In 1951, physicist Huang Kun formulated the Huang equations, which elucidate the coupling between long-wavelength optical vibrations, macroscopic polarization, and electromagnetic fields. These equations provided the theoretical bedrock for phonon polaritons and became a pivotal reference point for subsequent research. Over seven decades later, with the advent of nanotechnology, a central question has arisen: when phonon polaritons are confined to the nanoscale, what novel behaviors emerge in their light-matter coupling characteristics? And can these behaviors offer vital insights for new technologies and applications?

Phonon polaritons are quasiparticles that arise from the strong coupling of photons and optical phonons in polar crystals. They can compress light fields to scales far below the wavelength, thereby breaking through the traditional optical diffraction limit. At the nanoscale, their light-matter coupling characteristics exhibit a range of novel behaviors:

First, ultra-low loss and extended lifetime. By enriching α-MoO₃ with molybdenum isotopes, the lifetime of phonon polaritons is significantly prolonged, presenting promising candidates for ultra-low-loss polaritonic devices.

Second, dynamic tuning and reconfigurability. Through electrical methods and the construction of heterostructures, dynamic control over the propagation behavior of phonon polaritons can be achieved. For example, utilizing a graphene/α-MoO₃ heterostructure enables topological transformations of the polariton’s equal-frequency dispersion contours. Furthermore, electrical reconstruction of Bloch modes showcases the potential for dynamically tunable nanophotonic devices.

Third, high-sensitivity sensing. Capitalizing on the sensitivity of phonon polaritons to their surrounding environment, highly sensitive detection of picometer-scale deformations becomes feasible. For instance, based on STEM-EELS technology, frequency shifts of phonon polaritons allow for picometer-scale sensing, offering a new approach to detect atomic-scale mechanical behaviors in buried interfaces, two-dimensional material heterostructures, and quantum devices.

These novel behaviors provide crucial insights for advancing fields such as nanophotonics, integrated optoelectronics, and terahertz technology, driving the development of new technologies such as on-chip sensors and ultra-high-speed optical communication modulators.