Transverse hypercrystals formed by periodically modulated phonon-polaritons

TL;DR

This paper reports the experimental creation and analysis of nanoscale hypercrystals formed by periodically modulated phonon-polaritons, revealing unique dispersion features like negative group velocity and density of states peaks, advancing light manipulation technologies.

Contribution

First experimental realization of hypercrystals with detailed dispersion analysis, demonstrating their complex bandstructure and potential for nanoscale light-matter interaction engineering.

Findings

Observation of negative group velocity indicating bandfolding

Detection of sharp density of states peaks unique to hypercrystals

Validation of theoretical predictions of hypercrystal bandstructure

Abstract

Photonic crystals and metamaterials are two overarching paradigms for manipulating light. Combining the two approaches leads to hypercrystals: hyperbolic dispersion metamaterials that undergo periodic modulation and mix photonic-crystal-like aspects with hyperbolic dispersion physics. So far, there has been limited experimental realization of hypercrystals due to various technical and design constraints. Here, we create nanoscale hypercrystals with lattice constants ranging from 25 nm to 160 nm and measure their collective Bloch modes and dispersion with scattering nearfield microscopy. We demonstrate for the first time dispersion features such as negative group velocity, indicative of bandfolding, and signatures of sharp density of states peaks, expected for hypercrystals (and not for ordinary polaritonic crystals). These density peaks connect our findings to the theoretical prediction of an extremely rich hypercrystal bandstructure emerging even in geometrically simple lattices. These features make hypercrystals both fundamentally interesting, as well as of potential use to engineering nanoscale light-matter interactions.