Deep subwavelength topological edge state in a hyperbolic medium.

Topological photonics offers the opportunity to control light propagation in a way that is robust from fabrication disorders and imperfections. However, experimental demonstrations have remained on the order of the vacuum wavelength. Theoretical proposals have shown topological edge states that can propagate robustly while embracing deep subwavelength confinement that defies diffraction limits. Here we show the experimental proof of these deep subwavelength topological edge states by implementing periodic modulation of hyperbolic phonon polaritons within a van der Waals heterostructure composed of isotopically pure hexagonal boron nitride flakes on patterned gold films. The topological edge state is confined in a subdiffraction volume of 0.021 µm3, which is four orders of magnitude smaller than the free-space excitation wavelength volume used to probe the system, while maintaining the resonance quality factor above 100. This finding can be directly extended to and hybridized with other van der Waals materials to broadened operational frequency ranges, streamline integration of diverse polaritonic materials, and compatibility with electronic and excitonic systems.

High-quality nanocavities through multimodal confinement of hyperbolic polaritons in hexagonal boron nitride.

Compressing light into nanocavities substantially enhances light-matter interactions, which has been a major driver for nanostructured materials research. However, extreme confinement generally comes at the cost of absorption and low resonator quality factors. Here we suggest an alternative optical multimodal confinement mechanism, unlocking the potential of hyperbolic phonon polaritons in isotopically pure hexagonal boron nitride. We produce deep-subwavelength cavities and demonstrate several orders of magnitude improvement in confinement, with estimated Purcell factors exceeding 108 and quality factors in the 50-480 range, values approaching the intrinsic quality factor of hexagonal boron nitride polaritons. Intriguingly, the quality factors we obtain exceed the maximum predicted by impedance-mismatch considerations, indicating that confinement is boosted by higher-order modes. We expect that our multimodal approach to nanoscale polariton manipulation will have far-reaching implications for ultrastrong light-matter interactions, mid-infrared nonlinear optics and nanoscale sensors.

Transverse Hypercrystals Formed by Periodically Modulated Phonon Polaritons.

Photonic crystals and metamaterials are two overarching paradigms for manipulating light. By combining these approaches, hypercrystals can be created, which are hyperbolic dispersion metamaterials that undergo periodic modulation and mix photonic-crystal-like aspects with hyperbolic dispersion physics. Despite several attempts, there has been limited experimental realization of hypercrystals due to technical and design constraints. In this work, hypercrystals with nanoscale lattice constants ranging from 25 to 160 nm were created. The Bloch modes of these crystals were then measured directly using scattering near-field microscopy. The dispersion of the Bloch modes was extracted from the frequency dependence of the Bloch modes, revealing a clear switch from positive to negative group velocity. Furthermore, spectral features specific to hypercrystals were observed in the form of sharp density of states peaks, which are a result of intermodal coupling and should not appear in ordinary polaritonic crystals with an equivalent geometry. These findings are in agreement with theoretical predictions that even simple lattices can exhibit a rich hypercrystal bandstructure. This work is of both fundamental and practical interest, providing insight into nanoscale light-matter interactions and the potential to manipulate the optical density of states.