  Measuring terahertz (THz) conductivity on an ultrafast time scale is an
excellent way to observe charge-carrier dynamics in semiconductors as a
function of time after photoexcitation. However, a conductivity measurement
alone cannot separate the effects of charge-carrier recombination from
effective mass changes as charges cool and experience different regions of the
electronic band structure. Here we present a form of time-resolved magneto-THz
spectroscopy which allows us to measure cyclotron effective mass on a
picosecond time scale. We demonstrate this technique by observing electron
cooling in the technologically-significant narrow-bandgap semiconductor indium
antimonide (InSb). A significant reduction of electron effective mass from
0.032$m_\mathrm{e}$ to 0.017$m_\mathrm{e}$ is observed in the first 200ps after
injecting hot electrons. Measurement of electron effective mass in InSb as a
function of photo-injected electron density agrees well with conduction band
non-parabolicity predictions from ab initio calculations of the quasiparticle
band structure.
