Taming mid-infrared light-matter interaction with van der Waals nanostructures

Mid-infrared radiation plays a profound role in a variety of applications such as thermal imaging, molecular and bio-sensing, radiative cooling, among others. Here we study light-materials interaction within hexagonal boron nitride (hBN), a layered van der Waals polar dielectric which exhibits intrinsically sharp phonon resonance in mid-infrared wavelength range. Unlike previously reported results which focused on accessing resonances using hBN’s polaritonic features (within Reststrahlen band where material in-plane permittivity ε_⊥< 0), in our study we pay attention to the high material index of refraction outside Reststrahlen band (where ε_⊥> 0), which offers avenues for sustaining all-dielectric modes. By patterning bulk hBN flakes into nano-disk resonator arrays, we demonstrate multiple resonances in the mid-IR range both within and outside Reststrahlen band with Q-factor up to ~90. In addition, by varying the disk radius, we manage to shift the position of resonances to cover an over 1 μm band. Our study reveals that enhanced light-materials interactions can be achieved within hBN at both sides of the hyperbolic transition, allowing access to multiple resonances across a broad wavelength range and thus enabling efficient control and manipulation of mid-infrared radiation and absorption.