A ${\mathrm{VO}}_2$ film was grown on a sapphire(0001) substrate using pulsed laser deposition. The film showed a first-order metal-insulator (MI) transition and its dc conductivity started to increase drastically near 68 °C and changed by three orders of magnitude. Mid-infrared transmittance and reflectance spectra of the ${\mathrm{VO}}_2$ film were measured between 20 °C and 90 °C. Using the intensity transfer-matrix method, the frequency-dependent dielectric constant ${\mathrm{\epsilon }}_{\mathit{f}}$(ω) and the conductivity ${\mathrm{\sigma }}_{\mathit{f}}$(ω) of the film were obtained between 1600 and 4000 ${\mathrm{cm}}^{\mathrm{-}1}$ from the measured transmittance and reflectance spectra. With the ${\mathrm{\epsilon }}_{\mathit{f}}$(ω) and ${\mathrm{\sigma }}_{\mathit{f}}$(ω) spectra, mid-infrared properties of the ${\mathrm{VO}}_2$ film near the MI transition region were investigated in detail. Above 78 °C, ${\mathrm{\epsilon }}_{\mathit{f}}$(ω)0, which is a typical metallic behavior. In particular, ${\mathrm{\epsilon }}_{\mathit{f}}$(ω) and ${\mathrm{\sigma }}_{\mathit{f}}$(ω) at 88 °C were analyzed in terms of extended Drude model in which the frequency-dependent scattering rate and the effective mass could be obtained. The mean free path of charge carriers in the dc limit was estimated to be larger by an order of magnitude than the previously reported value, i.e., 4 Å. Below 74 °C, ${\mathrm{\epsilon }}_{\mathit{f}}$(ω)>0 and d${\mathrm{\epsilon }}_{\mathit{f}}$/dω≊0, which is characteristic of an insulator. Interestingly, ${\mathrm{\epsilon }}_{\mathit{f}}$ in the insulating region increased as the temperature approached the MI transition temperature. To explain this anomalous behavior, the MI transition of the ${\mathrm{VO}}_2$ film was modeled with coexistence of metallic and insulating domains and their dynamic evolution. Then the behaviors of ${\mathrm{\epsilon }}_{\mathit{f}}$(ω) and ${\mathrm{\sigma }}_{\mathit{f}}$(ω) were explained using the effective medium approximation, which is a mean-field theory predicting a percolation transition. This work clearly demonstrates that the transport and optical properties near the MI transition region are strongly influenced by the connectivity of the metallic domains. © 1996 The American Physical Society.

Temperature dependent optical conductivity spectra of a ${\mathrm{La}}_{0.7}{\mathrm{Ca}}_{0.3}{\mathrm{MnO}}_3$ (LCMO) sample were measured. In the metallic regime at very low temperatures, they clearly showed two types of absorption features, i.e., a sharp Drude peak and a broad midinfrared absorption band, which could be explained as coherent and incoherent bands of a large lattice polaron. Electrodynamic analyses suggest that the elementary excitation in LCMO should be in a strong coupling regime and have interactions with the spin degree of freedom.

The optical magnetoelectric (ME) effect, i.e., the change of optical absorption upon the reversal of the light propagation direction, has been investigated for a polar ferrimagnet ${\mathrm{G}\mathrm{a}\mathrm{F}\mathrm{e}\mathrm{O}}_3$. For dipole- and spin-forbidden $d-d$ transition bands located at 1.2–2.3 eV, a clear signal of the optical ME effect ($\Delta \alpha t\sim 3\times {10}^{-3}$) is observed with an applied magnetic field as low as 500 Oe and a sample thickness ($t$) of $50\mu \mathrm{m}$. The observation of a large ME effect in the present compound suggests a possible route to magnification of this novel phenomenon for application.