The first successful operation of a AgGaSe2 infrared parametric oscillator is reported. Continuous tuning ranges of 1.6–1.7 μm, 6.7–6.9 μm, and 2.65–9.02 μm were achieved using 1.34‐μm neodymium and 2.05‐μm holmium pump lasers. Pulse energies exceeding 3 mJ, peak powers near 100 kW, and conversion efficiencies of 18% were obtained. Operation of the parametric oscillator was possible well below the 13–40 MW/cm2 surface damage threshold of this nonlinear material.

Broadly tunable infrared parametric oscillator using AgGaSe2

Silver gallium sulfide (AgGaS2), silver gallium selenide (AgGaSe2), and silver gallium indium selenide (AgGaxIn(1 − x)Se2) are unique nonlinear crystals suitable for a wide range of three-wave mixing applications. They combine strong nonlinear coupling with phase matching across a wide (0.5 − 12.5 μm) transmission range. These properties provide the basis for CO2 laser harmonic-generator (HG), visible (VIS), and near-infrared (NIR) pumped opticalparametric-oscillator (OPO) and sumand difference-frequency-generator (SFG/DFG) systems for producing tunable laser radiation from approximately 0.65 to 12 μm. A wide range of pump lasers is now available, including diode lasers capable of direct pumping of AgGaS2 and efficient diode-pumped lasers for pumping any of these crystals. Significant progress in material characteristics and nonlinear optical (NLO) performance has been accomplished. Bulk absorption for production-quality AgGaS2 is ~0.01 − 0.02 cm across the 0.8−9-μm region. Exceptional crystals of AgGaS2 have absorption <0.0005 cm−1 at 1.064 μm, and even at 0.633 μm, absorption is ~0.015 cm−1. Figure 1 illustrates the late 1980s vintage and present transmission of AgGaS2. For AgGaSe2, absorption for production material ranges from 0.010 cm to 0.018 cm−1 from 1 μm to 11 μm. Certain crystals can have 0.007−0.008 cm−1 absorption at 10.6 μm. The NIR transmission curves for AgGaSe2 and AgGaxIn(1 − x)Se2 appear in Figure 2. This combination of optical properties, along with their good physical characteristics, make these two crystals versatile for NLO applications spanning the VIS to the infrared (ir).

Crystal Growth and Optical Properties of AgGaS2 and AgGaSe2

In recent years, chalcopyrites have distinguished themselves as the nonlinear optical materials of choice for mid- to far-infrared (ir) laser applications. In particular, AgGaSe2, ZnGeP2, and CdGeAs2 have demonstrated the highest conversion efficiencies and output powers in the wavelength range beyond 4 μm. 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-μm 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 CO2 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 high-power 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 photons (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.

Nonlinear Frequency Conversion Performance of AgGaSe2, ZnGeP2, and CdGeAs2

CO2 laser frequency doubling in ZnGeP2

Thermodynamic description of the binary Ag–Ga system

AgGaSe2 crystals for nonlinear infrared applications are being grown reproducibly. Using high‐quality, 2‐cm‐long crystals, 14% energy and 60% peak intensity conversion efficiency have been demonstrated for second harmonic generation of the pulsed output of a CO2 miniature transversely excited atmospheric laser.

Jan van der Laan

Papers

Efficient second harmonic generation of 10‐μm radiation in AgGaSe2

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