Manipulating Coherence of Near-Field Thermal Radiation in Time-Modulated Systems

We show that the spatial coherence of thermal radiation can be manipulated in time-modulated photonic systems supporting surface polaritons. We develop a fluctuational electrodynamics formalism for such systems to calculate the cross-spectral density tensor of the emitted thermal electromagnetic fields in the near-field regime. Our calculations indicate that, due to time-modulation, spatial coherence can be transferred between different frequencies, and correlations between different frequency components become possible. All these effects are unique to time-modulated systems. We also show that the decay rate of optical emitters can be controlled in the proximity of such time-modulated structure. Our findings open a promising avenue toward coherence control in thermal radiation, dynamical thermal imaging, manipulating energy transfer among thermal or optical emitters, efficient near-field radiative cooling, and engineering spontaneous emission rates of molecules.