We present data obtained by time-resolved terahertz spectroscopy in 
selected semiconducting and molecular systems exhibiting sub-picosecond
far-infrared dynamics. We use a frequency-domain method which
eliminates the influence of instrumental functions and artifacts due
to frequency mixing and yields a two-dimensional transient conductivity
of the photo-excited sample. This technique enables improving the 
attainable experimental time resolution and allows a simple qualitative
interpretation of the results without a priori modeling. The quantitative
interpretation is based on the time-dependent Drude and damped harmonic
oscillator models.