Ultra‐Directional Transition Radiation From Deep‐Subwavelength Epsilon‐Near‐Zero Metamaterials

Abstract Transition radiation occurs whenever free electrons penetrate an optical sample and offers a powerful route to create light emission for arbitrary frequencies, which is crucial to numerous interdisciplinary applications. However, this type of light emission generally has a broad angular distribution and weak intensity, especially when the sample thickness is subwavelength. Accordingly, ultra‐thick samples are required in practical applications and fundamentally inhibit enticing on‐chip applications. Here, a mechanism is theoretically revealed to create ultra‐directional transition radiation with enhanced intensity from epsilon‐near‐zero metamaterials with a deep‐subwavelength thickness, down to one hundredth of the light wavelength. The underlying mechanism lies in the extreme photonic density of states that can couple to free space within a predefined angular range through the judicious design of metamaterials’ anisotropy. Specifically, by tailoring one component of metamaterials’ relative permittivity to be near zero and the other much larger than unity, the radiation peak can exhibit a narrow angular spread with full width at half maximum much smaller than one degree, along with a three‐order‐of‐magnitude enhancement in intensity. Moreover, this ultra‐high directionality is robust to variations in electron velocity and can be achieved by low‐energy electrons. The finding may pave the way toward the development of novel on‐chip photonic devices.