Abstract
Thermal analysis of the monoclinic solid state laser host KLu(WO4)2 is presented. The specific heat was measured by the relaxation method in the temperature range from 1.9 to 385 K: its value at room temperature is 0.324 J/gK. The Debye temperature and the sound velocity amount to 303+/-3 K and 3734 m/s. The linear thermal expansion tensor was measured by X-ray powder diffraction from room temperature up to 773 K. The eigenvalues of this tensor are alpha'(11)=8.98 x 10(-6) K(-1), alpha'(22)=3.35 x 10(-6) K(-1), and alpha'(33)=16.72 x 10(-6) K(-1), with the maximum value in the a-b crystallographic plane, at 31.94 degrees from the N(g) principal optical axis. The thermal diffusivity and its anisotropy in the temperature range between 300 and 500 K were measured by the pyroelectric method to determine the thermal conductivity tensor. The eigenvalues of the thermal conductivity are kappa'(11)=2.95 Wm(-1)K(-1), kappa'(22)=2.36 Wm-1K-1, and kappa'(33)=4.06 Wm(-1)K(-1), with the maximum value along a direction again in the a-b crystallographic plane, at 40.75 degrees from the N(g) principal optical axis. Simulation of the temperature distribution in a bulk sample of KLu(WO4)2 with dimensions 3 x 3 x 3 mm(3) shows that pump and laser beam directions along the N(p) principal optical axis in terms of thermal effects are preferable because the propagation is along a quasi-isothermal path.
Highlights
KLu(WO4)2 belongs to a class of compounds known as monoclinic potassium double tungstates, KREW, and similar to the other inert compounds of this kind, KGdW and KYW, it is well suited as a host for active rare-earth (RE) ion dopants [1]
The linear thermal expansion coefficients in a given crystallographic direction are α=(ΔL/ΔT)/LRT, where LRT is the initial parameter at 298 K
The volume linear thermal expansion of KLuW is αV=29.2 10-6 K1 and the average anisotropy of this property is lower in KLuW than in the other members of the KREW family of monoclinic laser hosts
Summary
KLu(WO4) (hereafter KLuW) belongs to a class of compounds known as monoclinic potassium double tungstates, KREW, and similar to the other inert compounds of this kind, KGdW and KYW, it is well suited as a host for active rare-earth (RE) ion dopants [1]. The non-uniform temperature distribution inside the laser crystal results in a distortion of the laser beam due to temperature and stress-dependent variation of the index of refraction These changes produce thermal lensing and thermal stress induced birefringence, which depends on the thermo-optic coefficient, dn/dT, and the thermal conductivity of the material. In more complicated situations (non-uniform heating, anisotropic thermal conductivity, etc), the radial temperature dependence is not parabolic and the focal length at the center of the active element is not the same as near the edges. In such cases, precise knowledge of the temperature distribution inside the crystal is needed in order to reduce the thermally induced optical distortions. A detailed thermal characterisation of the KLuW host will be given, with special emphasis on the anisotropy of the different parameters
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