Split Cherenkov radiation in isotropic chiral matter

TL;DR

This paper investigates electromagnetic radiation modifications in isotropic chiral matter with a time-varying axion field, revealing stable Cherenkov radiation with a split cone structure that could enable circularly polarized THz light sources.

Contribution

It introduces a Green's function method to analyze radiation in chiral matter, demonstrating Cherenkov cone splitting and stability despite imaginary wave modes.

Findings

Cherenkov cone splits into two with opposite circular polarizations for n>1

System remains stable with no exponentially growing fields at large times

Potential to generate circularly polarized THz radiation

Abstract

Chiral matter exhibits unique electromagnetic responses due to the macroscopic manifestation of the chiral anomaly as anomalous transport currents. Here, we study the modification of electromagnetic radiation in isotropic chiral matter characterized by an axion coupling that varies linearly over time $\theta(t) = b_0 t$. Using Carroll-Field-Jackiw electrodynamics, we derive the causal Green's function to investigate the stability and radiation properties of the system. Even though the plane-wave modes of isotropic chiral matter exhibit imaginary frequencies for long wavelengths, which might suggest instability in the system, we show that their contribution is confined to the near-field region. Also we find no exponentially growing fields at arbitrarily large times, so that stability is preserved. Under these conditions the radiation yields a positive energy flux, although this is not an inherent property of the general definition. In the case of a fast-moving charge, we confirm the existence of vacuum Cherenkov radiation and show that, for refractive indices $n > 1$, the Cherenkov cone can split into two concentric cones with opposite circular polarizations. This split, governed by the speed of the particle $v$, $n$ and $b_0$, resembles the optical spin-Hall effect and offers potential applications for creating circularly polarized terahertz (THz) light sources. Our Green's function approach provides a general method for analyzing radiation in chiral matter, from Weyl semimetals to quark-gluon plasmas, and can be extended to systems such as oscillating dipoles and accelerated charges.