- Email: [email protected]
Crystal growth, optical and luminescence properties of Pr-doped Y2SiO5 single crystals
Optical Materials 29 (2007) 1381–1384 www.elsevier.com/locate/optmat
Crystal growth, optical and luminescence properties of Pr-doped Y2SiO5 single crystals A. Novoselov a
a,*
, H. Ogino a, A. Yoshikawa a, M. Nikl b, J. Pejchal b, A. Beitlerova b, T. Fukuda a
Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, Katahira 2-1-1, Aoba-ku, Sendai 980-8577, Japan b Institute of Physics AS CR, Cukrovarnicka 10, Prague 6 162-53, Czech Republic Received 15 January 2006; accepted 2 April 2006 Available online 21 August 2006
Abstract Using the micro-pulling-down method, Pr3+-doped Y2SiO5 single crystals have been grown and their optical and luminescence properties have been investigated. The position of the lowest 5d absorption level was found at about 246 nm and an intense and fast 5d–4f luminescence peaking round 275 nm with a shoulder round 315 nm has been observed at room temperature. Photoluminescence decay time at room temperature was obtained to be of about 17 ns in the leading decay component. 2006 Elsevier B.V. All rights reserved. PACS: 78.55.m; 78.47.+p Keywords: Micro-pulling-down method; Oxides; Pr-doping; Scintillator materials
1. Introduction Single crystal scintillators possessing high density, and fast and efficient luminescence characteristics are under intense development due to strong demands of various modern applications in medicine, industry or science [1]. The luminescence and scintillation properties together with the polycrystalline powder preparation and single crystal growth of the Ce3+-doped Y2SiO5 (YSO) [2] and Lu2SiO5 [3] or (Lu,Y)2SiO5 [4] structures were the subject of numerous studies in the recent years because of the efficient and fast 5d–4f luminescence of the Ce3+ center in these hosts and their possible applications in the field of luminescent phosphors and scintillators. At the same time, some of the Cedoped fluorides were studied due to their potential to be used as active laser media for ultrafast tunable UV lasers [5]. *
Corresponding author. Tel.: +81 22 217 5167; fax: +81 22 217 5102. E-mail address: [email protected] (A. Novoselov).
0925-3467/$ - see front matter 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.optmat.2006.04.017
Recently, allowed 5d–4f transitions of Pr3+ have become of interest due to even faster 5d–4f luminescence shifted by about 1.5 eV towards higher energies with respect to Ce3+ ion in the same host under the assumption of sufficiently strong crystal field, which shifts the lowest 5d state below the 1S0 state situated round 210–215 nm. Related UV emission band becomes broader with respect to that of Ce3+ due to several disposable transitions to the 3Hx ground state multiplet. However, the relative positions of the 4f and 5d levels strongly influence the speed and efficiency of the luminescence response because of phonon-induced crossing into lower lying 4f levels and ionization of 5d state may occur as well due to thermally induced escape of the electron into a conduction band [6]. Some of the Pr-doped single crystal hosts such as Y3Al5O12, YAlO3 and YSO have been already briefly studied in the light of their possible usage for fast scintillators [7,8], while Yb-doped YSO was proposed as a promising laser crystal for tunable laser applications as well as for ultrashort laser pulses generation [9].
A. Novoselov et al. / Optical Materials 29 (2007) 1381–1384
In this work we have grown single crystals of Pr-doped YSO using the micro-pulling-down (l-PD) method and discuss its optical and luminescence properties.
Starting materials were prepared from the stoichiometric mixture of 4N purity Y2O3, Pr6O11 and SiO2 powders. Undoped and Pr-doped Y2SiO5 shaped single crystals were grown by the micro-pulling-down method using an iridium crucible with a die [10]. The crucible was heated inductively at a frequency of 20 kHz. Pulling rate was 0.05–0.1 mm/ min. The growth atmosphere was mixture of N2 + O2 (1 vol.%). To identify the obtained phase, powder X-ray diffraction (XRD) analysis was carried out in air at room temperature (RT) with a RIGAKU RINT Ultima diffractometer using CuKa X-ray source (40 kV, 40 mA). The chemical composition was analyzed by electron probe micro-analysis (EPMA) using an electron probe 10 lm with JEOL JXA8621MX analyzer. Plates of 8 · 4 · 1 mm were cut and polished for the optical experiments. Absorption spectra were measured by the UV–VIS–NIR spectrophotometer Shimadzu UV3101PC at RT. Measurements of photoluminescence characteristics were performed within 80–450 K using liquid nitrogen bath optical cryostat of Oxford Instruments. The Spectrofluorometer 199S (Edinburgh Instruments) was used for the luminescence experiment. It was equipped with a steady-state hydrogen flashlamp and nanosecond hydrogen-filled flashlamp serving as the excitation sources. Single grating emission monochromator and Peltier-cooled TBX-04 detection module (IBH Scotland) working in photon counting mode were used in the detection part. All the spectra were corrected for experimental distortions. Luminescence decay kinetics within ns to ms time scale was measured at the same set-up using the nanosecond hydrogen-filled coaxial flashlamp and time-correlated single photon counting method. Deconvolution procedures (Spectra-Solve software of LASTEK) were used to extract true decay times in the situation where the decay curves were distorted due to a finite width of the instrumental response.
3
0.4 c, mol%
2. Experimental
0.5 2
0.3 0.2 0.1 0
1 0
0.2
0.4
0.8
1.0
Fig. 1. Axial (1) central part and (2) outer part, and radial (3) dopant distribution in YSO:Pr(0.25%).
leads to increase the meniscus volume and, consequently, strongly increases thermocapillary Marangoni convection. In these conditions heavier Pr-ions of bigger ionic radius are rejected to the edge of the crystal and the effective segregation coefficient is not unity. However, high pulling rates applied in practice of the single crystal growth by the l-PD method reduce this effect significantly and the overall effective segregation coefficient is finally kept close to unity. Analysis of this phenomenon is given in Chapter 2 Ref. [10] and in Ref. [11]. It should be noted here that for the Czochralski grown Ce-doped YSO and Lu2SiO5 crystals, Ce-ion has low distribution coefficient and distribution itself is not uniform [12,13]. Doping Pr-ion into YSO host results in an intense absorption spectra, shown in Fig. 2, with the peaks at 246 and 219 nm followed by another absorption increase towards VUV spectral region, see also Fig. 2. Taking into account that the lowest 4f ! 5d absorption/excitation band of Ce3+ in LSO is located at 356 nm [14] and the above mentioned energy difference [6] between the position of the lowest 5d states of Pr3+ and Ce3+ in the same host, the absorption bands around 246 and 219 nm in Fig. 2 can be ascribed to the absorption transition from 3H4 ground state to the lowest 5d1 and 5d2 levels of Pr3+, respectively. Between 450 and 500 nm of the absorption spectrum there are also the visible Pr3+ 4f–4f transitions of much lower amplitude. The excitation within the 246 nm bands at RT
3. Results and discussion
3
3
3
3
H4→ P2 1I P1 P0 6
absorption [a.u.]
We have succeeded to grow transparent and crack-free undoped and Pr-doped (0.25 and 1%) Y2SiO5 single crystal rods of the diameter of 4 mm and length 15–20 mm by the micro-pulling-down method. Powder XRD patterns of ground crystals revealed that all samples under investigation belong to high temperature monoclinic structure socalled X2 phase with no impurity phases detected. Results of EPMA demonstrated uniform axial distribution of the Pr-ion in the samples, but the dopant radial concentration gradient was found out, see Fig. 1. Employing of crucibles with a die for the l-PD growth of shaped crystals
0.6
crystal length/diameter
4
2
1.00 0.75
×20
1 3
0.50
2 0.25
200
300 400 wavelength [nm]
intensity [rel.u.]
1382
0 500
Fig. 2. Absorption (1) excitation (2) kem = 281 nm, and emission (3) kex = 246 nm, spectra of YSO:Pr(1%) at RT.
A. Novoselov et al. / Optical Materials 29 (2007) 1381–1384
intensity [a.u.]
1000
way and demonstrate relatively high scintillator efficiency of the Pr-doped YSO.
instrumental response
100
4. Conclusions
10 1 0
50
100
150
time [ns] Fig. 3. Photoluminescence decay of YSO:Pr(0.25%) at RT, kex = 253 nm, kem = 281 nm. Solid line is the convolution of the instrumental response (dashed line) with the function I(t) = 1813exp[t/17.2 ns] + 21exp[t/ 36.4 ns] + 0.1.
results in an intense structured emission with maxima at 270–280 nm and around 315 nm, also shown in Fig. 2. The position of the lowest 4f–5d absorption band at 246 nm is very close to the 249 nm KrF laser line and the emission band extends between 255–345 nm (10% of the peak intensity at 276 nm) at room temperature. The Pr-doped YSO luminescence decay under excitation at 253 nm at RT is plotted in Fig. 3. Two-exponential approximation (solid line in Fig. 3) yields the decay times of about 17 and 36 ns, and about 90% of the emission intensity is released with the former decay time. Such a fast decay provides an additional argument to an idea that the structured emission spectrum in Fig. 2 is due to the 5d ! 4f radiative transition of Pr3+. The observed emission structure can be explained as due to the radiative transitions from the lowest 5d level to the 3H4,5,6 ground state 4f-levels. Emission structure and the decay in Fig. 3 is probably somewhat complicated by the occurrence of two nonequivalent Pr3+-centers confirmed by the high resolution absorption and emission spectroscopy of 3H4 ! 1D2 f ! f transition of Pr3+ in Lu2SiO5 and YSO hosts [15]. The overall scintillator efficiency of the Pr-doped YSO is compared with commercially available Bi4Ge3O12 (BGO) and Ce-doped (Lu0.97Y0.03)2SiO5 scintillators using the measurements of radioluminescence (Mo cathode, 35 kV) spectra in Fig. 4. Spectra could be compared in absolute 6
1.5x10
intensity [a.u.]
1383
1
2
6
1.0x10
3
6
0.5x10
0 200
300
400
500
600
700
wavelength [nm] Fig. 4. Comparison of radioluminescence spectra (overall scintillator efficiency) of Pr-doped (0.25%) YSO (1) with those of Ce-doped (Lu0.97Y0.03)2SiO5 (2) and BGO (3) (intensity · 10).
Undoped and Pr-doped (0.25 and 1%) Y2SiO5 shaped single crystals have been grown by the micro-pulling-down method. Crystal samples have uniform axial dopant distribution with some radial dopant concentration gradient from the centre to the edge of the crystals and the overall effective segregation coefficient close to unity. The onset of the Pr3+ 4f ! 5d absorption transitions was found around 260 nm i.e. well below the position of the 1S0 level around 215 nm, which is the necessary condition to obtain the fast 5d ! 4f luminescence transition. The position of the lowest 4f–5d absorption band at 246 nm is very close to the 249 nm KrF laser line and the emission band extends between 255345 nm (10% of the peak intensity at 276 nm). At room temperature the dominant decay time is about 17 ns. Acknowledgements The authors thank Mr. Y. Shoji, Fukuda X’tal Laboratory Ltd., for big practical help with crystal growth and Mr. Y. Murakami, IMR, Tohoku University, for his assistance with EPMA. This work was partially supported by the Ministry of Education, Culture, Sports, Science and Technology of Japan, Grant-in-Aid for Young Scientists (A), 15686001, 2003 (AY), the Industrial Technology Research Grant Program in 03A26014a from New Energy and Industrial Technology Development Organization of Japan (NEDO) and the Japan Society for the Promotion of Science (JSPS), the Postdoctoral Fellowship for Foreign Researches Program (AN). Partial support of Czech MSMT KONTAKT 1P2004ME716 project is gratefully acknowledged. References [1] W.W. Moses, Nucl. Instrum. Methods Phys. Res. A. 487 (2002) 123. [2] P.J. Marsh, J. Silver, A. Vecht, A. Newport, J. Lumin. 97 (2002) 229. [3] C.L. Melcher, J.S. Schweitzer, Nucl. Instrum. Methods Phys. Res. A. 314 (1992) 212. [4] D.W. Cooke, K.J. McClellan, B.L. Bennett, J.M. Roper, M.T. Whittaker, R.E. Muenchausen, R.C. Sze, J. Appl. Phys. 88 (2000) 7360. [5] V.K. Castillo, G.J. Quarles, J. Cryst. Growth 174 (1997) 337. [6] P. Dorenbos, J. Lumin. 91 (2000) 155. [7] E.G. Gumanskaya, M.V. Korzhik, S.A. Smirnova, V.B. Pavlenko, A.A. Fedorov, Opt. Spectrosc. 72 (1992) 155, in Russian. [8] C.W.E. van Eijk, P. Dorenbos, R. Visser, IEEE Trans. Nucl. Sci. 41 (1994) 738. [9] R. Gaume, P.H. Haumesser, B. Viana, D. Vivien, B. Ferrend, G. Aka, Opt. Mater. 19 (2002) 81. [10] T. Fukuda, in: T. Fukuda, P. Rudolph, S. Uda (Eds.), Fiber Crystal Growth from the Melt, Springer, Berlin, 2004, pp. 255–281. [11] B.M. Eppelbaum, G. Schierning, A. Winnacker, J. Cryst. Growth 275 (2005) e867. [12] C.D. Brandle, A.J. Valentino, G.W. Berkstresser, J. Cryst. Growth 79 (1986) 308.
1384
A. Novoselov et al. / Optical Materials 29 (2007) 1381–1384
[13] P. Dorenbos, C.W.E. van Eijk, A.J.J. Bos, C.L. Melcher, J. Phys.: Condens. Matter 6 (1994) 4167. [14] H. Suzuki, T.A. Tombrello, C.L. Melcher, J.S. Schweitzer, Nucl. Instrum. Methods A. 320 (1992) 263.
[15] N.V. Znamenskii, E.A. Manykin, Yu.V. Orlov, E.A. Petrenko, T.G. Yukina, Yu.V. Malyukin, R.S. Borisov, P.N. Zhmurin, A.N. Lebedenko, B.V. Grinev, J. Exp. Theor. Phys. 93 (2001) 372.
Crystal growth, optical and luminescence properties of Pr-doped Y2SiO5 single crystals A. Novoselov a
a,*
, H. Ogino a, A. Yoshikawa a, M. Nikl b, J. Pejchal b, A. Beitlerova b, T. Fukuda a
Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, Katahira 2-1-1, Aoba-ku, Sendai 980-8577, Japan b Institute of Physics AS CR, Cukrovarnicka 10, Prague 6 162-53, Czech Republic Received 15 January 2006; accepted 2 April 2006 Available online 21 August 2006
Abstract Using the micro-pulling-down method, Pr3+-doped Y2SiO5 single crystals have been grown and their optical and luminescence properties have been investigated. The position of the lowest 5d absorption level was found at about 246 nm and an intense and fast 5d–4f luminescence peaking round 275 nm with a shoulder round 315 nm has been observed at room temperature. Photoluminescence decay time at room temperature was obtained to be of about 17 ns in the leading decay component. 2006 Elsevier B.V. All rights reserved. PACS: 78.55.m; 78.47.+p Keywords: Micro-pulling-down method; Oxides; Pr-doping; Scintillator materials
1. Introduction Single crystal scintillators possessing high density, and fast and efficient luminescence characteristics are under intense development due to strong demands of various modern applications in medicine, industry or science [1]. The luminescence and scintillation properties together with the polycrystalline powder preparation and single crystal growth of the Ce3+-doped Y2SiO5 (YSO) [2] and Lu2SiO5 [3] or (Lu,Y)2SiO5 [4] structures were the subject of numerous studies in the recent years because of the efficient and fast 5d–4f luminescence of the Ce3+ center in these hosts and their possible applications in the field of luminescent phosphors and scintillators. At the same time, some of the Cedoped fluorides were studied due to their potential to be used as active laser media for ultrafast tunable UV lasers [5]. *
Corresponding author. Tel.: +81 22 217 5167; fax: +81 22 217 5102. E-mail address: [email protected] (A. Novoselov).
0925-3467/$ - see front matter 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.optmat.2006.04.017
Recently, allowed 5d–4f transitions of Pr3+ have become of interest due to even faster 5d–4f luminescence shifted by about 1.5 eV towards higher energies with respect to Ce3+ ion in the same host under the assumption of sufficiently strong crystal field, which shifts the lowest 5d state below the 1S0 state situated round 210–215 nm. Related UV emission band becomes broader with respect to that of Ce3+ due to several disposable transitions to the 3Hx ground state multiplet. However, the relative positions of the 4f and 5d levels strongly influence the speed and efficiency of the luminescence response because of phonon-induced crossing into lower lying 4f levels and ionization of 5d state may occur as well due to thermally induced escape of the electron into a conduction band [6]. Some of the Pr-doped single crystal hosts such as Y3Al5O12, YAlO3 and YSO have been already briefly studied in the light of their possible usage for fast scintillators [7,8], while Yb-doped YSO was proposed as a promising laser crystal for tunable laser applications as well as for ultrashort laser pulses generation [9].
A. Novoselov et al. / Optical Materials 29 (2007) 1381–1384
In this work we have grown single crystals of Pr-doped YSO using the micro-pulling-down (l-PD) method and discuss its optical and luminescence properties.
Starting materials were prepared from the stoichiometric mixture of 4N purity Y2O3, Pr6O11 and SiO2 powders. Undoped and Pr-doped Y2SiO5 shaped single crystals were grown by the micro-pulling-down method using an iridium crucible with a die [10]. The crucible was heated inductively at a frequency of 20 kHz. Pulling rate was 0.05–0.1 mm/ min. The growth atmosphere was mixture of N2 + O2 (1 vol.%). To identify the obtained phase, powder X-ray diffraction (XRD) analysis was carried out in air at room temperature (RT) with a RIGAKU RINT Ultima diffractometer using CuKa X-ray source (40 kV, 40 mA). The chemical composition was analyzed by electron probe micro-analysis (EPMA) using an electron probe 10 lm with JEOL JXA8621MX analyzer. Plates of 8 · 4 · 1 mm were cut and polished for the optical experiments. Absorption spectra were measured by the UV–VIS–NIR spectrophotometer Shimadzu UV3101PC at RT. Measurements of photoluminescence characteristics were performed within 80–450 K using liquid nitrogen bath optical cryostat of Oxford Instruments. The Spectrofluorometer 199S (Edinburgh Instruments) was used for the luminescence experiment. It was equipped with a steady-state hydrogen flashlamp and nanosecond hydrogen-filled flashlamp serving as the excitation sources. Single grating emission monochromator and Peltier-cooled TBX-04 detection module (IBH Scotland) working in photon counting mode were used in the detection part. All the spectra were corrected for experimental distortions. Luminescence decay kinetics within ns to ms time scale was measured at the same set-up using the nanosecond hydrogen-filled coaxial flashlamp and time-correlated single photon counting method. Deconvolution procedures (Spectra-Solve software of LASTEK) were used to extract true decay times in the situation where the decay curves were distorted due to a finite width of the instrumental response.
3
0.4 c, mol%
2. Experimental
0.5 2
0.3 0.2 0.1 0
1 0
0.2
0.4
0.8
1.0
Fig. 1. Axial (1) central part and (2) outer part, and radial (3) dopant distribution in YSO:Pr(0.25%).
leads to increase the meniscus volume and, consequently, strongly increases thermocapillary Marangoni convection. In these conditions heavier Pr-ions of bigger ionic radius are rejected to the edge of the crystal and the effective segregation coefficient is not unity. However, high pulling rates applied in practice of the single crystal growth by the l-PD method reduce this effect significantly and the overall effective segregation coefficient is finally kept close to unity. Analysis of this phenomenon is given in Chapter 2 Ref. [10] and in Ref. [11]. It should be noted here that for the Czochralski grown Ce-doped YSO and Lu2SiO5 crystals, Ce-ion has low distribution coefficient and distribution itself is not uniform [12,13]. Doping Pr-ion into YSO host results in an intense absorption spectra, shown in Fig. 2, with the peaks at 246 and 219 nm followed by another absorption increase towards VUV spectral region, see also Fig. 2. Taking into account that the lowest 4f ! 5d absorption/excitation band of Ce3+ in LSO is located at 356 nm [14] and the above mentioned energy difference [6] between the position of the lowest 5d states of Pr3+ and Ce3+ in the same host, the absorption bands around 246 and 219 nm in Fig. 2 can be ascribed to the absorption transition from 3H4 ground state to the lowest 5d1 and 5d2 levels of Pr3+, respectively. Between 450 and 500 nm of the absorption spectrum there are also the visible Pr3+ 4f–4f transitions of much lower amplitude. The excitation within the 246 nm bands at RT
3. Results and discussion
3
3
3
3
H4→ P2 1I P1 P0 6
absorption [a.u.]
We have succeeded to grow transparent and crack-free undoped and Pr-doped (0.25 and 1%) Y2SiO5 single crystal rods of the diameter of 4 mm and length 15–20 mm by the micro-pulling-down method. Powder XRD patterns of ground crystals revealed that all samples under investigation belong to high temperature monoclinic structure socalled X2 phase with no impurity phases detected. Results of EPMA demonstrated uniform axial distribution of the Pr-ion in the samples, but the dopant radial concentration gradient was found out, see Fig. 1. Employing of crucibles with a die for the l-PD growth of shaped crystals
0.6
crystal length/diameter
4
2
1.00 0.75
×20
1 3
0.50
2 0.25
200
300 400 wavelength [nm]
intensity [rel.u.]
1382
0 500
Fig. 2. Absorption (1) excitation (2) kem = 281 nm, and emission (3) kex = 246 nm, spectra of YSO:Pr(1%) at RT.
A. Novoselov et al. / Optical Materials 29 (2007) 1381–1384
intensity [a.u.]
1000
way and demonstrate relatively high scintillator efficiency of the Pr-doped YSO.
instrumental response
100
4. Conclusions
10 1 0
50
100
150
time [ns] Fig. 3. Photoluminescence decay of YSO:Pr(0.25%) at RT, kex = 253 nm, kem = 281 nm. Solid line is the convolution of the instrumental response (dashed line) with the function I(t) = 1813exp[t/17.2 ns] + 21exp[t/ 36.4 ns] + 0.1.
results in an intense structured emission with maxima at 270–280 nm and around 315 nm, also shown in Fig. 2. The position of the lowest 4f–5d absorption band at 246 nm is very close to the 249 nm KrF laser line and the emission band extends between 255–345 nm (10% of the peak intensity at 276 nm) at room temperature. The Pr-doped YSO luminescence decay under excitation at 253 nm at RT is plotted in Fig. 3. Two-exponential approximation (solid line in Fig. 3) yields the decay times of about 17 and 36 ns, and about 90% of the emission intensity is released with the former decay time. Such a fast decay provides an additional argument to an idea that the structured emission spectrum in Fig. 2 is due to the 5d ! 4f radiative transition of Pr3+. The observed emission structure can be explained as due to the radiative transitions from the lowest 5d level to the 3H4,5,6 ground state 4f-levels. Emission structure and the decay in Fig. 3 is probably somewhat complicated by the occurrence of two nonequivalent Pr3+-centers confirmed by the high resolution absorption and emission spectroscopy of 3H4 ! 1D2 f ! f transition of Pr3+ in Lu2SiO5 and YSO hosts [15]. The overall scintillator efficiency of the Pr-doped YSO is compared with commercially available Bi4Ge3O12 (BGO) and Ce-doped (Lu0.97Y0.03)2SiO5 scintillators using the measurements of radioluminescence (Mo cathode, 35 kV) spectra in Fig. 4. Spectra could be compared in absolute 6
1.5x10
intensity [a.u.]
1383
1
2
6
1.0x10
3
6
0.5x10
0 200
300
400
500
600
700
wavelength [nm] Fig. 4. Comparison of radioluminescence spectra (overall scintillator efficiency) of Pr-doped (0.25%) YSO (1) with those of Ce-doped (Lu0.97Y0.03)2SiO5 (2) and BGO (3) (intensity · 10).
Undoped and Pr-doped (0.25 and 1%) Y2SiO5 shaped single crystals have been grown by the micro-pulling-down method. Crystal samples have uniform axial dopant distribution with some radial dopant concentration gradient from the centre to the edge of the crystals and the overall effective segregation coefficient close to unity. The onset of the Pr3+ 4f ! 5d absorption transitions was found around 260 nm i.e. well below the position of the 1S0 level around 215 nm, which is the necessary condition to obtain the fast 5d ! 4f luminescence transition. The position of the lowest 4f–5d absorption band at 246 nm is very close to the 249 nm KrF laser line and the emission band extends between 255345 nm (10% of the peak intensity at 276 nm). At room temperature the dominant decay time is about 17 ns. Acknowledgements The authors thank Mr. Y. Shoji, Fukuda X’tal Laboratory Ltd., for big practical help with crystal growth and Mr. Y. Murakami, IMR, Tohoku University, for his assistance with EPMA. This work was partially supported by the Ministry of Education, Culture, Sports, Science and Technology of Japan, Grant-in-Aid for Young Scientists (A), 15686001, 2003 (AY), the Industrial Technology Research Grant Program in 03A26014a from New Energy and Industrial Technology Development Organization of Japan (NEDO) and the Japan Society for the Promotion of Science (JSPS), the Postdoctoral Fellowship for Foreign Researches Program (AN). Partial support of Czech MSMT KONTAKT 1P2004ME716 project is gratefully acknowledged. References [1] W.W. Moses, Nucl. Instrum. Methods Phys. Res. A. 487 (2002) 123. [2] P.J. Marsh, J. Silver, A. Vecht, A. Newport, J. Lumin. 97 (2002) 229. [3] C.L. Melcher, J.S. Schweitzer, Nucl. Instrum. Methods Phys. Res. A. 314 (1992) 212. [4] D.W. Cooke, K.J. McClellan, B.L. Bennett, J.M. Roper, M.T. Whittaker, R.E. Muenchausen, R.C. Sze, J. Appl. Phys. 88 (2000) 7360. [5] V.K. Castillo, G.J. Quarles, J. Cryst. Growth 174 (1997) 337. [6] P. Dorenbos, J. Lumin. 91 (2000) 155. [7] E.G. Gumanskaya, M.V. Korzhik, S.A. Smirnova, V.B. Pavlenko, A.A. Fedorov, Opt. Spectrosc. 72 (1992) 155, in Russian. [8] C.W.E. van Eijk, P. Dorenbos, R. Visser, IEEE Trans. Nucl. Sci. 41 (1994) 738. [9] R. Gaume, P.H. Haumesser, B. Viana, D. Vivien, B. Ferrend, G. Aka, Opt. Mater. 19 (2002) 81. [10] T. Fukuda, in: T. Fukuda, P. Rudolph, S. Uda (Eds.), Fiber Crystal Growth from the Melt, Springer, Berlin, 2004, pp. 255–281. [11] B.M. Eppelbaum, G. Schierning, A. Winnacker, J. Cryst. Growth 275 (2005) e867. [12] C.D. Brandle, A.J. Valentino, G.W. Berkstresser, J. Cryst. Growth 79 (1986) 308.
1384
A. Novoselov et al. / Optical Materials 29 (2007) 1381–1384
[13] P. Dorenbos, C.W.E. van Eijk, A.J.J. Bos, C.L. Melcher, J. Phys.: Condens. Matter 6 (1994) 4167. [14] H. Suzuki, T.A. Tombrello, C.L. Melcher, J.S. Schweitzer, Nucl. Instrum. Methods A. 320 (1992) 263.
[15] N.V. Znamenskii, E.A. Manykin, Yu.V. Orlov, E.A. Petrenko, T.G. Yukina, Yu.V. Malyukin, R.S. Borisov, P.N. Zhmurin, A.N. Lebedenko, B.V. Grinev, J. Exp. Theor. Phys. 93 (2001) 372.