Far-infrared conversion materials: Gallium selenide for far-infrared conversion applications

Executive summary

Northrop Grumman Corporation performed the experimental and theoretical studies to synthesize, grow and characterize centimeter size single crystals of an extremely efficient nonlinear optical material gallium selenide (GaSe). Gallium selenide compound was synthesized by reacting purified gallium and selenium in stoichiometric ratio. The mixture was prepared by raising the temperature to 1050°C in steps. We designed a two-zone furnace and grew crystals in quartz tube in a Bridgman geometry. We varied the process parameters such as growth velocity and temperature gradient to study their effect on crystal quality. We used cylindrical, tapered, pyrolyzed and encapsulated ampoules to reduce the stress induced cracking. A capillary was used to seed the crystal. A numerical method involving finite volume technique was used to optimize crystal growth furnace configuration. The modeling equations that described the axial and radial segregation in the solid and solid-liquid interface are separated into four parts: the first part refers to field equation in the GaSe liquid phase, the second part to the equations in the encapsulant phase, the third part to the field equations in the solid phase and the fourth part to the ampoule itself. Single crystals of pure GaSe and doped with indium (In) and silver (Ag) were grown and doping significantly affected the cleaving tendency of pure GaSe crystals. Crystals were characterized by using X-ray laue and rocking curve, and etchpit techniques to understand the point and line defects. Crystals were fabricated by cutting with string saw and polished with diamond powder. Optical quality was evaluated by studying the bulk transparency and transmittance. Crystals transmitted between 0.65 to 18 μm and doping did not affect the transmission characteristics. Measurements of second harmonic conversion efficiency “d” for 9.6 μm wavelength yielded values between 62 to 75 pm/V depending on the quality of crystals. The optical damage threshold for GaSe was observed to be very high. GaSe samples could not be damaged with the 1.3 J/pulse, which corresponds to an energy density of 3.7 J/cm² within the focused spot area giving an intensity of 180 MW/cm². In a separate damage threshold experiment we operated laser at 30 kHz and an output of 21 W and the beam was focused through the crystal corresponding to an average power density of 60 kW/cm². GaSe crystal was held stationary without beam scanning and did not show any sign of damage.