Near-field thermal emission in low-dimensional epsilon-near-zero materials

Thermal and blackbody radiations are tightly entangled with the material’s quantum properties for any finite temperature above 0 K. Such radiation is incoherent, both spatially and temporally. However, this characteristic change abruptly in the optical near-field region where it is related to the local density of photonic states (LDOS) [1]. Near-field thermal emission experiences huge amplification close to the epsilon-near-zero (ENZ) spectral region, i.e. at those specific LDOS phonon resonances where the material’s complex dielectric permittivity ε approaches zero. It may lead to narrow-band emission, directionality and coherence properties. Here, we investigate the near-field of anisotropic two-dimensional ENZ materials (Hexagonal boron nitride and α-Molybdenum Trioxide) with Synchrotron Infrared Nano-Spectroscopy and s-SNOM imaging using quantum cascade lasers. Theoretical and numerical investigations confirm the observed enhanced oscillating behaviour of the LDOS around the ENZ frequency. Tuning of the emission properties of 2D ENZ material via external control of the temperature is also demonstrated using a setup for nano-imaging at low temperature.