  We present a comprehensive optical study of the narrow gap $FeSb_2$
semiconductor. From the optical reflectivity, measured from the far infrared up
to the ultraviolet spectral range, we extract the complete absorption spectrum,
represented by the real part $\sigma_1(\omega)$ of the complex optical
conductivity. With decreasing temperature below 80 K, we find a progressive
depletion of $\sigma_1(\omega)$ below $E_g\sim 280$ cm$^{-1}$, the
semiconducting optical gap. The suppressed (Drude) spectral weight within the
gap is transferred at energies $\omega>E_g$ and also partially piles up over a
continuum of excitations extending in the spectral range between zero and
$E_g$. Moreover, the interaction of one phonon mode with this continuum leads
to an asymmetric phonon shape. Even though several analogies between $FeSb_2$
and $FeSi$ were claimed and a Kondo-insulator scenario was also invoked for
both systems, our data on $FeSb_2$ differ in several aspects from those of
$FeSi$. The relevance of our findings with respect to the Kondo insulator
description will be addressed.
