offers infrared crystals such as AgGaS2, AgGaSe2, GaSe, ZnGeP2. Due to their unique features, nonlinear IR crystals like ZnGeP2, AgGaSe2, AgGaS2, GaSe have gained tremendous interest for the middle and deep infrared applications. The IR crystals have large effective optical nonlinearity, wide spectral and angular acceptance, broad transparency range, non-critical requirements for temperature stabilization and vibration control, and are easy to mechanically process (except for GaSe).
IR crystal ZnGeP2 (ZGP) has band edges at 0.74 and 12 µm. Its useful transmission range lies from 1.9 to 10.6 µm. ZnGeP2 (ZGP) crystal has the largest nonlinear optical coefficient and relatively high laser damage threshold. ZGP crystal is successfully used in diverse applications: up-conversion of CO2 laser light to near IR range via mixing with 10.6 µm; sum frequency generation of CO and CO2 laser radiation; efficient SHG of pulsed CO, CO2 and chemical DF-laser; OPO light generation in mid infrared when pumped by erbium and holmium lasers.
Crystals with the lowest absorption α < 0.04 cm-1 @ 2.1 µm are available, best suited for OPO or OPA application when pumped by 2.05-2.1 µm, with antireflection coatings.
IR crystal AgGaSe2 has band edges at 0.73 and 18 µm. Its useful transmission range within 0.9–16 µm and wide phase matching capability provide excellent potential for OPO applications when pumped by a variety of currently available lasers. Tuning within 2.5–12 µm has been obtained when pumping by Ho:YLF laser at 2.05 µm; as well as non-critical phase matching (NCPM) operation within 1.9–5.5 µm when pumping at 1.4–1.55 µm. AgGaSe2 (AgGaSe) has been demonstrated to be an efficient frequency doubling crystal for infrared CO2 lasers radiation.
IR crystal AgGaS2 (AGS) is transparent from 0.53 to 12 µm. Although its nonlinear optical coefficient is the lowest among the above mentioned infrared crystals, high short wavelength transparency edging at 550 nm is made use of in OPOs pumped by Nd:YAG laser; in numerous difference frequency mixing experiments with diode, Ti:Sapphire, Nd:YAG and IR dye lasers covering 3–12 µm range; in direct infrared countermeasure systems, and for SHG of CO2 laser. Thin AgGaS2 (AGS) crystal plates are popular for ultrashort pulse generation in mid IR range by difference frequency generation employing NIR wavelength pulses.
IR crystal GaSe has band edges at 0.65 and 18 µm. GaSe has been successfully used for efficient SHG of CO2 laser; SHG of pulsed CO, CO2 and chemical DF-laser (λ = 2.36 µm) radiation; up conversion of CO and CO2 laser radiation into the visible range; infrared pulses generation via difference frequency mixing of neodymium and infrared dye laser or (F-)-centre laser pulses; OPG light generation within 3.5–18 µm. Thin GaSe crystal plates are also used for fsec THz generation. It is impossible to cut the crystals for certain phase matching angles because of material structure (cleave along (001) plane), what limits the application areas .