Spectral dependencies of terahertz emission from femtosecond laser excited surfaces of germanium crystals

Pulsed terahertz emission excitation spectra from germanium crystals are being presented. The most intense terahertz pulses from germanium crystals are emitted at quanta energies coinciding with technologically significant telecommunication wavelengths. The terahertz generation mechanisms are an interplay of the photocurrent surge in the surface electric field and the photo-Dember effect. Remarkably, the terahertz emission is also observed at quanta energies below the direct bandgap of this material even when photoexcited at a surface normal. This is the result of a broken symmetry of effective electron mass in the L valleys.Pulsed terahertz emission excitation spectra from germanium crystals are being presented. The most intense terahertz pulses from germanium crystals are emitted at quanta energies coinciding with technologically significant telecommunication wavelengths. The terahertz generation mechanisms are an interplay of the photocurrent surge in the surface electric field and the photo-Dember effect. Remarkably, the terahertz emission is also observed at quanta energies below the direct bandgap of this material even when photoexcited at a surface normal. This is the result of a broken symmetry of effective electron mass in the L valleys.