  Thermal emission is typically associated with a blackbody at a temperature
above absolute zero, which exchanges energy with its environment in the form of
radiation. Blackbody thermal emission is largely incoherent both spatially and
temporally. Using principles in nanophotonics, thermal emission with
characteristics that differ considerably from those of a blackbody have been
demonstrated. In particular, by leveraging intrinsic properties of emerging
materials or via nanostructuring at the wavelength or sub-wavelength scale, one
can gain control over the directionality, temporal coherence, and other more
exotic properties of thermal radiation. Typically, however, these are fixed at
the time of fabrication. Gaining dynamic control of thermal emission requires
exploiting external mechanisms that actively modulate radiative properties.
Numerous applications can benefit from such thermal emission control, for
example in solar energy harvesting, thermo-photovoltaic energy conversion,
radiative cooling, sensing, spectroscopy, imaging and thermal camouflage. In
this tutorial, we introduce thermal emission in two domains: the far-field, and
the near-field, and we outline experimental approaches for probing thermal
radiation in both ranges. We discuss ways for tailoring the spatial and
temporal coherence of thermal emission and present available mechanisms to
actively tune these characteristics.
