Abstract

We report the first observation of terahertz emission from unbiased YBa₂Cu₃O₇ thin films at room temperature excited by 1.5 eV femtosecond laser pulses. The strength of the radiated terahertz field depends on the incident angle (θ) and depends linearly on incident power. The terahertz emission also depends strongly on the rotation of the film about its surface normal (azimuthal angle, (ϕ). The strong anisotropy, with 4ϕ symmetry is demonstrated for different polarizations (incident and detected) and found to be consistent with optical rectification. Because YBCO is centrosymmetric, the secord order nonlinear susceptibility vanishes. The next order term in the nonlinear polarizations is the electric quadrupole source term which describes the observed data quite well. The terahertz radiation was found to decrease with decreasing oxygen content, indicating that the carriers in the doped sample enhance the nonlinearity responsible for the terahertz emission.

© 1997 Optical Society of America