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

Xiangdong Guo, Chenchen Wu, Shu Zhang, Debo Hu, Shunping Zhang, Qiao Jiang, Xiaokang Dai, Yu Duan, Xiaoxia Yang, Zhipei Sun, Shuang Zhang, Hongxing Xu and Qing Dai

Nature Communications 14, 2532 (2023)
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., α-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 α-MoO3, which can be further tuned in the heterostructures based on α-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.