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Spectral and kinetic properties of LiNbO3:Cr3+ crystals
Volume 78, number 3,4
OPTICSCOMMUNICATIONS
1 September 1990
Spectral and kinetic properties of LiNbO3" Cr 3+ crystals V.G. B a b a d j a n y a n , E.P. K o k a n y a n , R.B. K o s t a n y a n and V.R. N i k o g o s y a n Institute for Physical Research. Armenian Academy of Sciences, 378440 Ashrarak, Armenian SSR, USSR
Received 28 November 1989;revised manuscript received 9 March 1990
Spectral and kinetic properties of LiNbO3:Cr3+ crystalswith different concentrations (0.01, 0.1 and 1wt%) of doping impurity Cr203 in the initial melt have been investigated. The cross-sectionof the 4T2-4A2transition and the pumpingthreshold energyat the excitation wavelength 690 nm are estimated.
Trivalent chromium ions are most common activating additives ensuring stimulated emission both at discrete wavelengths (0.69 ~tm in ruby) and within a broad spectral range (0.7-1.0 ~tm) [ 1,2]. To design tunable lasers based on the Cr 3+ impurity one needs a medium where the excitation of the impurity results in the generation of broad band radiation related to the transition from the electronic-vibrational 4T2 state to the 4A 2 ground state. This condition is satisfied for the media where the energy gap AE between 2E and 4T2 states of Cr 3+ ion is relatively small thus enabling thermally activated population of the upper laser level a T 2 via the 2E metastable level. According to the Tanabe-Sugano diagrams [ 3 ] such a situation may occur in lithium niobate crystals with trivalent chromium impurity. In recently published papers [4,5 ] the useful laser properties of Nd 3+ activators are combined with nonlinear and electrooptical properties of lithium niobate crystals to design compact laser devices with a wide range of operating modes (continuous, Qswitched and self-doubling of generation frequency). LiNbO3:Cr 3÷ crystals with different concentrations of doping impurity in the initial components were studied in refs. [6-9]. The absorption spectrum shows two broad bands with maxima at 476 and 654 nm (corresponding to the transitions from the 4A 2 ground state to the 4T 1 and 4T 2 triplet states), which is convenient for pumping the impurity by different lasers and exciting lamps. The luminescence spectrum consists of one broad band with peaks at different wavelengths from 840 to 920 nm in dif-
ferent reports, which is probably due to the use of different radiation receivers and monochromators with varying spectral responses in the range of luminescence detection [ 8 ]. Thus the investigation of LiNbO3:Cr 3+ crystals is of certain interest. In this work the results of kinetic and spectroscopic investigations of LiNbO3:Cr 3÷ crystals with different concentrations of doping impurity in the initial components are presented for temperatures from 80 to 300 K. All crystals were grown in air by the Czochralski method using the congruent composition of components (RLi/N b = 0 . 9 5 ) obtained by a solid state reaction. The specially purified Li2CO3 and Nb205 were used for the synthesis. Platinum vessels of 50 mm diameter were used as melt contailaers. The pulling and rotating velocities during the growth were 3 m m / h and 20 rpm, respectively. The concentrations of doping chromium introduced into the initial melt as Cr203, were 0.01, 0.1 and I wt%. During the growth process across the crystal-melt system dc with density ,~ 10 A / m 2 was applied to make the growing crystals single-domained and to distribute the dopant homogeneously, preventing the formation of strip inhomogeneities [10]. The positive electrode was applied to the seed crystal and the negative one to the vessel. The crystals obtained were of light- to deep-green colors. The transmission spectra of oriented plates with dimensions of 1 5 × 1 0 × 1 (x, y, z) mm [9] were taken on Specord M-40 (200-900 nm), SF-8 (9002500 nm) and Specord M-80 (2500-5500 rim)
0030-4018/90/$03.50 © 1990 - Elsevier Science Publishers B.V. (North-Holland)
247
Volume 78, number 3,4
OPTICS COMMUNICATIONS
spectrometers. Fig. 1 shows the a b s o r p t i o n spectrum o f crystalline L i N b O 3 : C r 3+ ( ~ 0 . 0 1 wt%) at r o o m temperature. The peaks at 484, 657, 725 n m corres p o n d to the transitions from the 2A2 ground state to 4Tl, aT 2 and 2E, respectively, a n d the small absorption b a n d near 2864 n m to the O H group absorption. The luminescence investigations were carried out using 6 × 6 × 7 (x, y, z) m m 3 oriented samples. The nitrogen (,~g=337 n m ) , H e - N e ( 2 g = 6 3 2 n m ) a n d ruby ( 2 g = 6 9 4 n m ) lasers radiations were used as excitation sources. The spectra were recorded on a c o m b i n e d s p e c t r a l - c o m p u t i n g device K S V U - 2 , s p e c t r o m e t e r S D L - 1 and s p e c t r a l - a n a l y t i c a l complex based on the "Electronica D3-28" m i c r o c o m puter, M D R - 3 m o n o c h r o m a t o r and a tie-in unit [11 ]. The spectral sensitivities o f detectors and m o n o c h r o m a t o r s o b t a i n e d by use of a reference tungsten l a m p were taken into account to get the real spectrum o f luminescence o f the investigated crystals. Such a spectrum for L i N b O 3 : C r 3÷ ( ~ 0 . 1 wt% o f Cr203) excited by a H e - N e laser at r o o m temperature is given in fig. 2. This b r o a d b a n d r a d i a t i o n with peak near 870 nm and half-width ~ 120 n m corresponds to the transition 4T2-4A2 o f Cr 3+ ions in trigonal crystalline field o f LiNbO3 lattice. The large Stokes shift ( ~ 3800 c m - ~) between a b s o r p t i o n and luminescence peaks in the 4T2-4A 2 transition permits the hope that tunable generation in a wide spectral range can be o b t a i n e d using the investigated crystals in a four-level scheme.
1 September 1990
"-~ 1,0
f I
700
[
i
i
800
900
1000
WAVELENGTI-I(nm j Fig. 2. Luminescence spectrum of LiNbO3:Cr 3÷ (~0.1 wt%) crystal at room temperature (632 nm excitation ).
l0
J ~5
' 360 100 200 TEMPERATURE(oK) ,,,--,,,.
Fig. 3. Temperature dependence of luminescence decay time from 2T2 level of Cr3+ ion in LiNbO3 crystal. ©, data for a crystal with ,,~0.01 wt% concentration of CrzO3 in the initial components; D, 0.1 wt%; A, 1 wt%; ×, data from ref. [ 8 ].
-
~10 .5 i
~0,55O
z5 z'0
i5
10z,5 z,5
WAVELENGTH(10 3, c m - ' ) Fig. 1. Absorption spectrum ofLiNbO3: Cr3+ (.~0.01wt%) crystal at room temperature). 248
The t e m p e r a t u r e d e p e n d e n c e o f the luminescence decay rate for all the investigated crystals is shown in fig. 3. F o r c o m p a r i s o n the d a t a from ref. [8] for the same crystal with very low d o p a n t concentration ( 1 . 2 7 × 1017 cm - 9 ) are also shown. The analysis o f the results o b t a i n e d shows that the decay of the aT 2 level o f Cr 3+ ion in LiNbO3 up to < 1 wt% is susceptible to weak concentrational quenching. It is also obvious that at temperatures below ~ 100 K the lifetime o f the aT 2 state is essentially constant ( ~ 10 ~ts). Taking this as the value of the radiational lifetime o f
Volume 78, number 3,4
OPTICS COMMUNICATIONS
4T 2 level ( r r) one can evaluate the effective crosssection of the induced transition in the channel 4T24A2 at 870 nm, using the formula from ref. [12], 24 iT= 87CCF12Z~ef.~r ,
(
1)
where 2 is the wavelength, c is the light velocity in vacuum, n is the refractive index of the m e d i u m at 2 wavelength, A2ef is the effective b a n d w i d t h of luminescence equal to the ratio of b a n d area to the m a x i m u m intensity. Substituting the corresponding quantities into eq. ( 1 ) we obtain t r = 2 × 10 -19 cm 2. Using the obtained value one can evaluate the threshold absorbed energy at the wavelength of the chosen excitation, needed to initiate the laser emission Pth-- h v o r n -2 -2 o"cf 2 2 ( O ) p ' ~ - O ) g ) ,
(2)
where up is the radiation frequency of the excitation source, zf is decay time of luminescence, ~ is the loss during one pass within the laser resonator at the oscillation wavelength, o~p and ~Og are the radii of excitation and laser beams, respectively, averaged over the crystal length: ~p.g=(2pgl/121/2nn, p,g,~/2, l is length of active crystal, np,g are the refractive indices at the respective wavelengths. In the case of ruby laser p u m p i n g ( v p = 4 . 3 2 × 10 t4 s -~) of a 1 cm long sample at a temperature of the investigated crystal of about 300 K, rf-~ 1 ms assuming 8 = 0.1 c m - ~, we obtain Pth= 1.5 W cm -~. It is k n o w n that the lithium niobate crystals are subject to the "optical damage" effect when irradiated at wavelengths below 800 n m [ 13 ]. The effect of light-induced change of the refractive index
1 September 1990
( m a i n l y extraordinary) results in large losses at the radiation wavelength a n d suppression of the oscillation. To overcome this effect one must either introduce additional MgO impurity ( ~ 6 mol%) during the crystal growth or choose pulse excitation sources with small repetition rates [4,5 ].
References [ 1] P.F. Moulton, Laser Focus 19 (1983) 83. [2] R.Y. Abdulsabirov, M.A. Dubinskij, S.L. Korableva, M.V. Mityagin,N.I. Silkin, G.A. Skripko, A.P. Shkadarevich and M.V. Yagudin, Report of All-Union Conference on KiNO, 26-29.8.1985, part 1, Moscow, p. 139. [3] D.T. Sviridov, P.K. Sviridova and Y.F. Smirnov, Optical spectra of transition group ions in the crystals (Nauka, Moscow, 1976) p. 139. [4] T.Y. Fan, A. Cordova-Plaza, M.J.F. Digonnet, R.L. Byer and H.J. Shaw, J. Opt. Soc. Amer. B 3 (1986) 140. [5] A. Cordova-Plaza, T.Y. Fan, M.J.F. Digonnet, R.L. Byer and H.J. Shaw, Optics Lett. 13 (1988) 209. [ 6 ] G. Bums, D.F. O'Kane and R.S. Title, Phys. Lett. 23 ( 1966) 56. [ 7 ] A.M. Glass, J. Chem. Phys. 50 ( 1969) 1501. [8] R.C. Powelland E.E. Freed, J. Chem. Phys. 70 (1979) 4681. [91P.A. Arsenev and B.A. Baranov, Phys. Stat. Sol. (a) 9 (1972) 673. [ 10] R.N. Balasanyan, V.T. Gabrielyan, I.P. Zapasskaya, E.P. Kokanyan, A.O. Krylov, A.Ya. Nieman and A.P. Sharafutdinov, The Processes Accompanied the Lithium Niobate Crystals Growth in Electrical Field, Reprint, IFI86-119, Yerevan, (1988) p. 32. [ 11 ] V.G. Badadjanyan,E.V. Barsegyanand S.V. Oganesyan,The Spectral-analytical Complex Based on Electronica D3-28 Microcomputer, Reprint, IFI-88-131, Yerevan, 91988) p. 24. [ 12"]F. Kachmarek, Introduction to laser physics (Mir, Moscow, 1981) p. 74. [13] Y.S. Kuz'minov, Lithium niobate and tantalate (Nauka, Moscow, 1975).
249
OPTICSCOMMUNICATIONS
1 September 1990
Spectral and kinetic properties of LiNbO3" Cr 3+ crystals V.G. B a b a d j a n y a n , E.P. K o k a n y a n , R.B. K o s t a n y a n and V.R. N i k o g o s y a n Institute for Physical Research. Armenian Academy of Sciences, 378440 Ashrarak, Armenian SSR, USSR
Received 28 November 1989;revised manuscript received 9 March 1990
Spectral and kinetic properties of LiNbO3:Cr3+ crystalswith different concentrations (0.01, 0.1 and 1wt%) of doping impurity Cr203 in the initial melt have been investigated. The cross-sectionof the 4T2-4A2transition and the pumpingthreshold energyat the excitation wavelength 690 nm are estimated.
Trivalent chromium ions are most common activating additives ensuring stimulated emission both at discrete wavelengths (0.69 ~tm in ruby) and within a broad spectral range (0.7-1.0 ~tm) [ 1,2]. To design tunable lasers based on the Cr 3+ impurity one needs a medium where the excitation of the impurity results in the generation of broad band radiation related to the transition from the electronic-vibrational 4T2 state to the 4A 2 ground state. This condition is satisfied for the media where the energy gap AE between 2E and 4T2 states of Cr 3+ ion is relatively small thus enabling thermally activated population of the upper laser level a T 2 via the 2E metastable level. According to the Tanabe-Sugano diagrams [ 3 ] such a situation may occur in lithium niobate crystals with trivalent chromium impurity. In recently published papers [4,5 ] the useful laser properties of Nd 3+ activators are combined with nonlinear and electrooptical properties of lithium niobate crystals to design compact laser devices with a wide range of operating modes (continuous, Qswitched and self-doubling of generation frequency). LiNbO3:Cr 3÷ crystals with different concentrations of doping impurity in the initial components were studied in refs. [6-9]. The absorption spectrum shows two broad bands with maxima at 476 and 654 nm (corresponding to the transitions from the 4A 2 ground state to the 4T 1 and 4T 2 triplet states), which is convenient for pumping the impurity by different lasers and exciting lamps. The luminescence spectrum consists of one broad band with peaks at different wavelengths from 840 to 920 nm in dif-
ferent reports, which is probably due to the use of different radiation receivers and monochromators with varying spectral responses in the range of luminescence detection [ 8 ]. Thus the investigation of LiNbO3:Cr 3+ crystals is of certain interest. In this work the results of kinetic and spectroscopic investigations of LiNbO3:Cr 3÷ crystals with different concentrations of doping impurity in the initial components are presented for temperatures from 80 to 300 K. All crystals were grown in air by the Czochralski method using the congruent composition of components (RLi/N b = 0 . 9 5 ) obtained by a solid state reaction. The specially purified Li2CO3 and Nb205 were used for the synthesis. Platinum vessels of 50 mm diameter were used as melt contailaers. The pulling and rotating velocities during the growth were 3 m m / h and 20 rpm, respectively. The concentrations of doping chromium introduced into the initial melt as Cr203, were 0.01, 0.1 and I wt%. During the growth process across the crystal-melt system dc with density ,~ 10 A / m 2 was applied to make the growing crystals single-domained and to distribute the dopant homogeneously, preventing the formation of strip inhomogeneities [10]. The positive electrode was applied to the seed crystal and the negative one to the vessel. The crystals obtained were of light- to deep-green colors. The transmission spectra of oriented plates with dimensions of 1 5 × 1 0 × 1 (x, y, z) mm [9] were taken on Specord M-40 (200-900 nm), SF-8 (9002500 nm) and Specord M-80 (2500-5500 rim)
0030-4018/90/$03.50 © 1990 - Elsevier Science Publishers B.V. (North-Holland)
247
Volume 78, number 3,4
OPTICS COMMUNICATIONS
spectrometers. Fig. 1 shows the a b s o r p t i o n spectrum o f crystalline L i N b O 3 : C r 3+ ( ~ 0 . 0 1 wt%) at r o o m temperature. The peaks at 484, 657, 725 n m corres p o n d to the transitions from the 2A2 ground state to 4Tl, aT 2 and 2E, respectively, a n d the small absorption b a n d near 2864 n m to the O H group absorption. The luminescence investigations were carried out using 6 × 6 × 7 (x, y, z) m m 3 oriented samples. The nitrogen (,~g=337 n m ) , H e - N e ( 2 g = 6 3 2 n m ) a n d ruby ( 2 g = 6 9 4 n m ) lasers radiations were used as excitation sources. The spectra were recorded on a c o m b i n e d s p e c t r a l - c o m p u t i n g device K S V U - 2 , s p e c t r o m e t e r S D L - 1 and s p e c t r a l - a n a l y t i c a l complex based on the "Electronica D3-28" m i c r o c o m puter, M D R - 3 m o n o c h r o m a t o r and a tie-in unit [11 ]. The spectral sensitivities o f detectors and m o n o c h r o m a t o r s o b t a i n e d by use of a reference tungsten l a m p were taken into account to get the real spectrum o f luminescence o f the investigated crystals. Such a spectrum for L i N b O 3 : C r 3÷ ( ~ 0 . 1 wt% o f Cr203) excited by a H e - N e laser at r o o m temperature is given in fig. 2. This b r o a d b a n d r a d i a t i o n with peak near 870 nm and half-width ~ 120 n m corresponds to the transition 4T2-4A2 o f Cr 3+ ions in trigonal crystalline field o f LiNbO3 lattice. The large Stokes shift ( ~ 3800 c m - ~) between a b s o r p t i o n and luminescence peaks in the 4T2-4A 2 transition permits the hope that tunable generation in a wide spectral range can be o b t a i n e d using the investigated crystals in a four-level scheme.
1 September 1990
"-~ 1,0
f I
700
[
i
i
800
900
1000
WAVELENGTI-I(nm j Fig. 2. Luminescence spectrum of LiNbO3:Cr 3÷ (~0.1 wt%) crystal at room temperature (632 nm excitation ).
l0
J ~5
' 360 100 200 TEMPERATURE(oK) ,,,--,,,.
Fig. 3. Temperature dependence of luminescence decay time from 2T2 level of Cr3+ ion in LiNbO3 crystal. ©, data for a crystal with ,,~0.01 wt% concentration of CrzO3 in the initial components; D, 0.1 wt%; A, 1 wt%; ×, data from ref. [ 8 ].
-
~10 .5 i
~0,55O
z5 z'0
i5
10z,5 z,5
WAVELENGTH(10 3, c m - ' ) Fig. 1. Absorption spectrum ofLiNbO3: Cr3+ (.~0.01wt%) crystal at room temperature). 248
The t e m p e r a t u r e d e p e n d e n c e o f the luminescence decay rate for all the investigated crystals is shown in fig. 3. F o r c o m p a r i s o n the d a t a from ref. [8] for the same crystal with very low d o p a n t concentration ( 1 . 2 7 × 1017 cm - 9 ) are also shown. The analysis o f the results o b t a i n e d shows that the decay of the aT 2 level o f Cr 3+ ion in LiNbO3 up to < 1 wt% is susceptible to weak concentrational quenching. It is also obvious that at temperatures below ~ 100 K the lifetime o f the aT 2 state is essentially constant ( ~ 10 ~ts). Taking this as the value of the radiational lifetime o f
Volume 78, number 3,4
OPTICS COMMUNICATIONS
4T 2 level ( r r) one can evaluate the effective crosssection of the induced transition in the channel 4T24A2 at 870 nm, using the formula from ref. [12], 24 iT= 87CCF12Z~ef.~r ,
(
1)
where 2 is the wavelength, c is the light velocity in vacuum, n is the refractive index of the m e d i u m at 2 wavelength, A2ef is the effective b a n d w i d t h of luminescence equal to the ratio of b a n d area to the m a x i m u m intensity. Substituting the corresponding quantities into eq. ( 1 ) we obtain t r = 2 × 10 -19 cm 2. Using the obtained value one can evaluate the threshold absorbed energy at the wavelength of the chosen excitation, needed to initiate the laser emission Pth-- h v o r n -2 -2 o"cf 2 2 ( O ) p ' ~ - O ) g ) ,
(2)
where up is the radiation frequency of the excitation source, zf is decay time of luminescence, ~ is the loss during one pass within the laser resonator at the oscillation wavelength, o~p and ~Og are the radii of excitation and laser beams, respectively, averaged over the crystal length: ~p.g=(2pgl/121/2nn, p,g,~/2, l is length of active crystal, np,g are the refractive indices at the respective wavelengths. In the case of ruby laser p u m p i n g ( v p = 4 . 3 2 × 10 t4 s -~) of a 1 cm long sample at a temperature of the investigated crystal of about 300 K, rf-~ 1 ms assuming 8 = 0.1 c m - ~, we obtain Pth= 1.5 W cm -~. It is k n o w n that the lithium niobate crystals are subject to the "optical damage" effect when irradiated at wavelengths below 800 n m [ 13 ]. The effect of light-induced change of the refractive index
1 September 1990
( m a i n l y extraordinary) results in large losses at the radiation wavelength a n d suppression of the oscillation. To overcome this effect one must either introduce additional MgO impurity ( ~ 6 mol%) during the crystal growth or choose pulse excitation sources with small repetition rates [4,5 ].
References [ 1] P.F. Moulton, Laser Focus 19 (1983) 83. [2] R.Y. Abdulsabirov, M.A. Dubinskij, S.L. Korableva, M.V. Mityagin,N.I. Silkin, G.A. Skripko, A.P. Shkadarevich and M.V. Yagudin, Report of All-Union Conference on KiNO, 26-29.8.1985, part 1, Moscow, p. 139. [3] D.T. Sviridov, P.K. Sviridova and Y.F. Smirnov, Optical spectra of transition group ions in the crystals (Nauka, Moscow, 1976) p. 139. [4] T.Y. Fan, A. Cordova-Plaza, M.J.F. Digonnet, R.L. Byer and H.J. Shaw, J. Opt. Soc. Amer. B 3 (1986) 140. [5] A. Cordova-Plaza, T.Y. Fan, M.J.F. Digonnet, R.L. Byer and H.J. Shaw, Optics Lett. 13 (1988) 209. [ 6 ] G. Bums, D.F. O'Kane and R.S. Title, Phys. Lett. 23 ( 1966) 56. [ 7 ] A.M. Glass, J. Chem. Phys. 50 ( 1969) 1501. [8] R.C. Powelland E.E. Freed, J. Chem. Phys. 70 (1979) 4681. [91P.A. Arsenev and B.A. Baranov, Phys. Stat. Sol. (a) 9 (1972) 673. [ 10] R.N. Balasanyan, V.T. Gabrielyan, I.P. Zapasskaya, E.P. Kokanyan, A.O. Krylov, A.Ya. Nieman and A.P. Sharafutdinov, The Processes Accompanied the Lithium Niobate Crystals Growth in Electrical Field, Reprint, IFI86-119, Yerevan, (1988) p. 32. [ 11 ] V.G. Badadjanyan,E.V. Barsegyanand S.V. Oganesyan,The Spectral-analytical Complex Based on Electronica D3-28 Microcomputer, Reprint, IFI-88-131, Yerevan, 91988) p. 24. [ 12"]F. Kachmarek, Introduction to laser physics (Mir, Moscow, 1981) p. 74. [13] Y.S. Kuz'minov, Lithium niobate and tantalate (Nauka, Moscow, 1975).
249