GaSe is one of the most promising semiconductor crystals for wide frequency-tunable terahertz (THz) wave generation by photo mixing. The ε-type monocrystalline GaSe crystals were successfully grown by the liquid-phase epitaxy at constant and low (530-590 °C) growth temperatures under the controlled different selenium (Se) vapor pressures (PSe∼0-7.75 Torr). From the coherent THz-wave spectroscopy, the absorption spectra have shown different resonant frequencies and absorption coefficients due to the stoichiometry-dependent point defects which depend on the applied PSe. It is shown that the resonance in GaSe under PSe∼0 Torr shifts towards the lower THz frequencies compared with those under high PSe, maybe due to the degraded intermolecular interactions caused by the introduction of Se 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 PSe is improved by an amount of 25% compared with that of Bridgman-grown crystals. By using Bridgman-grown GaSe crystals, coherent-THz wave was generated by the difference frequency method (DFM) in a wide frequency range of 0.1-70 THz. Coherent-THz spectroscopy is a revolutionary method for the evaluation of molecular structures and defects in organic molecule also could be analyzed.
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