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3The correlation of SHG responses with the (M) cation in TbM3(BO3)4 (M = Al, Sc, Ga) orthoborates
A.Y. Jamous1'2*. A.B. Kuznetsov2, V.A. Svetlichnyi1, A.E. Kokh2
1- Tomsk State University, Tomsk 634050, Russia 2- Sobolev Institute of Geology and Mineralogy SB RAS, Novosibirsk 630090, Russia
The widespread use of laser devices in various industries, medicine, communication systems, etc., up to small household appliances, requires the improvement and optimization of the physical parameters and structural qualities of known optical materials that have already proven themselves in practice, with further control of their properties. Borate-based crystals due to their unique properties, such as a wide range of transparency, high threshold of laser destruction, physical and chemical stability, high thermal conductivity, etc., are widely used for nonlinear optic and laser technology [1]. Moreover, borates are characterized by an extremely wide variety of chemical composition and crystal structure, which provides a wide opportunity to search for and create new nonlinear optical crystals based on them. Among them one can highlight the complex rare-earth orthoborates with the general formula LnM3(BO3)4 (Ln = La-Lu, Y; M = Al, Ga, Sc, Cr, Fe), which for some Ln and M belong to the huntite family (huntite CaMg3(CO3)4, space group R32). Pure and activated LnM3(BO3)4 compounds as well as their-based solid solutions are promising materials for lasers, nonlinear optics, spintronics, and photonics, which are characterized by multifunctional properties depending on a composition and crystal structure [2].
In this work, the second harmonic generation (SHG) response in new terbium orthoborates TbM3(BO3)4 (M = Al, Ga, Sc) crystals are investigated. In particular the influence of the nature of the M cation on SHG efficiency. Both TbAl3(BO3)4 and TbGa3(BO3)4 are uniaxial trigonal crystals related to R32 space group. Since the ionic radius of scandium (rSc = 0.74 A) is large comparing with the ionic radii of aluminum (rAl = 0.53 A) and gallium (raa = 0.62 A), and is close to the ionic radius of terbium (rTb = 0.92 A) there is no R32 modification for TbSc3(BO3)4 [3]. However, the R32 modification can be obtained by lanthanum doping in TbSc3(BO3)4 matrix (rAl = 1.02 A) [4], thus the La:TbSc3(BO3)4 crystal was used. The SHG response was investigated using Kurtz-Perry powder test [5]. Crystals were crushed and then sieved and distributed using calibrated sieves into successive particle size ranges (fractions) from 20 to 200 ^m. To illuminate the powdered samples Q-switched YAG:Nd laser radiation (1064 nm, 7 ns) was used. Test was carried out under various pump power densities (Ipump) up to 70 MW/cm2 for each powder fraction.
The Kurtz-Perry powder test showed the typical quadratic dependence of the SHG response on the pump power density for all powder fraction of the TbAl3(BO3)4, La:TbSc3(BO3)4 and TbGa3(BO3)4 crystals. However, for Ipump > 60 MW/cm2 there is a deviation of the SHG intensity value from the quadratic function for TbGa3(BO3)4 powders, that is due to laser-induced damage. Thus, the 50 MW/cm2 power density was chosen from the SHG fitted curves to determine the dependence of SHG on particle size and calculate the effective nonlinearity coefficient (def). For all studied crystals, the increase in the size of powder fractions leads to higher SHG signal, that is characteristic of crystals with phase matching. The results also show that there is a direct correlation between the atomic mass of the cation (M) and the SHG efficiency, and therefore the effective nonlinearity coefficient: df(TbGa3(BO3)4) = 1.19xdef(La:TbSc3(BO3)4) = 1.33xdf(TbAl3(BO3)4).
This work was supported by the RSF project (№ 23-19-00617).
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[4] A. Kuznetsov, K. Kokh, N. Kononova, et al, New scandium borates RxLayScz(BO3)4 (x+y+z=4, R=Sm, Tb): Synthesis, growth, structure and optical properties, Mater. Res. Bull., 126, 110850, (2020).
[5] S.K. Kurtz and T.T. Perry, A Powder Technique for the Evaluation of Nonlinear Optical Materials, J. Appl. Phys., 39, 3798-3813, (1968).
