  Electronic transport in highly doped but still insulating silicon at low
temperatures is dominated by hopping between localized states; it serves as a
model system of a disordered solid for which the electronic interaction can be
investigated. We have studied the frequency-dependent conductivity of
phosphorus-doped silicon in the THz frequency range (30 GHz to 3 THz) at low
temperatures $T\geq 1.8$ K. The crossover in the optical conductivity from a
linear to a quadratic frequency dependence as predicted by Efros and Shklovskii
is observed qualitatively; however, the simple model does not lead to a
quantitative agreement. Covering a large range of donor concentration, our
temperature- and frequency-dependent investigations reveal that electronic
correlation effects between the localized states play an important and complex
role at low temperatures. In particular we find a super-linear frequency
dependence of the conductivity that highlights the influence of the density of
states, i.e. the Coulomb gap, on the optical conductivity. When approaching the
metal-to-insulator transition by increasing doping concentration, the
dielectric constant and the localization length exhibit critical behavior.
