Achromatic interfacial Cherenkov radiation from photonic crystals

Interfacial Cherenkov radiation originates from the interaction of charged particles with sequential interfaces inside photonic crystals and is crucial to many applications, ranging from particle detectors and lasers to electron microscopy and spectroscopy, since it can create directional light emission at arbitrary spectra. However, the interfacial Cherenkov radiation suffers from chromatic issues and is featured with frequency-sensitive radiation angles due to the intrinsic structural dispersion of photonic crystals. Counterintuitively, here we reveal a universal mechanism to create the broadband achromatic interfacial Cherenkov radiation from photonic crystals. Our mechanism exploits the anomalous Maxwell-Garnett theory beyond the long-wavelength limit via the Brewster effect and introduces it for the first time into the context of Cherenkov radiation. Specifically, we present photonic crystal designs that mimic homogeneous achromatic media in terms of their interfacial Cherenkov radiation, emitted precisely at the Brewster angle. We find the radiation peak angle could be frequency-insensitive, even over the entire visible spectrum.