Tunable Infrared Generation from Four-Wave Mixing in Room-Temperature Germanium

Abstract

The third-order nonlinear susceptibility [1] of Germanium enables two incident laser beams (υ₁, υ₂ wavenumbers) to generate combination frequencies υ₃ = ²υ₁ - υ₂, υ₄ = ²υ₂ - υ_1∘ Two efficient and powerful gas lasers (CO₂, NH₃), operating between 900 and 1100 cm^-1 (υ₁) and near 780 cm^-1 (υ₂) respectively, enable combination frequencies to be generated in the photochemically important regions 1420 - 1020 cm^-1 (υ₃) and 660 - 460 cm^-1 (υ₄). Such mixing can be phase-matched in room temperature Germanium if the incident laser beams υ₁ υ₂ are very slightly inclined at an angle θ (inside the crystal) (Fig. 1). Also shown in Fig. 1 are multiphonon absorption coefficients for Germanium at various temperatures.