In recent years, there has been a growing demand for specific visible and ultraviolet laser sources in medicine, industrial processing, remote sensing, laser printing, optical displays, and other areas. At this time, the availability of laser frequencies in the visible and UV is limited by laser materials and pump sources. Frequency conversion of solid-state lasers operating in the near infrared range by nonlinear optical (NLO) crystals has become the most available method to obtain shorter wavelength lasers with high beam stability, low cost and compactness. Thus, the reliance on nonlinear methods of frequency generation demonstrates the need for new nonlinear harmonic crystals with the ability to frequency convert a wide variety of laser wavelengths.
     YCa₄O(BO₃)₃ (YCOB) has attracted great attention as a new NLO crystal for frequency generation since its earliest development [1]. YCOB is a congruent melting non linear material allowing the growth of large dimensions and high optical quality crystals to be used as frequency converters in solid-state laser systems [1-4]. Our previous researches [5-7] showed that in YCOB crystal, the Y³⁺ ions can be partially substituted by smaller radius ions Sc³⁺ or Lu³⁺ in order to tune the chemical composition of the crystal. By changing the compositional parameter x of Y_1-xR_xCa₄O(BO₃)₃ (R = Lu, Sc) crystals, their optical birefringence can be controlled in order to perform non critical phase matching (NCPM) second harmonic generation (SHG) of specific near infrared laser emission wavelengths shorter than phase matching cutoff wavelength of YCOB crystal (724 nm along Y axis and 832 nm along Z axis at room temperature [8]).
     For biaxial crystals like YCOB family compounds, NCPM is the phase matching along one principal axis of the crystal, and for frequency conversion applications, NCPM is advantageous because of its large angular acceptance and because it eliminates walk-off between fundamental and harmonic radiations which leads to the highest efficiency.
     Since NCPM is determined by the optical birefringence and is accomplished to an unique wavelength for each NLO process, the objective of this work is to evaluate the potential of Y_1-xR_xCa₄O(BO₃)₃ crystals as frequency converters for the laser emissions around 800 nm (AlGaAs laser diodes and the strongest emission of Ti: Sapphire laser) and about 700 nm (red laser diodes) in order to obtain visible or near-UV laser radiations by type-I NCPM SHG processes at room temperature. In this aim, crystal growth and NCPM frequency conversion properties of Y_1-xR_xCa₄O(BO₃)₃ new nonlinear crystals are reported.
     Five new NLO crystals of Y_1-xLu_xCa₄O(BO₃)₃ and Y_1-xSc_xCa₄O(BO₃)₃, with x = 0.19, 0.29, 0.39 and x = 0.07, 0.11, respectively, of good quality with no cracks and bubbles have been grown by Czochralski method, and their NCPM properties were investigated. It was demonstrated that efficient room temperature type-I NCMP SHG of any wavelength from 692.6 - 724 nm and 791.4 - 832 nm spectral ranges, can be achieved in Y_1-xR_xCa₄O(BO₃)₃ crystals by tuning the composition. These results have very important implications for many of today’s tunable solid-state lasers (Ti: Sapphire, Cr: LiSAF, Cr: LiCAF, Alexandrite) and laser diodes (AlGaAs, AlGaInP) with emission in these spectral ranges, in order to obtain specific blue and/or near-UV laser emissions.

References:

[1] M. Iwai, T. Kobayshi, H. Furuya, Y. Mori, T. Sasaki, Jpn. J. Appl. Phys. 36, L276 (1997).
[2] Q. Ye, B.H.T. Chai, J. Cryst. Growth 197, 228 (1999).
[3] D. Vivien, G. Aka, A. Kahn-Harari, A. Aron, F. Mougel, J.M. Bénitez, B. Ferrand, R. Klein, G. Kugel, N. Le Nain, M. Jacquet, J. Cryst. Growth 237-239, 621 (2002).
[4] Y. Fei, B.H.T. Chai, C.A. Ebbers, Z.M. Liao, K.I. Schaffers, P. Thelin, J. Cryst. Growth 290, 301 (2006).
[5] L. Gheorghe, A. Achim, F. Voicu, C. Ghica, J. Optoelectron. Adv. Mater. 12, 1680 (2010).
[6]. L. Gheorghe, A. Achim, B. Sbarcea, S. Mitrea, Optoelectron. Adv. Mater.-Rapid Commun. 4, 318 (2010).
[7]. A. Achim, L. Gheorghe, S. Georgescu, F. Voicu, Optoelectron. Adv. Mater.-Rapid Commun. 4, 1977 (2010).
[8] N. Umemura, K. Yoshida, H. Nakao, H. Furuya, M. Yoshimura, Y. Mori, T. Sasaki, K. Kato, Jpn. J. Appl. Phys. 40, 596 (2001).

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