Probing thin-film ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$ superconductors by second-harmonic generation with the use of femtosecond laser pulses

High-temperature superconducting thin films of ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$ are examined by measuring the second-harmonic generated (SHG) signal from an incident 60-fs laser pulse of 2.0 eV (620 nm) in air at room temperature. As oxygen stoichiometry is varied (0.00<δ<0.50), large changes in the SHG intensity are observed. The oxygen content of the films is independently determined by measuring (i) the c-axis lattice parameter derived from x-ray diffraction and (ii) the stoichiometry of the films determined by elastic backscattering spectrometry. We report a negative correlation of the SHG intensity with the c-axis parameter indicating that a known compression of the c axis leads to a larger SHG signal. An additional experiment in which the SHG intensity is monitored as a function of temperature (300–20 K) in vacuum is performed. A gradual increase of ∼50% in the SHG intensity is observed as the temperature is lowered, which can also be correlated to a decrease in the length of the c-axis parameter. Other experiments including rotation anisotropy and polarization of the incident and analyzer beams are also discussed.