Terahertz radiation properties of underdoped YBa2Cu3O7-δ thin films

Terahertz (THz)-wave pulse radiation properties by femtosecond optical pulse excitation were investigated on high-T-c superconductors. For the measurements three kinds of under-doped YBa2Cu3O7-delta (YBCO) thin films with superconducting transition temperatures of T-c = 84.0 K, 61.5 K and 52.4 K were prepared by annealing with a bulk YBCO sample under different reduced oxygen conditions. It was found that the amplitude of the radiated THz pulse strongly depends on the hole-doping level. The maximum radiation efficiency was obtained for a middle underdoped sample with. T-c = 61.5 K. The observed hole-doping dependence of radiation efficiency is qualitatively explained by the relation between the transmittance of YBCO and supercarrier density.