CdSiP2 (CSP) is a nonlinear optical chalcopyrite semiconductor developed as a wider-band-gap analog of ZnGeP2 (ZGP) to enable mid-infrared generation with widely-available 1- and 1.55-micron pump laser sources. CSP has an even higher nonlinear coefficient (d36=84.5 pm/V) than ZGP (d36=79 pm/V), and its lower thermal conductivity (13.6 W/mK vs 35 W/mK for ZGP) is more than offset by nearly 10-fold lower absorption losses in the 1.06- to 2.1-micron wavelength range, making CSP an attractive alternative to ZGP even for power-scaling 2-micron-pumped OPOs.
CSP growth presents significant crystal growth challenges compared to ZGP including: a higher melting point and vapor pressure that push the limits of fused silica based growth technology, a higher reactivity with boat materials and fused silica ampoules, an increased incidence of twin formation, and a negative c-axis thermal expansion coefficient (which makes it prone to cracking). Despite these difficulties, recent advances in crystal growth from stoichiometric melts using the horizontal gradient freeze (HGF) technique have resulted in scaling boule diameters from 19 to 28 millimeters. Improved refractive index data was recently measured at AFRL over a wide temperature range from 85K through 450K and fit to a temperature-dependent Sellmeier equation. More accurate thermal conductivity measurements along the major crystallographic axes are also reported. Finally, we report the first multi-watt CSP OPO: over 5 Watts of average power output (near-degenerate signal plus idler) were achieved at a slope efficiency of 60% from tandem, walk-off-compensated CSP crystals in a linear resonator pumped by a q-switched 32-nanosecond 1.94-micron Tm:YAP laser.
Kevin T. Zawilski, Peter G. Schunemann, John C. McCarthy, and Leonard A. Pomeranz, "Advances in growth and power scaling of the nonlinear crystal CdSiP2 (Conference Presentation)," Proc. SPIE 10516, Nonlinear Frequency Generation and Conversion: Materials and Devices XVII, 105160U (Presented at SPIE LASE: January 31, 2018; Published: 14 March 2018); https://doi.org/10.1117/12.2291334.5751419446001.
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