LINEAR OPTICAL-PROPERTIES AND 2ND-HARMONIC GENERATION FROM ULTRATHIN NIOBIUM FILMS - A SEARCH FOR QUANTIZATION EFFECTS

We present the experimental investigations of the optical properties o f ultrathin niobium films embedded in a dielectric that form quantum w ell structures, The linear optical properties of the Al2O3-Nb-Al2O3 st ructures in the near infrared and visible spectral ranges exhibit peri odic dependencies on the Nb layer thickness that can be explained by t he influence of quantum size effects on the electron scattering rate, The absorption bands that can be ascribed to the intersubband transiti ons have not been found in the linear optical spectra, The investigati ons of the optical second-harmonic generation from the Nb films show t hat the thickness dependencies-of the second-harmonic intensity reveal resonant behavior that can be accounted for by the intersubband trans itions in the niobium quantum wells, The microscopic local-field calcu lations qualitatively explain the thickness dependence of the SHG obse rved in the experiment.