Mid-infrared analogue polaritonic reversed Cherenkov radiation in natural anisotropic crystals

Abstract

Here, the authors demonstrate an analogue reversed Cherenkov radiation at mid-infrared frequencies in MoO3, a natural hyperbolic material, and show that the radiation angle and the quality factor can be increased by stacking hBN layers on the MoO3 surface.Cherenkov radiation (CR) excited by fast charges can serve as on-chip light sources with a nanoscale footprint and broad frequency range. The reversed CR, which usually occurs in media with the negative refractive index or negative group-velocity dispersion, is highly desired because it can effectively separate the radiated light from fast charges thanks to the obtuse radiation angle. However, reversed CR at the mid-infrared remains challenging due to the significant loss of conventional artificial structures. Here we observe mid-infrared analogue polaritonic reversed CR in a natural van der Waals (vdW) material (i.e., & alpha;-MoO3), whose hyperbolic phonon polaritons exhibit negative group velocity. Further, the real-space image results of analogue polaritonic reversed CR indicate that the radiation distributions and angles are closely related to the in-plane isofrequency contours of & alpha;-MoO3, which can be further tuned in the heterostructures based on & alpha;-MoO3. This work demonstrates that natural vdW heterostructures can be used as a promising platform of reversed CR to design on-chip mid-infrared nano-light sources.