Optical and Electro-Optical Properties of RFe$_{\mathrm{2}}$O$_{\mathrm{4}}$ (R $=$ Y and Yb) Thin Films*

We present optical, and electro-optical properties of RFe$_{\mathrm{2}}$O$_{\mathrm{4\thinspace }}$(R $=$ Y and Yb) thin films deposited on sapphire and YSZ substrates by reactive electron-beam deposition. In order to investigate the electronic transitions and optical properties, we measured transmittance and reflectance of the RFe$_{\mathrm{2}}$O$_{\mathrm{4\thinspace }}$thin films in the 1 -- 6 eV photon energy range and at temperatures from 10 to 400 K. The optical spectra of RFe$_{\mathrm{2}}$O$_{\mathrm{4\thinspace }}$show several electronic peaks arising from Fe$^{\mathrm{2+}} \quad d $to $d $on-site and O 2$p $to Fe 3$d$, Y 4$d$ and Y 5$s$ (Yb 5$d$ and 6$s)$ charge-transfer transitions. Interestingly, the electronic excitations display strong temperature dependence with an anomaly between 170 K and 190 K, indicating a structural distortion. Moreover, the electro-optical effects up to \textpm 10 {\%} have been observed in the RFe$_{\mathrm{2}}$O$_{\mathrm{4\thinspace }}$thin films at 10 K for applied electric fields below 1 kV/cm. These electro-optical effects mostly occur between the photon energy of 1 and 3 eV, vary almost linearly with applied fields, and the effects disappear above 150 K. We will discuss the driving mechanism for the observed electro-optical effects of these compounds.