Linear optical properties and second-harmonic generation from ultrathin niobium films: A search for quantization effects

We present the experimental investigations of the optical properties of ultrathin niobium films embedded in a dielectric that form quantum well structures. The linear optical properties of the Al₂ O₃-Nb-Al₂O₃ structures in the near infrared and visible spectral ranges exhibit periodic dependencies on the Nb layer thickness that can be explained by the influeuce of quantum size effects on the electron scattering rate. The absorption bands that can be ascribed to the intersubband transitions have not been found in the linear optical spectra. The investigations of the optical second-harmonic generation from the Nb films show that the thickness dependencies of the second-harmonic intensity reveal resonant behavior that can be accounted for by the intersubband transitions in the niobium quantum wells. The microscopic local-field calculations qualitatively explain the thickness dependence of the SHG observed in the experiment