Thermal Photonics with Low-Dimensional Materials

The blackbody spectrum for room temperature peaks at mid-infrared (IR) frequencies, making this spectral range of prominent importance for both fundamental science and technology. Examples include mid-IR thermal energy harvesting and radiative cooling, spectroscopy, molecular sensing and detection, thermal camouflage, and mid-IR free-space communications. Despite the relevance of mid-IR photonics, however, phase retardation, the process of rotating light's polarization and converting linear to circularly polarized light is limited to wavelengths up to approximately ten micrometers. Beyond this wavelength, due to the lack of strongly birefringent crystals and the prominent resonances in all polar media due to lattice vibrations (phonons), there exists no commercial phase retarder. In this talk, I will discuss how, by leveraging the strong frequency dispersion of a low-dimensional van der Waals crystal, a-molybdenum trioxide (a-MoO3), we built the first deeply subwavelength phase retarders at mid-IR frequencies, achieving 90 degrees polarization rotation within a micrometer of material.<br/><br/>Using the strong frequency dispersion due to optical phonons in crystals, I will also discuss how once can built fully pattern-free heterostructures that can achieve directional control of thermal emission and absorption at mid-IR frequencies. By considering hexagonal boron nitride (hBN) as a thermal emitter, we theoretically predict thermal emission lobes thare are comparable to those achievable with conventional gratings, yet without any lithography.<br/><br/>Furthermore, I will discuss approaches to leverage the strong anisotropy of several low-dimensional crystals to achieve deep-subwavelength control of optical chirality, without any nanopatterning. Chirality up to 0.8 is estimated with a-MoO3 at mid-IR frequencies. I will discuss our current efforts to measure mid-IR chirality in the laboratory.<br/><br/>Furthermore, mid-IR-relevant low-dimensional materials, such as a-MoO3 or hBN, are typically exfoliated in the laboratory, rather than grown in large scale. Exfoliated flakes are typically too small for mid-IR far-field optical experiments, due to the size mismatch between an IR beam and a flake. This introduces a major challenge: rather than being directly measured experimentally, for example via spectroscopic ellipsometry, the dielectric function of most low-dimensional materials is approximated by numerical fitting in experiments performed in the near-field, via scanning microscopy. I will introduce a robust method for retrieving the mid-IR dielectric properties of exfoliated flakes using far-field optics.