Nonlinear Frequency Conversion Performance of AgGaSe 2 , ZnGeP 2 , and CdGeAs 2
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Nonlinear Frequency Conversion Performance of AgGaSe2, ZnGeP2, and CdGeAs2 P.G. Schunemann, K.L. Schepler, and P.A. Budni
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Introduction In recent years, chalcopyrites have distinguished themselves as the nonlinear optical materials of choice for mid- to farinfrared (ir) laser applications. In particular, AgGaSe2, ZnGeP2, and CdGeAs2 have demonstrated the highest conversion efficiencies and output powers in the wavelength range beyond 4 μ-m. The superior performance of these crystals arises from their high nonlinear optical coefficients (39 pm/V, 75 pm/V, and 236 pm/V, respectively), their relatively large birefringence (sufficient for phase matching), and advances in crystal growth and processing that have improved transparency and eliminated cracking. . The two most direct approaches to generating laser output in the mid-ir (in particular the 3-5-^,m atmospheric transmission window) are (1) shifting the output of a solid-state laser to longer wavelengths via optical parametric oscillation (OPO), or (2) doubling the frequency of a CO 2 laser (9-11 μm) via second-harmonic generation (SHG). The OPO approach offers the advantage of tunability combined with potentially more compact and efficient solid-state lasers, whereas the SHG approach benefits from the greater maturity of highpower CO2 laser technology. A third process, difference-frequency generation (DFG), is also a 3-wave interaction similar to OPO that can be used to mix two photons (from two lasers or from an OPO) to produce longer wavelength pho-
MRS BULLETIN/JULY 1998
tons (corresponding to the small difference frequency) over a large spectral range. The optimum chalcopyrite crystal (AgGaSe2, ZnGeP2, or CdGeAs2) for a given approach depends on a complex combination of material parameters described in the sections that follow.
OPO Performance
AgGaSe2 AgGaSe2 was identified in the early 1970s as a potentially useful nonlinear optical material1 for phase-matched OPO and SHG interactions. However crystals of sufficient size and quality were not available for a number of years because of severe cracking and clouding of crystals during cooling from the melt. A negative thermal-expansion coefficient along the c axis caused cracking during cooldown. Clouding of crystals was caused by slightly nonstoichiometric crystal growth and precipitation of a second phase during cooldown. Route et al.2 at Stanford University developed key growth and annealing techniques to remove microscopic scattering centers and make optical-quality, useful-sized AgGaSe2 crystals possible. The first AgGaSe2 optical parametric oscillator was also demonstrated by Eckardt et al.3 in 1986. With a 2.05-/um Ho:YLF (1-50 Hz, 30 mj, 50 ns) pump laser, continuous frequency tuning over a very wide range (2.65-9.02 /xm) was 3 observed from a single 10 x 10 X 21 mm
antireflective (AR)-coated sample. Opticalparametric-oscillation output energies (3.55-/i.m signal plus 4.84-/n.m idler) of 3 mj/pulse at conversion efficiencies as high as 18
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