Thermal properties of Nd:SBN through the ferroeletric phase transition using the thermal lens spectroscopy C. Jacinto,1 D. Jaque,2 T. Catunda,3 A. A. Kaminskii,4 J. García Sole 2 1
Instituto de Física - Universidade Federal de Alagoas, CEP 57072-970, Maceió – AL, Brazil 2 Departamento de Física de Materiales, Faculdad de Ciencias, Universidad Autónoma de Madrid, Cantoblanco, 28049 Madrid, Spain. 3 Instituto de Física de São Carlos – Universidade de São Paulo, CEP 13560-970, São Carlos – SP, Brazil 4 Institute of Crystallography, Russian Academy of Sciencies, Leninskii Prospek 59, 117333 Moscow, Russia.
[email protected]
Abstract Thermal lens spectroscopy has been used to investigate the optical properties of Nd:SBN crystals during the ferro-to-para electric phase transition. Both crystal composition and polarization of interacting radiation have been found to be critical.
Strontium Barium Niobate crystal (SrxBa1-xNb2O6, hereafter SBN) is nowadays regarded as one of the most important and promising systems in the field of optoelectronics. It has been widely studied in the past because of its high non-linear, pyroelectric, acousto-optic and electro optical coefficients. It is a ferroelectric crystal with a phase transition temperature close to room temperature (15-250 ºC depending on the composition). This fact makes phase transition easily achieved. Some of the most recent applications of SBN crystals, such as beam fanning reversals and polarization based adjustable memories, are related to these relative low phase transition temperatures. It also shows the advantage of easy doping with rare earth and metal transition ions. When optically activated with luminescent ions, a rich variety of phenomena related to the Ferro-to-Para electric phase transition arises. As for example, doping SBN with Cr3+ ions has shown the sensibility of its luminescence to the phase transition, by displaying a thermally induced bistable emission related to the thermal hysteresis of the phase transition. When doped with Nd3+ ions it becomes a multi-frequency converter laser system operating simultaneously in the infrared and visible domains [1]. Furthermore, due to the intrinsic disorder characteristic of SBN crystals, Nd3+ ions show broad emission and absorption band allowing for continuous wave tunable laser light generation in the infrared and visible [2]. Additionally, the broad gain bandwidth makes the Nd3+ doped SBN system a promising candidate for ultra-short pulse generation by the Mode-Locking technique. When a Nd3+ doped SBN crystal is operating under strong or moderate pumping intensities, the pump induced thermal loading could be strong enough to cause a local temperature higher than transition temperature, therefore driving the active volume from the Ferro to the Para-electric phase [3]. Consequently, a complete knowledge and understanding of how phase transition affects the parameters of relevance in laser dynamics becomes necessary. Among the different parameters limiting the laser performance of ferroelectric laser crystals, the thermo optical properties are among the most important. The influence of phase transition on the thermo optical properties of Nd:SBN crystals is not only interesting from an applied point of view but also from a fundamental one, as it could provide additional information about the solid state physics involved in a phase transition. Thermal Lens Spectroscopy (TLS) has been proved to be a valuable tool to study the thermo-optical properties of transparent materials, including Neodymium doped laser crystals. It allows for the determination of thermal diffusivity, the thermal conductivity, the temperature coefficient of optical path length, optical absorption coefficient and fluorescence quantum efficiency. There are several experimental configurations for TLS. Among all of them, the two-beam mode-mismatched experimental arrangement has been demonstrated to be the most sensitive one. Indeed the theoretical model for the thermal lens effect in this configuration has been already used to obtain the behavior of the optical and thermal properties during phase transition of a wide range of materials including glasses, polymers and liquid crystals. In this work we present the results obtained from the application the two-beam mode-mismatched TLS technique to determine the thermo-optical properties of the Nd:SBN system through the ferro-to-para electric phase transition. The experimental set-up used is schematically drawn in Figure 1 (a). The unpolarized laser radiation at 808 nm from a modulated fiber coupled diode was used as the pump beam. The time dependence of
pump radiation was monitored by using a broad area Si detector (D1 in Figure 1(a)). As a probe beam we used a 2 mW He-Ne laser providing a 100 % polarized radiation. The on-axis intensity of the probe beam after passing through the Nd:SBN crystal was measured by using a 1 mm2 Si detector (D2 in Fig.1(a)). The time dependence of the on-axis intensity of the probe beam was registered by using the average facilities of a LeCroy digital oscilloscope. When pump is switched on, it causes the deposition of heat into crystal lattice via the non-radiative decay processes departing from pumped states. Since pump intensity is not homogeneous, the pumping rate and, therefore, the amount of heat generated, follows an inhomogeneous spatial distribution inside the crystal. In this situation a transverse temperature gradient is established, and owing to the temperature dependence of the optical path length, a lens-like optical element is created. The propagation of the non-absorbed He-Ne probe laser beam through this pump induced TL results in a variation of its on-axis intensity with respect to the non-pumped crystal. If pump is suddenly switched on, then the on-axis intensity of the probe beam varies with time until it reaches a steady value. In this “transient regime”. The Nd:SBN crystal was placed inside an oven in such a way that its temperature was controlled. Figure 1(b) shows a typical transient curve obtained at room temperature. From the analysis of this curve it is possible to access to the value of thermal diffusivity as well as to the value of the probe beam phase shift induced by the thermal lens which is proportional to the temperature coefficient of the optical path length.
20 mW @ 808 nm
Oven
L1
1.00
Diode Laser
SBN:47 L2
Nd:SBN
He-Ne
D1
E2 E3
I(t)/I(0)
E1
0.98
0.96
D2
(a)
0.94
Digital Oscilloscope
(b)
signal trigger
0
30
60
90
120
Time (µs) Figure 1: (a) Schematic diagram of the mode-mismatched thermal lens experimental apparatus, where, M’s are mirrors, P’s are photodetectors and L’s are convergent lenses. (b) Typical TL transient obtained for both Nd:SBN47
Experiments have been performed on two samples with different Sr contents and for crystal temperatures ranging from room temperature up to 120 ºC. We have found that the sign of the pump induced TL is strongly dependent on the Sr composition changing from negative to positive when the Sr composition is decreased from 0.61 down to 0.47. This change in the sign of the TL has been explained in terms of the different behavior observed in the thermal expansion along the optical axis for the two compositions under study. Additionally we have observed that, for temperatures close to the phase transition, the probe beam phase shift induced by the thermal lens effect displays a cups-like behavior. This is discussed in terms of the temperature dependence of the refractive indices and also in terms of the anomalous thermal expansion of SBN crystals along the optical axis for temperatures around the phase transition.
Acknowledgements The authors are thankful to CNPq and FAPESP for the financial support of this work. This work has been supported by the Comunidad Autónoma de Madrid (projects 07N/0020/2002 and GR/MAT/0110/2004) and by Spanish Ministerio de Ciencia y Tecnología (MAT2004-03347). Authors thank the group of Prof. Luo Zundu (Fuzhou, China) for providing us the NAB sample. __________________________________
References [1] J.J. Romero, D,Jaque, J.García Solé and A.A. Kaminskii. “Diffuse multi-self frequency conversion processes in the blue and green by quasicylindrical ferroelectric domains in Nd:SBN laser crystal” Appl. Phys. Lett. 78, 1961 (2001).
[2] M.O. Ramirez, P. Molina, J.J. Romero and L.E. Bausá. “Near infrared and visible tunability from a diode pumped Nd activated SBN laser” Appl. Phys. B 81, 827 (2005). [3] M.O. Ramirez, D.Jaque, L.E. Bausa, J.García Solé and A.A. Kamisnkii. “Coherent Light Generation from a Nd:SBN Nonlinear Laser Crystal through its Ferroelectric Phase Transition” Phys. Rev. Lett. 95, 267401 (2005).
