Far field excitation of Polaritons in Photonic Hypercrystals of van der Waals Materials

This study presents a pioneering contribution to the field of polariton physics, focusing on the behavior of hyperbolic phonon polaritons (HPhPs) emerging from the coupling of crystal vibrations and light in biaxial van der Waals (vdW) materials. Unlike uniaxial counterparts, these materials exhibit isofrequency contours with a hyperbolic geometry, offering unprecedented opportunities for nanoscale manipulation of infrared light waves. While prior studies have been constrained to the near-field regime, this research introduces the concept of photonic hypercrystals, showcasing the potential to excite in-plane HPhPs in the far field. By utilizing α-MoO3 as a model material, we demonstrate that periodic structural modulations enable the tuning of far-field responses by twisting the hypercrystal lattice relative to the lattice of α-MoO3. Additionally, employing a cylindrical hole geometry affords an extra degree of freedom for light manipulation. Experimental and simulation results detail the fabrication process and provide a comprehensive analysis of the far-field responses, highlighting the tunability of HPhP resonance frequencies through hypercrystal periodicity adjustments. Moreover, the rotational adjustment of α-MoO₃ hypercrystals allows for the excitation of HPhPs with varying orientations within the crystal's basal plane. This research represents a significant advancement in harnessing the potential of in-plane hyperbolic vdW crystals, with broad implications for the development of advanced photonic devices across nanophotonics and twist photonics applications.