The Fluctuation Induced Pseudogap in the Infrared Optical Conductivity of High Temperature Superconductors

We study the effect of fluctuations on the {\bf ac} conductivity of a layered superconductor both for $c$-axis and $ab$-plane electromagnetic wave polarizations. The fluctuation contributions of different physical nature and signs (paraconductivity, Maki-Thompson anomalous contribution, one-electron density of states renormalization) are found to be suppressed by the external field at different characterisitic frequencies ($ \omega_{\rm AL}\sim T-T_{\rm c}$, $\omega_{\rm MT} \sim \max\{ T-T_{\rm c}, \tau_{\varphi}^{-1}\}$, $ \omega_{\rm DOS} \sim \min\{T,\tau ^{-1}\}$ for the $2D$ case). As a result the appearance of the nonmonotonic frequency dependence (pseudogap) in the infrared optical conductivity of HTS film is predicted. The effect has to be especially pronounced in the case of the electromagnetic field polarization along $c$-axis.