| Summary: | Cherenkov radiation is generally believed to be threshold-free in hyperbolicmetamaterials owing to the extremely large photonic density of states in classical localframework. Although recent advances in nonlocal and quantum plasmonics extend ourunderstanding of light-matter interactions in metamaterials, how such effects influenceCherenkov radiation in hyperbolic metamaterials still remains unknown. Here, it isdemonstrated that effects of nonlocality add a new degree of freedom to engineerCherenkov thresholds in hyperbolic metamaterials. The interplay between finite structuraldimensions and nonlocal nature of metallic electrons results in a nonzero Cherenkovthreshold. Counterintuitively, such nonlocality-induced Cherenkov threshold can besignificantly smaller than the classically predicted one if the metamaterial is designed to workaround the epsilon-near-zero frequency. This phenomenon is attributed to the excitation oflongitudinal plasmon modes which are absent in classical electromagnetic framework. Thesefindings apply to a general class of hyperbolic materials, including metallodielectric layeredstructures, nanorod/nanoribbon arrays, hyperbolic van der Waals crystals, etc.
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