Experimental example of isotropic surface second-harmonic generation: dc-sputtered air-exposed aluminum thin films

Abstract

We have determined the amplitudes and relative phases of the allowed components of the effective surface second-harmonic susceptibility, ${\chi }^{\mathrm{}\left(2\right)\mathrm{}},$ for a set of air-exposed dc-sputtered Al thin films. The second-harmonic intensity and polarization state was measured as a function of linear input polarization state and azimuthal orientation of the sample using a Ti:sapphire laser at 810 nm. The films are found to be optically isotropic with respect to azimuthal rotation angle, consistent with an isotropic ${\chi }^{\mathrm{}\left(2\right)\mathrm{}}$ with seven elements in three families. Using measured linear optical properties, the elements of ${\chi }^{\mathrm{}\left(2\right)\mathrm{}}$ were determined from the data. In terms of the dimensionless parameters $a,\hspace{0.5em}b,$ and $d$ [J. Rudnick and E. A. Stern, Phys. Rev. B 4, 4274 (1971)], we find $|a|=2.30\mathrm{}\pm 0.72,$ in rough agreement with previous measurements of the surface current perpendicular to the surface on oxidized Al surface, $|b|=0.013\mathrm{}\pm 0.004$ for the surface current parallel to the surface, and $|d|=0.009\mathrm{}\pm 0.002$ for the bulk contribution. The relative phases of a and b with respect to d are found to be $(-0.4\pm 6)°$ and $(63\mathrm{}\pm 12)°,$ respectively. The measurement indicates a large discrepancy between measured values and the universal theoretical expectation that $b=-1$ and $d=1.\mathrm{}$