Probing near-field thermal radiation of van der Waals thin films at room temperature

Near-field thermal radiation (NFTR) contains important information on the dielectric and charge transport properties of materials but occurs only extremely close to the surface. Scattering thermal infrared near-field microscopy enables the probing of spectral contributions and addresses limitations of current near-field thermal probing techniques, which only measure total energy flux. However, studies on spectral NFTR remain hindered due to the challenges of experiments. Here, we report the NFTR measurements at a wavelength of 14.1 µm on van der Waals WTe 2 thin films under thermal equilibrium, addressing the discrepancy between prior experiments conducted under nonequilibrium conditions and the thermal equilibrium-based theory. Despite its metallic nature at room temperature, we found that NFTR of WTe 2 at an off-resonant frequency is governed by low-energy interband transitions between electron and hole pockets rather than by free carriers or phonons. Furthermore, we revealed an unexplored hybrid NFTR that is substrate- and thickness-dependent and demonstrated significant enhancement on thin films. This previously unrecognized hybridization of electromagnetic modes between the film and the substrate dominates the detected signal via tip-mediated scattering in the extreme near field, in good agreement with fluctuational electrodynamics calculations. Our work advances the fundamental understanding of NFTR and paves the way for leveraging high-sensitivity thermal spectroscopy for probing the surface properties of low-dimensional materials.