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Crystal growth and laser properties of new RAl3(BO3)4 (R = Yb, Er) crystals
Optical Materials 30 (2007) 161–163 www.elsevier.com/locate/optmat
Crystal growth and laser properties of new RAl3(BO3)4 (R = Yb, Er) crystals N.I. Leonyuk a, V.V. Maltsev a, E.A. Volkova a,*, O.V. Pilipenko a, E.V. Koporulina a, V.E. Kisel b, N.A. Tolstik b, S.V. Kurilchik b, N.V. Kuleshov b b
a Lomonosov Moscow State University, Geological Faculty, Moscow 119992, Russia Institute for Optical Materials and Technologies, Belarusian National Technical University, Minsk 220013, Belarus
Available online 22 December 2006
Abstract (Er, Yb):YAl3(BO3)4 ((Er, Yb):YAB) single crystals were grown from seeds using K2Mo3O10–B2O3 fluxed melts. Thin films of Yb:YAl3(BO3)4 (Yb – 5 and 10 at.%) (Yb:YAB) were obtained by LPE method on substrates of YAB crystals. Spectroscopic properties of (Er, Yb):YAB crystals were investigated, the CW (continuous-wave) and Q-switched lasing with maximal output power of 245 mW in CW regime was demonstrated under continuous-wave laser diode pumping. 2006 Published by Elsevier B.V. PACS: 81.10.Dn; 42.70.Hj; 42.55.Xi Keywords: Borates; Flux growth; Diode-pumped lasers; Q-switched lasers
1. Introduction Lasers emitting at 1.5 lm are of great interest for several industrial applications: detectors in range-finding, environmental sensing, telecommunications, surgery, etc. due to their eye-safety, high transparency in atmosphere and fused-silica waveguides. From the spectroscopic point of view the (Er, Yb)-codoped phosphate glass shows a short 4I11/2 level lifetime, high luminescence quantum yield and high Yb3+ ! Er3+ energy transfer efficiency. However, they are characterized by poor thermal and mechanical stabilities that limit the laser power down to a few hundred milliwatts. For these
* Corresponding author. Address: Department of Crystallography and Crystal Chemistry, Geological Faculty, Moscow State University, GSP-2, 119992 Moscow, Russian Federation. Tel./fax: +7 495 939 2980. E-mail address: [email protected] (E.A. Volkova).
0925-3467/$ - see front matter 2006 Published by Elsevier B.V. doi:10.1016/j.optmat.2006.11.017
reasons, (Er, Yb)-codoped crystalline matrices are still intensively investigated. To date, the most efficient 1.5 lm diode-pumped laser action in a crystalline medium has been demonstrated with (Yb, Er)-activated borates [1,2]. Non-centrosymmetric borate crystals RAl3(BO3)4 (R = Y, Pr–Lu) with possibility of wide isomorphous substitutions can be considered as polyfunctional solids having device potential due to their high non-linear optical coefficient, good mechanical hardness and unique thermal conductivity (14–15 W/m K) [3]. Solid solutions YAl3(BO3)4–RAl3(BO3)4, where R = Nd, Er, Yb, Dy and Tm, are of particular interest as promising media for selffrequency-doubling lasers. Among them the Yb:YAl3(BO3)4 (Yb:YAB) is a good candidate for planar waveguides as the diode-pumped device for laser sources, and for power scaling and finally also for microchip operation [4]. In this paper, recent results on the growth of (Er, Yb):YAB single crystals and measurements of their spectroscopic and laser characteristics are presented. Also, an attempt is made to obtain Yb:YAB epitaxial layers as a first step towards their growth technology.
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2. Crystal growth
3. Optical characteristics
(Er, Yb):YAB crystals were grown from seed using K2Mo3O10–B2O3 based fluxed melt. The starting solvents were chosen as 88.1 wt.% K2Mo3O10–8.6 wt.% B2O3– 3.3 wt.% Y2O3 and 91.0 wt.% K2Mo3O10–8.0 wt.% B2O3– 1.0 wt.% Y2O3 for Yb:YAB and (Er, Yb):YAB. Following experimental data on the solubility of YAB in these systems [5], the 17 wt.% – concentrations of crystalline substance is acceptable for the RE:YAB crystal growth. Initial chemicals (not lower than 3N purity in all the runs) were Y2O3, Yb2O3, Er2O3, Al2O3 and B2O3. In these experiments, K2Mo3O10 was previously sintered of K2MoO4 and H2MoO4 at 650 C according to the scheme:
The absorption spectra of the crystals were measured using Cary-500 spectrophotometer with spectral resolution of 0.4 nm. An optical parametric oscillator LOTIS LT-2214OPO pumped by Nd:YAG laser was used as an excitation source for lifetime measurements. Room temperature polarized absorption spectra of (Er, Yb):YAB crystal around 980 nm are shown in Fig. 2. A strong absorption band corresponding to Yb3+ ions in (Er, Yb):YAB crystal is centred at 976 nm in r-polarization, with a maximum absorption cross-section of about 2.75 · 1020 cm2 and a bandwidth of 17 nm (FWHM). A number of local maxima are observed in the 1480– 1605 nm range of (Er, Yb):YAB absorption spectra in both polarizations. The strongest maximum with cross-section of about 3.8 · 1020 cm2 is located at 1530 nm in rpolarization. The luminescence decay time of 4I13/2 level was measured to be of about 350 ls. The measured lifetime is significantly shorter than that calculated from the Judd–Offelt analysis (4.41 ms [7]). Thus the luminescence quantum efficiency for the 4I13/2 manifold of Er:YAB is of about 8%. Unfortunately, it was impossible to measure the luminescence decay of 4I11/2 level of erbium directly because of the absence of YAB single crystals doped with Er3+ ions. But during our measurements, we did not see any up-converted green luminescence. That gives an indirect evidence of rather short 4I11/2 level lifetime (not more than few microseconds). Ytterbium 2F5/2 level lifetime in Yb:YAB crystal was measured to be 480 ± 5 ls, whereas it is of about 60 ± 5 ls in (Er, Yb):YAB. The shortening of Yb3+ radiative lifetime in the erbium–ytterbium codoped crystal in comparison with the ytterbium-doped one indicates a strong non-radiative energy transfer from Yb to Er ions. The energy transfer efficiency was obtained to be of about 88%.
K2 MoO4 þ 2H2 MoO3 ¼ K2 Mo3 O10 þ H2 O " : The cooling rate was gradually increased from 0.06 to 0.12 C/h in the temperature range 1060–1000 C. The epitaxial layers of Yb:YAB with 5 and 10 at.% of Yb and YAB substrates were obtained in the same systems. Composition, homogeneity and external morphology of the grown samples were studied by analytical scanning electron microscope JSM-5300 + Link ISIS. The electron microprobe analysis was performed with an accuracy of 0.2–0.3 wt.%. (Er, Yb):YAB single crystals with size of 8 · 8 · 12 mm3 were grown (Fig. 1). They are characterized by well-developed f11 20g, f2 1 10g and f10 11g faces. Ytterbium and erbium are uniformly distributed over the entire volume of the crystals grown. The growth rate of Yb:YAB crystal layers was estimated to be within the range of 0.07–0.61 lm/mm. These values are higher in comparison with the growth rate of the NAB epilayers obtained from PbO–PbF2 based flux system on the Gd0.59La0.41Al3(BO3)4 [6]. On the other hand, experimental data of this work agree with earlier results on YAB bulk crystal growth from K2Mo3O10 based high-temperature solutions [5]. From the ASEM data, it was found that Yb concentration in the grown films slightly increases from 0.09 to 0.11 at yttrium position with an increase of supersaturation.
Fig. 1. (Er, Yb):YAB crystal.
Fig. 2. Room temperature polarized absorption spectra of (Er, Yb):YAB crystal in the 1 lm spectral range.
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5. Conclusions
Fig. 3. Input–output diagrams of Er(1 at.%), Yb(11 at.%):YAB CW laser with output coupler transmittance of 0.8% (circles) and 1.5% (squares).
4. Laser performance The CW laser experiments were carried out with the nearly hemispherical cavity consisting of a 50-mm radiusof-curvature output coupler and plane high reflector mirror. A CW fiber-coupled (B = 100 lm, NA = 0.22) laser diode with maximum output power of 5.5 W emitting near 980 nm was used as a pump source. The 1.5-mm-thick laser element was mounted on a copper heatsink (without active cooling) inside the resonator close to the plane high reflector mirror. The cavity-mode diameter at the active element was close to the pump beam waist. The Q-switching experiments were performed with the same cavity configuration. A 110-lm-thick Co2+:MgAl2O4 crystal was used with initial transmission of about 99.5% at 1604 nm as a passive shutter. Input–output diagrams for Er(1 at.%), Yb(11 at.%):YAB CW laser are shown in Fig. 3. A maximum output power of about 245 mW with a slope efficiency of 14% was obtained at 1604 nm for r-polarization. In the Q-switched mode with Co2+:MgAl2O4 saturable absorber, Er(1 at.%), Yb(11 at.%):YAB laser produced up to 108 mW of total output power with a slope efficiency of 9.6% at 1604 nm. The Q-switched pulse duration at the maximum output power was about 135 ns and pulse energy of 4.3 lJ.
(Er, Yb):YAB single crystals were grown from seed using K2Mo3O10–B2O3 flux. The obtained results from the liquid phase epitaxy of Yb:YAB can be considered as the first stage towards development of thin films for modern applications. A major problem that requires clarification concerns the growth control of these layers with optimum composition and thickness. Evaluation of the potential of rare earth doped YAB thin films, to be used as miniature optical components, is in progress. Polarized absorption cross-section spectra of a (Er, Yb):YAB crystal were determined at room temperature. Lifetimes of Er and Yb ions levels and Yb ! Er energy transfer efficiency were measured. The CW and Q-switched laser operations of this material under continuous-wave laser diode pumping at 980 nm were demonstrated. A CW output power of 245 mW with slope efficiency of 14% at 1604 nm was achieved. In the Q-switched regime an average output power of 108 mW with slope efficiency of 9.6%, repetition rate of 20 kHz and pulse duration of 135 ns was obtained. Acknowledgements This research was supported, in part, by the grants of RFBR NN 04-05-64709, 05-05-08021, 06-05-08103 and the Russian President Grants for Young Scientists 2794.2005.5 and 4456.2006.5 and MNP MSU-2005 No. 15. References [1] P. Burns, J. Dawes, P. Dekker, J. Piper, H. Jiang, H. Jiang, J. Wang, Proc. SPIE Int. Soc. Opt. Eng. 79 (2003) 4968. [2] B. Denker, B. Galagan, L. Ivleva, V. Osiko, S. Sverchkov, I. Voronina, J.E. Hellstrom, G. Karlsson, F. Laurel, Appl. Phys. B 79 (2004) 577. [3] L.M. Dorozhkin, I.I. Kuratev, N.I. Leonyuk, T.I. Timchenko, A.V. Shestakov, Pis’ma v zhurnal experimentalnoi i teoreticheskoi physiki 7 (1981) 1297 (in Russian). [4] P. Dekker, J.M. Dawes, J.A. Piper, Y. Liu, J.Y. Wang, Opt. Commun. 195 (2001) 431. [5] N.I. Leonyuk, L.I. Leonyuk, Prog. Cryst. Growth. Charact. 31 (1995) 179. [6] F. Lutz, M. Leiss, J. Mu¨ller, J. Crystal Growth. 47 (1979) 130. [7] W. You, Y. Lin, Y. Chen, Z. Luo, Y. Huang, Opt. Mater. 29 (2007) 488.
Crystal growth and laser properties of new RAl3(BO3)4 (R = Yb, Er) crystals N.I. Leonyuk a, V.V. Maltsev a, E.A. Volkova a,*, O.V. Pilipenko a, E.V. Koporulina a, V.E. Kisel b, N.A. Tolstik b, S.V. Kurilchik b, N.V. Kuleshov b b
a Lomonosov Moscow State University, Geological Faculty, Moscow 119992, Russia Institute for Optical Materials and Technologies, Belarusian National Technical University, Minsk 220013, Belarus
Available online 22 December 2006
Abstract (Er, Yb):YAl3(BO3)4 ((Er, Yb):YAB) single crystals were grown from seeds using K2Mo3O10–B2O3 fluxed melts. Thin films of Yb:YAl3(BO3)4 (Yb – 5 and 10 at.%) (Yb:YAB) were obtained by LPE method on substrates of YAB crystals. Spectroscopic properties of (Er, Yb):YAB crystals were investigated, the CW (continuous-wave) and Q-switched lasing with maximal output power of 245 mW in CW regime was demonstrated under continuous-wave laser diode pumping. 2006 Published by Elsevier B.V. PACS: 81.10.Dn; 42.70.Hj; 42.55.Xi Keywords: Borates; Flux growth; Diode-pumped lasers; Q-switched lasers
1. Introduction Lasers emitting at 1.5 lm are of great interest for several industrial applications: detectors in range-finding, environmental sensing, telecommunications, surgery, etc. due to their eye-safety, high transparency in atmosphere and fused-silica waveguides. From the spectroscopic point of view the (Er, Yb)-codoped phosphate glass shows a short 4I11/2 level lifetime, high luminescence quantum yield and high Yb3+ ! Er3+ energy transfer efficiency. However, they are characterized by poor thermal and mechanical stabilities that limit the laser power down to a few hundred milliwatts. For these
* Corresponding author. Address: Department of Crystallography and Crystal Chemistry, Geological Faculty, Moscow State University, GSP-2, 119992 Moscow, Russian Federation. Tel./fax: +7 495 939 2980. E-mail address: [email protected] (E.A. Volkova).
0925-3467/$ - see front matter 2006 Published by Elsevier B.V. doi:10.1016/j.optmat.2006.11.017
reasons, (Er, Yb)-codoped crystalline matrices are still intensively investigated. To date, the most efficient 1.5 lm diode-pumped laser action in a crystalline medium has been demonstrated with (Yb, Er)-activated borates [1,2]. Non-centrosymmetric borate crystals RAl3(BO3)4 (R = Y, Pr–Lu) with possibility of wide isomorphous substitutions can be considered as polyfunctional solids having device potential due to their high non-linear optical coefficient, good mechanical hardness and unique thermal conductivity (14–15 W/m K) [3]. Solid solutions YAl3(BO3)4–RAl3(BO3)4, where R = Nd, Er, Yb, Dy and Tm, are of particular interest as promising media for selffrequency-doubling lasers. Among them the Yb:YAl3(BO3)4 (Yb:YAB) is a good candidate for planar waveguides as the diode-pumped device for laser sources, and for power scaling and finally also for microchip operation [4]. In this paper, recent results on the growth of (Er, Yb):YAB single crystals and measurements of their spectroscopic and laser characteristics are presented. Also, an attempt is made to obtain Yb:YAB epitaxial layers as a first step towards their growth technology.
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2. Crystal growth
3. Optical characteristics
(Er, Yb):YAB crystals were grown from seed using K2Mo3O10–B2O3 based fluxed melt. The starting solvents were chosen as 88.1 wt.% K2Mo3O10–8.6 wt.% B2O3– 3.3 wt.% Y2O3 and 91.0 wt.% K2Mo3O10–8.0 wt.% B2O3– 1.0 wt.% Y2O3 for Yb:YAB and (Er, Yb):YAB. Following experimental data on the solubility of YAB in these systems [5], the 17 wt.% – concentrations of crystalline substance is acceptable for the RE:YAB crystal growth. Initial chemicals (not lower than 3N purity in all the runs) were Y2O3, Yb2O3, Er2O3, Al2O3 and B2O3. In these experiments, K2Mo3O10 was previously sintered of K2MoO4 and H2MoO4 at 650 C according to the scheme:
The absorption spectra of the crystals were measured using Cary-500 spectrophotometer with spectral resolution of 0.4 nm. An optical parametric oscillator LOTIS LT-2214OPO pumped by Nd:YAG laser was used as an excitation source for lifetime measurements. Room temperature polarized absorption spectra of (Er, Yb):YAB crystal around 980 nm are shown in Fig. 2. A strong absorption band corresponding to Yb3+ ions in (Er, Yb):YAB crystal is centred at 976 nm in r-polarization, with a maximum absorption cross-section of about 2.75 · 1020 cm2 and a bandwidth of 17 nm (FWHM). A number of local maxima are observed in the 1480– 1605 nm range of (Er, Yb):YAB absorption spectra in both polarizations. The strongest maximum with cross-section of about 3.8 · 1020 cm2 is located at 1530 nm in rpolarization. The luminescence decay time of 4I13/2 level was measured to be of about 350 ls. The measured lifetime is significantly shorter than that calculated from the Judd–Offelt analysis (4.41 ms [7]). Thus the luminescence quantum efficiency for the 4I13/2 manifold of Er:YAB is of about 8%. Unfortunately, it was impossible to measure the luminescence decay of 4I11/2 level of erbium directly because of the absence of YAB single crystals doped with Er3+ ions. But during our measurements, we did not see any up-converted green luminescence. That gives an indirect evidence of rather short 4I11/2 level lifetime (not more than few microseconds). Ytterbium 2F5/2 level lifetime in Yb:YAB crystal was measured to be 480 ± 5 ls, whereas it is of about 60 ± 5 ls in (Er, Yb):YAB. The shortening of Yb3+ radiative lifetime in the erbium–ytterbium codoped crystal in comparison with the ytterbium-doped one indicates a strong non-radiative energy transfer from Yb to Er ions. The energy transfer efficiency was obtained to be of about 88%.
K2 MoO4 þ 2H2 MoO3 ¼ K2 Mo3 O10 þ H2 O " : The cooling rate was gradually increased from 0.06 to 0.12 C/h in the temperature range 1060–1000 C. The epitaxial layers of Yb:YAB with 5 and 10 at.% of Yb and YAB substrates were obtained in the same systems. Composition, homogeneity and external morphology of the grown samples were studied by analytical scanning electron microscope JSM-5300 + Link ISIS. The electron microprobe analysis was performed with an accuracy of 0.2–0.3 wt.%. (Er, Yb):YAB single crystals with size of 8 · 8 · 12 mm3 were grown (Fig. 1). They are characterized by well-developed f11 20g, f2 1 10g and f10 11g faces. Ytterbium and erbium are uniformly distributed over the entire volume of the crystals grown. The growth rate of Yb:YAB crystal layers was estimated to be within the range of 0.07–0.61 lm/mm. These values are higher in comparison with the growth rate of the NAB epilayers obtained from PbO–PbF2 based flux system on the Gd0.59La0.41Al3(BO3)4 [6]. On the other hand, experimental data of this work agree with earlier results on YAB bulk crystal growth from K2Mo3O10 based high-temperature solutions [5]. From the ASEM data, it was found that Yb concentration in the grown films slightly increases from 0.09 to 0.11 at yttrium position with an increase of supersaturation.
Fig. 1. (Er, Yb):YAB crystal.
Fig. 2. Room temperature polarized absorption spectra of (Er, Yb):YAB crystal in the 1 lm spectral range.
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5. Conclusions
Fig. 3. Input–output diagrams of Er(1 at.%), Yb(11 at.%):YAB CW laser with output coupler transmittance of 0.8% (circles) and 1.5% (squares).
4. Laser performance The CW laser experiments were carried out with the nearly hemispherical cavity consisting of a 50-mm radiusof-curvature output coupler and plane high reflector mirror. A CW fiber-coupled (B = 100 lm, NA = 0.22) laser diode with maximum output power of 5.5 W emitting near 980 nm was used as a pump source. The 1.5-mm-thick laser element was mounted on a copper heatsink (without active cooling) inside the resonator close to the plane high reflector mirror. The cavity-mode diameter at the active element was close to the pump beam waist. The Q-switching experiments were performed with the same cavity configuration. A 110-lm-thick Co2+:MgAl2O4 crystal was used with initial transmission of about 99.5% at 1604 nm as a passive shutter. Input–output diagrams for Er(1 at.%), Yb(11 at.%):YAB CW laser are shown in Fig. 3. A maximum output power of about 245 mW with a slope efficiency of 14% was obtained at 1604 nm for r-polarization. In the Q-switched mode with Co2+:MgAl2O4 saturable absorber, Er(1 at.%), Yb(11 at.%):YAB laser produced up to 108 mW of total output power with a slope efficiency of 9.6% at 1604 nm. The Q-switched pulse duration at the maximum output power was about 135 ns and pulse energy of 4.3 lJ.
(Er, Yb):YAB single crystals were grown from seed using K2Mo3O10–B2O3 flux. The obtained results from the liquid phase epitaxy of Yb:YAB can be considered as the first stage towards development of thin films for modern applications. A major problem that requires clarification concerns the growth control of these layers with optimum composition and thickness. Evaluation of the potential of rare earth doped YAB thin films, to be used as miniature optical components, is in progress. Polarized absorption cross-section spectra of a (Er, Yb):YAB crystal were determined at room temperature. Lifetimes of Er and Yb ions levels and Yb ! Er energy transfer efficiency were measured. The CW and Q-switched laser operations of this material under continuous-wave laser diode pumping at 980 nm were demonstrated. A CW output power of 245 mW with slope efficiency of 14% at 1604 nm was achieved. In the Q-switched regime an average output power of 108 mW with slope efficiency of 9.6%, repetition rate of 20 kHz and pulse duration of 135 ns was obtained. Acknowledgements This research was supported, in part, by the grants of RFBR NN 04-05-64709, 05-05-08021, 06-05-08103 and the Russian President Grants for Young Scientists 2794.2005.5 and 4456.2006.5 and MNP MSU-2005 No. 15. References [1] P. Burns, J. Dawes, P. Dekker, J. Piper, H. Jiang, H. Jiang, J. Wang, Proc. SPIE Int. Soc. Opt. Eng. 79 (2003) 4968. [2] B. Denker, B. Galagan, L. Ivleva, V. Osiko, S. Sverchkov, I. Voronina, J.E. Hellstrom, G. Karlsson, F. Laurel, Appl. Phys. B 79 (2004) 577. [3] L.M. Dorozhkin, I.I. Kuratev, N.I. Leonyuk, T.I. Timchenko, A.V. Shestakov, Pis’ma v zhurnal experimentalnoi i teoreticheskoi physiki 7 (1981) 1297 (in Russian). [4] P. Dekker, J.M. Dawes, J.A. Piper, Y. Liu, J.Y. Wang, Opt. Commun. 195 (2001) 431. [5] N.I. Leonyuk, L.I. Leonyuk, Prog. Cryst. Growth. Charact. 31 (1995) 179. [6] F. Lutz, M. Leiss, J. Mu¨ller, J. Crystal Growth. 47 (1979) 130. [7] W. You, Y. Lin, Y. Chen, Z. Luo, Y. Huang, Opt. Mater. 29 (2007) 488.