Abstract-Terahertz-Pulse Radiation Properties of Oxygen-Deficient YBa2Cu3O7−δ Thin Films

• Yukihiro Tominari, 
• Ryotaro Inoue, 
• Iwao Kawayama, 
• Hironaru Murakami, 
• Masayoshi Tonouchi

http://link.springer.com/article/10.1007%2Fs10762-013-0016-1

By measuring the detailed characteristics of terahertz-pulse (THz-pulse) radiation and applying THz time-domain spectroscopy, we investigate the generation mechanism of THz-pulse radiation from biased YBa₂Cu₃O_7−δ (YBCO) thin films with various carrier concentrations. To vary the carrier concentration, the oxygen deficiency of the YBCO samples was systematically varied. With decreasing carrier concentration, the THz-pulse width increases and the response speed of supercarriers to illumination by femtosecond optical pulses tends to decrease. By examining the radiated waveforms, we clarify the influence of carrier doping not only on the relaxation process of quasiparticles but also on the excitation process. A suitably underdoped YBCO film results in maximum efficiency for generating THz radiation. This tendency is difficult to explain if we only consider the variation in transmittance, which has been the approach till now. Instead, we suggest that a variation in quasiparticle excitation efficiency due to carrier doping should also be considered.