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Optical damage resistance in Zn:Nd:LiNbO3 laser crystals
Materials Chemistry and Physics 80 (2003) 11–14
Material science communication
Optical damage resistance in Zn:Nd:LiNbO3 laser crystals Xi-He Zhen a,∗ , Rui Wang b , Mei-Cheng Li a , Lian-Cheng Zhao a , Yu-Heng Xu b a
School of Material Science and Engineering, Harbin Institute of Technology, Harbin 150001, China b Department of Applied Chemistry and Electro Optics Research Center, Harbin Institute of Technology, Harbin 150001, China Received 29 April 2002; received in revised form 29 August 2002; accepted 10 September 2002
Abstract A series of Zn, Nd co-doped LiNbO3 crystals were grown by Czochralski technique with 0.5 mol% of Nd2 O3 and with 0, 4.0, and 7.0 mol% of ZnO, respectively. Their optical damage resistance was characterized by measurement of the photo-induced birefringence change. The optical damage resistance of Zn (7.0 mol%):Nd:LiNbO3 was much higher than that of Nd:LiNbO3 . The ultraviolet-visible (UV-Vis) absorption spectra and the infrared (IR) transmission spectra of Nd:Zn:LiNbO3 crystals were measured and investigated. The defects were discussed in this paper to explain the optical damage resistance in the Zn:Nd:LiNbO3 crystals. © 2002 Elsevier Science B.V. All rights reserved. Keywords: Zn:Nd:LiNbO3 ; Optical damage resistance; UV-Vis absorption spectra; IR transmission spectra
1. Introduction LiNbO3 is a widely studied optoelectronic material because of its technological applications. Doping with foreign ions modifies the optical properties of the matrix and makes the system useful for a great variety of application such as photorefractive devices [1], solid-state lasers [2] or optical waveguides [3]. Nowadays, LiNbO3 is the only optical material with a degree of development and so extensively investigated to serve as the basis for development of integrated optics, playing a similar role as Si did in the development of integrated electronics. Another potential application of LiNbO3 is in Nd3+ based compact diode-pumped self-frequency-doubled lasers which emit green radiation, useful for applications in optical data storage, undersea imaging, diagnosis in medicine, excitation sources to replace ion gas lasers for science and pumping of parametric oscillators and amplifiers [4]. Mg:Nd:LiNbO3 was the first system to be operated as a continuous wave self-frequency-doubled laser [5]. It is known that pure LiNbO3 presents a low threshold for photorefractive damage [6]. When LiNbO3 devices are used at high laser intensity, their performance is severely ∗ Corresponding author. Tel.: +86-451-6413-551; fax: +86-451-6221-048. E-mail address: [email protected] (X.-H. Zhen).
limited by the optical damage effect, which induces birefringence change and deforms the laser beams [7]. Some damage resistant impurities have been discovered, such as Mg [8], Zn [9], In [10,11], and Sc [12], which lead to a strong decrease of the photo damage in LiNbO3 and have been receiving a lot of interesting. The LiNbO3 crystal with 6.0 mol% concentration ZnO has the highest optical damage resistance property [13]. However, devices operating at room temperature are preferred. Although Nd3+ based lasers and amplifiers have been shown to reduce the optical quality and make difficult the growth of crystals sharing high optical quality and high Nd3+ concentration. For this reason, research on alternative codopant impurities and ways of avoiding photorefractive damage and still allow for stable and efficient laser operation at room temperature, are of major importance in the field and constitute an active field of research. The efficient and stable laser oscillation was obtained at room temperature and the long-term stability green radiation was obtained by self-frequency-doubling at room temperature in the Mg:Nd:LiNbO3 crystal [4]. In this paper, both doping with damage-resistant Zn and increasing Zn concentration are used to enhance the optical damage resistance in Zn:Nd:LiNbO3 crystals. The dependence of optical damage resistance on the concentration of ZnO is discussed. The UV-Vis absorption spectra and the IR transmission spectra of Nd:Zn:LiNbO3 crystals were measured to investigated the structure of the crystals.
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Table 1 Composition of raw materials of the LiNbO3 crystals the size of the samples Crystal
1#
2#
3#
4#
[ZnO] (mol%) [Nd2 O3 ] (mol%) Crystal size, Ø (mm)
0 0 30 × 50
0 0.50 30 × 40
4.0 0.50 30 × 35
7.0 0.50 30 × 40
Wafer size (mm)
12 × 12 × 1 12 × 12 × 5
12 × 12 × 1 12 × 12 × 5
12 × 12 × 1 12 × 12 × 5
12 × 12 × 1 12 × 12 × 5
2. Materials and methods 2.1. Crystal preparation The congruent LiNbO3 crystals with 0.5 mol% of Nd2 O3 and various concentration of ZnO were grown in air along c-direction in diameter-controlled Czochralski equipment using a SiC heater furnace. The raw materials used for crystal growth are Li2 CO3 (4 N purity), Nb2 O5 (4 N purity), ZnO (3 N purity), and Nd2 O3 (4 N purity). The compositions of raw material are shown in Table 1. To prepare doped LiNbO3 polycrystalline materials, all thoroughly mixed raw materials were put into a platinum (Pt) crucible (70 mm in diameter and 50 mm in height), and calcined at 700 and 1150 ◦ C for 2 h, respectively. The crystals were grown under the optimum technology conditions: temperature gradient of 35–40 ◦ C cm−1 , rotating rate of 18 rpm, and pulling rate of 1.5–2.0 mm h−1 . About 70% of melts were crystallized. After growth, the crystals were cooled to room temperature at a speed of 150 ◦ C h−1 . All of the crystals were clear, transparent, and inclusion-free. The crystals were placed in a furnace where the temperature gradient is below 5 ◦ C cm−1 for polarizing. After be kept temperature at 1200 ◦ C for 6 h, the crystals were polarized with 5 mA cm−2 current density. To make the test samples, the crystals were cut from the middle of them, and polished to optical grade smoothness. The size of the samples was listed in Table 1. 2.2. Measurements The optical damage resistance was measured by monitoring the change in the magnitude of the induced birefringence as a function of irradiation time, along the (1 0 0) direction. The Senarmont’s method [14,15] was used with a low power He–Ne laser (λ = 632.8 nm, 1.0 mW, 1.5 mm in diameter) as a probe beam. Optical damage was induced with an Ar-laser (λ = 488.0 nm) at incident power levels of 300 mW. The laser spot size was 1.5 mm in diameter. The change in the transmitted He–Ne beam intensity was monitored with a photodetector as a function of time. The ultraviolet-visible absorption spectra of Nd:Zn: LiNbO3 crystals were measured by using CARYIE style UV-Vis spectrophotometer. The measurement range was from 300 to 900 nm. The infrared transmission spectra of the
crystals were obtained with a Fourier infrared spectrophotometer in the 3000–4000 cm−1 wavenumber range at room temperature. The crystal samples of 12 mm ×12 mm ×1 mm were used in the measurement of the infrared transmission spectra and the UV-Vis absorption spectra.
3. Results and discussion Fig. 1 shows the optically induced change in the birefringence of the undoped LiNbO3 (1#), Nd:LiNbO3 (2#), and Zn:Nd:LiNbO3 (3# and 4#) versus irradiation time. The saturated change value in the birefringence of the Zn (7.0 mol):Nd (1.0 mol):LiNbO3 was approximately four and two times lower than that of Nd (1.0 mol):LiNbO3 , and undoped LiNbO3 , respectively. The saturated induced change in the birefringence of the Zn (4.0 mol):Nd (1.0 mol):LiNbO3 is similar to that of undoped LiNbO3 . Fig. 2 shows the distortion of the transmitted light spot when the samples were irradiated with the Ar-laser at the same power level. The Nd:LiNbO3 (2#) showed a severe distortion along the c-axis in the beam (Fig. 2(b)). The undoped LiNbO3 and Zn (4.0 mol):Nd (1.0 mol):LiNbO3 displayed slight distortion in the beam (Fig. 2(c) and (d)). The Zn (7.0 mol):Nd (1.0 mol):LiNbO3 crystal (4#) was found to be able to withstand the laser power density without noticeable distortion in the beam (Fig. 2(e)). Although the reason for the optical damage resistance was a vexed problem, the following explanation is considered. According to expression δ n ≈ RkαI /(σd + σp ) (σd σp ), where R is the generalized electro-optical coefficient, k Glass constant, α the optical absorption coefficient, σ d the dark conductivity, σ p the photoconductivity, and I the light intensity [13,15]. With an increase of cation-vacancy photoconductivity, the photorefraction decreases because of an increase in photoconductivity, whereas the photovoltaic current is almost unchanged. If an iron impurity is present, then an abrupt decrease in the capture cross section of Fe3+ acceptors is responsible for the observed increase in σ p [15]. [NbLi 4+ ] is the most probable electron acceptor in a Li-deficient LiNbO3 host. Thus, a reduced NbLi concentration should result in an increase of photoconductivity if the concentration of the concurrent Fe3+ acceptor is negligible [16]. In LiNbO3 , we may relate an increase of σ p to a decrease of the NbLi concentration owing to its replacement by
X.-H. Zhen et al. / Materials Chemistry and Physics 80 (2003) 11–14
13
Fig. 1. Optically induced birefringence change of the crystals irradiated with Ar-laser beam (λ = 488.0 nm, 300 mW, beam spot size = 1.5 mm in diameter).
the impurities. Because Nd3+ substitutes some of the NbLi and Nb5+ at the same time in the Nd (1.0 mol%):LiNbO3 . The role of NbLi becomes negligible, and σ p is governed by the electron acceptor Nd3+ . The Nd3+ plays a same role as Fe3+ in the LiNbO3 . So, the optical damage resistance of the Nd (1.0 mol%):LiNbO3 is lower than that of LiNbO3 , and similar to that of Fe:LiNbO3 . In Zn:Nd:LiNbO3 crystals, the Zn2+ substitutes the NbLi , and Nd3+ substitutes some of the NbLi and Nb5+ at the same time. For Zn doping exceeding thresholds, NbLi is canceled and Nd3+ only substitutes Nb5+ . This results in σ p increases rapidly. So the optical damage resistance of the Zn (7.0 mol%):Nd :: LiNbO3 is much higher than that of Zn (4.0 mol%):Nd:LiNbO3 . Fig. 3 shows that there are some absorption peaks appears in the absorption spectra of Nd:LiNbO3 . Compared with that of Nd:LiNbO3 crystal, there is no new absorption
peak appears in the absorption spectra of Nd:Zn:LiNbO3 . The ground state spectrum item of Nd3+ is 4 I9/2 . The transition wavelengths of the Nd:Zn:LiNbO3 can be used as pumping light. Compared with that of Nd:LiNbO3 crystal, the absorption edge of Nd:Zn:LiNbO3 crystal shifts to ultraviolet band. In crystal with oxygen octahedron structure, the basal optical absorption edge is decided by valence electron transition energy from 2p orbits of O2− to 4d orbits of Nb5+ . Therefore the valence electronic state of O2− directly affected the position of absorption edge. When Zn2+ entered into lattice, it takes priority of replacing antisite Nb (NbLi )4+ , and reduces the polarization of O2− . So the absorption edge of Zn:Nd: LiNbO3 crystal shifts to ultraviolet band. Fig. 4 shows the IR optical transmission spectra of Livarious Zn-doped LiNbO3 crystals. The pure congruent
Fig. 2. Transmitted laser beam distortions with Ar-laser irradiation at steady state for equal irradiation times: (a) Ar-laser beam (no crystal); (b) undoped LiNbO3 (1#); (c) Nd (1.0 mol%):LiNbO3 (2#); (d) Zn (4.0 mol) Nd (1.0 mol%):LiNbO3 (3#); (e) Zn (7.0 mol) Nd (1.0 mol%):LiNbO3 (4#).
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defect groups were formed. So, the double peaks appear in the IR absorption spectrum.
4. Conclusion
Fig. 3. The absorption spectra in the near ultraviolet and visible region of the doped LiNbO3 crystals. The absorption bands correspond to transitions from the 4 I9/2 ground state of Nd3+ ions to the different excited multiplets, such as 4 G9/2 , 4 G7/2 (∼529 nm), 2 G7/2 (∼590 nm), 4 F7/2 (∼750 nm) and 4F 5/2 (∼810 nm).
Zn2+ takes priority of replacing antisite Nb (NbLi )4+ in Zn:Nd:LiNbO3 crystals, when the concentration of Zn2+ exceeds its threshold, Zn2+ replaces both Nb5+ and Li+ . Nd3+ substitutes some of the NbLi and Nb5+ at the same time. The optical damage resistance of the Zn (7.0 mol%):Nd :: LiNbO3 is much higher than that of Zn (4.0 mol%):Nd:LiNbO3 and Nd:LiNbO3 .
Acknowledgements This work is financially supported by National Research for Fundamental Key Projects no. 973 (G19990330) and 863 Program (863-715-001-0100). References
Fig. 4. The IR transmission spectra of the LiNbO3 crystals (1#, 2#, 3#, 4#).
LiNbO3 crystal (1#) presents a broad non-symmetrical OH− absorption band at approximately 3478 cm−1 . The absorption peak position of Nd:LiNbO3 crystal (2#) shifts to 3488 cm−1 . We can see from Fig. 4 that the absorption peak position of Zn (4.0 mol%):Nd:LiNbO3 crystal (3#) is similar to that of the Nd:LiNbO3 crystal (2#). A weak peak and a strong peak located at 3522 and 3537 cm−1 , respectively, are observed in the IR transmission spectrum of Zn (7.0 mol%):Nd:LiNbO3 crystals (4#). When Nd3+ replaces the Li+ , occupies Li site and combines with OH− , the (NdLi )2+ –OH− defect group is formed [17]. Its stretch vibration is corresponding to the peak that is located at 3488 cm−1 . In Zn (7.0 mol%):Nd: LiNbO3 crystal (4#), the concentration of Zn2+ exceeds its threshold, a part of Zn2+ replaces Nb5+ instead of entering into Li site. Both (NdLi )2+ –OH− defect group and (NdLi )2+ –OH− –(ZnNb )3−
[1] E. Kratzig, O.F. Schirmer, Photorefractive Materials and their Applications. I. Springer Topics in Applied Physics, vol. 61, Springer, Berlin, 1988. [2] L.F. Johnson, A.A. Ballman, J. Appl. Phys. 40 (1969) 297. [3] M.N. Armenise, C. Canali, M. De Sario, E. Zanoni, Mater. Chem. Phys. 9 (1983) 267. [4] J. Capmany, D. Jaque, J.A. Sanz Garc´ıa, J. Garc´ıa Solé, Opt. Commun. 161 (1999) 253. [5] T.Y. Fan, A. Cordova-Plaza, M.J.F. Digonnet, R.L. Byer, H.J. Shaw, J. Opt. Soc. Am. B 3 (1) (1986) 140. [6] A. Ashkin, G.D. Boyd, J.M. Dziedzik, R.G. Smith, A.A. Ballman, K. Nassaau, Appl. Phys. Lett. 9 (1966) 72. [7] M.H. Li, Y.H. Xu, R. Wang, X.H. Zhen, C.Z. Zhao, Cryst. Res. Technol. 36 (2) (2001) 191. [8] G. Zhong, J. Jian, Z. Wu, in: Proceedings of the 11th International Conference on Quantum Electronics, IEEE, New York, 1990, p. 631. [9] T.R. Volk, V.I. Pryalkin, N.M. Rubinina, Opt. Lett. 15 (18) (1990) 996. [10] T.R. Volk, N.M. Rubinina, Ferroelectr. Lett. 14 (1992) 37. [11] Y. Kong, J. Wen, H. Wang, Appl. Phys. Lett. 66 (1995) 280. [12] J.K. Yamanoto, K. Kitamura, N. Iyi, S. Kimura, Y. Furukawa, M. Sato, Appl. Phys. Lett. 61 (1992) 2156. [13] Y. Zhang, Y.H. Xu, M.H. Li, Y.Q. Zhao, J. Cryst. Growth 233 (2001) 537. [14] Y. Furukawa, M. Sato, K. Kitamura, Y. Yajima, M. Minakata, J. Appl. Phys. 72 (8) (1992) 3250. [15] T. Volk, N. Rubinina, M. Wöhlecke, J. Opt. Soc. Am. B 11 (9) (1994) 1681. [16] H. Donnerberg, S.M. Tomlinson, S.R.A. Catlow, O.F. Schirmer, Phys. Rev. B 40 (1989) 1909. [17] J. Garc´ıa-Solé, T. Petit, H. Jaffrezic, Europhys. Lett. 24 (1993) 719.
Material science communication
Optical damage resistance in Zn:Nd:LiNbO3 laser crystals Xi-He Zhen a,∗ , Rui Wang b , Mei-Cheng Li a , Lian-Cheng Zhao a , Yu-Heng Xu b a
School of Material Science and Engineering, Harbin Institute of Technology, Harbin 150001, China b Department of Applied Chemistry and Electro Optics Research Center, Harbin Institute of Technology, Harbin 150001, China Received 29 April 2002; received in revised form 29 August 2002; accepted 10 September 2002
Abstract A series of Zn, Nd co-doped LiNbO3 crystals were grown by Czochralski technique with 0.5 mol% of Nd2 O3 and with 0, 4.0, and 7.0 mol% of ZnO, respectively. Their optical damage resistance was characterized by measurement of the photo-induced birefringence change. The optical damage resistance of Zn (7.0 mol%):Nd:LiNbO3 was much higher than that of Nd:LiNbO3 . The ultraviolet-visible (UV-Vis) absorption spectra and the infrared (IR) transmission spectra of Nd:Zn:LiNbO3 crystals were measured and investigated. The defects were discussed in this paper to explain the optical damage resistance in the Zn:Nd:LiNbO3 crystals. © 2002 Elsevier Science B.V. All rights reserved. Keywords: Zn:Nd:LiNbO3 ; Optical damage resistance; UV-Vis absorption spectra; IR transmission spectra
1. Introduction LiNbO3 is a widely studied optoelectronic material because of its technological applications. Doping with foreign ions modifies the optical properties of the matrix and makes the system useful for a great variety of application such as photorefractive devices [1], solid-state lasers [2] or optical waveguides [3]. Nowadays, LiNbO3 is the only optical material with a degree of development and so extensively investigated to serve as the basis for development of integrated optics, playing a similar role as Si did in the development of integrated electronics. Another potential application of LiNbO3 is in Nd3+ based compact diode-pumped self-frequency-doubled lasers which emit green radiation, useful for applications in optical data storage, undersea imaging, diagnosis in medicine, excitation sources to replace ion gas lasers for science and pumping of parametric oscillators and amplifiers [4]. Mg:Nd:LiNbO3 was the first system to be operated as a continuous wave self-frequency-doubled laser [5]. It is known that pure LiNbO3 presents a low threshold for photorefractive damage [6]. When LiNbO3 devices are used at high laser intensity, their performance is severely ∗ Corresponding author. Tel.: +86-451-6413-551; fax: +86-451-6221-048. E-mail address: [email protected] (X.-H. Zhen).
limited by the optical damage effect, which induces birefringence change and deforms the laser beams [7]. Some damage resistant impurities have been discovered, such as Mg [8], Zn [9], In [10,11], and Sc [12], which lead to a strong decrease of the photo damage in LiNbO3 and have been receiving a lot of interesting. The LiNbO3 crystal with 6.0 mol% concentration ZnO has the highest optical damage resistance property [13]. However, devices operating at room temperature are preferred. Although Nd3+ based lasers and amplifiers have been shown to reduce the optical quality and make difficult the growth of crystals sharing high optical quality and high Nd3+ concentration. For this reason, research on alternative codopant impurities and ways of avoiding photorefractive damage and still allow for stable and efficient laser operation at room temperature, are of major importance in the field and constitute an active field of research. The efficient and stable laser oscillation was obtained at room temperature and the long-term stability green radiation was obtained by self-frequency-doubling at room temperature in the Mg:Nd:LiNbO3 crystal [4]. In this paper, both doping with damage-resistant Zn and increasing Zn concentration are used to enhance the optical damage resistance in Zn:Nd:LiNbO3 crystals. The dependence of optical damage resistance on the concentration of ZnO is discussed. The UV-Vis absorption spectra and the IR transmission spectra of Nd:Zn:LiNbO3 crystals were measured to investigated the structure of the crystals.
0254-0584/02/$ – see front matter © 2002 Elsevier Science B.V. All rights reserved. PII: S 0 2 5 4 - 0 5 8 4 ( 0 2 ) 0 0 4 8 4 - 4
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Table 1 Composition of raw materials of the LiNbO3 crystals the size of the samples Crystal
1#
2#
3#
4#
[ZnO] (mol%) [Nd2 O3 ] (mol%) Crystal size, Ø (mm)
0 0 30 × 50
0 0.50 30 × 40
4.0 0.50 30 × 35
7.0 0.50 30 × 40
Wafer size (mm)
12 × 12 × 1 12 × 12 × 5
12 × 12 × 1 12 × 12 × 5
12 × 12 × 1 12 × 12 × 5
12 × 12 × 1 12 × 12 × 5
2. Materials and methods 2.1. Crystal preparation The congruent LiNbO3 crystals with 0.5 mol% of Nd2 O3 and various concentration of ZnO were grown in air along c-direction in diameter-controlled Czochralski equipment using a SiC heater furnace. The raw materials used for crystal growth are Li2 CO3 (4 N purity), Nb2 O5 (4 N purity), ZnO (3 N purity), and Nd2 O3 (4 N purity). The compositions of raw material are shown in Table 1. To prepare doped LiNbO3 polycrystalline materials, all thoroughly mixed raw materials were put into a platinum (Pt) crucible (70 mm in diameter and 50 mm in height), and calcined at 700 and 1150 ◦ C for 2 h, respectively. The crystals were grown under the optimum technology conditions: temperature gradient of 35–40 ◦ C cm−1 , rotating rate of 18 rpm, and pulling rate of 1.5–2.0 mm h−1 . About 70% of melts were crystallized. After growth, the crystals were cooled to room temperature at a speed of 150 ◦ C h−1 . All of the crystals were clear, transparent, and inclusion-free. The crystals were placed in a furnace where the temperature gradient is below 5 ◦ C cm−1 for polarizing. After be kept temperature at 1200 ◦ C for 6 h, the crystals were polarized with 5 mA cm−2 current density. To make the test samples, the crystals were cut from the middle of them, and polished to optical grade smoothness. The size of the samples was listed in Table 1. 2.2. Measurements The optical damage resistance was measured by monitoring the change in the magnitude of the induced birefringence as a function of irradiation time, along the (1 0 0) direction. The Senarmont’s method [14,15] was used with a low power He–Ne laser (λ = 632.8 nm, 1.0 mW, 1.5 mm in diameter) as a probe beam. Optical damage was induced with an Ar-laser (λ = 488.0 nm) at incident power levels of 300 mW. The laser spot size was 1.5 mm in diameter. The change in the transmitted He–Ne beam intensity was monitored with a photodetector as a function of time. The ultraviolet-visible absorption spectra of Nd:Zn: LiNbO3 crystals were measured by using CARYIE style UV-Vis spectrophotometer. The measurement range was from 300 to 900 nm. The infrared transmission spectra of the
crystals were obtained with a Fourier infrared spectrophotometer in the 3000–4000 cm−1 wavenumber range at room temperature. The crystal samples of 12 mm ×12 mm ×1 mm were used in the measurement of the infrared transmission spectra and the UV-Vis absorption spectra.
3. Results and discussion Fig. 1 shows the optically induced change in the birefringence of the undoped LiNbO3 (1#), Nd:LiNbO3 (2#), and Zn:Nd:LiNbO3 (3# and 4#) versus irradiation time. The saturated change value in the birefringence of the Zn (7.0 mol):Nd (1.0 mol):LiNbO3 was approximately four and two times lower than that of Nd (1.0 mol):LiNbO3 , and undoped LiNbO3 , respectively. The saturated induced change in the birefringence of the Zn (4.0 mol):Nd (1.0 mol):LiNbO3 is similar to that of undoped LiNbO3 . Fig. 2 shows the distortion of the transmitted light spot when the samples were irradiated with the Ar-laser at the same power level. The Nd:LiNbO3 (2#) showed a severe distortion along the c-axis in the beam (Fig. 2(b)). The undoped LiNbO3 and Zn (4.0 mol):Nd (1.0 mol):LiNbO3 displayed slight distortion in the beam (Fig. 2(c) and (d)). The Zn (7.0 mol):Nd (1.0 mol):LiNbO3 crystal (4#) was found to be able to withstand the laser power density without noticeable distortion in the beam (Fig. 2(e)). Although the reason for the optical damage resistance was a vexed problem, the following explanation is considered. According to expression δ n ≈ RkαI /(σd + σp ) (σd σp ), where R is the generalized electro-optical coefficient, k Glass constant, α the optical absorption coefficient, σ d the dark conductivity, σ p the photoconductivity, and I the light intensity [13,15]. With an increase of cation-vacancy photoconductivity, the photorefraction decreases because of an increase in photoconductivity, whereas the photovoltaic current is almost unchanged. If an iron impurity is present, then an abrupt decrease in the capture cross section of Fe3+ acceptors is responsible for the observed increase in σ p [15]. [NbLi 4+ ] is the most probable electron acceptor in a Li-deficient LiNbO3 host. Thus, a reduced NbLi concentration should result in an increase of photoconductivity if the concentration of the concurrent Fe3+ acceptor is negligible [16]. In LiNbO3 , we may relate an increase of σ p to a decrease of the NbLi concentration owing to its replacement by
X.-H. Zhen et al. / Materials Chemistry and Physics 80 (2003) 11–14
13
Fig. 1. Optically induced birefringence change of the crystals irradiated with Ar-laser beam (λ = 488.0 nm, 300 mW, beam spot size = 1.5 mm in diameter).
the impurities. Because Nd3+ substitutes some of the NbLi and Nb5+ at the same time in the Nd (1.0 mol%):LiNbO3 . The role of NbLi becomes negligible, and σ p is governed by the electron acceptor Nd3+ . The Nd3+ plays a same role as Fe3+ in the LiNbO3 . So, the optical damage resistance of the Nd (1.0 mol%):LiNbO3 is lower than that of LiNbO3 , and similar to that of Fe:LiNbO3 . In Zn:Nd:LiNbO3 crystals, the Zn2+ substitutes the NbLi , and Nd3+ substitutes some of the NbLi and Nb5+ at the same time. For Zn doping exceeding thresholds, NbLi is canceled and Nd3+ only substitutes Nb5+ . This results in σ p increases rapidly. So the optical damage resistance of the Zn (7.0 mol%):Nd :: LiNbO3 is much higher than that of Zn (4.0 mol%):Nd:LiNbO3 . Fig. 3 shows that there are some absorption peaks appears in the absorption spectra of Nd:LiNbO3 . Compared with that of Nd:LiNbO3 crystal, there is no new absorption
peak appears in the absorption spectra of Nd:Zn:LiNbO3 . The ground state spectrum item of Nd3+ is 4 I9/2 . The transition wavelengths of the Nd:Zn:LiNbO3 can be used as pumping light. Compared with that of Nd:LiNbO3 crystal, the absorption edge of Nd:Zn:LiNbO3 crystal shifts to ultraviolet band. In crystal with oxygen octahedron structure, the basal optical absorption edge is decided by valence electron transition energy from 2p orbits of O2− to 4d orbits of Nb5+ . Therefore the valence electronic state of O2− directly affected the position of absorption edge. When Zn2+ entered into lattice, it takes priority of replacing antisite Nb (NbLi )4+ , and reduces the polarization of O2− . So the absorption edge of Zn:Nd: LiNbO3 crystal shifts to ultraviolet band. Fig. 4 shows the IR optical transmission spectra of Livarious Zn-doped LiNbO3 crystals. The pure congruent
Fig. 2. Transmitted laser beam distortions with Ar-laser irradiation at steady state for equal irradiation times: (a) Ar-laser beam (no crystal); (b) undoped LiNbO3 (1#); (c) Nd (1.0 mol%):LiNbO3 (2#); (d) Zn (4.0 mol) Nd (1.0 mol%):LiNbO3 (3#); (e) Zn (7.0 mol) Nd (1.0 mol%):LiNbO3 (4#).
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defect groups were formed. So, the double peaks appear in the IR absorption spectrum.
4. Conclusion
Fig. 3. The absorption spectra in the near ultraviolet and visible region of the doped LiNbO3 crystals. The absorption bands correspond to transitions from the 4 I9/2 ground state of Nd3+ ions to the different excited multiplets, such as 4 G9/2 , 4 G7/2 (∼529 nm), 2 G7/2 (∼590 nm), 4 F7/2 (∼750 nm) and 4F 5/2 (∼810 nm).
Zn2+ takes priority of replacing antisite Nb (NbLi )4+ in Zn:Nd:LiNbO3 crystals, when the concentration of Zn2+ exceeds its threshold, Zn2+ replaces both Nb5+ and Li+ . Nd3+ substitutes some of the NbLi and Nb5+ at the same time. The optical damage resistance of the Zn (7.0 mol%):Nd :: LiNbO3 is much higher than that of Zn (4.0 mol%):Nd:LiNbO3 and Nd:LiNbO3 .
Acknowledgements This work is financially supported by National Research for Fundamental Key Projects no. 973 (G19990330) and 863 Program (863-715-001-0100). References
Fig. 4. The IR transmission spectra of the LiNbO3 crystals (1#, 2#, 3#, 4#).
LiNbO3 crystal (1#) presents a broad non-symmetrical OH− absorption band at approximately 3478 cm−1 . The absorption peak position of Nd:LiNbO3 crystal (2#) shifts to 3488 cm−1 . We can see from Fig. 4 that the absorption peak position of Zn (4.0 mol%):Nd:LiNbO3 crystal (3#) is similar to that of the Nd:LiNbO3 crystal (2#). A weak peak and a strong peak located at 3522 and 3537 cm−1 , respectively, are observed in the IR transmission spectrum of Zn (7.0 mol%):Nd:LiNbO3 crystals (4#). When Nd3+ replaces the Li+ , occupies Li site and combines with OH− , the (NdLi )2+ –OH− defect group is formed [17]. Its stretch vibration is corresponding to the peak that is located at 3488 cm−1 . In Zn (7.0 mol%):Nd: LiNbO3 crystal (4#), the concentration of Zn2+ exceeds its threshold, a part of Zn2+ replaces Nb5+ instead of entering into Li site. Both (NdLi )2+ –OH− defect group and (NdLi )2+ –OH− –(ZnNb )3−
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