Abstract

The generation of subpicosecond terahertz bandwidth radiation via the illumination of nonlinear materials with visible femtosecond pulses is of interest both as a spectroscopic source and as a probe of the electronic and vibrational properties of materials. Previous studies have included terahertz emission from semiconductors¹ where such emission is the result of optical rectification and/or the evolution of a transient photogenerated current.²³ Terahertz emission has recently been observed from superconducting (T < T_c) YBa₂Cu₃O₇ thin films configured as biased antennas⁴ and unbiased in a magnetic field.⁵ We describe here the first observation of terahertz emission from unbiased YBa₂Cu₃O_7−δ (YBCO) thin films. These films are excited by 150 fs, 1.5 eV, and 3.0 eV, optical pulses for a range of temperatures, T, 4 K < T < 300 K and for a range of oxygen doping from optimally doped (δ = 0) to insulating (δ = 0.8). We demonstrate that this emission is generated by optical rectification due to the bulk electric quadrupole source term, comprising the first observation of such a source term in terahertz emission.

© 1998 Optical Society of America