Far Field excitation of Polaritons in Photonic Hypercrystals of Van der Waals Materials

This study delves into the realm of in-plane Hyperbolic Phonon polaritons (HPhPs), intriguing quasiparticles arising from the strong interaction between photons and optical phonons in hyperbolic materials. While these hold immense promise in various domains such as sensing, thermal emission, and high-resolution imaging, their technological application has been hampered by a substantial momentum mismatch between photons and in-plane HPhPs. Previous experimental demonstrations have primarily relied on intricate and costly near-field detection schemes.<br/><br/>In this work, we present a pioneering approach, exemplified by α−MoO₃, showcasing the potential of constructing photonic hypercrystals. This innovative concept not only enables the far-field excitation of in-plane HPhPs but also offers the ability to fine-tune the far-field response by introducing controlled twists in the hypercrystal lattice relative to the lattice of α−MoO₃. These findings represent a significant stride forward, promising the development of practical in-plane HPhP devices while also providing access to novel fundamental physics of such materials via conventional and well-established far-field measurement techniques.<br/><br/>The study systematically explores biaxial van der Waals (vdW) crystals, focusing on α-MoO₃, wherein HPhPs exhibit pronounced sensitivity to the crystal's basal plane direction. The isofrequency contours in these cases take on a hyperbolic geometry, a stark departure from the circular contours observed in uniaxial vdW materials like h-BN. This biaxial hyperbolicity in vdW crystals opens up unprecedented opportunities for manipulating and tailoring infrared light waves and energy flow at the nanoscale, offering intriguing optical phenomena such as negative refraction, topological transitions, light canalization, and twist photonics.<br/><br/>The research confronts the longstanding challenge of momentum mismatch for far-field excitation of phonon polaritons. Previous studies have proposed patterning the vdW crystal with various materials to surmount this limitation. This work, however, introduces the concept of a photonic hypercrystal, involving periodic structural modulations in a hyperbolic material. By employing α-MoO₃ as a model system, we demonstrate the feasibility of far-field excitation of in-plane HPhPs, showcasing a new dimension of control over light-matter interactions.<br/><br/>In essence, this study represents a transformative advancement in the understanding and utilization of in-plane hyperbolic vdW crystals. The results not only deepen our comprehension of the intricate interplay between light and these hypercrystals but also lay the foundation for the practical realization of phonon polariton-based devices. These findings hold broad implications, extending from the realm of nanophotonics to the burgeoning field of twist photonics, opening up a new chapter in the exploration of advanced photonic technologies.