Broadband Achromatic Anomalous Mirror in Near-IR and Visible Frequency Ranges

An anomalous achromatic mirror operating in the near-IR and visible frequency ranges was designed using an array of metal−insulator−metal (MIM) resonators. An incident wave interacting with the MIM resonator experiences phase shift that is equal to the optical path traveled by the gap plasmon excited by the wave. A phase gradient along the mirror surface is created through the difference in plasmon optical paths in the resonators of varying lengths. In a frequency region well below the plasma frequency of metal, the phase gradient is a linear function of frequency, and thus the mirror operates in the achromatic regime; that is, the reflection angle does not depend on the radiation frequency. Using silver−air−silver resonators, we predicted that the mirror can steer a normally incident beam to angles as large as 40° with high radiation efficiency (exceeding 98%) and small Joule losses (below 10%). Our study indicates that it is feasible to create an efficient broadband anomalous mirror.