  The terahertz (THz) frequency range (0.1-10 THz) fills the gap between the
microwave and optical parts of the electromagnetic spectrum. Recent progress in
the generation and detection of the THz radiation has made it a powerful tool
for fundamental research and resulted in a number of applications. However,
some important components necessary to effectively manipulate THz radiation are
still missing. In particular, active polarization and phase control over a
broad THz band would have major applications in science and technology. It
would, e.g., enable high-speed modulation for wireless communications and
real-time chiral structure spectroscopy of proteins and DNA. In physics, this
technology can be also used to precisely measure very weak Faraday and Kerr
effects, as required, for instance, to probe the electrodynamics of topological
insulators. Phase control of THz radiation has been demonstrated using various
approaches. They depend either on the physical dimensions of the phase plate
(and hence provide a fixed phase shift) or on a mechanically controlled time
delay between optical pulses (and hence prevent fast modulation). Here, we
present data that demonstrate the room temperature giant Faraday effect in HgTe
can be electrically tuned over a wide frequency range (0.1-1 THz). The
principle of operation is based on the field effect in a thin HgTe semimetal
film. These findings together with the low scattering rate in HgTe open a new
approach for high-speed amplitude and phase modulation in the THz frequency
range.
