Terahertz emission from YBa2Cu3O7-(delta) thin films via bulk electric-quadrupole-magnetic-dipole optical rectification

We report the observation of terahertz emission from YBa2Cu3O7-delta thin f ilms excited by 1.5-eV, 150-fs pulses in the absence of external electric o r magnetic fields. This emission is characterized for optimally doped delta = 0 films and underdoped delta = 0.5 films, over a range of temperatures T , 4 K < T < 300 K, where the films change from superconductors to correlate d metals. It exhibits a pronounced 4 phi dependence with respect to the azi muthal angle phi. We demonstrate that this emission is generated by optical rectification due to the bulk electric quadrupole and magnetic dipole sour ce terms. A model of this emission, assuming that the nonlinearity results from transport of carriers with different masses and relaxation times in th e three crystallographic directions, is consistent with the data. A fit of the data to this model yields values for the anisotropic masses and relaxat ion times in YBa2Cu3O7-delta thin films.