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Optical properties and lasing of BaLaGa3O7 single crystals doped with neodymium
1. fhys. Chem. Solids Vol. 49, No. 2, pp. 19!%203, 1988 Printed in Great Britain.
0022-3697/88 53.00 + 0.00 Pergamon Press plc
OPTICAL PROPERTIES AND LASING OF BaLaGa,O, SINGLE CRYSTALS DOPED WITH NEODYMIUM W. RYBA-ROMANOWSKI, B. JE~OWSKA-TRZEBIATOWSKA Institute of Low Temperature and Structure Research, Polish Academy of Sciences, SO-950 Wroclaw 2. Poland
W. PIEKARCZYK and M. BERKOWSKI Institute of Physics, Polish Academy of Sciences, Warsaw, Poland (Received 23 March 1987; accepted 17 June 1987) Abstract-Large single crystafs of E%aLaGa,O, containing Nd were prepared by the Czochralski method and studied experimentally by absorption and emission spectroscopy. The dependence of the spectroscopic properties of the crystals on temperature, Nd concentration and Iigbt polarization was determined. Room temperature laser action associated with the 4F,,2-411,,2transition of Nd3+ was achieved. Keywords: Laser crystal, luminescence, ne~~ium,
INTRODUCTION BaLaGa30, was obtained for the first time by Ismatov et al. [l] in polycrystalline form by sintering
the stoichiometric mixture of the components at high temperature. Melting point, refractive indices and unit cell parameters of this compound were then determined. The authors reported that the crystals were of tetragonal symmetry, space group PifZ,m-D,,3, hence without a centre of symmetry. Recently, large single crystals of BaLaGaXO, were produced by Piekarczyk et al. [2] by the Czochralski method using the automatic control of crystal diameter. Measurements of absorption in polarized light, of refractive indices and birefringence of these crystals were performed and reported by Berkowski et al. [3]. In particular it was found that the crystal is transparent in the 15~crn-‘~l,~~-’ region and slightly birefringent with refractive indices: n, = 1.845 and n,, = 1.850. The elastic, piezoelectric and dielectric constants were determined by Soluch et al. [4] from the measured parameters of thin-plate resonators.
The reported
values are: elastic constants
(in 10” N/m’): c;,= 1.78, cB= 1.03, cf,=O.88, c$ = 1.17, c& = 0.39, & = 0.54; piezoelectric constants (in C/m*): e,4 = 0.28, e36= 0.10; dielectric constants: tT,/co = 12.7, t&/c, = 9.5. The absorption spectra of BaLaGa,O, single crystal doped with Nd were analyzed within the framework of the Judd-Ofelt theory in our previous work [SJ. In this paper we present the results of a detailed study of the optical propertiers of BaLaGa,O, crystals over a wide temperature range and for several Nd
concentrations. Preliminary laser experiments are reported in the last section of this paper. EXPERIMENTAL Spectroscopic
measurements
were made
on sam-
ples in the form of plane parallel plates of the desired PC S 4912-F
luminescence lifetime, luminescence quenching.
orientation. Laser experiments and measurements of thermal conductivity were made on samples in the form of rods with the axis parallel to the optical axis of the crystal. Polarized absorption spectra were measured at room temperature with a Varian Model 2300 spectrophotometer. Luminescence was excited by a high-pressure mercury lamp HBO-200, analyzed by a Carl Zeiss Model GDM 1000 grating monochromator and detected by a photomultiplier connected to a phase-sensitive detection system. For luminescence decay time measurements an arrangement consisting of pulsed rhodamine 6G dye laser, prism monochroamator and a ZWG Model BCI 280 boxcar integrator was used. For low-tem~rature measurements the samples were placed in a continuous flow liquid helium cryostat. RESULTS
dependence of the transition intensities on the polarization of the light The BaLaGa,O, : Nd3+ crystal is optically uniaxial, and therefore the intensities of the optical transitions are not the same for different orientations of the E field vector relative to the optical axis. The effect of crystal anisotropy is significant both in absorption and emission. In Fig. 1 we present the survey absorption spectrum recorded at room temperature with light polarized parallel (11) and perpendicular (I) to the c-axis of the crystal. The corresponding quantitative data evaluated by numerical integration of absorption bands are given in Table 1. The intensities of transitions observed in the //-spectrum are roughly twice as weak as those observed in the I-spectrum, except for the less altered transition to the 4G,,z, ‘G,,* group of levels. In Fig. 2 we present the polarized luminescence 199
W. RYBA-ROMANOWSKI
200
et al.
lnml
Fig. I. Room temperature absorption spectra of BaLaGa,O,: Nd3+ measured with light polarized parallel (II) and perpendicular (I) to the c-axis of the crystal. C,, = 5.6. lOI crnme3.
Table I. Oscillator strengths~and absorption coefficients k,, at band maxima evaluated from absorption spectra measured with light polarized parallel (11)and perpendicular (I) to the c-axis of the crystal. C,, = 5.6. lOI9cm-3 Excited state “F,,, 4F,,, %,, .+F,, , %z 4F92
“G,i,, 2G,/, ‘K,,i, 34%z, 4%, 2P,,* %Z> %,,,, 4D,,,, 4D~,2
Spectral region (nm)
(cm-’
853-915 778-835 725-773 665-705 568-605 494-544 429-441 346-368
0.59 I .39 0.79 0.11 I.96 0.47 0.12 1.18
(x
106)
(cm-‘)
(x/;O’)
2.88 6.85 5.85 0.45 15.35 6.32 0.63 13.37
0.19 0.64 0.56 0.04 I .55 0.54 0.05 0.36
1.33 3.89 3.39 0.26 12.34 3.22 0.26 5.41
9200 Wavenumber fcm-')
Fig. 2. Polarized luminescence spectra corresponding
to the 4F,i,-41,,,, transition recorded at 300 K.
Optical properties and lasing of BaLaGa,O,
spectrum measured at room temperature and correspondmg to the 4F,,,-41,1,2 transition. It should be noted that the shape of the luminescence band does not depend on the polarization in a significant manner. The 4F,,r level is depopulated by four radiative transitions to the 41,,,, ‘I,,,,, 41,,,, and “I,,,, levels. The intensity distributions of these transitions are characterized by the luminescence branching ratios & defined in the case of isotropic media as
under the the values evaluated polarized
201
corresponding luminescence bands. Using of & = 0.47 and trad = 3 12 ps, which were by the Judd&felt analysis of the unabsorption spectra [S], and putting s,=f0q+/q>,
(3)
we obtain the following values of cr.&and flirt for the 4F~,~-41iin laser transition: a:, = 8.72. 10-20cm2, a& = 2.7.1O-*O cm2.
where A, denotes the probability of radiative transition from the initial level i to terminal level j, rrad is the radiative lifetime of the initial level. In the case of uniaxial crystals, the introduction of orientational branching ratios /3$ and /?i is convenient for the determination of stimulated emission cross-sections a& and rr& according to the relation:
(2) where A, is the mean wavelength of the transition, n is the index of refraction at the wavelength of transition and Av,e is the effective bandwidth determined from the luminescence spectrum. The relation between /3; and #?i is found by comparing the areas
These values are, respectively, the highest and the lowest stimulated emission cross-sections of the laser transition in these crystals. The output of lasers employing rods with c-axis orientations away from the rod axis will be linearly polarized. Efect of temperature on luminescence spectra and Byetime Temperature has little effect on the bandwidths of the transitions. The observed changes in the luminescence spectra upon lowering the temperature of the sample may be explained by changes in the population distribution of the 4F,2 level which is split by the crystal field into two components separated by 18Ocm-‘, In Fig. 3 we present the spectra corre-
11400
116Ot
Wavenu’mber km” 1
Fig. 3. Luminescence band ~o~~nding
to the ‘F,,,-41,,z transition recorded at 77 K and 300 K.
W. RYBA-ROMANOWSKI et al.
202
nescence lifetime are plotted vs the temperature of the sample. It can be seen that the role of nonradiative transitions in depopulating the 4F,,, level is negligible. Figure 4 also shows that the coefficients j?$ and @I are practically independent of temperature.
100
200
300
400 TIKI
Fig. 4. Dependence of luminescence lifetime (0) and of
integrated luminescence intensity polarized parallel ( x ) and perpendicular (a) to the c-axis on temperature of the sample. C,, = 5.6. lOI cm-‘.
sponding to the 4F~,2-419,z transition, recorded at 77 K and at 300 K. Crystal field splitting of the 419,2ground state as determined from the decomposition of the spectrum recorded at 77 K is only 361 cm-‘. Three remaining components are situated at 120cm-‘, 148 cm-’ and 320 cm-‘. The effect of temperature on the luminescence intensity originating from the ‘F,,, level has been investigated in the 13 K-300 K temperature region. In Fig. 4 the integrated intensity of the 4F,,r41,,,, transition and the measured lumi-
Unit cell parameters of BaLaGa,O, and BaNdGaSO, do not differ si~ifi~ntly and thus, the Nd concentration should not influence the spectral bands. On the other hand, deviations from stoichiometry have been observed in crystals grown in the 001 direction [4]. Therefore, we examined carefully the optical spectra obtained with samples containing Nd up to 20 at%. No detectable effects of Nd concentration on the absorption spectra were found. Concentration quenching of the Nd luminescence was investigated at room temperature on the basis of lifetime measurements. The data obtained were used to evaluate the probabilities k, of concentration quenching of luminescence according to the formula: k,=i_L, ~ium
(4) %ad
where T]“,,,is the measured luminescence lifetime. The dependence of k, and z,“,,, on the concentration N is plotted in Fig. 5. The slope of the line k,(N) is close to 2, therefore the probability of concentration
- lo4 K&-9
-103
I
1020
m I ;;121
lo2
N km-3l
Fig. 5. Dependence of the luminescence lifetime z,um(0) and of the probability k,. of concentration quenching of the luminescence (0) on the Nd concentration.
Optical properties and lasing of BaLaGa,O, Table
2.
5.45 312
flashlamp. The reflectivity of the output mirror was then 90% and the length of pumping pulse was 600~s. Slightly above the threshold the laser oscillation lasted for approximately 200 PCS.
176 close to 1
SUMMARY
Optical and physical properties BaLaGa,O,: Nd3+ single crystals
Density (g cm--‘) Radiative lifetime o(s) Effective bandwidth of the 4F,,2-41,,,z transition at 300 K (cm-‘) Quantum efficiency of the 4F3,, level Stimulated emission cross-section for the 4F3jz-411 iir transition at 300 K (cm’) Upper laser level splitting (cm-‘) Lasing wavelength (pm) The&al conductivity at 297 K (alone c-axis) 0%’ cn-’ K-‘)
203
of
8.72. lo-*” 180 1.059 0.1127
quenching of luminescence depends quadratically the Nd concentration: k, - N*.
on
Preliminary investigation of stimulated emission Laser action was achieved in BaLaGa,O, : Nd3+ at room temperature. A sample rod containing Nd at a concentration of 5.6. lOI cmm3 was prepared with dimensions 6.3 x 40 mm. The axis was parallel to the c-axis of the crystal. The laser rod and one linear flashlamp 10 mm in diameter were wrapped with aluminium foil and placed in a water solution of sodium nitrite. In this configuration 28mm of rod length was pumped. The resonator was formed by two plane-parallel dielectric mirrors separated by 40cm. The threshold for laser oscillation was observed at 90 J of electrical input energy to thi
In Table 2 we gathered the data which may be used to characterize the BaLaGa,0,:Nd3+ crystal as a laser-active material. These data indicate that this material possesses a combination of properties favourable for laser operation. We believe that further laser experiments which are in preparation will confirm
this conclusion.
Acknowledgement-This work was carried Research programme CPBR 8.14.
out
under
REFERENCES 1. Ismatov A. A., Kolesova V. A. and Piryutko N. M., Izu. Akad. Nauk SSSR Neorn. Mater. 6. 1361 (1970). 2. Piekarczyk W., BerkoGki M. and JasioIek G., J. Crystal Growth 71, 395 (1985). 3. Berkowski M., Borowiec M. T., Pataj K., Piekarczyk W. and Wardzynski W., Physica lUB, 215 (1984). 4. Soluch W., Ksiezopolski R., Piekarczyk W., Berkowski M., Goodberlet M. A. and Vetelino J. F., J. Appl. Phys. 58, 2285 (1985). 5. Ryba-Romanowski
W., Gutowska M. U., Piekarczyk W., Berkowski M., Mazurak Z. and JeiowskaTrzebiatowska B., J. Luminesc. 36, 369 (1987).
0022-3697/88 53.00 + 0.00 Pergamon Press plc
OPTICAL PROPERTIES AND LASING OF BaLaGa,O, SINGLE CRYSTALS DOPED WITH NEODYMIUM W. RYBA-ROMANOWSKI, B. JE~OWSKA-TRZEBIATOWSKA Institute of Low Temperature and Structure Research, Polish Academy of Sciences, SO-950 Wroclaw 2. Poland
W. PIEKARCZYK and M. BERKOWSKI Institute of Physics, Polish Academy of Sciences, Warsaw, Poland (Received 23 March 1987; accepted 17 June 1987) Abstract-Large single crystafs of E%aLaGa,O, containing Nd were prepared by the Czochralski method and studied experimentally by absorption and emission spectroscopy. The dependence of the spectroscopic properties of the crystals on temperature, Nd concentration and Iigbt polarization was determined. Room temperature laser action associated with the 4F,,2-411,,2transition of Nd3+ was achieved. Keywords: Laser crystal, luminescence, ne~~ium,
INTRODUCTION BaLaGa30, was obtained for the first time by Ismatov et al. [l] in polycrystalline form by sintering
the stoichiometric mixture of the components at high temperature. Melting point, refractive indices and unit cell parameters of this compound were then determined. The authors reported that the crystals were of tetragonal symmetry, space group PifZ,m-D,,3, hence without a centre of symmetry. Recently, large single crystals of BaLaGaXO, were produced by Piekarczyk et al. [2] by the Czochralski method using the automatic control of crystal diameter. Measurements of absorption in polarized light, of refractive indices and birefringence of these crystals were performed and reported by Berkowski et al. [3]. In particular it was found that the crystal is transparent in the 15~crn-‘~l,~~-’ region and slightly birefringent with refractive indices: n, = 1.845 and n,, = 1.850. The elastic, piezoelectric and dielectric constants were determined by Soluch et al. [4] from the measured parameters of thin-plate resonators.
The reported
values are: elastic constants
(in 10” N/m’): c;,= 1.78, cB= 1.03, cf,=O.88, c$ = 1.17, c& = 0.39, & = 0.54; piezoelectric constants (in C/m*): e,4 = 0.28, e36= 0.10; dielectric constants: tT,/co = 12.7, t&/c, = 9.5. The absorption spectra of BaLaGa,O, single crystal doped with Nd were analyzed within the framework of the Judd-Ofelt theory in our previous work [SJ. In this paper we present the results of a detailed study of the optical propertiers of BaLaGa,O, crystals over a wide temperature range and for several Nd
concentrations. Preliminary laser experiments are reported in the last section of this paper. EXPERIMENTAL Spectroscopic
measurements
were made
on sam-
ples in the form of plane parallel plates of the desired PC S 4912-F
luminescence lifetime, luminescence quenching.
orientation. Laser experiments and measurements of thermal conductivity were made on samples in the form of rods with the axis parallel to the optical axis of the crystal. Polarized absorption spectra were measured at room temperature with a Varian Model 2300 spectrophotometer. Luminescence was excited by a high-pressure mercury lamp HBO-200, analyzed by a Carl Zeiss Model GDM 1000 grating monochromator and detected by a photomultiplier connected to a phase-sensitive detection system. For luminescence decay time measurements an arrangement consisting of pulsed rhodamine 6G dye laser, prism monochroamator and a ZWG Model BCI 280 boxcar integrator was used. For low-tem~rature measurements the samples were placed in a continuous flow liquid helium cryostat. RESULTS
dependence of the transition intensities on the polarization of the light The BaLaGa,O, : Nd3+ crystal is optically uniaxial, and therefore the intensities of the optical transitions are not the same for different orientations of the E field vector relative to the optical axis. The effect of crystal anisotropy is significant both in absorption and emission. In Fig. 1 we present the survey absorption spectrum recorded at room temperature with light polarized parallel (11) and perpendicular (I) to the c-axis of the crystal. The corresponding quantitative data evaluated by numerical integration of absorption bands are given in Table 1. The intensities of transitions observed in the //-spectrum are roughly twice as weak as those observed in the I-spectrum, except for the less altered transition to the 4G,,z, ‘G,,* group of levels. In Fig. 2 we present the polarized luminescence 199
W. RYBA-ROMANOWSKI
200
et al.
lnml
Fig. I. Room temperature absorption spectra of BaLaGa,O,: Nd3+ measured with light polarized parallel (II) and perpendicular (I) to the c-axis of the crystal. C,, = 5.6. lOI crnme3.
Table I. Oscillator strengths~and absorption coefficients k,, at band maxima evaluated from absorption spectra measured with light polarized parallel (11)and perpendicular (I) to the c-axis of the crystal. C,, = 5.6. lOI9cm-3 Excited state “F,,, 4F,,, %,, .+F,, , %z 4F92
“G,i,, 2G,/, ‘K,,i, 34%z, 4%, 2P,,* %Z> %,,,, 4D,,,, 4D~,2
Spectral region (nm)
(cm-’
853-915 778-835 725-773 665-705 568-605 494-544 429-441 346-368
0.59 I .39 0.79 0.11 I.96 0.47 0.12 1.18
(x
106)
(cm-‘)
(x/;O’)
2.88 6.85 5.85 0.45 15.35 6.32 0.63 13.37
0.19 0.64 0.56 0.04 I .55 0.54 0.05 0.36
1.33 3.89 3.39 0.26 12.34 3.22 0.26 5.41
9200 Wavenumber fcm-')
Fig. 2. Polarized luminescence spectra corresponding
to the 4F,i,-41,,,, transition recorded at 300 K.
Optical properties and lasing of BaLaGa,O,
spectrum measured at room temperature and correspondmg to the 4F,,,-41,1,2 transition. It should be noted that the shape of the luminescence band does not depend on the polarization in a significant manner. The 4F,,r level is depopulated by four radiative transitions to the 41,,,, ‘I,,,,, 41,,,, and “I,,,, levels. The intensity distributions of these transitions are characterized by the luminescence branching ratios & defined in the case of isotropic media as
under the the values evaluated polarized
201
corresponding luminescence bands. Using of & = 0.47 and trad = 3 12 ps, which were by the Judd&felt analysis of the unabsorption spectra [S], and putting s,=f0q+/q>,
(3)
we obtain the following values of cr.&and flirt for the 4F~,~-41iin laser transition: a:, = 8.72. 10-20cm2, a& = 2.7.1O-*O cm2.
where A, denotes the probability of radiative transition from the initial level i to terminal level j, rrad is the radiative lifetime of the initial level. In the case of uniaxial crystals, the introduction of orientational branching ratios /3$ and /?i is convenient for the determination of stimulated emission cross-sections a& and rr& according to the relation:
(2) where A, is the mean wavelength of the transition, n is the index of refraction at the wavelength of transition and Av,e is the effective bandwidth determined from the luminescence spectrum. The relation between /3; and #?i is found by comparing the areas
These values are, respectively, the highest and the lowest stimulated emission cross-sections of the laser transition in these crystals. The output of lasers employing rods with c-axis orientations away from the rod axis will be linearly polarized. Efect of temperature on luminescence spectra and Byetime Temperature has little effect on the bandwidths of the transitions. The observed changes in the luminescence spectra upon lowering the temperature of the sample may be explained by changes in the population distribution of the 4F,2 level which is split by the crystal field into two components separated by 18Ocm-‘, In Fig. 3 we present the spectra corre-
11400
116Ot
Wavenu’mber km” 1
Fig. 3. Luminescence band ~o~~nding
to the ‘F,,,-41,,z transition recorded at 77 K and 300 K.
W. RYBA-ROMANOWSKI et al.
202
nescence lifetime are plotted vs the temperature of the sample. It can be seen that the role of nonradiative transitions in depopulating the 4F,,, level is negligible. Figure 4 also shows that the coefficients j?$ and @I are practically independent of temperature.
100
200
300
400 TIKI
Fig. 4. Dependence of luminescence lifetime (0) and of
integrated luminescence intensity polarized parallel ( x ) and perpendicular (a) to the c-axis on temperature of the sample. C,, = 5.6. lOI cm-‘.
sponding to the 4F~,2-419,z transition, recorded at 77 K and at 300 K. Crystal field splitting of the 419,2ground state as determined from the decomposition of the spectrum recorded at 77 K is only 361 cm-‘. Three remaining components are situated at 120cm-‘, 148 cm-’ and 320 cm-‘. The effect of temperature on the luminescence intensity originating from the ‘F,,, level has been investigated in the 13 K-300 K temperature region. In Fig. 4 the integrated intensity of the 4F,,r41,,,, transition and the measured lumi-
Unit cell parameters of BaLaGa,O, and BaNdGaSO, do not differ si~ifi~ntly and thus, the Nd concentration should not influence the spectral bands. On the other hand, deviations from stoichiometry have been observed in crystals grown in the 001 direction [4]. Therefore, we examined carefully the optical spectra obtained with samples containing Nd up to 20 at%. No detectable effects of Nd concentration on the absorption spectra were found. Concentration quenching of the Nd luminescence was investigated at room temperature on the basis of lifetime measurements. The data obtained were used to evaluate the probabilities k, of concentration quenching of luminescence according to the formula: k,=i_L, ~ium
(4) %ad
where T]“,,,is the measured luminescence lifetime. The dependence of k, and z,“,,, on the concentration N is plotted in Fig. 5. The slope of the line k,(N) is close to 2, therefore the probability of concentration
- lo4 K&-9
-103
I
1020
m I ;;121
lo2
N km-3l
Fig. 5. Dependence of the luminescence lifetime z,um(0) and of the probability k,. of concentration quenching of the luminescence (0) on the Nd concentration.
Optical properties and lasing of BaLaGa,O, Table
2.
5.45 312
flashlamp. The reflectivity of the output mirror was then 90% and the length of pumping pulse was 600~s. Slightly above the threshold the laser oscillation lasted for approximately 200 PCS.
176 close to 1
SUMMARY
Optical and physical properties BaLaGa,O,: Nd3+ single crystals
Density (g cm--‘) Radiative lifetime o(s) Effective bandwidth of the 4F,,2-41,,,z transition at 300 K (cm-‘) Quantum efficiency of the 4F3,, level Stimulated emission cross-section for the 4F3jz-411 iir transition at 300 K (cm’) Upper laser level splitting (cm-‘) Lasing wavelength (pm) The&al conductivity at 297 K (alone c-axis) 0%’ cn-’ K-‘)
203
of
8.72. lo-*” 180 1.059 0.1127
quenching of luminescence depends quadratically the Nd concentration: k, - N*.
on
Preliminary investigation of stimulated emission Laser action was achieved in BaLaGa,O, : Nd3+ at room temperature. A sample rod containing Nd at a concentration of 5.6. lOI cmm3 was prepared with dimensions 6.3 x 40 mm. The axis was parallel to the c-axis of the crystal. The laser rod and one linear flashlamp 10 mm in diameter were wrapped with aluminium foil and placed in a water solution of sodium nitrite. In this configuration 28mm of rod length was pumped. The resonator was formed by two plane-parallel dielectric mirrors separated by 40cm. The threshold for laser oscillation was observed at 90 J of electrical input energy to thi
In Table 2 we gathered the data which may be used to characterize the BaLaGa,0,:Nd3+ crystal as a laser-active material. These data indicate that this material possesses a combination of properties favourable for laser operation. We believe that further laser experiments which are in preparation will confirm
this conclusion.
Acknowledgement-This work was carried Research programme CPBR 8.14.
out
under
REFERENCES 1. Ismatov A. A., Kolesova V. A. and Piryutko N. M., Izu. Akad. Nauk SSSR Neorn. Mater. 6. 1361 (1970). 2. Piekarczyk W., BerkoGki M. and JasioIek G., J. Crystal Growth 71, 395 (1985). 3. Berkowski M., Borowiec M. T., Pataj K., Piekarczyk W. and Wardzynski W., Physica lUB, 215 (1984). 4. Soluch W., Ksiezopolski R., Piekarczyk W., Berkowski M., Goodberlet M. A. and Vetelino J. F., J. Appl. Phys. 58, 2285 (1985). 5. Ryba-Romanowski
W., Gutowska M. U., Piekarczyk W., Berkowski M., Mazurak Z. and JeiowskaTrzebiatowska B., J. Luminesc. 36, 369 (1987).