  Adoption of terahertz technologies is hindered by the lack of cost-effective
THz sources. Here we demonstrate a fundamentally new way to generate and
control THz radiation, via spatio-temporal emissivity modulation. By patterning
the optical photoexcitation of a surface-passivated silicon wafer, we locally
control the free-electron density, and thereby pattern the wafer's emissivity
in the THz part of the electromagnetic spectrum. We show how this
unconventional source of controllable THz radiation enables a new form of
incoherent computational THz imaging. We use it to image various concealed
objects, demonstrating this scheme has the penetrating capability of
state-of-the-art THz imaging approaches, without the requirement of
femto-second pulsed laser sources. Furthermore, the incoherent nature of
thermal radiation also ensures the obtained images are free of interference
artifacts. Our spatio-temporal emissivity control paves the way towards a new
family of long-wavelength structured illumination, imaging and spectroscopy
systems.
