Controlling Cherenkov threshold with nonlocality

TL;DR

This paper demonstrates theoretically that nonlocal effects in hyperbolic metamaterials can be used to engineer and lower Cherenkov radiation thresholds, enabling new applications in free-electron radiation sources.

Contribution

It reveals how nonlocality introduces a lower-bound velocity cutoff for Cherenkov radiation in layered metamaterials, a novel insight into threshold control.

Findings

Nonlocality sets a lower velocity threshold comparable to electron Fermi velocity.

Threshold can be significantly reduced near epsilon-near-zero frequencies.

Control over Cherenkov thresholds enables new free-electron radiation applications.

Abstract

Cherenkov radiation is generally believed to be threshold-free in hyperbolic metamaterials owing to the extremely large photonic density of states in classical local framework. While recent advances in nonlocal and quantum effects extend our understanding of light-matter interactions in metallic nanostructures, the influence of nonlocality on threshold-free Cherenkov radiation still remains elusive. Here we theoretically demonstrate that the nonlocality provides an indispensable way to flexibly engineer Cherenkov thresholds in metallodielectric layered structures. Particularly, the nonlocality results in a lower-bound velocity cutoff, whose value is comparable to the electron Fermi velocity. Surprisingly, this lower-bound threshold can be significantly smaller than the classically predicted one if the metamaterial works around epsilon-near-zero frequencies. The capability to control Cherenkov thresholds opens numerous prospects for practical applications of Cherenkov radiation, in particular, for integrated free-electron radiation sources.