Ultra-high-Q resonances in plasmonic metasurfaces

Resonant cavities play a crucial role in many aspects of science and engineering. A resonator with a large quality-factor (Q-factor) is essential to many applications in photonics, such as filtering, delay lines and memories, lasing, switching, spectroscopy, sensing, and nonlinear optical processes, among others. Recently, the rapid development of nanofabrication technology has increased the appeal of nanostructured metasurfaces. A metasurface with a large Q-factor could be used as a cavity for an ultra-flat nano-laser with a large transverse mode size, or to increase light-matter interactions (e.g., for THz-wave generation, entangled photon-pair generation, or deterministic single-photon sources). However, to date, experimental demonstrations of meta-surface nanoresonators have yet to produce Q-factors in excess of the order of 10^2, even for low-loss dielectric platforms. In this Letter, we report the experimental observation of a metasurface nanoresonator with a Q-factor of 2400 in the telecommunication C band. This value is an order of magnitude higher than the previously reported values for any metasurface platform, and is enabled by surface lattice resonances aided by the proper choice of nanostructure dimension, large array sizes, and the use of a strongly collimated light source. Moreover, it was achieved with an array of plasmonic nanostructures, usually considered too lossy to support high Q-factor resonances due to high optical absorption in metals. Our results demonstrate that surface lattice resonances provide an exciting and unexplored method to achieve ultra-high-Q resonances in metasurfaces, and could pave the way to flexible wavelength-scale devices for any optical resonating application.