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ZnSe:Co2+—nonlinear optical absorber for giant-pulse eye-safe lasers
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Optics & Laser Technology 35 (2003) 169 – 172 www.elsevier.com/locate/optlastec
ZnSe : Co2+— nonlinear optical absorber for giant-pulse eye-safe lasers Z. Mierczyka , A. Majchrowskib , I.V. Kitykc;∗ , W. Gruhnc a Institute
b Institute
of Optoelectronics, Military University of Technology, 2 Kaliskiego Str., 00-908 Warsaw, Poland of Applied Physics, Military University of Technology, 2 Kaliskiego Str., 00-908 Warsaw, Poland c Institute of Physics, WSP, Al. Armii Krajowej 13/15, 42-201, Cze / stochowa, Poland
Received 16 November 2001; received in revised form 14 August 2002; accepted 1 October 2002
Abstract ZnSe : Co2+ crystals were grown by the Bridgeman technique. Optical absorption measurements showed non-uniform distribution of Co2+ ions along the as-grown crystals. Using Pfann formula the distribution coeAcient of Co2+ ions between crystal and melt was estimated to be 0.5. Transmission dependence on laser power at = 1535 nm was investigated for diCerent Co2+ ions concentration. Maximal Enal transmission was found for ZnSe samples containing 1:6 × 1019 cm−3 of Co2+ ions. Obtained ZnSe : Co2+ samples were used as saturable absorbers to generate giant-pulse eye-safe laser radiation. KIGRE QE-7S rod placed in resonator cavity was used as active element. ? 2002 Elsevier Science Ltd. All rights reserved. Keywords: ZnSe : Co2+ ; Nonlinear optical absorber; Eye-safe lasers
1. Introduction
2. Crystal growth
Saturable absorbers containing Co2+ or U2+ ions have been arising much of interest recently due to their potential applications in giant-pulse laser generation [1]. Several crystals containing Co2+ ions allow to obtain such generation near 1:5 m, what is of great importance because this radiation is “eye-safe” [2]. Among these crystals one can distinguish two groups of absorbers: slow absorbers like LaGaO3 : Co2+ with relaxation time of about 12 s [3], and fast absorbers like YAG : Co2+ having relaxation time around 1 ns [4]. Comparing with other nonlinear absorbers, Co2+ doped materials are characterized by the highest absorption cross-section. This property is of great importance when materials for applications in lasers operating in the giant-pulse regime at the wavelength of 1:5 m are considered. The paper describes our eCorts to obtain crystalline ZnSe : Co2+ and investigations of its nonlinear absorption dependence on Co2+ concentration. Band structure simulations of the basic spectroscopic properties of ZnSe : Co2+ crystals will be reported in the other paper.
Bridgeman technique was used to grow ZnSe : Co2+ crystals from graphite crucibles under argon ambient atmosphere (5 MPa). The details of the technology were described by Demianiuk [5]. As-grown transparent crystals had diameter and length of 10 and 60 mm, respectively. They were cut with use of wire saw into 1 mm thick slices perpendicularly to the growth direction, ground and optically polished. Co2+ ions concentration in ZnSe : Co2+ samples was calculated by means of optical measurements, which results are described in the next paragraph. The distribution of Co2+ ions in ZnSe : Co2+ ingots proved to be very non-uniform along the growth direction, changing from 1:3 × 1019 cm−3 in the initial part of the crystal up to 2:7 × 1019 cm−3 at the end part of the crystal. Using Pfann formula [6]:
∗
Corresponding author. E-mail address: [email protected] (I.V. Kityk).
C = kc0 (1 − g)k−1 ;
(1)
where c0 is the initial concentration of the dopant in the melt, k the eCective coeAcient of dopant distribution between crystal and melt, g the fraction of crystallization, C the dopant concentration in the sample corresponding to g fraction, we were able to estimate the eCective distribution coeAcient of Co2+ ions in ZnSe to be near 0.5. This low value of the coeAcient caused strong repulsion of Co2+ ions from the growing crystal. As the result the concentration of Co2+
0030-3992/03/$ - see front matter ? 2002 Elsevier Science Ltd. All rights reserved. doi:10.1016/S0030-3992(02)00167-6
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Z. Mierczyk et al. / Optics & Laser Technology 35 (2003) 169 – 172
Fig. 1. Schematic diagram of energy bands of Co2+ ions in ZnSe : Co2+ and absorption spectrum of ZnSe : Co2+ (0:05 at%) crystal.
ions in the melt increased during the crystallization, and in consequence it also increased in the growing crystal. 3. Optical investigations Absorption spectra of three ZnSe : Co2+ samples were measured with use of the Perkin Elmer Lamda-900 (R = 1 nm) and 1725-X FT-IR (1=R=1 cm−1 ) spectrometers in the range 200 –3000 and 1500 –2000 nm, respectively. Fig. 1 shows absorption spectrum of ZnSe :Co2+ crystal containing 0:05 at% of Co2+ . One can see three bands which correspond with the following transitions: 4 A2 → 4 T1 (4 P) (700 –800 nm), 4 A2 → 4 T1 (1500 –2100 nm), and 4 A2 → 4 T2 (2500 –2800 nm). Broad absorption band 1500 –2100 nm was investigated from the point of view of nonlinear absorption in ZnSe : Co2+ . The changes of light transmission vs. the laser probe power were investigated. As a source of probe laser beam giant pulse KIGRE MR-253 laser generating on the erbium glass was used. Mechanical chopper (rotating prism) allowed to generate 25 ns pulses having 8 mJ energy. The pump density power of the laser beam was varied from 0:17 MW=cm2 to 1:26 GW=cm2 , what corresponds to the energy power densities in the range of 0.004 –31:2 J=cm2 . In Fig. 2 the measurement setup, while in Fig. 3 the results of transmission changes versus the laser beam power at 1535 nm for three ZnSe : Co2+ having different Co2+ concentrations are presented, respectively. The experimentally obtained parameters of transmission vs. the energy density E were compared with the Frantz-Nodvik [7] dependence: E Es ln 1 + exp T= − 1 · T0 ; (2) E Es where E is the energy density incident on the absorber, T the transmission of the absorber for the given energy power
E, T0 the incident transmission (for the low power of light), and Es the saturation energy: Es =
h ;
(3)
where is the actual laser beam cross-section. An analysis of the nonlinear absorber that is characterized by additional absorption maximum (the second excited level) with the relatively short lifetime (assuming the long-lived feature of the ground state) can be done analogously to the Avizonis and Grotbeck models [8] integrating the transport equation together with the material equations of the medium [9,10]. The variations of the pump powers in the nonlinear absorbers may be described by equation [11]: dE
2 E
2
1 E = −h N0 + 1− 1 − exp − dz
1 h h −E;
(4)
where E is the energy density, N0 the total number of the absorption centres N0 = N1 + N2 , N1 the population of the Erst excited level, N2 the population of the second excited level, h the photon energy, 1 ≡ GSA the absorption cross-section for the optical transitions from the ground state, 2 ≡ ESA the absorption cross-section for the transitions from the excited state, and the non-active losses coeAcient. The analysis of the spectroscopic parameters of nonlinear absorbers described by the diCerential Eq. (4) was done by the method of nonlinear optimization with limitations, resolving the inverse task consisting in the evaluation of optical constants such as absorption cross-sections GSA , ESA , and concentration of absorption centres N0 . At the same time we have taken into account the scattering losses obtained from the measurements of pump dependences of the transmission for the incident probe light. In Fig. 3 the theoretical calculations are compared with measured dependences.
Z. Mierczyk et al. / Optics & Laser Technology 35 (2003) 169 – 172
171
Fig. 2. Experimental setup for measurements of transmission changes dependence on energy density of incident laser radiation: ME—energy meter Laser Precission Co., DP—piroelectric detector, OSC—oscilloscope LeCroy 9350AM (500 MHz), FD—germanium photodiode HAMAMATSU, F—Elter, L1, L2, and D—optical set with diaphragm.
Fig. 3. Changes of transmission of ZnSe : Co2+ samples under 1535 nm laser radiation. Experimental result (EXP) were compared with Frantz–Nodvik dependence (F–N): A—concentration of Co2+ ions; 1:3 · 1019 cm−3 ; B—concentration of Co2+ ions; 1:7 · 1019 cm−3 ; C—concentration of Co2+ ions; 2:8 · 1019 cm−3 . Table 1 ZnSe : Co2+ parameters
Parameter
ZnSe : Co2+ (A)
ZnSe : Co2+ (B)
ZnSe : Co2+ (C)
Initial transmission Absorption cross-section
61.4%
GSA = 6:5 · 10−19 cm2
ESA = 0:5 · 10−19 cm2 = 2:2 · 10−3 cm−1 N0 = 1:3 · 1019 cm−3
57.0%
GSA = 6:5 · 10−19 cm2
ESA = 0:5 · 10−19 cm2 = 0:2 · 10−3 cm−1 N0 = 1:6 · 1019 cm−3
41.7%
GSA = 6:5 · 10−19 cm2
ESA = 0:5 · 10−19 cm2 = 2:7 · 10−3 cm−1 N0 = 2:7 · 1019 cm−3
Es = 0:1 J=cm2
Es = 0:1 J=cm2
Es = 0:1 J=cm2
CoeAcient of non-active losses Concentration of absorbing centres Saturation energy
Measured and estimated parameters for ZnSe : Co2+ are also compared in Table 1. Also generation characteristics of erbium laser (KIGRE QE-7S rod of 4 mm in diameter and 55 mm long) and passive ZnSe : Co2+ modulator were measured. Investigations
were carried out in CDDN laser head, the scheme of which is shown in Fig. 4. Fig. 5 illustrates results of investigations of free-running generation in the CDDN resonator system with the KIGRE QE-7S rod.
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Z. Mierczyk et al. / Optics & Laser Technology 35 (2003) 169 – 172
4. Conclusion
Fig. 4. Scheme of CDDN laser resonator used for investigation of free-running and giant-pulse generation of 1535 nm radiation.
ZnSe : Co2+ single crystals grown by us were found to be good nonlinear optical absorbers, which can be used as saturable absorber in giant-pulse “eye-safe” lasers. The distribution of Co2+ ions along the as-grown ZnSe : Co2+ ingots was non-uniform, with distribution coeAcient between crystal and melt near 0.5. Changes of transmission of ZnSe : Co2+ samples under 1535 nm laser radiation showed strong dependence on concentration of Co2+ ions, with maximal Enal transmission for Co2+ ions concentration equal to 1:6 × 10−19 cm−3 . Generation of giant-pulse laser radiation ( = 30 ns, output energy 8 mJ) in the CDDN resonator system with the KJGRE QE-7S rod was demonstrated. Acknowledgements This work was supported by Polish State Committee on Science—project No. 0 T00A 013 22. References
Fig. 5. Results of investigations of free-running generation in the CDDN resonator system with KIGRE QE-7S rod.
In the CDDN resonator with KIGRE QE-7S erbium glass rod of 4 mm diameter and 55 mm length with ZnSe : Co2+ modulator of initial transmission of T0 = 57:7%, the generation of the giant pulses with pulse duration 30 ns and output energy 8 mJ was obtained.
[1] Burshtein Z, Shimony Y, Feldman R, Krupkin V, Glushko A, Galun E. Opt Mater 2001;15:285. [2] Birnbaum M, Camargo MB, Lee S, Unlu F, Stultz RD. OSA TOPS, Vol. 10, Advanced Solid State Lasers, 1997. p. 148. [3] Golab S, Mierczyk Z, Ryba-Romanowski W. Phys Stat Sol A 2000;179:463. [4] Stultz RD, Camargo MD, Birnbaum M. OSA Proceedings on Advanced Solid-State Lasers, Vol. 24, 1995. p. 460. [5] Demianiuk M. Mater Res Bull 1990;25:337. [6] Pfann WG. Principles of zone melting. Trans Am Inst Mining Met Eng 1952;194:747. [7] Frantz L, Nodvik JS. J Appl Phys 1963;34:2346. [8] Avizonis PV, Grotbeck RL. J Appl Phys 1996;37:687. [9] Mokhailov VP, Zhavoronkov NI, Kuleshov NV, Avtuch AS, Shcherbitsky VG, Minkov BI. Opt Quantum Electron 1995;27:767. [10] Buzelis R, Dementev A, Kosenko E, Murauskas E, Ivanauskas F, Radzionas M. Lithuanian J Phys 1997;37:291. [11] Kuo YK, Huang MF, Bidram M. IEEE J Quantum Electron 1995;31:657.
Optics & Laser Technology 35 (2003) 169 – 172 www.elsevier.com/locate/optlastec
ZnSe : Co2+— nonlinear optical absorber for giant-pulse eye-safe lasers Z. Mierczyka , A. Majchrowskib , I.V. Kitykc;∗ , W. Gruhnc a Institute
b Institute
of Optoelectronics, Military University of Technology, 2 Kaliskiego Str., 00-908 Warsaw, Poland of Applied Physics, Military University of Technology, 2 Kaliskiego Str., 00-908 Warsaw, Poland c Institute of Physics, WSP, Al. Armii Krajowej 13/15, 42-201, Cze / stochowa, Poland
Received 16 November 2001; received in revised form 14 August 2002; accepted 1 October 2002
Abstract ZnSe : Co2+ crystals were grown by the Bridgeman technique. Optical absorption measurements showed non-uniform distribution of Co2+ ions along the as-grown crystals. Using Pfann formula the distribution coeAcient of Co2+ ions between crystal and melt was estimated to be 0.5. Transmission dependence on laser power at = 1535 nm was investigated for diCerent Co2+ ions concentration. Maximal Enal transmission was found for ZnSe samples containing 1:6 × 1019 cm−3 of Co2+ ions. Obtained ZnSe : Co2+ samples were used as saturable absorbers to generate giant-pulse eye-safe laser radiation. KIGRE QE-7S rod placed in resonator cavity was used as active element. ? 2002 Elsevier Science Ltd. All rights reserved. Keywords: ZnSe : Co2+ ; Nonlinear optical absorber; Eye-safe lasers
1. Introduction
2. Crystal growth
Saturable absorbers containing Co2+ or U2+ ions have been arising much of interest recently due to their potential applications in giant-pulse laser generation [1]. Several crystals containing Co2+ ions allow to obtain such generation near 1:5 m, what is of great importance because this radiation is “eye-safe” [2]. Among these crystals one can distinguish two groups of absorbers: slow absorbers like LaGaO3 : Co2+ with relaxation time of about 12 s [3], and fast absorbers like YAG : Co2+ having relaxation time around 1 ns [4]. Comparing with other nonlinear absorbers, Co2+ doped materials are characterized by the highest absorption cross-section. This property is of great importance when materials for applications in lasers operating in the giant-pulse regime at the wavelength of 1:5 m are considered. The paper describes our eCorts to obtain crystalline ZnSe : Co2+ and investigations of its nonlinear absorption dependence on Co2+ concentration. Band structure simulations of the basic spectroscopic properties of ZnSe : Co2+ crystals will be reported in the other paper.
Bridgeman technique was used to grow ZnSe : Co2+ crystals from graphite crucibles under argon ambient atmosphere (5 MPa). The details of the technology were described by Demianiuk [5]. As-grown transparent crystals had diameter and length of 10 and 60 mm, respectively. They were cut with use of wire saw into 1 mm thick slices perpendicularly to the growth direction, ground and optically polished. Co2+ ions concentration in ZnSe : Co2+ samples was calculated by means of optical measurements, which results are described in the next paragraph. The distribution of Co2+ ions in ZnSe : Co2+ ingots proved to be very non-uniform along the growth direction, changing from 1:3 × 1019 cm−3 in the initial part of the crystal up to 2:7 × 1019 cm−3 at the end part of the crystal. Using Pfann formula [6]:
∗
Corresponding author. E-mail address: [email protected] (I.V. Kityk).
C = kc0 (1 − g)k−1 ;
(1)
where c0 is the initial concentration of the dopant in the melt, k the eCective coeAcient of dopant distribution between crystal and melt, g the fraction of crystallization, C the dopant concentration in the sample corresponding to g fraction, we were able to estimate the eCective distribution coeAcient of Co2+ ions in ZnSe to be near 0.5. This low value of the coeAcient caused strong repulsion of Co2+ ions from the growing crystal. As the result the concentration of Co2+
0030-3992/03/$ - see front matter ? 2002 Elsevier Science Ltd. All rights reserved. doi:10.1016/S0030-3992(02)00167-6
170
Z. Mierczyk et al. / Optics & Laser Technology 35 (2003) 169 – 172
Fig. 1. Schematic diagram of energy bands of Co2+ ions in ZnSe : Co2+ and absorption spectrum of ZnSe : Co2+ (0:05 at%) crystal.
ions in the melt increased during the crystallization, and in consequence it also increased in the growing crystal. 3. Optical investigations Absorption spectra of three ZnSe : Co2+ samples were measured with use of the Perkin Elmer Lamda-900 (R = 1 nm) and 1725-X FT-IR (1=R=1 cm−1 ) spectrometers in the range 200 –3000 and 1500 –2000 nm, respectively. Fig. 1 shows absorption spectrum of ZnSe :Co2+ crystal containing 0:05 at% of Co2+ . One can see three bands which correspond with the following transitions: 4 A2 → 4 T1 (4 P) (700 –800 nm), 4 A2 → 4 T1 (1500 –2100 nm), and 4 A2 → 4 T2 (2500 –2800 nm). Broad absorption band 1500 –2100 nm was investigated from the point of view of nonlinear absorption in ZnSe : Co2+ . The changes of light transmission vs. the laser probe power were investigated. As a source of probe laser beam giant pulse KIGRE MR-253 laser generating on the erbium glass was used. Mechanical chopper (rotating prism) allowed to generate 25 ns pulses having 8 mJ energy. The pump density power of the laser beam was varied from 0:17 MW=cm2 to 1:26 GW=cm2 , what corresponds to the energy power densities in the range of 0.004 –31:2 J=cm2 . In Fig. 2 the measurement setup, while in Fig. 3 the results of transmission changes versus the laser beam power at 1535 nm for three ZnSe : Co2+ having different Co2+ concentrations are presented, respectively. The experimentally obtained parameters of transmission vs. the energy density E were compared with the Frantz-Nodvik [7] dependence: E Es ln 1 + exp T= − 1 · T0 ; (2) E Es where E is the energy density incident on the absorber, T the transmission of the absorber for the given energy power
E, T0 the incident transmission (for the low power of light), and Es the saturation energy: Es =
h ;
(3)
where is the actual laser beam cross-section. An analysis of the nonlinear absorber that is characterized by additional absorption maximum (the second excited level) with the relatively short lifetime (assuming the long-lived feature of the ground state) can be done analogously to the Avizonis and Grotbeck models [8] integrating the transport equation together with the material equations of the medium [9,10]. The variations of the pump powers in the nonlinear absorbers may be described by equation [11]: dE
2 E
2
1 E = −h N0 + 1− 1 − exp − dz
1 h h −E;
(4)
where E is the energy density, N0 the total number of the absorption centres N0 = N1 + N2 , N1 the population of the Erst excited level, N2 the population of the second excited level, h the photon energy, 1 ≡ GSA the absorption cross-section for the optical transitions from the ground state, 2 ≡ ESA the absorption cross-section for the transitions from the excited state, and the non-active losses coeAcient. The analysis of the spectroscopic parameters of nonlinear absorbers described by the diCerential Eq. (4) was done by the method of nonlinear optimization with limitations, resolving the inverse task consisting in the evaluation of optical constants such as absorption cross-sections GSA , ESA , and concentration of absorption centres N0 . At the same time we have taken into account the scattering losses obtained from the measurements of pump dependences of the transmission for the incident probe light. In Fig. 3 the theoretical calculations are compared with measured dependences.
Z. Mierczyk et al. / Optics & Laser Technology 35 (2003) 169 – 172
171
Fig. 2. Experimental setup for measurements of transmission changes dependence on energy density of incident laser radiation: ME—energy meter Laser Precission Co., DP—piroelectric detector, OSC—oscilloscope LeCroy 9350AM (500 MHz), FD—germanium photodiode HAMAMATSU, F—Elter, L1, L2, and D—optical set with diaphragm.
Fig. 3. Changes of transmission of ZnSe : Co2+ samples under 1535 nm laser radiation. Experimental result (EXP) were compared with Frantz–Nodvik dependence (F–N): A—concentration of Co2+ ions; 1:3 · 1019 cm−3 ; B—concentration of Co2+ ions; 1:7 · 1019 cm−3 ; C—concentration of Co2+ ions; 2:8 · 1019 cm−3 . Table 1 ZnSe : Co2+ parameters
Parameter
ZnSe : Co2+ (A)
ZnSe : Co2+ (B)
ZnSe : Co2+ (C)
Initial transmission Absorption cross-section
61.4%
GSA = 6:5 · 10−19 cm2
ESA = 0:5 · 10−19 cm2 = 2:2 · 10−3 cm−1 N0 = 1:3 · 1019 cm−3
57.0%
GSA = 6:5 · 10−19 cm2
ESA = 0:5 · 10−19 cm2 = 0:2 · 10−3 cm−1 N0 = 1:6 · 1019 cm−3
41.7%
GSA = 6:5 · 10−19 cm2
ESA = 0:5 · 10−19 cm2 = 2:7 · 10−3 cm−1 N0 = 2:7 · 1019 cm−3
Es = 0:1 J=cm2
Es = 0:1 J=cm2
Es = 0:1 J=cm2
CoeAcient of non-active losses Concentration of absorbing centres Saturation energy
Measured and estimated parameters for ZnSe : Co2+ are also compared in Table 1. Also generation characteristics of erbium laser (KIGRE QE-7S rod of 4 mm in diameter and 55 mm long) and passive ZnSe : Co2+ modulator were measured. Investigations
were carried out in CDDN laser head, the scheme of which is shown in Fig. 4. Fig. 5 illustrates results of investigations of free-running generation in the CDDN resonator system with the KIGRE QE-7S rod.
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Z. Mierczyk et al. / Optics & Laser Technology 35 (2003) 169 – 172
4. Conclusion
Fig. 4. Scheme of CDDN laser resonator used for investigation of free-running and giant-pulse generation of 1535 nm radiation.
ZnSe : Co2+ single crystals grown by us were found to be good nonlinear optical absorbers, which can be used as saturable absorber in giant-pulse “eye-safe” lasers. The distribution of Co2+ ions along the as-grown ZnSe : Co2+ ingots was non-uniform, with distribution coeAcient between crystal and melt near 0.5. Changes of transmission of ZnSe : Co2+ samples under 1535 nm laser radiation showed strong dependence on concentration of Co2+ ions, with maximal Enal transmission for Co2+ ions concentration equal to 1:6 × 10−19 cm−3 . Generation of giant-pulse laser radiation ( = 30 ns, output energy 8 mJ) in the CDDN resonator system with the KJGRE QE-7S rod was demonstrated. Acknowledgements This work was supported by Polish State Committee on Science—project No. 0 T00A 013 22. References
Fig. 5. Results of investigations of free-running generation in the CDDN resonator system with KIGRE QE-7S rod.
In the CDDN resonator with KIGRE QE-7S erbium glass rod of 4 mm diameter and 55 mm length with ZnSe : Co2+ modulator of initial transmission of T0 = 57:7%, the generation of the giant pulses with pulse duration 30 ns and output energy 8 mJ was obtained.
[1] Burshtein Z, Shimony Y, Feldman R, Krupkin V, Glushko A, Galun E. Opt Mater 2001;15:285. [2] Birnbaum M, Camargo MB, Lee S, Unlu F, Stultz RD. OSA TOPS, Vol. 10, Advanced Solid State Lasers, 1997. p. 148. [3] Golab S, Mierczyk Z, Ryba-Romanowski W. Phys Stat Sol A 2000;179:463. [4] Stultz RD, Camargo MD, Birnbaum M. OSA Proceedings on Advanced Solid-State Lasers, Vol. 24, 1995. p. 460. [5] Demianiuk M. Mater Res Bull 1990;25:337. [6] Pfann WG. Principles of zone melting. Trans Am Inst Mining Met Eng 1952;194:747. [7] Frantz L, Nodvik JS. J Appl Phys 1963;34:2346. [8] Avizonis PV, Grotbeck RL. J Appl Phys 1996;37:687. [9] Mokhailov VP, Zhavoronkov NI, Kuleshov NV, Avtuch AS, Shcherbitsky VG, Minkov BI. Opt Quantum Electron 1995;27:767. [10] Buzelis R, Dementev A, Kosenko E, Murauskas E, Ivanauskas F, Radzionas M. Lithuanian J Phys 1997;37:291. [11] Kuo YK, Huang MF, Bidram M. IEEE J Quantum Electron 1995;31:657.