Synthesis and characterization of composition-spread (Sr,Ca)2CuO3 thin films with high third-order optical nonlinearity

Abstract

The optical properties of composition-spread $({\mathrm{Sr}}_x{\mathrm{Ca}}_{\text{1−x}}{)}_2{\mathrm{CuO}}_3\text{(0⩽x⩽1)}$ films showing high third-order optical nonlinearity are systematically investigated. Desired orientations and high crystallinity are obtained for the entire composition $x$ by employing a composition-spread $\mathrm{[(}{\mathrm{Sr}}_2{\mathrm{TiO}}_4{)}_x({\mathrm{LaSrAlO}}_4{)}_{\text{1−x}}]$ buffer layer and a temperature-gradient method in order to compensate the change in lattice mismatch and growth temperature depending on $\mathrm{x.}$ Linear absorption spectra and third-harmonic intensity with an exciting wavelength of 1.56 μm are measured as a function of $\mathrm{x.}$ As the Cu–O bond length increases with $\mathrm{x,}$ the charge-transfer gap decreases and the third-harmonic intensity increases, and both the charge-transfer gap and third-harmonic intensity change almost linearly with $x$ between ${\mathrm{Ca}}_2{\mathrm{CuO}}_3$ and ${\mathrm{Sr}}_2{\mathrm{CuO}}_3.$