Aspects of point defects in coherent terahertz-wave spectroscopy

The ε-type monocrystalline GaSe crystals were successfully grown by liquid phase epitaxy at constant and low (530-590 °C) growth temperatures under different Se vapor pressures (P_Se∼0-7.75 Torr). From coherent terahertz (THz)-wave spectroscopy, the absorption spectra have different resonant frequencies and absorption coefficients due to the stoichiometry-dependent point defects, which depend on the applied P_Se. It is shown that the resonance in GaSe under P_Se∼0 Torr shifts toward lower THz frequencies compared with those under high P_Se, maybe due to the degraded intermolecular interactions caused by the introduction of selenium vacancy-related defects. The absorption coefficients (1-5 THz) decreased according to the increase of Se vapor pressure, thus the transparency of GaSe under higher P_Se is improved by 25% compared with that of Bridgeman-grown crystals. It is revolutional method, in which value of Q could be as high as 3 million, for the molecular structure and defects in organic molecule also could be already analyzed.