Nanoring color filters based on Fabry-Perot cavities

Coaxial nanoring structures have attracted extensive attention in recent years due to their peculiar optical properties. In this article, we investigate two different types of resonances in plasmonic Fabry-Perot cavities, planar surface plasmon and propagating surface plasmon. Using nanoring arrays with the same periodicity and different gaps, we can tune propagating surface plasmons and finally filter individual colors out. With large periodicities, planar surface plasmon resonance can be fixed in the near infrared range to avoid any disturbance on propagating surface plasmon resonance which is located in visible frequencies. In this work, we filter a broadband white source into different colors by using nanoring arrays with a fixed periodicity of 1200 nm and varying gaps range from 10 nm to 180 nm (in steps of 10 nm). Compared with one-dimensional nano slits or metal-insulator-metal (MIM) nanogratings, nanoring structures present polarization independence to the incident light, leading to more functional devices and broader applications (applicable to natural light, for instance). Finite-difference time-domain (FDTD) simulations accord well with measurements, which confirms our conclusions and supports our explanations.