Spectroscopic ellipsometry study on doped SrTiO$_{3}$ superlattice films

We report on the spectroscopic ellipsometry study on the low-dimensional confinement of chemical doping in SrTiO$_{3}$. We fabricated superlattice films composed of the stacking of insulating SrTiO$_{3}$ (STO) and metallic La-doped SrTiO$_{3}$ (SLTO) layers. As the dimensionality is varied from three to two dimensions by changing the thickness of the SrTiO$_{3}$ layers, phase transition from metal to insulator occurred through interplay of charge, spin, orbital, and lattice degrees of freedom. The optical conductivity spectra obtained from the spectrosocopic ellipsometry show a significant change below the charge transfer gap near 3 eV through the insulator-metal transition. We detail our spectroscopic finding on the STO/SLTO superlattice, and compare them with the transport and structural properties of the films.