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Cr3+ luminescence quenching in stoichiometric lithium niobate crystals
Journal of Non-Crystalline Solids 352 (2006) 2395–2398 www.elsevier.com/locate/jnoncrysol
Cr3+ luminescence quenching in stoichiometric lithium niobate crystals V.V. Galutskiy a, B.V. Ignatyev a, V.A. Isaev a, V.A. Lebedev a,*, A.G. Avanesov a, A.L. Mihaylenko a, E.V. Stroganova a, M.G. Brik b,c b
a Experimental Physics Department, Kuban State University, Stavropolskaya 149, Krasnodar, 350040, Russia Fukui Institute for Fundamental Chemistry, Kyoto University, 34-4, Takano Nishihiraki-cho, Sakyo-ku, Kyoto 606-8103, Japan c School of Science and Technology, Kwansei Gakuin University, 2-1 Gakuen, Sanda, Hyogo 669-1337, Japan
Available online 16 May 2006
Abstract Cr3+-doped stoichiometric LiNbO3 crystals were grown and detailed spectroscopic investigations were performed. The samples are characterized by extremely low frequency factor of the luminescence thermal quenching. Its numerical value 1.7 · 108 s1 is about 104 times lower than in crystals of lithium-aluminum fluorides. Under such conditions, radiative transitions of Cr3+ ions compete successfully with non-radiative ones, resulting in a relatively high quantum yield of the broadband luminescence at room temperature. The quenching of luminescence is counteracted by the effect of the dynamic removal of the radiative transition exclusion, and as a result, Cr3+ radiative lifetime in LiNbO3 decreases from 8 to 4 ls when temperature grows up from 77 to 300 K. Therefore quantum yield of the broadband luminescence at room temperature is not 5–10%, as reported previously, but about 30%. The results obtained in the present study show the stoichiometric lithium niobate doped with Cr3+ ions to be a potential active media for tunable lasers. 2006 Elsevier B.V. All rights reserved. PACS: 42.70.a; 42.70.Hj Keywords: Crystal growth; Optical spectroscopy; Absorption; Lasers; Luminescence
1. Introduction Stoichiometric LiNbO3 crystals are more attractive as the active medium for solid state lasers in comparison to congruent ones due to their high photorefractive stability [1]. Besides, the coercive force in stoichiometric crystals decreases from 24 to 2 jV/mm and less [2] what is very attractive for the advance of poly-functional optoelectronic device. They possess increased thermal conductivity and irradiation resistance also. Cr3+ ions are very perspective as the activators in this host. Very often laser generation of rare-earth and transition metal ions is hampered by effective luminescence thermal quenching. It should be pointed out, that the luminescence quenching processes related to the broadband Cr3+ luminescence *
Corresponding author. E-mail addresses: [email protected] (V.A. Lebedev), brik@ fukui.kyoto-u.ac.jp (M.G. Brik). 0022-3093/$ - see front matter 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2006.03.017
have not been studied so far [3]. This circumstance was the reason to choose the broadband luminescence of the Cr3+-doped stoichiometric crystals of LiNbO3 and its thermal dependence as the subject of investigation in the present work.
2. Single crystal growth and samples preparation Single crystals of stoichiometric LiNbO3 of 25 mm in diameter were grown in the air atmosphere by the Czochralski technique from the platinum crucibles of 60 mm in diameter and 60 mm in height. The pulling rate was 1 mm/h in the three-fold axis direction at 10–12 rev/min. The technique of melt feeding was similar to that one described in papers [4,5]. The composition of the stoichiometric crystals Li/(Li + Nb) = 0.495 was determined using the shape of the water absorption band at around 3440–3540 cm1 by comparing the samples spectra with
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V.V. Galutskiy et al. / Journal of Non-Crystalline Solids 352 (2006) 2395–2398
spectra of the regularly ordered crystals obtained by the vapor phase equilibrium method [6]. The sample which was used for the spectroscopic and kinetic investigations was a single crystals slab with dimensions of 2 · 6 · 6 mm3; with the c axis being in the 6 · 6 mm2 plate. The Cr3+ ions concentration was 2 · 1019 cm3. 3. Spectroscopic measurements Fig. 1 shows the absorption luminescent spectrum of the stoichiometric LiNbO3:Cr3+ crystal obtained by using the grating monochromator MDR 23 at 300 K. The lumines-
cence spectrum consists of one broad electron-vibronic band with a maximum at around 890 nm (11 236 cm1). It was obtained using the 510.6 nm emission from a copper laser as the excitation source and is also shown in Fig. 1. To determine the parameters of the Cr3+ luminescence quenching in the studied crystal we plotted the dependence of the lifetime on temperature (Fig. 2; experimental values are shown by circles). Cr3+ luminescence lifetime was determined using the long-time kinetic part; at liquid nitrogen temperature luminescence lifetime is about 8 ls, and at room temperature – about 1.25 ls. 4. Evaluation of parameters of electron–phonon interaction Luminescence spectra were recorded for Cr3+:LiNbO3 in the temperature range 77–500 K. First and second moments of these spectra were calculated at each temperature from the relations [7]
Cross-section, *10-19 cm2
4
3
E_|_c
Absorption
Emission
2
N 1 ¼ hEi;
ð1Þ 2
N 2 ¼ hðE hEiÞ i; Z 1 hf ðEÞi ¼ dE f ðEÞ IðEÞ;
1
0 350
450
550
650
750
850
ð2Þ ð3Þ
1
950
Wavelength, nm 12000 11800
First moment, cm-1
Cross-section, *10-19 cm2
2
E||c
Absorption
Emission
1
11600 11400 11200 11000
0 350
450
550
650
750
850
950
10800
Wavelength, nm
250
Fig. 1. Polarized absorption and emission cross-section of stoichiometric Cr3+:LiNbO3 at room temperature.
12
400
450
500
Fig. 3. Temperature dependence of the first moments of the 4T2g–4A2g luminescence band in LiNbO3:Cr3+.
6
0. 5
4
τr
0. 25
τ
2
0
100
200
300
Q
8
400
0
Temperature, K Fig. 2. Temperature dependence of Cr3+ lifetime s, luminescence quantum output Q and radiative lifetime sr in stochiometric Cr:LiNbO3.
Second moment, cm-2
Q 0. 75
Lifetime, μs
350
Temperature, K
1
10
0
300
9, E+05
7, E+05
5, E+05 70
170
270
370
470
Temperature, K Fig. 4. Temperature dependence of the second moments of the 4T2g–4A2g luminescence band in LiNbO3:Cr3+.
V.V. Galutskiy et al. / Journal of Non-Crystalline Solids 352 (2006) 2395–2398
2397
Table 1 Spectroscopic properties of Cr3+ in some doped crystals
sr0 a Æ 103, K1 S0, s1 DEnr r, p, 1020 cm2 T(Q = 0.1), Kc a b c
LiNbO3
CeSc3(BO3)4 (CSB) [9]
ScBO3a
Emeraldb
LiSrAlF6 (LiSAF) [12]
LiCaAlF6 (LiCAF) [12]
9.5 2.5 170 · 106 1160 32 370
66 1.3 50 · 106 2050 3.1 550
211 1.3 25 · 106 2100 1.2 500
58 0.4 3.5 · 109 6200 – 900
64 0 6 · 1012 5125 3.5 390
68 0 8 · 1013 8500 3.4 590
Values obtained from [10]. Values obtained from [11]. The temperature of 90% decrease in the quantum yield.
where E is the photon energy and I(E) is the normalized line-shape function. These moments are plotted as functions of temperature in Figs. 3 and 4, respectively. Parameters of electron–phonon interaction in terms of a single-configuration-coordinate linear-coupling model are determined by fitting the first and second moments to the expressions (solid lines in Figs. 3 and 4) [7] N1 ¼ hX0 þ Shx0 aT ; 2
hx0 Þ cothð hx0 =2kT Þ: N 2 ¼ Sð
ð4Þ ð5Þ
We obtained the following values of parameters: the Huang-Rhys factor S = 2, phonon energy hx0 = 540 cm1, zero-phonon transition energy hX0 = 13 020 cm1, a = 1.5 1 cm /K. 5. Discussion of the results 5.1. Quenching of luminescent and radiation constants The observed values of the Cr3+ lifetime in Fig. 2 are approximated by one-frequency Mott’s curve [8]: W NR ¼ S 0 expðDEq =kT Þ;
ð6Þ
where WNR stands for the non-radiative relaxation rate, S0 is the frequency factor which defines how many times per one second an electron ‘attempts’ to undergo the non-radiative transition and DEq is the energy barrier between the bottom of the 4T2g potential curve and the points of intersection of the potential parabolas corresponding to the 4 T2g and 4A2g states. Dynamical removal of the transition interdiction was accounted for by means of an empirical equation: sr ¼ sr0 expðaT Þ;
ð7Þ
where sr0 is the radiative lifetime of the excited state at 0 K, a is the thermal coefficient which is proportional to the number of vibrations which continuously interact with the excited state when the temperature grows up. This allows to represent the excited state luminescent lifetime s as s ¼ sr þ 1=W NR :
ð8Þ
Fig. 3 also shows the thermal dependence of the excited state luminescence lifetime (Eq. (8)). The best fit of the experimental and theoretical dependencies was obtained
if sr0 = 9.5 ls, DE = 1160 cm1, S0 = 1.7 · 108 s1, a = 2.5 · 103 K1. If we assume the quantum luminescence yield Q as a ratio of the excited state luminescent lifetime s to the radiative lifetime sr Q ¼ s=sr ;
ð9Þ
then starting from 150 K and below the luminescence quantum yield is constant and equals 1 (Fig. 2). All changes of the lifetime in this temperature range are caused by the dynamical removal of the ban for the radiative transition. Table 1 contains the values of parameters characterizing the radiative and non-radiative processes in some laser crystals doped with Cr3+ ions. The data in Table 1 show that lithium niobate possesses extremely low luminescence quenching frequency factor 1.7 · 108 s1; what is more than 104 times smaller than in fluoride crystals. Such a luminescence quenching probability is a special feature of the heterodesmic crystals [9] (such as borates and silicates, Table 1), which lithium niobate also belongs to. The obtained values of the luminescence quantum yield at room temperature – of about 30% – and probability of the radiative 4T2g–4A2g laser transition W = 1/sr = 250,000 s1 (sr = 4 ls) are approximately 2.5–3 times higher than reported previously [13]. We think that this is related to not taking into account dynamical removal of the radiative transition ban, which is an essential property of the heterodesmic compounds (unlike fluorides crystals, for example). 6. Conclusion Complete and consistent study of the LiNbO3 crystals doped with Cr3+ ions, starting from the crystal growth and ending up with detailed spectroscopic investigations, is presented. Dependence of the first and second spectral moments of the luminescence band on temperature is studied; it is shown in the framework of the single-coordinate one-frequency harmonic model that Cr3+ ions effectively interact with 540 cm1 phonons. The Huang-Rhys parameter was determined to be 2.0. In spite of rather high phonon energy and low value of the Huang-Rhys parameter, the thermal quenching of luminescence is weak. At the same time, this crystal has extremely low value of the
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V.V. Galutskiy et al. / Journal of Non-Crystalline Solids 352 (2006) 2395–2398
luminescence quenching frequency factor of about 1.7 · 108 s1. Increase of temperature is accompanied by a pronounced dynamical removal of the ban for the 4 T2g–4A2g radiative transition. The radiative lifetime equals 8 ls at 77 K and decreases to 4 ls at room temperature. This is the reason for the luminescence quantum yield to be not less than 30% at room temperature, what is about 2.5–3 times greater than the previously reported value. The obtained results show the LiNbO3:Cr3+ crystal to be perspective active medium for the tunable solid state lasers. Acknowledgement Financial support coming from the RFFR-South 03-0296557-r2003yug_a is gratefully acknowledged. References [1] Y. Furucawa, K. Kitamura, S. Takekawa, A. Miyamoto, M. Terao, N. Suda, Appl. Phys. Lett. 77 (2000) 2494. [2] V. Bermudez, L. Huang, D. Hui, S. Field, E. Diguez, Appl. Phys. A 70 (2000) 591.
[3] R.K. Sviridov, L.N. Rashkovich, I.N. Voronina, In: ‘Spectroscopy of crystals’ (‘Spektroskopiya kristallov’), M., Nauka, 1970, p. 270 (in Russian). [4] K. Kitamura, J.K. Yamamoto, N. Iyi, S. Kimura, J. Cryst. Growth 116 (1992) 327. [5] Y. Zheng, E. Shi, Z. Lu, S. Cui, S. Wang, W. Zhong, Crys. Res. Technol. 39 (2004) 387. ´ . Pe´ter, L. Kova´cs, G. Corradi, Zs. Szaller, J. Cryst. [6] K. Polga´r, A Growth 177 (1997) 211. [7] L.J. Andrews, A. Lempicki, B.C. McCollum, C.J. Giunta, Phys. Rev. B 34 (1986) 2735. [8] N.F. Mott, Proc. R. Soc. London, Ser. A 167 (1937) 384. [9] E.V. Stroganova, V.A. Lebedev, I.V. Voroshilov, A. De Backer, I.M. Razdobreev, M.G. Brik, in: Martin E. Fermann, Larry R. Marshall (Eds.), OSA Trends in Optics and Photonics v.68, Advanced SolidState Lasers, Optical society of America, Washington, DC, 2002, p. 260. [10] S.T. Lai, B.H.T. Chai, M. Long, R.C. Morris, IEEE J. Quantum Electr. 22 (1986) 1931. [11] S.T. Lai, J. Opt. Soc. Am. B 4 (1987) 1286. [12] M. Stadler, M. Bass, B.H.T. Chai, J. Opt. Soc. Am. B 9 (1992) 2271. [13] G.A. Torchia, J.A. Munoz, F. Cusso, F. Jaque, J.O. Tocho, J. Lumin. 92 (2001) 317.
Cr3+ luminescence quenching in stoichiometric lithium niobate crystals V.V. Galutskiy a, B.V. Ignatyev a, V.A. Isaev a, V.A. Lebedev a,*, A.G. Avanesov a, A.L. Mihaylenko a, E.V. Stroganova a, M.G. Brik b,c b
a Experimental Physics Department, Kuban State University, Stavropolskaya 149, Krasnodar, 350040, Russia Fukui Institute for Fundamental Chemistry, Kyoto University, 34-4, Takano Nishihiraki-cho, Sakyo-ku, Kyoto 606-8103, Japan c School of Science and Technology, Kwansei Gakuin University, 2-1 Gakuen, Sanda, Hyogo 669-1337, Japan
Available online 16 May 2006
Abstract Cr3+-doped stoichiometric LiNbO3 crystals were grown and detailed spectroscopic investigations were performed. The samples are characterized by extremely low frequency factor of the luminescence thermal quenching. Its numerical value 1.7 · 108 s1 is about 104 times lower than in crystals of lithium-aluminum fluorides. Under such conditions, radiative transitions of Cr3+ ions compete successfully with non-radiative ones, resulting in a relatively high quantum yield of the broadband luminescence at room temperature. The quenching of luminescence is counteracted by the effect of the dynamic removal of the radiative transition exclusion, and as a result, Cr3+ radiative lifetime in LiNbO3 decreases from 8 to 4 ls when temperature grows up from 77 to 300 K. Therefore quantum yield of the broadband luminescence at room temperature is not 5–10%, as reported previously, but about 30%. The results obtained in the present study show the stoichiometric lithium niobate doped with Cr3+ ions to be a potential active media for tunable lasers. 2006 Elsevier B.V. All rights reserved. PACS: 42.70.a; 42.70.Hj Keywords: Crystal growth; Optical spectroscopy; Absorption; Lasers; Luminescence
1. Introduction Stoichiometric LiNbO3 crystals are more attractive as the active medium for solid state lasers in comparison to congruent ones due to their high photorefractive stability [1]. Besides, the coercive force in stoichiometric crystals decreases from 24 to 2 jV/mm and less [2] what is very attractive for the advance of poly-functional optoelectronic device. They possess increased thermal conductivity and irradiation resistance also. Cr3+ ions are very perspective as the activators in this host. Very often laser generation of rare-earth and transition metal ions is hampered by effective luminescence thermal quenching. It should be pointed out, that the luminescence quenching processes related to the broadband Cr3+ luminescence *
Corresponding author. E-mail addresses: [email protected] (V.A. Lebedev), brik@ fukui.kyoto-u.ac.jp (M.G. Brik). 0022-3093/$ - see front matter 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2006.03.017
have not been studied so far [3]. This circumstance was the reason to choose the broadband luminescence of the Cr3+-doped stoichiometric crystals of LiNbO3 and its thermal dependence as the subject of investigation in the present work.
2. Single crystal growth and samples preparation Single crystals of stoichiometric LiNbO3 of 25 mm in diameter were grown in the air atmosphere by the Czochralski technique from the platinum crucibles of 60 mm in diameter and 60 mm in height. The pulling rate was 1 mm/h in the three-fold axis direction at 10–12 rev/min. The technique of melt feeding was similar to that one described in papers [4,5]. The composition of the stoichiometric crystals Li/(Li + Nb) = 0.495 was determined using the shape of the water absorption band at around 3440–3540 cm1 by comparing the samples spectra with
2396
V.V. Galutskiy et al. / Journal of Non-Crystalline Solids 352 (2006) 2395–2398
spectra of the regularly ordered crystals obtained by the vapor phase equilibrium method [6]. The sample which was used for the spectroscopic and kinetic investigations was a single crystals slab with dimensions of 2 · 6 · 6 mm3; with the c axis being in the 6 · 6 mm2 plate. The Cr3+ ions concentration was 2 · 1019 cm3. 3. Spectroscopic measurements Fig. 1 shows the absorption luminescent spectrum of the stoichiometric LiNbO3:Cr3+ crystal obtained by using the grating monochromator MDR 23 at 300 K. The lumines-
cence spectrum consists of one broad electron-vibronic band with a maximum at around 890 nm (11 236 cm1). It was obtained using the 510.6 nm emission from a copper laser as the excitation source and is also shown in Fig. 1. To determine the parameters of the Cr3+ luminescence quenching in the studied crystal we plotted the dependence of the lifetime on temperature (Fig. 2; experimental values are shown by circles). Cr3+ luminescence lifetime was determined using the long-time kinetic part; at liquid nitrogen temperature luminescence lifetime is about 8 ls, and at room temperature – about 1.25 ls. 4. Evaluation of parameters of electron–phonon interaction Luminescence spectra were recorded for Cr3+:LiNbO3 in the temperature range 77–500 K. First and second moments of these spectra were calculated at each temperature from the relations [7]
Cross-section, *10-19 cm2
4
3
E_|_c
Absorption
Emission
2
N 1 ¼ hEi;
ð1Þ 2
N 2 ¼ hðE hEiÞ i; Z 1 hf ðEÞi ¼ dE f ðEÞ IðEÞ;
1
0 350
450
550
650
750
850
ð2Þ ð3Þ
1
950
Wavelength, nm 12000 11800
First moment, cm-1
Cross-section, *10-19 cm2
2
E||c
Absorption
Emission
1
11600 11400 11200 11000
0 350
450
550
650
750
850
950
10800
Wavelength, nm
250
Fig. 1. Polarized absorption and emission cross-section of stoichiometric Cr3+:LiNbO3 at room temperature.
12
400
450
500
Fig. 3. Temperature dependence of the first moments of the 4T2g–4A2g luminescence band in LiNbO3:Cr3+.
6
0. 5
4
τr
0. 25
τ
2
0
100
200
300
Q
8
400
0
Temperature, K Fig. 2. Temperature dependence of Cr3+ lifetime s, luminescence quantum output Q and radiative lifetime sr in stochiometric Cr:LiNbO3.
Second moment, cm-2
Q 0. 75
Lifetime, μs
350
Temperature, K
1
10
0
300
9, E+05
7, E+05
5, E+05 70
170
270
370
470
Temperature, K Fig. 4. Temperature dependence of the second moments of the 4T2g–4A2g luminescence band in LiNbO3:Cr3+.
V.V. Galutskiy et al. / Journal of Non-Crystalline Solids 352 (2006) 2395–2398
2397
Table 1 Spectroscopic properties of Cr3+ in some doped crystals
sr0 a Æ 103, K1 S0, s1 DEnr r, p, 1020 cm2 T(Q = 0.1), Kc a b c
LiNbO3
CeSc3(BO3)4 (CSB) [9]
ScBO3a
Emeraldb
LiSrAlF6 (LiSAF) [12]
LiCaAlF6 (LiCAF) [12]
9.5 2.5 170 · 106 1160 32 370
66 1.3 50 · 106 2050 3.1 550
211 1.3 25 · 106 2100 1.2 500
58 0.4 3.5 · 109 6200 – 900
64 0 6 · 1012 5125 3.5 390
68 0 8 · 1013 8500 3.4 590
Values obtained from [10]. Values obtained from [11]. The temperature of 90% decrease in the quantum yield.
where E is the photon energy and I(E) is the normalized line-shape function. These moments are plotted as functions of temperature in Figs. 3 and 4, respectively. Parameters of electron–phonon interaction in terms of a single-configuration-coordinate linear-coupling model are determined by fitting the first and second moments to the expressions (solid lines in Figs. 3 and 4) [7] N1 ¼ hX0 þ Shx0 aT ; 2
hx0 Þ cothð hx0 =2kT Þ: N 2 ¼ Sð
ð4Þ ð5Þ
We obtained the following values of parameters: the Huang-Rhys factor S = 2, phonon energy hx0 = 540 cm1, zero-phonon transition energy hX0 = 13 020 cm1, a = 1.5 1 cm /K. 5. Discussion of the results 5.1. Quenching of luminescent and radiation constants The observed values of the Cr3+ lifetime in Fig. 2 are approximated by one-frequency Mott’s curve [8]: W NR ¼ S 0 expðDEq =kT Þ;
ð6Þ
where WNR stands for the non-radiative relaxation rate, S0 is the frequency factor which defines how many times per one second an electron ‘attempts’ to undergo the non-radiative transition and DEq is the energy barrier between the bottom of the 4T2g potential curve and the points of intersection of the potential parabolas corresponding to the 4 T2g and 4A2g states. Dynamical removal of the transition interdiction was accounted for by means of an empirical equation: sr ¼ sr0 expðaT Þ;
ð7Þ
where sr0 is the radiative lifetime of the excited state at 0 K, a is the thermal coefficient which is proportional to the number of vibrations which continuously interact with the excited state when the temperature grows up. This allows to represent the excited state luminescent lifetime s as s ¼ sr þ 1=W NR :
ð8Þ
Fig. 3 also shows the thermal dependence of the excited state luminescence lifetime (Eq. (8)). The best fit of the experimental and theoretical dependencies was obtained
if sr0 = 9.5 ls, DE = 1160 cm1, S0 = 1.7 · 108 s1, a = 2.5 · 103 K1. If we assume the quantum luminescence yield Q as a ratio of the excited state luminescent lifetime s to the radiative lifetime sr Q ¼ s=sr ;
ð9Þ
then starting from 150 K and below the luminescence quantum yield is constant and equals 1 (Fig. 2). All changes of the lifetime in this temperature range are caused by the dynamical removal of the ban for the radiative transition. Table 1 contains the values of parameters characterizing the radiative and non-radiative processes in some laser crystals doped with Cr3+ ions. The data in Table 1 show that lithium niobate possesses extremely low luminescence quenching frequency factor 1.7 · 108 s1; what is more than 104 times smaller than in fluoride crystals. Such a luminescence quenching probability is a special feature of the heterodesmic crystals [9] (such as borates and silicates, Table 1), which lithium niobate also belongs to. The obtained values of the luminescence quantum yield at room temperature – of about 30% – and probability of the radiative 4T2g–4A2g laser transition W = 1/sr = 250,000 s1 (sr = 4 ls) are approximately 2.5–3 times higher than reported previously [13]. We think that this is related to not taking into account dynamical removal of the radiative transition ban, which is an essential property of the heterodesmic compounds (unlike fluorides crystals, for example). 6. Conclusion Complete and consistent study of the LiNbO3 crystals doped with Cr3+ ions, starting from the crystal growth and ending up with detailed spectroscopic investigations, is presented. Dependence of the first and second spectral moments of the luminescence band on temperature is studied; it is shown in the framework of the single-coordinate one-frequency harmonic model that Cr3+ ions effectively interact with 540 cm1 phonons. The Huang-Rhys parameter was determined to be 2.0. In spite of rather high phonon energy and low value of the Huang-Rhys parameter, the thermal quenching of luminescence is weak. At the same time, this crystal has extremely low value of the
2398
V.V. Galutskiy et al. / Journal of Non-Crystalline Solids 352 (2006) 2395–2398
luminescence quenching frequency factor of about 1.7 · 108 s1. Increase of temperature is accompanied by a pronounced dynamical removal of the ban for the 4 T2g–4A2g radiative transition. The radiative lifetime equals 8 ls at 77 K and decreases to 4 ls at room temperature. This is the reason for the luminescence quantum yield to be not less than 30% at room temperature, what is about 2.5–3 times greater than the previously reported value. The obtained results show the LiNbO3:Cr3+ crystal to be perspective active medium for the tunable solid state lasers. Acknowledgement Financial support coming from the RFFR-South 03-0296557-r2003yug_a is gratefully acknowledged. References [1] Y. Furucawa, K. Kitamura, S. Takekawa, A. Miyamoto, M. Terao, N. Suda, Appl. Phys. Lett. 77 (2000) 2494. [2] V. Bermudez, L. Huang, D. Hui, S. Field, E. Diguez, Appl. Phys. A 70 (2000) 591.
[3] R.K. Sviridov, L.N. Rashkovich, I.N. Voronina, In: ‘Spectroscopy of crystals’ (‘Spektroskopiya kristallov’), M., Nauka, 1970, p. 270 (in Russian). [4] K. Kitamura, J.K. Yamamoto, N. Iyi, S. Kimura, J. Cryst. Growth 116 (1992) 327. [5] Y. Zheng, E. Shi, Z. Lu, S. Cui, S. Wang, W. Zhong, Crys. Res. Technol. 39 (2004) 387. ´ . Pe´ter, L. Kova´cs, G. Corradi, Zs. Szaller, J. Cryst. [6] K. Polga´r, A Growth 177 (1997) 211. [7] L.J. Andrews, A. Lempicki, B.C. McCollum, C.J. Giunta, Phys. Rev. B 34 (1986) 2735. [8] N.F. Mott, Proc. R. Soc. London, Ser. A 167 (1937) 384. [9] E.V. Stroganova, V.A. Lebedev, I.V. Voroshilov, A. De Backer, I.M. Razdobreev, M.G. Brik, in: Martin E. Fermann, Larry R. Marshall (Eds.), OSA Trends in Optics and Photonics v.68, Advanced SolidState Lasers, Optical society of America, Washington, DC, 2002, p. 260. [10] S.T. Lai, B.H.T. Chai, M. Long, R.C. Morris, IEEE J. Quantum Electr. 22 (1986) 1931. [11] S.T. Lai, J. Opt. Soc. Am. B 4 (1987) 1286. [12] M. Stadler, M. Bass, B.H.T. Chai, J. Opt. Soc. Am. B 9 (1992) 2271. [13] G.A. Torchia, J.A. Munoz, F. Cusso, F. Jaque, J.O. Tocho, J. Lumin. 92 (2001) 317.