:MMA 50:50) P(TMSMA:MMA 30:70) P(TMSMA:MMA 10:90)
560 567 565 562 562 565 561 561 560 561
49 12 8 6 4 3 5 14 12 11
70 85 50 30 40 6 100 70 30
Dye concentration: 1:5 103 M. Nd:KGW pump energy: 5.5 mJ/pulse, repetition rate 1 Hz. a kmax : peak of the laser emission; Eff: energy conversion efficiency; I10;000 (%): intensity of the dye laser output after 10,000 pump pulses referred to initial intensity I0 . b TMS: tetramethylsilane; MMA: methyl methacrylate; TMSMA: trimethylsilyl-methacrylate; EGDMA: ethylene glycol dimethacrylate.
A. Costela et al. / Optics Communications 201 (2002) 437–445
ciency in the polymeric matrices are to be expected with enhanced optical quality of their surface since, as indicated in the experimental section, the finishing of the surfaces of the solid samples relevant to the laser operation was not laser-grade. In addition, the reflectivity of the output coupler in the oscillator cavity has not been optimized for laser operation in any case. Our main concern in this work is not so much to optimize the laser output as to study the photostability and relative efficiency of operation in different polymeric materials under otherwise identical conditions. Although our materials exhibit good optical homogenity, it is unlikely that the sole effect of the poor polishing of the samples surfaces could explain the important differences in efficiency between solid and liquid solutions. A more important source of distorsions could be the thermal lensing effect experienced at the solid-state gain media under our hard excitation regime [33]. Efficiencies of the 50–60% range, higher than those reported in this work, have been reported for the more conventional solid media rhodamine doped MPMMA [34]. For properly assessing our results obtained with pyrromethene dyes when comparing them with those obtained with rhodamine doped polymers, the notes caution indicated above on the optical quality of the surfaces of our samples not being laser-grade and the parameters of the laser cavity not being optimized for laser operation, in addition to our pumping arrangement being transversal, should be taken into account. In the aforementioned work on rhodamine doped MPMMA the reflectivity of the output coupler was optimized and the pumping was longitudinal. Improvement on the lasing efficiency of our samples is to be expected with enhanced optical quality of the surfaces, optimization of the output coupling and longitudinal pumping. Thus, the laser efficiency herein reported should be considered as the worst case. The evolution of lasing efficiency and stability with the degree of cross-linking is compared in Fig. 2. From these experimental results, it is clear that the proportion of EGDMA plays an important role in the effectiveness of laser operation. The increase of the rigidity of the polymeric matrix by cross-linking reduces both the lasing effi-
441
Fig. 2. Dependence on the percentage of the cross-linking monomer EGDMA added to TMSMA of the efficiency and stability of the laser output of PM567 after 10,000 pump pulses referred to the initial intensity I0 [In ð%Þ ¼ ðIn =I0 Þ 100]. Dye concentration: 1:5 103 M. Pump energy and repetition rate: 5.5 mJ/pulse and 1 Hz, respectively.
ciency and lifetime with respect to that exhibited by PM567 embedded in linear PMMA, with the exception of the TMSMA:EGDMA 1% where a slight improvement of the lasing stability was observed, with a drop of the initial laser output of 15% after 10,000 pump pulses at 1 Hz repetition rate. Although our previous experiments had demonstrated that the controlled reduction of the polymeric free volume was a promising route for further improving lasing properties, [25,35] the present results reveal that the effect of structural changes on the laser performance, by increasing the rigidity of the polymeric host, is not a simple one but follows a complex mechanism not completely understood yet, implying other properties and interactions of the dye-polymer matrix as well. In spite of the soft thermal treatment selected to carry out the polymerization process, the presence of the cross-linking monomer EGDMA induces, inside the material, strains bearing to the generation of microfractures that, finally, along the time, result in the break of the solid sample. For this
442
A. Costela et al. / Optics Communications 201 (2002) 437–445
reason, we tried to improve the structural and mechanical properties of TMSMA homopolymer by copolymerization with MMA. This monomer was chosen as the pivotal component in the formulations developed because the excellent optical transparency and relatively high laser-damage resistance of PMMA make this material an obligated reference in any strategy directed to improve lasing performance in polymeric solid-state dye lasers. To study the effect of internally increasing the rigidity of TMSMA by copolymerization with MMA, the lasing properties of PM567 dissolved in a number of different P(TMSMA:MMA) copolymers, in proportions 1:9, 3:7, 5:5 and 7:3 vol/vol, were investigated. The influence of the composition of the polymeric matrix on laser action (emission wavelength maximum, efficiency and photostability) is reported as well in Table 1. In addition, the evolution of the laser output is illustrated, in Fig. 3, as a function of the number of pump pulses for two of these copolymers investigated. It should be noticed that for some samples after an initial decrease of the efficiency it starts to increase slightly. This phenomenon seems to be complicated and at the present time we cannot offer any non-speculative explanation on this effect. A first effect clearly apparent is the blue-shift of the maxima of the laser spectra induced by the
Fig. 3. Normalized laser output as a function of the number of pump pulses for PM567 dissolved in P(TMSMA:MMA 50:50) (A) and P(TMSMA:MMA 10:90) (B). Dye concentration: 1:5 103 M. Pump energy and repetition rate: 5.5 mJ/pulse and 1 Hz, respectively.
presence of MMA. These shifts, which correlate with similar shifts in the fluorescence spectra, are probably related with changes in the polarity of the media [36]. Also, as can be observed in Fig. 4, both lasing efficiency and photostability increase with the presence of MMA in the matrix, reaching an optimum value for the 50:50 molar proportion. Increasing the concentration of the monomer beyond this point results in a progressive worsening of the laser action. In our previous work with rhodamine and cumarine dyes, an apparent direct relationship between lasing efficiency and photostability had been observed: the higher the efficiency, the lower the rate of degradation [37,38]. In the present case this heuristic rule is maintained and the best photostability and highest lasing efficiency are obtained with PM567 dissolved in copolymer P(TM SMA:MMA 50:50), improving the laser performance reached with this dye doped in PMMA homopolymer. It has to be remarked that a high useful lifetime is obtained with this material
Fig. 4. Dependence on the percentage of the lineal monomer MMA added to TMSMA of the efficiency and stability of the laser output of PM567 after 10,000 pump pulses referred to the initial intensity I0 [In ð%Þ ¼ ðIn =I0 Þ 100]. Dye concentration: 1:5 103 M. Pump energy and repetition rate: 5.5 mJ/pulse and 1 Hz, respectively.
A. Costela et al. / Optics Communications 201 (2002) 437–445
since no sign of degradation is observed after 10,000 pump laser pulses. In order to test the photostability of the PM567 dye when doped in TMS MA:MMA 50:50, this sample was pumped at 10 Hz repetition rate for extended periods of time. When the pump repetition rate increases, the degradation rate increases significatively, since the dissipation channels of the energy released to the medium as heat are not fast enough and as a result the thermal and photochemical degradation of the dye is enhanced [39]. However, the laser emission of PM567 dissolved in P(TMSMA:MMA 50:50) remained at 45% of its initial value after 40,000 pump pulses. Up to this moment, this result is one of the highest stabilities reported for the PM567 dye doped in acrylic and methacrylic matrices under transversal pumping. The lifetime reached from this TMSMA:MMA copolymer is only overcome by matrices P(MMA:TMPTMA 99:1) and P(MM A:PETRA 95:5) which, when pumped under the same experimental conditions, remained at 40% and 80% of the initial laser output after 50,000 and 45,000 pump laser pulses, respectively [35]. Photostabilities higher than those reached in the present paper have been reported by Allik et al. [15] for PM567 doped in an undefined high temperature plastic material, in which case a 78% of the initial lasing efficiency was retained after 95,000 pump pulses. In addition, Ahmad et al. [23] reported high photostability for PM567 in pure PMMA, with 270,000 pulses emitted before the conversion efficiency fell to half its initial value for a pump fluence of 0.16 J/cm2 . When PMMA was modified with 1,4-diazobicyclo[2,2,2]octane as singlet oxygen quencher, the longevity increased to 550,000 pump pulses. In a previous paper, Rhan et al. [9] had reported a drop in the laser output to 60% of the initial value after 7000 pulses from PM567 in a nitrogen-saturated MPMMA matrix and a drop to 90% of the initial value after 1000 shots. In all the above referred cases pumping was longitudinal at 10 Hz repetition rate while, in the present case, the solid samples were transversely pumped. It is unlikely that the large changes observed in the laser performance obtained in the different materials studied here are only due to differences in the mechanical and optical quality of the sam-
443
ples. Rather differences in microstructure, polarity and free volume should play a central part in the performance of the various materials. In fact, the behaviour of the copolymers TMSMA:MMA could be understood in terms of a rigidization– plasticization mechanism. It has been shown [40] that for good damage resistance the material’s induced elastic limit must be smaller than its brittle-fracture limit. Plasticization lowers the material’s induced elastic limit, and it was observed [40] that modifying PMMA by copolymerization with appropriate aliphatic acrylic monomers (internal plasticization) can reduce the induced elastic limit to below the brittle-fracture point, improving the material’s resistance by orders of magnitude. Although the detailed mechanism of the process is not completely understood, it had been argued much earlier [41] that the damage resistance of a polymer is related to its ability to scatter excitons and to facilitate the thermal micro-Brownian motions of the molecular chains. In our case, the polymeric material becomes more plastic as the TMSMA comonomer concentration increases, resulting in a decrease of the elastic limit and, thus, in better damage resistance. On the other hand, as the internal plasticization increases beyond a certain limit, the protecting ‘‘polymer cage’’ that surrounds the dye groups weakens (the polymer becomes less rigid) and, as a result, the chromophore is more easily bleached [42]. Once a certain value of rigidity is attained, a further increase of the MMA content in the matrix results in laser-induced stresses exceeding the material’s brittle-fracture limit, which causes damage of the matrix with the corresponding losses in efficiency and stability. From the data of Fig. 4, it is concluded that, in the present case, a 50:50 (vol/vol) copolymer composition is the best compromise between these two conflicting requirements. Similar results were obtained in experiments performed with xanthene and cumarin dyes [36,43]. The above results indicate that, in order to obtain the best efficiency and photostability of a dye/polymer system, a balance between matrix and dye resistances must be reached by changing the copolymer composition for each specific matrix/ dye combination. In addition, the promising
444
A. Costela et al. / Optics Communications 201 (2002) 437–445
results reached with these solid materials based on TMSMA monomer allow one to define a priori the silicate-based inorganic–organic hybrid polymers as ideal candidates for laser matrices, since, due to their inorganic Si–O–Si backbone, they can present improved thermal and mechanical properties as compared with common organic polymers. Work in progress is directed to design the synthesis of hybrid materials obtained from organically modified silicon alkoxides.
Acknowledgements This work was supported by Project no. MAT2000-1361-C04-01 of the Spanish CICYT.
References [1] A. Costela, I. Garcıa-Moreno, R. Sastre, in: H.S. Nalwa (Ed.), Handbook of Advanced Electronic and Photonic Materials and Devices, vol. 7, Academic Press, San Diego, 2001, p. 161. [2] T.G. Pavlopoulos, J.H. Boyer, M. Shah, K. Thangaraj, M.L. Soong, Appl. Opt. 29 (1990) 3885. [3] T.G. Pavlopoulos, J.H. Boyer, K. Thangaraj, G. Sathyamoorthi, M.P. Shah, M.L. Soong, Appl. Opt. 31 (1992) 7089. [4] S.C. Guggenheimer, J.H. Boyer, K. Thangaraj, M.P. Shah, M.L. Soong, T.G. Pavlopoulos, Appl. Opt. 32 (1993) 3942. [5] J.H. Boyer, A.M. Haag, G. Sathyamoorthi, H.L. Soong, K. Thangaraj, T.G. Pavlopoulos, Heteroatom Chem. 4 (1993) 39. [6] W.P. Partridge Jr., N.M. Laurendeau, C.C. Johnson, R.N. Steppel, Opt. Lett. 19 (1994) 1630. [7] M.D. Rahn, T.A. King, Appl. Opt. 34 (1995) 8260. [8] M. Canva, P. Georges, J.F. Perelgritz, A. Brun, F. Chaput, J.P. Boilot, Appl. Opt. 34 (1995) 428. [9] M.D. Rahn, T.A. King, A. Gorman, I. Hamblett, Appl. Opt. 36 (1997) 5862. [10] M. Faloss, M. Canva, P. Georges, A. Brun, F. Chaput, J.P. Boilot, Appl. Opt. 36 (1997) 6760. [11] E. Yariv, R. Reisfeld, Opt. Mater. 13 (1999) 49. [12] M.J. Cazeca, X.L. Jiang, J. Kumar, S.K. Tripathy, Appl. Opt. 36 (1997) 4965. [13] D.P. Pacheco, H.R. Aldag, SPIE Proc. 3265 (1998) 2. [14] G. Jones II, O. Klueva, S. Linares-Samaniego, D.P. Pacheco, SPIE Proc. 3265 (1998) 51. [15] T.H. Allik, S. Chandra, T.R. Robinson, J.A. Hutchinson, G. Sathyamoorthi, J.H. Boyer, Mater. Res. Soc. Proc. 329 (1994) 291.
[16] C.K. Pearce, T.F. Ewanizky, in: V.J. Corcoran, T.A. Goldman (Eds.), Proc. Int. Conf. on LASERS ’95, STS, McLean, 1996, p. 383. [17] D.P. Pacheco, J.G. Burke, H.R. Aldag, in: V.J. Corcoran, T.A. Goldman (Eds.), Proc. Int. Conf. on LASERS ’95, STS, McLean, 1996, p. 791. [18] T.H. Allik, S. Chandra, R. Utano, T.R. Robinson, J.H. Boyer, SPIE Proc. 2698 (1996) 30. [19] S.W. Minning, D.P. Pacheco, J.G. Burke, P.J. Dunleavy, H.R. Aldag, SPIE Proc. 2698 (1996) 55. [20] A. Mandl, A. Zavriyev, D.E. Klimek, IEEE J. Quantum Electron. 32 (1996) 1723. [21] G. Jones II, J. Morais, J. Jimenez, D.P. Pacheco, H.R. Aldag, in: V.J. Corcoran, T.A. Goldman (Eds.), Proc. Int. Conf. on LASERS’97, STS, McLean, 1997, p. 293. [22] A.J. Finlayson, N. Peters, P.V. Kolinsky, M.R.W. Venner, Appl. Phys. Lett. 75 (1999) 457. [23] M. Ahmad, M.D. Rahn, T.A. King, Appl. Opt. 38 (1999) 6337. [24] A. Costela, I. Garcıa-Moreno, J. Barroso, R. Sastre, Appl. Phys. B 70 (2000) 367. [25] A. Costela, I. Garcıa-Moreno, R. Sastre, F. L opezArbeloa, T. L opez-Arbeloa, I. L opez-Arbeloa, Appl. Phys. B 73 (2001) 19. [26] B.M. Novak, Adv. Mater. 5 (1993) 422. [27] C. Sanchez, F. Ribot, New J. Chem. 18 (1994) 1007. [28] U. Schubert, N. H€ using, A. Lorenz, Chem. Mater. 7 (1995) 2010. [29] M. Rodrıguez, A. Costela, I. Garcıa-Moreno, F. Florido, J.M. Figuera, R. Sastre, Meas. Sci. Technol. 6 (1995) 971. [30] F. L opez Arbeloa, T. L opez Arbeloa, I. L opez Arbeloa, I. Garcıa-Moreno, A. Costela, R. Sastre, F. Amat-Guerri, Chem. Phys. 236 (1998) 331. [31] F. L opez Arbeloa, T. L opez Arbeloa, I. L opez Arbeloa, I. Garcıa-Moreno, A. Costela, R. Sastre, F. Amat-Guerri, Chem. Phys. Lett. 299 (1999) 315. [32] S.K. Saha, S.K. Dogra, J. Photochem. Photobiol. A 110 (1997) 257. [33] F.J. Duarte, A. Costela, I. Garcıa-Moreno, R. Sastre, J.J. Ehrlich, T.S. Taylor, Opt. Quantum Electron. 29 (1997) 461. [34] A. Maslyukov, S. Sokolov, M. Kaivola, K. Nyholm, S. Popov, Appl. Opt. 34 (1995) 1516. [35] A. Costela, I. Garcıa-Moreno, C. Gomez, O. Garcıa, R. Sastre, J. Appl. Phys. 90 (2001) 3159. [36] K.H. Drexhage, in: F.P. Sch€aefer (Ed.), Dye Lasers, third ed., Top. Appl. Phys., vol. 1, Springer, Berlin, 1990, p. 155. [37] A. Costela, F. Florido, I. Garcıa-Moreno, R. Duchowicz, F. Amat-Guerri, J.M. Figuera, R. Sastre, Appl. Phys. B 60 (1995) 383. [38] A. Costela, I. Garcıa-Moreno, J.M. Figuera, F. AmatGuerri, J. Barroso, R. Sastre, Opt. Commun. 130 (1996) 44. [39] R. Duchowicz, L.B. Scaffardi, A. Costela, I. GarcıaMoreno, R. Sastre, A.U. Acu~ na, Appl. Opt. 39 (2000) 4959.
A. Costela et al. / Optics Communications 201 (2002) 437–445 [40] K.M. Dyumaev, A.A. Manenkov, A.P. Maslyukov, G.A. Matyusin, V.S. Nechitailo, A.S. Tsaprilov, Sov. J. Quantum Electron. 12 (1982) 838. [41] G.M. Emelyanova, T.F. Ivanova, M.P. Votinov, V.M. Ovchinikov, V.D. Piterkin, Z.A. Smirnova, Sov. Tech. Phys. Lett. 3 (1968) 238.
445
[42] N.N. Barashkov, O.A. Gunder, in: Fluorescent Polymers, Ser. Polym. Sci. Technol., Ellis Horwood, Chischester, UK, 1994, p. 190. [43] A. Costela, I. Garcıa-Moreno, J. Barroso, R. Sastre, Appl. Phys. B 67 (1998) 167.
Optics Communications 201 (2002) 437–445 www.elsevier.com/locate/optcom
Studies on laser action from polymeric matrices based on trimethylsilyl methacrylate doped with pyrromethene 567 dye A. Costela a, I. Garcıa-Moreno a,*, M.L. Carrascoso b, R. Sastre b b
a Instituto de Quımica-Fısica ‘‘Rocasolano’’, CSIC, Serrano 119, 28006 Madrid, Spain Instituto de Ciencia y Technologyıa de Polımeros, CSIC, Juan de la Cierva 3, 28006 Madrid, Spain
Received 12 September 2001; received in revised form 8 November 2001; accepted 27 November 2001
Abstract The lasing properties of pyrromethene 567 (PM567) dissolved in solid poly-trimethylsilyl-methacrylate (TMSMA) cross-linked with ethylene glycol dimethacrylate (EGDMA) and copolymerized with methyl methacrylate (MMA) have been investigated. The vol/vol proportion of the different comonomers in each copolymer formulation was systematically varied, and the effect of each composition on the laser action of PM567 was evaluated. The laser samples were transversely pumped at 534 nm with 5.5 mJ/pulse from a frequency doubled Q-switched Nd:KGW laser. Lasing efficiencies of up to 14% and good stability with no sign of degradation after 10,000 pump pulses at 1 Hz in the copolymer P(TMSMA:MMA 50:50) were demonstrated. Pumping this sample at 10 Hz, the laser emission of PM567 remained at 45% of its initial value after 40,000 pulses. Ó 2002 Elsevier Science B.V. All rights reserved.
1. Introduction Over the last decade there has been a considerable interest in the development of solid-state dye lasers, where an organic dye is incorporated into an adequate solid matrix [1], as an attractive alternative to conventional liquid-state dye lasers. A solid dye laser, retaining the versatility of liquid dye lasers, allows the design of compact and selfcontained laser systems, which facilitates their utilization in industrial or medical environments.
* Corresponding author. Tel.: +34-91-561-9400; fax: +34-91564-2431. E-mail addresses: [email protected] (I. Garcıa-Moreno), [email protected] (R. Sastre).
The laser performance of organic dyes is ultimately limited by triplet–triplet absorption over their lasing spectral region. Over the late 1980s and early 1990s, Boyer and co-workers [2–5] synthesized a number of organic dyes belonging to a new class of laser dyes, namely, the pyrromethene– BF2 ðPM–BF2 Þ complexes, which exhibit reduced triplet–triplet absorption and high fluorescence quantum yields. These new dyes present laser emission over the spectral region from green/yellow to red, competing with the well-known rhodamine dyes. In particular, some of these new dyes outperform both in efficiency and tunability the widely employed rhodamine 6G and rhodamine B laser dyes in liquid solution [6]. On the other hand, the presence of aromatic amine groups in their structure renders these pyrromethene dyes
0030-4018/02/$ - see front matter Ó 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 3 0 - 4 0 1 8 ( 0 1 ) 0 1 7 2 3 - 0
438
A. Costela et al. / Optics Communications 201 (2002) 437–445
vulnerable to photochemical reactions with oxygen which makes them relatively unstable in air-saturated solutions [7]. Laser action from solid solutions of PM–BF2 complexes in materials such as sol–gel [7–11], epoxy resins [12–14], conventional polymers [7,9,13,15– 25], and glass–polymer composites [7,9,11] has been reported. Polymers, in particular, are very attractive materials to be used as solid hosts for lasing dyes, as they offer a number of advantages which include high optical homogeneity, good chemical compatibility with organic dyes and control over relevant properties such as free volume, chemical composition, molecular weight and viscoelasticity. This attractiveness prompted us to perform a systematic study on the effect on the laser performance of dye pyrromethene 567 (PM567) of the composition of the polymeric matrix, and good efficiency with reasonable photostability was demonstrated for some of the formulations prepared [24,25]. Thermal degradation under laser irradiation of the studied polymeric materials seemed to play an important role in their behaviour, impairing their lasing photostability. One way to improve the thermal resistance of the host material without losing the benefits provided by polymers is using inorganic–organic copolymers, composed of inorganic oxidic structures substituted or cross-linked by organic groups [26–28]. A previous step to develop these hybrid materials would be the synthesis of organic polymers containing silyl groups as pendant substituents of the main chain which can ulteriorly be involved in the hydrolysis and condensation steps in order to obtain cross-linked oxidic structures. To this aim, we have proceeded to explore the possibility of obtaining new solid materials based on trimethylsilyl-methacrylate (TMSMA) monomer. The plasticity and free volume of the matrix were modified by copolymerization with monomers such as methyl methacrylate (MMA) and ethylene glycol dimethacrylate (EGDMA), respectively. The vol/vol proportion of both monomers in each copolymer formulation was systematically varied, and the effect of each composition on the lasing properties of PM567 was evaluated and compared with the efficiency reached in liquid solution of tetramethylsilane (TMS), which is the solvent that mimics the
new polymer matrix. The preparation of these new materials is described in detail and their laser parameters under transversal pumping at 534 nm are characterized. Comparison with previous results obtained with PM567 incorporated into polymeric matrices shows significant increases in photostability for some of the new materials.
2. Experimental The laser system experimental setup was the same as previously described [24]. Solid laser samples in cylindrical shape, forming rods of 10 mm diameter and 10 mm length, were prepared with a PM567 concentration of 1:5 103 M so that the optical density of the samples at the 534 nm pump wavelength was about 18 for a 1 cm optical path. The ends of the laser rods were polished by hand to obtain reasonably flat surfaces. No attempt to produce laser grade flat surfaces was made. The air-equilibrated samples were transversely pumped at 534 nm with 5.5 mJ, 6 ns FWHM pulses from a frequency doubled Q-switched Nd:KGW (Potassium Gadolinium Tungstate) laser (Monocrom STR-2+) at a repetition rate of 1 Hz. A cut parallel to the axis of the laser rod defined a lateral flat surface onto which the exciting pulses were line-focused by using a combination of one spherical ðf ¼ 50 cmÞ and two cylindrical quartz lenses (f ¼ 15 cm and þ15 cm, respectively) perpendicularly arranged. The first cylindrical lens widened the spherical cross-section of the pump beam to illuminate the complete 1 cm length of the dye sample; then, the second cylindrical lens focused the pump pulses onto the input surface of the solid sample to form a line of 0:3 10 mm, so that the 2 pump fluence was 180 mJ=cm . The oscillator cavity consisted of a 90% reflectivity flat aluminium mirror and the end face of the solid sample as the output coupler, with a cavity length of 2 cm. The cavity was not optimized and did not have tuning elements to select the wavelength. The dye and pump laser pulses were characterized with the following instruments: GenTec ED-100A and ED-200 pyroelectric energy meters, ITL TF 1850 fast risetime photodiode, Tektronix 7934 storage oscilloscope and Tektronix 2430
A. Costela et al. / Optics Communications 201 (2002) 437–445
digital-storage oscilloscope, CVI CM110 monochromator, and EMI 9783B photomultiplier. In the photodegradation studies dye and pump laser signals were sampled with boxcars (Stanford Research, Model 250). All the integrated signals were digitized and processed using a PC computer via a Computerboard DASH-8 interface [29]. The estimated error of the energy measurements was 10%. Absorption and fluorescence spectra of the laser materials were obtained from thin discs cut out from the same samples used in the lasing experiments and measured, respectively, on a Shimadzu UV-265 FS spectrophotometer and on a PerkinElmer LS-50B luminescence spectrometer. 2.1. Materials 1,3,5,7,8-Pentamethyl-2,6-diethylpyrromethenedifluoroborate (PM567, laser grade from Exciton) was used as received. The purity of the dye was found to be > 99%, as determined by spectroscopic and chromatographic methods. The dye was incorporated into the different matrices following the methods described below. Tetramethylsilane (TMS, Aldrich), and methyl methacrylate (MMA, Merck) were washed three times with 10% vol. aqueous sodium hydroxide to remove the inhibitor and then twice with distilled water. The monomer, dried over anhydrous MgSO4 , was then distilled under reduced pressure before use. These experimental procedures allow the removal of impurities, especially stabilizers, which occur in commercial monomers. Ethylene glycol dimethacrylate (EGDMA, Merck), tetramethylsilane (TMS, Aldrich) and trimethylsilyl methacrylate (TMSMA, Aldrich) were used as received. A final degree of purity higher than 99.5% was obtained, as checked by spectroscopic and chromatograpic methods. 2.2. Preparation of polymer dye samples Freshly purified monomers were used to prepare the samples. The adequate amount of PM567 dye was dissolved in mixtures with different vol/vol proportions of TMSMA and the corresponding comonomers, and the resulting mixtures were placed in an ultrasonic bath until complete disso-
439
lution of the dye. After further addition of 0.015 g/l of 2,20 -azobis(isobutyronitrile) (AIBN), used as free radical initiator, the mixtures were again sonicated. AIBN is the thermal polymerization initiator of choice, since it leaves UV-transparent end-groups on the copolymer. The resulting solutions were filtered into appropriate cylindrical polypropylene moulds using a 0:45 lm pore size filter followed by a 0:2 lm pore size filter (Whatman Lab., PTFE disposable filters). After careful deaeration by bubbling dry argon during 10 min, the moulds were sealed. An inert atmosphere avoids the well-known oxygen inhibition of radical polymerization. Polymerization was performed in the dark at 40 °C over a period of two days and then at 45 °C for about one day. The temperature was then raised to 60 °C and increased slowly up to 80 °C over a period of 7 days, in order to decompose the residual AIBN. Finally, the temperature was reduced in steps of 5 °C per day until room temperature was reached, and only then the samples were unmoulded. This procedure was essential in order to reduce the buildup of stresses in the polymer samples due to thermal shock.
3. Results and discussion Laser emission with an efficiency of 49% (defined as the ratio between the energy of the dye laser output and the energy of the pump laser incident on the sample surface) was obtained from a 1:5 103 M air-equilibrated solution of PM567 dye in TMS. The spectral profiles of the stimulated emission together with the absorption and fluorescence spectra of PM567 in TMS are shown in Fig. 1. Laser emission peaked at 560 nm, with 6 nm bandwidth, much narrower than the fluorescence spectral width. The beam divergence of the laser emission was 5 mrad, the pulse duration was 5 ns FWHM, and the minimum pump energy producing laser emission was of the order of 50 lJ. In previous studies of PM567, we have properly characterized the lasing properties of this dye using a variety of solvents, which can be roughly classified as hydrogen-bond donor (HBD) solvents, dipolar non-HBD solvents and apolar
440
A. Costela et al. / Optics Communications 201 (2002) 437–445
Fig. 1. UV/Vis absorption, normalized fluorescence ðkexc ¼ 515 nmÞ and stimulated emission ðkexc ¼ 534 nmÞ spectra of PM567 dissolved in TMS.
non-HBD solvents [30,31]. From these results, it was inferred that very polar solvents, such as 2,2,2trifluoroethanol and methanol, are recommended as the best liquid media for the laser operation of PM567, with efficiencies as high as 56% and 52%, respectively, being observed in these solvents. This behaviour was related to the electrostatic stabilization of the chromophoric positive charge by the dielectric constant of the solvent, which decreases the non-radiative deactivation processes of the dye [32]. However, when a solution of PM567 in the non-polar HBD solvent TMS was pumped under the same experimental conditions, the high lasing efficiency reported above was observed. This effect could be a consequence of the high chemical stability of the TMS molecule which prevents possible interactions and/or dye/solvent aggregations. A full understanding of this behaviour requires a deeper analysis of the radiative and non-radiative deactivation mechanisms as well as dye-solvent interactions in both the ground and excited states of the chromophore. Nevertheless, we have previously observed that fluorescence quantum yield and lasing efficiency do not linearly correlate with any solvent parameters [30]. To mimic the TMS solvent, we selected first the homopolymer TMSMA as the host matrix for the PM567 dye. When the polymerization was carried out, a very soft material resulted, with a glass transition temperature (Tg ) lower than the room
temperature, unfit for subsequent mechanization and proper polishing. To overcome this problem, we developed solid polymeric matrices of TMSMA with different amounts of the cross-linking monomer EGDMA, up to 15%, which allowed us to obtain materials with the mechanical, optical and solvatation properties appropriate to sustain laser action in the solid state. The lasing efficiencies of PM567 in the different cross-linked polymeric compositions under study are tabulated in Table 1. For comparison, the laser parameters of PM567 doped in PMMA homopolymer pumped under identical experimental conditions are also included in Table 1. The dependence of the laser output on the number of pump pulses in the same position of the sample was studied for the different materials at a repetition rate of 1 Hz. In the last column of Table 1 are listed the lifetimes of the solid samples defined as the intensity of the laser output after 10,000 pump pulses referred to the initial intensity of the laser emission [In ð%Þ ¼ ðIn =I0 Þ 100, where I0 is the initial lasing intensity]. When comparing the efficiencies of the solid samples with those of the liquid solutions, it has to be remarked that improvements in the lasing effiTable 1 Laser performancea of PM567 dissolved in MSMA with different monomersb added in various proportions vol/vol Material
kmax (nm)
Eff (%)
I10;000 (%)
TMS PMMA P(TMSMA:EGDMA 99:1) P(TMSMA:EGDMA 95:5) P(TMSMA:EGDMA 90:10) P(TMSMA:EGDMA 85:15) P(TMSMA:MMA 70:30) P(TMSMA:MMA 50:50) P(TMSMA:MMA 30:70) P(TMSMA:MMA 10:90)
560 567 565 562 562 565 561 561 560 561
49 12 8 6 4 3 5 14 12 11
70 85 50 30 40 6 100 70 30
Dye concentration: 1:5 103 M. Nd:KGW pump energy: 5.5 mJ/pulse, repetition rate 1 Hz. a kmax : peak of the laser emission; Eff: energy conversion efficiency; I10;000 (%): intensity of the dye laser output after 10,000 pump pulses referred to initial intensity I0 . b TMS: tetramethylsilane; MMA: methyl methacrylate; TMSMA: trimethylsilyl-methacrylate; EGDMA: ethylene glycol dimethacrylate.
A. Costela et al. / Optics Communications 201 (2002) 437–445
ciency in the polymeric matrices are to be expected with enhanced optical quality of their surface since, as indicated in the experimental section, the finishing of the surfaces of the solid samples relevant to the laser operation was not laser-grade. In addition, the reflectivity of the output coupler in the oscillator cavity has not been optimized for laser operation in any case. Our main concern in this work is not so much to optimize the laser output as to study the photostability and relative efficiency of operation in different polymeric materials under otherwise identical conditions. Although our materials exhibit good optical homogenity, it is unlikely that the sole effect of the poor polishing of the samples surfaces could explain the important differences in efficiency between solid and liquid solutions. A more important source of distorsions could be the thermal lensing effect experienced at the solid-state gain media under our hard excitation regime [33]. Efficiencies of the 50–60% range, higher than those reported in this work, have been reported for the more conventional solid media rhodamine doped MPMMA [34]. For properly assessing our results obtained with pyrromethene dyes when comparing them with those obtained with rhodamine doped polymers, the notes caution indicated above on the optical quality of the surfaces of our samples not being laser-grade and the parameters of the laser cavity not being optimized for laser operation, in addition to our pumping arrangement being transversal, should be taken into account. In the aforementioned work on rhodamine doped MPMMA the reflectivity of the output coupler was optimized and the pumping was longitudinal. Improvement on the lasing efficiency of our samples is to be expected with enhanced optical quality of the surfaces, optimization of the output coupling and longitudinal pumping. Thus, the laser efficiency herein reported should be considered as the worst case. The evolution of lasing efficiency and stability with the degree of cross-linking is compared in Fig. 2. From these experimental results, it is clear that the proportion of EGDMA plays an important role in the effectiveness of laser operation. The increase of the rigidity of the polymeric matrix by cross-linking reduces both the lasing effi-
441
Fig. 2. Dependence on the percentage of the cross-linking monomer EGDMA added to TMSMA of the efficiency and stability of the laser output of PM567 after 10,000 pump pulses referred to the initial intensity I0 [In ð%Þ ¼ ðIn =I0 Þ 100]. Dye concentration: 1:5 103 M. Pump energy and repetition rate: 5.5 mJ/pulse and 1 Hz, respectively.
ciency and lifetime with respect to that exhibited by PM567 embedded in linear PMMA, with the exception of the TMSMA:EGDMA 1% where a slight improvement of the lasing stability was observed, with a drop of the initial laser output of 15% after 10,000 pump pulses at 1 Hz repetition rate. Although our previous experiments had demonstrated that the controlled reduction of the polymeric free volume was a promising route for further improving lasing properties, [25,35] the present results reveal that the effect of structural changes on the laser performance, by increasing the rigidity of the polymeric host, is not a simple one but follows a complex mechanism not completely understood yet, implying other properties and interactions of the dye-polymer matrix as well. In spite of the soft thermal treatment selected to carry out the polymerization process, the presence of the cross-linking monomer EGDMA induces, inside the material, strains bearing to the generation of microfractures that, finally, along the time, result in the break of the solid sample. For this
442
A. Costela et al. / Optics Communications 201 (2002) 437–445
reason, we tried to improve the structural and mechanical properties of TMSMA homopolymer by copolymerization with MMA. This monomer was chosen as the pivotal component in the formulations developed because the excellent optical transparency and relatively high laser-damage resistance of PMMA make this material an obligated reference in any strategy directed to improve lasing performance in polymeric solid-state dye lasers. To study the effect of internally increasing the rigidity of TMSMA by copolymerization with MMA, the lasing properties of PM567 dissolved in a number of different P(TMSMA:MMA) copolymers, in proportions 1:9, 3:7, 5:5 and 7:3 vol/vol, were investigated. The influence of the composition of the polymeric matrix on laser action (emission wavelength maximum, efficiency and photostability) is reported as well in Table 1. In addition, the evolution of the laser output is illustrated, in Fig. 3, as a function of the number of pump pulses for two of these copolymers investigated. It should be noticed that for some samples after an initial decrease of the efficiency it starts to increase slightly. This phenomenon seems to be complicated and at the present time we cannot offer any non-speculative explanation on this effect. A first effect clearly apparent is the blue-shift of the maxima of the laser spectra induced by the
Fig. 3. Normalized laser output as a function of the number of pump pulses for PM567 dissolved in P(TMSMA:MMA 50:50) (A) and P(TMSMA:MMA 10:90) (B). Dye concentration: 1:5 103 M. Pump energy and repetition rate: 5.5 mJ/pulse and 1 Hz, respectively.
presence of MMA. These shifts, which correlate with similar shifts in the fluorescence spectra, are probably related with changes in the polarity of the media [36]. Also, as can be observed in Fig. 4, both lasing efficiency and photostability increase with the presence of MMA in the matrix, reaching an optimum value for the 50:50 molar proportion. Increasing the concentration of the monomer beyond this point results in a progressive worsening of the laser action. In our previous work with rhodamine and cumarine dyes, an apparent direct relationship between lasing efficiency and photostability had been observed: the higher the efficiency, the lower the rate of degradation [37,38]. In the present case this heuristic rule is maintained and the best photostability and highest lasing efficiency are obtained with PM567 dissolved in copolymer P(TM SMA:MMA 50:50), improving the laser performance reached with this dye doped in PMMA homopolymer. It has to be remarked that a high useful lifetime is obtained with this material
Fig. 4. Dependence on the percentage of the lineal monomer MMA added to TMSMA of the efficiency and stability of the laser output of PM567 after 10,000 pump pulses referred to the initial intensity I0 [In ð%Þ ¼ ðIn =I0 Þ 100]. Dye concentration: 1:5 103 M. Pump energy and repetition rate: 5.5 mJ/pulse and 1 Hz, respectively.
A. Costela et al. / Optics Communications 201 (2002) 437–445
since no sign of degradation is observed after 10,000 pump laser pulses. In order to test the photostability of the PM567 dye when doped in TMS MA:MMA 50:50, this sample was pumped at 10 Hz repetition rate for extended periods of time. When the pump repetition rate increases, the degradation rate increases significatively, since the dissipation channels of the energy released to the medium as heat are not fast enough and as a result the thermal and photochemical degradation of the dye is enhanced [39]. However, the laser emission of PM567 dissolved in P(TMSMA:MMA 50:50) remained at 45% of its initial value after 40,000 pump pulses. Up to this moment, this result is one of the highest stabilities reported for the PM567 dye doped in acrylic and methacrylic matrices under transversal pumping. The lifetime reached from this TMSMA:MMA copolymer is only overcome by matrices P(MMA:TMPTMA 99:1) and P(MM A:PETRA 95:5) which, when pumped under the same experimental conditions, remained at 40% and 80% of the initial laser output after 50,000 and 45,000 pump laser pulses, respectively [35]. Photostabilities higher than those reached in the present paper have been reported by Allik et al. [15] for PM567 doped in an undefined high temperature plastic material, in which case a 78% of the initial lasing efficiency was retained after 95,000 pump pulses. In addition, Ahmad et al. [23] reported high photostability for PM567 in pure PMMA, with 270,000 pulses emitted before the conversion efficiency fell to half its initial value for a pump fluence of 0.16 J/cm2 . When PMMA was modified with 1,4-diazobicyclo[2,2,2]octane as singlet oxygen quencher, the longevity increased to 550,000 pump pulses. In a previous paper, Rhan et al. [9] had reported a drop in the laser output to 60% of the initial value after 7000 pulses from PM567 in a nitrogen-saturated MPMMA matrix and a drop to 90% of the initial value after 1000 shots. In all the above referred cases pumping was longitudinal at 10 Hz repetition rate while, in the present case, the solid samples were transversely pumped. It is unlikely that the large changes observed in the laser performance obtained in the different materials studied here are only due to differences in the mechanical and optical quality of the sam-
443
ples. Rather differences in microstructure, polarity and free volume should play a central part in the performance of the various materials. In fact, the behaviour of the copolymers TMSMA:MMA could be understood in terms of a rigidization– plasticization mechanism. It has been shown [40] that for good damage resistance the material’s induced elastic limit must be smaller than its brittle-fracture limit. Plasticization lowers the material’s induced elastic limit, and it was observed [40] that modifying PMMA by copolymerization with appropriate aliphatic acrylic monomers (internal plasticization) can reduce the induced elastic limit to below the brittle-fracture point, improving the material’s resistance by orders of magnitude. Although the detailed mechanism of the process is not completely understood, it had been argued much earlier [41] that the damage resistance of a polymer is related to its ability to scatter excitons and to facilitate the thermal micro-Brownian motions of the molecular chains. In our case, the polymeric material becomes more plastic as the TMSMA comonomer concentration increases, resulting in a decrease of the elastic limit and, thus, in better damage resistance. On the other hand, as the internal plasticization increases beyond a certain limit, the protecting ‘‘polymer cage’’ that surrounds the dye groups weakens (the polymer becomes less rigid) and, as a result, the chromophore is more easily bleached [42]. Once a certain value of rigidity is attained, a further increase of the MMA content in the matrix results in laser-induced stresses exceeding the material’s brittle-fracture limit, which causes damage of the matrix with the corresponding losses in efficiency and stability. From the data of Fig. 4, it is concluded that, in the present case, a 50:50 (vol/vol) copolymer composition is the best compromise between these two conflicting requirements. Similar results were obtained in experiments performed with xanthene and cumarin dyes [36,43]. The above results indicate that, in order to obtain the best efficiency and photostability of a dye/polymer system, a balance between matrix and dye resistances must be reached by changing the copolymer composition for each specific matrix/ dye combination. In addition, the promising
444
A. Costela et al. / Optics Communications 201 (2002) 437–445
results reached with these solid materials based on TMSMA monomer allow one to define a priori the silicate-based inorganic–organic hybrid polymers as ideal candidates for laser matrices, since, due to their inorganic Si–O–Si backbone, they can present improved thermal and mechanical properties as compared with common organic polymers. Work in progress is directed to design the synthesis of hybrid materials obtained from organically modified silicon alkoxides.
Acknowledgements This work was supported by Project no. MAT2000-1361-C04-01 of the Spanish CICYT.
References [1] A. Costela, I. Garcıa-Moreno, R. Sastre, in: H.S. Nalwa (Ed.), Handbook of Advanced Electronic and Photonic Materials and Devices, vol. 7, Academic Press, San Diego, 2001, p. 161. [2] T.G. Pavlopoulos, J.H. Boyer, M. Shah, K. Thangaraj, M.L. Soong, Appl. Opt. 29 (1990) 3885. [3] T.G. Pavlopoulos, J.H. Boyer, K. Thangaraj, G. Sathyamoorthi, M.P. Shah, M.L. Soong, Appl. Opt. 31 (1992) 7089. [4] S.C. Guggenheimer, J.H. Boyer, K. Thangaraj, M.P. Shah, M.L. Soong, T.G. Pavlopoulos, Appl. Opt. 32 (1993) 3942. [5] J.H. Boyer, A.M. Haag, G. Sathyamoorthi, H.L. Soong, K. Thangaraj, T.G. Pavlopoulos, Heteroatom Chem. 4 (1993) 39. [6] W.P. Partridge Jr., N.M. Laurendeau, C.C. Johnson, R.N. Steppel, Opt. Lett. 19 (1994) 1630. [7] M.D. Rahn, T.A. King, Appl. Opt. 34 (1995) 8260. [8] M. Canva, P. Georges, J.F. Perelgritz, A. Brun, F. Chaput, J.P. Boilot, Appl. Opt. 34 (1995) 428. [9] M.D. Rahn, T.A. King, A. Gorman, I. Hamblett, Appl. Opt. 36 (1997) 5862. [10] M. Faloss, M. Canva, P. Georges, A. Brun, F. Chaput, J.P. Boilot, Appl. Opt. 36 (1997) 6760. [11] E. Yariv, R. Reisfeld, Opt. Mater. 13 (1999) 49. [12] M.J. Cazeca, X.L. Jiang, J. Kumar, S.K. Tripathy, Appl. Opt. 36 (1997) 4965. [13] D.P. Pacheco, H.R. Aldag, SPIE Proc. 3265 (1998) 2. [14] G. Jones II, O. Klueva, S. Linares-Samaniego, D.P. Pacheco, SPIE Proc. 3265 (1998) 51. [15] T.H. Allik, S. Chandra, T.R. Robinson, J.A. Hutchinson, G. Sathyamoorthi, J.H. Boyer, Mater. Res. Soc. Proc. 329 (1994) 291.
[16] C.K. Pearce, T.F. Ewanizky, in: V.J. Corcoran, T.A. Goldman (Eds.), Proc. Int. Conf. on LASERS ’95, STS, McLean, 1996, p. 383. [17] D.P. Pacheco, J.G. Burke, H.R. Aldag, in: V.J. Corcoran, T.A. Goldman (Eds.), Proc. Int. Conf. on LASERS ’95, STS, McLean, 1996, p. 791. [18] T.H. Allik, S. Chandra, R. Utano, T.R. Robinson, J.H. Boyer, SPIE Proc. 2698 (1996) 30. [19] S.W. Minning, D.P. Pacheco, J.G. Burke, P.J. Dunleavy, H.R. Aldag, SPIE Proc. 2698 (1996) 55. [20] A. Mandl, A. Zavriyev, D.E. Klimek, IEEE J. Quantum Electron. 32 (1996) 1723. [21] G. Jones II, J. Morais, J. Jimenez, D.P. Pacheco, H.R. Aldag, in: V.J. Corcoran, T.A. Goldman (Eds.), Proc. Int. Conf. on LASERS’97, STS, McLean, 1997, p. 293. [22] A.J. Finlayson, N. Peters, P.V. Kolinsky, M.R.W. Venner, Appl. Phys. Lett. 75 (1999) 457. [23] M. Ahmad, M.D. Rahn, T.A. King, Appl. Opt. 38 (1999) 6337. [24] A. Costela, I. Garcıa-Moreno, J. Barroso, R. Sastre, Appl. Phys. B 70 (2000) 367. [25] A. Costela, I. Garcıa-Moreno, R. Sastre, F. L opezArbeloa, T. L opez-Arbeloa, I. L opez-Arbeloa, Appl. Phys. B 73 (2001) 19. [26] B.M. Novak, Adv. Mater. 5 (1993) 422. [27] C. Sanchez, F. Ribot, New J. Chem. 18 (1994) 1007. [28] U. Schubert, N. H€ using, A. Lorenz, Chem. Mater. 7 (1995) 2010. [29] M. Rodrıguez, A. Costela, I. Garcıa-Moreno, F. Florido, J.M. Figuera, R. Sastre, Meas. Sci. Technol. 6 (1995) 971. [30] F. L opez Arbeloa, T. L opez Arbeloa, I. L opez Arbeloa, I. Garcıa-Moreno, A. Costela, R. Sastre, F. Amat-Guerri, Chem. Phys. 236 (1998) 331. [31] F. L opez Arbeloa, T. L opez Arbeloa, I. L opez Arbeloa, I. Garcıa-Moreno, A. Costela, R. Sastre, F. Amat-Guerri, Chem. Phys. Lett. 299 (1999) 315. [32] S.K. Saha, S.K. Dogra, J. Photochem. Photobiol. A 110 (1997) 257. [33] F.J. Duarte, A. Costela, I. Garcıa-Moreno, R. Sastre, J.J. Ehrlich, T.S. Taylor, Opt. Quantum Electron. 29 (1997) 461. [34] A. Maslyukov, S. Sokolov, M. Kaivola, K. Nyholm, S. Popov, Appl. Opt. 34 (1995) 1516. [35] A. Costela, I. Garcıa-Moreno, C. Gomez, O. Garcıa, R. Sastre, J. Appl. Phys. 90 (2001) 3159. [36] K.H. Drexhage, in: F.P. Sch€aefer (Ed.), Dye Lasers, third ed., Top. Appl. Phys., vol. 1, Springer, Berlin, 1990, p. 155. [37] A. Costela, F. Florido, I. Garcıa-Moreno, R. Duchowicz, F. Amat-Guerri, J.M. Figuera, R. Sastre, Appl. Phys. B 60 (1995) 383. [38] A. Costela, I. Garcıa-Moreno, J.M. Figuera, F. AmatGuerri, J. Barroso, R. Sastre, Opt. Commun. 130 (1996) 44. [39] R. Duchowicz, L.B. Scaffardi, A. Costela, I. GarcıaMoreno, R. Sastre, A.U. Acu~ na, Appl. Opt. 39 (2000) 4959.
A. Costela et al. / Optics Communications 201 (2002) 437–445 [40] K.M. Dyumaev, A.A. Manenkov, A.P. Maslyukov, G.A. Matyusin, V.S. Nechitailo, A.S. Tsaprilov, Sov. J. Quantum Electron. 12 (1982) 838. [41] G.M. Emelyanova, T.F. Ivanova, M.P. Votinov, V.M. Ovchinikov, V.D. Piterkin, Z.A. Smirnova, Sov. Tech. Phys. Lett. 3 (1968) 238.
445
[42] N.N. Barashkov, O.A. Gunder, in: Fluorescent Polymers, Ser. Polym. Sci. Technol., Ellis Horwood, Chischester, UK, 1994, p. 190. [43] A. Costela, I. Garcıa-Moreno, J. Barroso, R. Sastre, Appl. Phys. B 67 (1998) 167.