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Pyrromethene doped silica laser glasses by sol–gel processes
Materials Chemistry and Physics 69 (2001) 95–98
Pyrromethene doped silica laser glasses by sol–gel processes Qinyuan Zhang a,∗ , Zhonghong Jiang b a
b
Pohl Institute of Solid State Physics, Tongji University, Shanghai 200092, PR China Shanghai Institute of Optics and Fine Mechanics, Academic Sinica, Shanghai 201800, PR China Received 13 April 2000; received in revised form 12 June 2000; accepted 15 June 2000
Abstract A new laser dye pyrromethene impregnated solid-state material has been developed for visible wavelength solid-state dye laser by means of a sol–gel process. The use of N,N0 -dimethylformamide (DMF) as a drying control chemical additive was found to be effective in order to obtain the best quality silica and organically modified silicate (Ormosil) in terms of monolithicity, transparency and density. The addition of ␥-glycidyl-propyl-trimethoxisilane (GPTMS) in Ormosil can efficiently increase the luminescent emission intensity. Using these new materials as laser gain media, an all-solid-state, efficient, compact, long-lifetime and tunable dye laser has been demonstrated. © 2001 Elsevier Science B.V. All rights reserved. Keywords: Laser glass; Silica; Pyrromethene; Dye laser; Structural properties
1. Introduction For many years, considerable interest has been given in the area of solid-state dye lasers [1–6]. Compared to the liquid host, the use of a solid-state host for laser dyes presents obvious technical advantages, such as compactness, manageability and free from toxicity. The concentration quenching in liquid solutions originating from molecule collisions is effectively reduced, when organic molecules are embedded in a rigid matrix [1,7–9]. For practical applications, it is extremely important that these structures can be impregnated by organic dye to very high concentration without a noticeable quenching of fluorescence. Although encouraging process has been achieved in dye-doped laser based on polymer materials, low thermal-conductivity and laser damage threshold still remain as main defect [6]. And the main problems of the solid-state xerogels dye lasers are related to the control of the material particle size, the mechanical processing and laser-conversion efficiency. In this paper, a new laser dye pyrromethene 597 (PM 597) doped solid-state silica and organically modified silicate (Ormosil) were prepared using the sol–gel method. A drying control chemical additive, N,N0 -dimethylformamide (DMF), was used in order to obtain the best quality silica and Ormosil. And ␥-glycidyl-propyl-trimethoxisilane (GPTMS) and methyl methacrylate monomers (MMA) were added to im∗ Corresponding author. Fax: +86-021-6598-6071. E-mail address: [email protected] (Q. Zhang).
prove the mechanical and fluorescence emission properties of dye-doped xerogels. The structural properties and laser performances of those material pumped by a laser-diode array side-pumping, frequency-doubled Nd:YAG Q-switch laser with a wavelength of 532 nm were measured and demonstrated.
2. Experimental details 2.1. Sample preparation The dye-doped silica, sample A, was prepared from the hydrolysis and condensation processes of tetraethoxysilane (TEOS). In the process, the molar ratio of TEOS:ethanol: water:DMF:hydrochloric acid was selected to be 1:6:10:3: 0.01. After stirring the mixed solution mentioned above for 1 h, organic dye PM 597 dissolved in DMF was added with a concentration of 2.0×10−4 mol l−1 . To obtain homogeneous sol, ultrasonic bathing was used to insure dye distribution. The mixture gelled within 48 h at 60◦ C. For the preparation of dye-doped Ormosil, sample B, TEOS and GPTMS were first mixed with half volume of ethanol; then water and hydrochloric acid dissolved in the remaining ethanol was added to the TEOS–GPTMS–EtOH solution slowly while stirring to avoid the occurrence of turbidity. The mixed solution was kept stirring at room temperature for about 1 h, and then certain amount of MMA was added. The typical molar ratio was TEOS:GPTMS:MMA:ethanol:water:DMF:hydrochloric
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Q. Zhang, Z. Jiang / Materials Chemistry and Physics 69 (2001) 95–98
Fig. 1. Optical configuration of the solid-state dye laser.
acid=1:0.5:0.5:6:10:3:0.01. Sample B has the same dye concentration as sample A. Laser dyes PM 597 is chosen as the laser gain medium because of its good chemical stability, high quantum fluorescence yields, and rather weak triplet–triplet absorption bands in its fluorescence (laser action) spectral region [10,11].
Table 1 Pore distribution parameters of the sol–gel materials Sample
Specific surface (m2 g−1 )
Pore volume (cm2 g−1 )
Average pore size (nm)
A B
0.37 0.06
0.0114 0.0015
4.4 3.1
2.2. Characterization The surface areas, pore volumes and pore size distributions of the samples were measured by nitrogen adsorption method with a Micromeritics Corp. Model Autosorb 2000 physical absorption analyzer. The special surface area was estimated by the Brunauer–Emmett–Teller (BET) method. The fluorescence spectra of dye-doped samples were measured with a Hitachi 850 spectrophotometer.
accounts for porosity in the 0.9∼20 nm range for sample A. The BJH desorption average pore diameter is only 4.4 nm. The BET specific surface areas, pore volumes and average pore size of the dye-doped xerogels are also given in Table 1. The smaller, more homogeneous pore size and the compact texture are mainly effected by DMF, a drying control chemical additive, which decreases the hydrolysis
2.3. Pump source for the organic dye laser Dye laser usually pumped by excimer laser, argon laser, nitrogen laser, or flash-lamp-pumped solid-state laser. All these pump sources are large in volume and are not all-solidstate lasers, which makes them difficult to operate. The advent and rapid improvement of laser diodes has caused great interest in the development of all-solid-state laser [12]. A laser-diode array, side-pumped, frequency-doubled Nd:YAG Q-switch laser with a 10 ns pulse width was selected as the pump source. The laser energy generated from the pumped xerogel slab was measured using a Molectron energy meter to an accuracy of 0.01 mJ. The experimental setup with a cavity length of 15 cm is presented schematically in Fig. 1. The input mirror (M3 ) was coated to be high reflective at the laser wavelength 560−650 nm and high transmission in the 532 nm spectral region (T=70%). The output coupler (M4 ) had transmission of 50−60% at the laser wavelength. The dye-doped solid-state materials sample is about 2 mm in thickness and 4 cm in diameter.
3. Experimental results and discussions Since N2 adsorption method is applicable to pores smaller than ∼100 nm, the BET surface areas and pore size distributions shown in Table 1 and Fig. 2 are related to the pores in that range. As can be seen in Fig. 2, N2 physisorption
Fig. 2. BET surface area (a), and pore size distribution (b), of the PM 597-doped silica (A) and Ormosil (B) slabs.
Q. Zhang, Z. Jiang / Materials Chemistry and Physics 69 (2001) 95–98
Fig. 3. Fluorescent emission spectra of two PM 597 doped xerogels.
and polycondensation reaction rates, greatly reduces the specific surface, pore volumes and average pore size of the dye-doped xerogels. The addition of DMF also decreases the magnitude of the capillary stress and minimizes the differential capillary stresses to protect the drying gel from crack generation [13,14]. In comparison with sample A, sample B has a smaller pore size, less pore volume and specific surface. And it has a denser structure, a better mechanical properties and flexibility. It is found that the use of GPTMS has great effect on the microstructure of the xerogels during the hydrolysis–condensation process. The organic and inorganic groups are bonded through C–O–Si bond and organic chains can be regarded as the extension of silica network, when organic modifier, GPTMS, was added [15]. Furthermore, the addition of MMA can efficiently improve the mechanical properties of Ormosil xerogel, due to the embedding of MMA in the micropores of silica matrix. Fig. 3 illustrates the fluorescence spectra of PM597 doped silica (sample A) and Ormosil (sample B). The maximum fluorescent intensity appears at 571 nm in both the samples. Compared with dye in silica slab, a slightly red shifted fluorescence spectrum was observed in dye-doped Ormosil. The maximum fluorescent emission intensity of sample B is stronger than that of sample A, due to the addition of GPTMS and MMA, this is in agreement with recent study in Ref. [15]. Laser oscillation was easily achieved in all hand-polished pyrromethene-doped samples with thresholds pump energy about 0.01 mJ. Two most important characteristics of the active elements in dye lasers are the pumping-conversion efficiency and lifetime, the critical number of pulses before photo-bleaching, which can be defined as the number of laser pulses at which the conversion efficiency decreases by more than 10% of the initial values. Fig. 4 indicates the linear dependence of the output laser energy on the input pump energy. A lasing threshold pumped energy of 0.01 mJ (532 nm) and a slope efficiency, which is defined as the ratio of output energy versus pump energy, of 61% from
97
Fig. 4. Dependence of the laser output energy on the pump pulse energy: the dye-doped silica slab (A) and the dye-doped Ormosil slab (B).
laser dye PM597 doped Ormosil slab was observed. Meanwhile, the slope efficiency for the dye-doped silica slab is 45%. The Ormosil-based laser wavelength is tunable from 550 to 600 nm, with a central wavelength of 570 nm and a bandwidth greater than 50 nm FWHM. The tuning curve with pump energy 0.5 mJ of the dye-doped Ormosil is illustrated in Fig. 5. The compact gel textures, better mechanical properties and flexibility of dye-doped Ormosil slab, sample B, which improve the polished-surface quality of dye laser xerogels and lessening scatters, is an essential factor for the improvement of laser efficiency of the dye-doped solid-state laser materials. To evaluate the laser output stability of the Ormosil laser materials, the laser output variation as the function of the number of the pump pulse shots were also measured, the pump repetition rate of Nd:YAG Q-switch laser was kept at 1 Hz. Fig. 6 shows the results corresponding to pump pulse energies of 1 and 10 mJ, respectively. The reduction in laser output energy is only 10% after 2.0×104 pulses at pump energy of 1 J cm−2 . When the laser output energy is decreased to its 10% initial value, we stopped the pump radiation and waited for a few minutes. The laser output energy regained its initial value when we restarted the pump. During experiments, a black conical-like region was formed in
Fig. 5. Tuning curve of the dye-doped Ormosil at 0.5 mJ pump energy.
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Q. Zhang, Z. Jiang / Materials Chemistry and Physics 69 (2001) 95–98
dye laser by means of a sol–gel process. More complete hydrolysis–condensation reactions and lower porosity of the resulting xerogels can be achieved, when using GPTMS and MMA as organic modifier. Using the new materials as laser gain media, we have demonstrated what we believe to be the first high efficiency pulsed emission of PM 597-doped solid-state materials pumped by a laser-diode side-pumping, Q-switched, and frequency-doubled Nd:YAG laser at room temperature. This should be made possible to obtain new laser output by low cost device based on the dye-doped solid-state laser materials. Fig. 6. Dependence of laser output energy on the number of shots at pump pulse energies of 1 and 10 mJ, respectively.
all samples after photo-stability measurements due to gradual degradation of organic dye molecules. Hu and Jiang [4], Rahn and King [16] mentioned similar phenomenon. When a dye-doped laser glass is excited, dye molecules gradually degrade. A new gain region is exposed to the laser pulse, and the excited region moves through the sample. As the center part is degraded quickly because of a thermal effect, a conical-like region is formed. The same result was obtained in the dye-doped silica slab, sample A. To eliminate the heat accumulation during the repetition pumping, we are going to design a rotary dye-doped cylinder laser, thus, the excitation area of the cylinder becomes alternatively pumped.
4. Conclusions In summary, laser dye PM 597 doped solid-state material has been developed for visible wavelength solid-state
References [1] D. Avnir, D. Levy, R. Reisfeld, J. Phys. Chem. 88 (1984) 5956. [2] M. Canva, P. Georges, J.F. Perelgritz, Appl. Opt. 34 (1995) 428. [3] E.T. Knobbe, B. Dunn, P.D. Fuqua, F. Nishida, Appl. Opt. 29 (1990) 2729. [4] L. Hu, Z. Jiang, Opt. Commun. 148 (1998) 275. [5] Q. Zhang, L. Hu, Z. Jiang, Chin. Phys. Lett. 16 (1999) 384. [6] R.E. Hermes, T.H. Alik, S. Chandrz, J.A. Hutchiuson, Appl. Phys. Lett. 62 (1993) 877. [7] W. Hu, H. Ye, Z. Jiang, Appl. Opt. 36 (1997) 5794. [8] A. Dubois, M. Canva, A. Brun, F. Chaput, J.P. Boilot, Appl. Opt. 35 (1996) 3139. [9] M. Faloss, M. Canva, P. Gerorges, A. Brun, Appl. Opt. 36 (1997) 6760. [10] T.G. Pavlopoulos, J.H. Boyer, M. Shah, Appl. Opt. 29 (1990) 3885. [11] T.G. Pavlopoulos, J.H. Boyer, K. Thangaraj, Appl. Opt. 31 (1992) 7089. [12] D.W. Hughes, J.R.M. Barr, J. Phys. D 25 (1992) 563. [13] A.V. Rao, H.M. Sakhare, A.K. Tamhankar, Mater. Chem. Phys. 60 (1999) 268. [14] S. Wu, C. Zhu, J. Mater. Sci. Lett. 18 (1999) 281. [15] Y. Zhang, M. Wang, Mater. Lett. 42 (2000) 86. [16] M.D. Rahn, T.A. King, Appl. Opt. 34 (1995) 8260.
Pyrromethene doped silica laser glasses by sol–gel processes Qinyuan Zhang a,∗ , Zhonghong Jiang b a
b
Pohl Institute of Solid State Physics, Tongji University, Shanghai 200092, PR China Shanghai Institute of Optics and Fine Mechanics, Academic Sinica, Shanghai 201800, PR China Received 13 April 2000; received in revised form 12 June 2000; accepted 15 June 2000
Abstract A new laser dye pyrromethene impregnated solid-state material has been developed for visible wavelength solid-state dye laser by means of a sol–gel process. The use of N,N0 -dimethylformamide (DMF) as a drying control chemical additive was found to be effective in order to obtain the best quality silica and organically modified silicate (Ormosil) in terms of monolithicity, transparency and density. The addition of ␥-glycidyl-propyl-trimethoxisilane (GPTMS) in Ormosil can efficiently increase the luminescent emission intensity. Using these new materials as laser gain media, an all-solid-state, efficient, compact, long-lifetime and tunable dye laser has been demonstrated. © 2001 Elsevier Science B.V. All rights reserved. Keywords: Laser glass; Silica; Pyrromethene; Dye laser; Structural properties
1. Introduction For many years, considerable interest has been given in the area of solid-state dye lasers [1–6]. Compared to the liquid host, the use of a solid-state host for laser dyes presents obvious technical advantages, such as compactness, manageability and free from toxicity. The concentration quenching in liquid solutions originating from molecule collisions is effectively reduced, when organic molecules are embedded in a rigid matrix [1,7–9]. For practical applications, it is extremely important that these structures can be impregnated by organic dye to very high concentration without a noticeable quenching of fluorescence. Although encouraging process has been achieved in dye-doped laser based on polymer materials, low thermal-conductivity and laser damage threshold still remain as main defect [6]. And the main problems of the solid-state xerogels dye lasers are related to the control of the material particle size, the mechanical processing and laser-conversion efficiency. In this paper, a new laser dye pyrromethene 597 (PM 597) doped solid-state silica and organically modified silicate (Ormosil) were prepared using the sol–gel method. A drying control chemical additive, N,N0 -dimethylformamide (DMF), was used in order to obtain the best quality silica and Ormosil. And ␥-glycidyl-propyl-trimethoxisilane (GPTMS) and methyl methacrylate monomers (MMA) were added to im∗ Corresponding author. Fax: +86-021-6598-6071. E-mail address: [email protected] (Q. Zhang).
prove the mechanical and fluorescence emission properties of dye-doped xerogels. The structural properties and laser performances of those material pumped by a laser-diode array side-pumping, frequency-doubled Nd:YAG Q-switch laser with a wavelength of 532 nm were measured and demonstrated.
2. Experimental details 2.1. Sample preparation The dye-doped silica, sample A, was prepared from the hydrolysis and condensation processes of tetraethoxysilane (TEOS). In the process, the molar ratio of TEOS:ethanol: water:DMF:hydrochloric acid was selected to be 1:6:10:3: 0.01. After stirring the mixed solution mentioned above for 1 h, organic dye PM 597 dissolved in DMF was added with a concentration of 2.0×10−4 mol l−1 . To obtain homogeneous sol, ultrasonic bathing was used to insure dye distribution. The mixture gelled within 48 h at 60◦ C. For the preparation of dye-doped Ormosil, sample B, TEOS and GPTMS were first mixed with half volume of ethanol; then water and hydrochloric acid dissolved in the remaining ethanol was added to the TEOS–GPTMS–EtOH solution slowly while stirring to avoid the occurrence of turbidity. The mixed solution was kept stirring at room temperature for about 1 h, and then certain amount of MMA was added. The typical molar ratio was TEOS:GPTMS:MMA:ethanol:water:DMF:hydrochloric
0254-0584/01/$ – see front matter © 2001 Elsevier Science B.V. All rights reserved. PII: S 0 2 5 4 - 0 5 8 4 ( 0 0 ) 0 0 3 8 0 - 1
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Q. Zhang, Z. Jiang / Materials Chemistry and Physics 69 (2001) 95–98
Fig. 1. Optical configuration of the solid-state dye laser.
acid=1:0.5:0.5:6:10:3:0.01. Sample B has the same dye concentration as sample A. Laser dyes PM 597 is chosen as the laser gain medium because of its good chemical stability, high quantum fluorescence yields, and rather weak triplet–triplet absorption bands in its fluorescence (laser action) spectral region [10,11].
Table 1 Pore distribution parameters of the sol–gel materials Sample
Specific surface (m2 g−1 )
Pore volume (cm2 g−1 )
Average pore size (nm)
A B
0.37 0.06
0.0114 0.0015
4.4 3.1
2.2. Characterization The surface areas, pore volumes and pore size distributions of the samples were measured by nitrogen adsorption method with a Micromeritics Corp. Model Autosorb 2000 physical absorption analyzer. The special surface area was estimated by the Brunauer–Emmett–Teller (BET) method. The fluorescence spectra of dye-doped samples were measured with a Hitachi 850 spectrophotometer.
accounts for porosity in the 0.9∼20 nm range for sample A. The BJH desorption average pore diameter is only 4.4 nm. The BET specific surface areas, pore volumes and average pore size of the dye-doped xerogels are also given in Table 1. The smaller, more homogeneous pore size and the compact texture are mainly effected by DMF, a drying control chemical additive, which decreases the hydrolysis
2.3. Pump source for the organic dye laser Dye laser usually pumped by excimer laser, argon laser, nitrogen laser, or flash-lamp-pumped solid-state laser. All these pump sources are large in volume and are not all-solidstate lasers, which makes them difficult to operate. The advent and rapid improvement of laser diodes has caused great interest in the development of all-solid-state laser [12]. A laser-diode array, side-pumped, frequency-doubled Nd:YAG Q-switch laser with a 10 ns pulse width was selected as the pump source. The laser energy generated from the pumped xerogel slab was measured using a Molectron energy meter to an accuracy of 0.01 mJ. The experimental setup with a cavity length of 15 cm is presented schematically in Fig. 1. The input mirror (M3 ) was coated to be high reflective at the laser wavelength 560−650 nm and high transmission in the 532 nm spectral region (T=70%). The output coupler (M4 ) had transmission of 50−60% at the laser wavelength. The dye-doped solid-state materials sample is about 2 mm in thickness and 4 cm in diameter.
3. Experimental results and discussions Since N2 adsorption method is applicable to pores smaller than ∼100 nm, the BET surface areas and pore size distributions shown in Table 1 and Fig. 2 are related to the pores in that range. As can be seen in Fig. 2, N2 physisorption
Fig. 2. BET surface area (a), and pore size distribution (b), of the PM 597-doped silica (A) and Ormosil (B) slabs.
Q. Zhang, Z. Jiang / Materials Chemistry and Physics 69 (2001) 95–98
Fig. 3. Fluorescent emission spectra of two PM 597 doped xerogels.
and polycondensation reaction rates, greatly reduces the specific surface, pore volumes and average pore size of the dye-doped xerogels. The addition of DMF also decreases the magnitude of the capillary stress and minimizes the differential capillary stresses to protect the drying gel from crack generation [13,14]. In comparison with sample A, sample B has a smaller pore size, less pore volume and specific surface. And it has a denser structure, a better mechanical properties and flexibility. It is found that the use of GPTMS has great effect on the microstructure of the xerogels during the hydrolysis–condensation process. The organic and inorganic groups are bonded through C–O–Si bond and organic chains can be regarded as the extension of silica network, when organic modifier, GPTMS, was added [15]. Furthermore, the addition of MMA can efficiently improve the mechanical properties of Ormosil xerogel, due to the embedding of MMA in the micropores of silica matrix. Fig. 3 illustrates the fluorescence spectra of PM597 doped silica (sample A) and Ormosil (sample B). The maximum fluorescent intensity appears at 571 nm in both the samples. Compared with dye in silica slab, a slightly red shifted fluorescence spectrum was observed in dye-doped Ormosil. The maximum fluorescent emission intensity of sample B is stronger than that of sample A, due to the addition of GPTMS and MMA, this is in agreement with recent study in Ref. [15]. Laser oscillation was easily achieved in all hand-polished pyrromethene-doped samples with thresholds pump energy about 0.01 mJ. Two most important characteristics of the active elements in dye lasers are the pumping-conversion efficiency and lifetime, the critical number of pulses before photo-bleaching, which can be defined as the number of laser pulses at which the conversion efficiency decreases by more than 10% of the initial values. Fig. 4 indicates the linear dependence of the output laser energy on the input pump energy. A lasing threshold pumped energy of 0.01 mJ (532 nm) and a slope efficiency, which is defined as the ratio of output energy versus pump energy, of 61% from
97
Fig. 4. Dependence of the laser output energy on the pump pulse energy: the dye-doped silica slab (A) and the dye-doped Ormosil slab (B).
laser dye PM597 doped Ormosil slab was observed. Meanwhile, the slope efficiency for the dye-doped silica slab is 45%. The Ormosil-based laser wavelength is tunable from 550 to 600 nm, with a central wavelength of 570 nm and a bandwidth greater than 50 nm FWHM. The tuning curve with pump energy 0.5 mJ of the dye-doped Ormosil is illustrated in Fig. 5. The compact gel textures, better mechanical properties and flexibility of dye-doped Ormosil slab, sample B, which improve the polished-surface quality of dye laser xerogels and lessening scatters, is an essential factor for the improvement of laser efficiency of the dye-doped solid-state laser materials. To evaluate the laser output stability of the Ormosil laser materials, the laser output variation as the function of the number of the pump pulse shots were also measured, the pump repetition rate of Nd:YAG Q-switch laser was kept at 1 Hz. Fig. 6 shows the results corresponding to pump pulse energies of 1 and 10 mJ, respectively. The reduction in laser output energy is only 10% after 2.0×104 pulses at pump energy of 1 J cm−2 . When the laser output energy is decreased to its 10% initial value, we stopped the pump radiation and waited for a few minutes. The laser output energy regained its initial value when we restarted the pump. During experiments, a black conical-like region was formed in
Fig. 5. Tuning curve of the dye-doped Ormosil at 0.5 mJ pump energy.
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Q. Zhang, Z. Jiang / Materials Chemistry and Physics 69 (2001) 95–98
dye laser by means of a sol–gel process. More complete hydrolysis–condensation reactions and lower porosity of the resulting xerogels can be achieved, when using GPTMS and MMA as organic modifier. Using the new materials as laser gain media, we have demonstrated what we believe to be the first high efficiency pulsed emission of PM 597-doped solid-state materials pumped by a laser-diode side-pumping, Q-switched, and frequency-doubled Nd:YAG laser at room temperature. This should be made possible to obtain new laser output by low cost device based on the dye-doped solid-state laser materials. Fig. 6. Dependence of laser output energy on the number of shots at pump pulse energies of 1 and 10 mJ, respectively.
all samples after photo-stability measurements due to gradual degradation of organic dye molecules. Hu and Jiang [4], Rahn and King [16] mentioned similar phenomenon. When a dye-doped laser glass is excited, dye molecules gradually degrade. A new gain region is exposed to the laser pulse, and the excited region moves through the sample. As the center part is degraded quickly because of a thermal effect, a conical-like region is formed. The same result was obtained in the dye-doped silica slab, sample A. To eliminate the heat accumulation during the repetition pumping, we are going to design a rotary dye-doped cylinder laser, thus, the excitation area of the cylinder becomes alternatively pumped.
4. Conclusions In summary, laser dye PM 597 doped solid-state material has been developed for visible wavelength solid-state
References [1] D. Avnir, D. Levy, R. Reisfeld, J. Phys. Chem. 88 (1984) 5956. [2] M. Canva, P. Georges, J.F. Perelgritz, Appl. Opt. 34 (1995) 428. [3] E.T. Knobbe, B. Dunn, P.D. Fuqua, F. Nishida, Appl. Opt. 29 (1990) 2729. [4] L. Hu, Z. Jiang, Opt. Commun. 148 (1998) 275. [5] Q. Zhang, L. Hu, Z. Jiang, Chin. Phys. Lett. 16 (1999) 384. [6] R.E. Hermes, T.H. Alik, S. Chandrz, J.A. Hutchiuson, Appl. Phys. Lett. 62 (1993) 877. [7] W. Hu, H. Ye, Z. Jiang, Appl. Opt. 36 (1997) 5794. [8] A. Dubois, M. Canva, A. Brun, F. Chaput, J.P. Boilot, Appl. Opt. 35 (1996) 3139. [9] M. Faloss, M. Canva, P. Gerorges, A. Brun, Appl. Opt. 36 (1997) 6760. [10] T.G. Pavlopoulos, J.H. Boyer, M. Shah, Appl. Opt. 29 (1990) 3885. [11] T.G. Pavlopoulos, J.H. Boyer, K. Thangaraj, Appl. Opt. 31 (1992) 7089. [12] D.W. Hughes, J.R.M. Barr, J. Phys. D 25 (1992) 563. [13] A.V. Rao, H.M. Sakhare, A.K. Tamhankar, Mater. Chem. Phys. 60 (1999) 268. [14] S. Wu, C. Zhu, J. Mater. Sci. Lett. 18 (1999) 281. [15] Y. Zhang, M. Wang, Mater. Lett. 42 (2000) 86. [16] M.D. Rahn, T.A. King, Appl. Opt. 34 (1995) 8260.