A review of near-field modulation of hyperbolic phonon polaritons

In practical applications, effective manipulation of polaritons brings great prospect for nanophotonic devices, subwavelength imaging, anomalous refraction and other fields of interest, but the modulation flexibility of polaritons in conventional dielectric materials is relatively low and cannot meet the broad needs of reality becomes a challenging problem. However, phonon polaritons, as a new type of quasiparticle with strong photon-phonon coupling, have stronger light-binding ability, longer lifetime and lower loss than other polaritons, and can play a crucial role in the field of sub wavelength-scale infrared light modulation. In recent years, with the research and reports on two-dimensional van der Waals crystals, dielectric materials capable of hosting hyperbolic phonon polaritons have come into the public eye, and with the support of ultra-high resolution nano-imaging technology, many novel near-field infrared optical phenomena have been explored by various manipulation methods, which greatly enriches the knowledge of polarization excitations. This review starts with the mechanism of hyperbolic phonon polaritons, introducing the concept of phonon polaritons, the dispersion relation and the hyperbolic media (h-BN and a-MoO3) that have recently received much attention. Subsequently, the different propagation properties of hyperbolic phonon polaritons in the above mentioned media and the analysis of near-field imaging under various dimensional modulations, such as changing the surrounding dielectric environment of van der Waals crystals, resonant cavities, in-plane modulation of metallic antennas, etc. , are summarized. Finally, we give an outlook on the study of phonon polaritons. The diverse modulation tools show the rich applications of phonon polaritons, which provide avenues for infrared nanophotonic devices such as nano-imaging, integrated optical circuits, and nano-lenses, and may also lead to more emerging fields in the future.