Thermal emission from finite photonic crystals

We present a microscopic theory of thermal emission from truncated photonic crystals and show that spectral emissivity and related quantities can be evaluated via standard bandstructure computations without any approximation. We then analyze the origin of thermal radiation enhancement and suppression inside photonic crystals and demonstrate that the central quantity that determines the thermal radiation characteristics such as intensity and emissive power is the area of the iso-frequency surfaces and not the density of states as is generally assumed. We also identify the physical mechanisms through which interfaces modify the potentially super-Planckian radiation flow inside infinite photonic crystals, such that thermal emission from finite-sized samples is consistent with the fundamental limits set by Planck's law. As an application, we further demonstrate that a judicious choice of a photonic crystal's surface termination facilitates considerable control over both the spectral and angular thermal emission properties. Finally, we outline design principles that allow the maximization of the radiation flux, including effects associated with the isotropy of the effective Brillouin zone, photonic band gap size and flatness of the band structure in the spectral range of interest.