Terahertz emission from a bulk GaSe crystal excited by above bandgap photons

Spectral dependences of the amplitudes of terahertz (THz) transients radiated from a GaSe surface after its excitation by femtosecond optical pulses with photon energies in the range from 1.8 eV to 3.8 eV were used for the study of electron energy band structure of this layered crystal. The energy separation of 0.21 eV between the main Γ valleys and the satellite K valleys in the conduction band was determined from the maximum position of THz excitation spectrum; the polarity of the THz transients became inverted at photon energies higher than 3 eV due to the onset of electron transitions from the second, lower lying valence band.