Thermal near-field: spectroscopy of the resonant enhancement of the local electromagnetic density of states

One of the most universal physical processes shared by all matter at finite temperature is the emission of thermal radiation.~Associated with the well understood far-field radiation and its spectral characteristics, recent theoretical work has shown that the corresponding near-field can exhibit distinct spectral, spatial, resonant, and coherence properties. The electromagnetic local density of states (EM-LDOS) is a fundamental quantity determining these properties. We demonstrate the technique of thermal infrared near-field spectroscopy (TINS) to characterize the unique spatial and spectral properties of the thermal near-field by scattering with a nanoscale probe. In particular, we discuss the observed vibrational and phonon-resonant enhancement and relationship with the underlying EM-LDOS. We also demonstrate the sensitivity of the emitted spectra to local dielectric environment, tip-sample coupling, and antenna effects of the scanning probe tip.