Plasmonic Anapole Mode in a Mid-Infrared Metasurface with Improved Quality Factor

Anapole presents a promising scheme for obtaining both high quality factor and large near-field enhancement for nanophotonic applications. Due to ohmic loss in metals and parasitic radiative moments, resonant properties of anapole mode are presently still far from the ideal nonradiant mode behaviors especially at visible and infrared frequencies. In this work, we design and demonstrate a plasmonic anapole mode with improved quality (Q) factor in the mid-infrared. By fabricating a metasurface consisting of a metallic split ring resonator array, we measured an anapole resonance at 37.87 THz with a Q factor of 35, which is more than two times of prior reported values in anapole metamaterials at infrared and visible frequencies. The improvement originated from suppression of parasitic magnetic quadrupole induced in the metasurface. Our measured narrowband anapole mode is switchable to a broadband electric dipolar mode by varying the incident polarization. In addition, our measured Q factor of the anapole mode is about one half of the theoretical value, i.e., 88. Spatial size inhomogeneity was shown to be a major factor accounting for the discrepancy. Our findings reveal great potential for improving the resonant properties of anapole, which could be beneficial for metamaterial applications in sensing, infrared camouflaging, and wavelength-selective devices, etc.