Fano-resonance-assisted metasurface for color routing

Controlling the phase of an electromagnetic field using plasmonic nanostructures provides a versatile way to manipulate light at the nanoscale.Broadband phase modulation has been demonstrated using inhomogeneous metasurfaces with different geometries;however,for many applications such as filtering,hyperspectral imaging and color holography,narrowband frequency selectivity is a key functionality.In this work,we demonstrate,both theoretically and experimentally,a narrowband metasurface that relies on Fano resonances to control the propagation of light.By geometrically tuning the sub-radiant modes with respect to a fixed super-radiant resonance,we can create a phase modulation along the surface within a narrow spectral range.The resulting anomalous reflection measured for such a Fano-resonant metasurface exhibits a 100 nm bandwidth and a color routing efficiency of up to 81%at a central wavelength ofλ=750 nm.The design flexibility provided by this Fano-assisted metasurface for colorselective light manipulation is further illustrated by demonstrating a highly directional color-routing effect between two channels,atλ=532 and 660 nm,without any crosstalk.

Revisiting Newton’s rings with a plasmonic optical flat for high-accuracy surface inspection

Two parallel optical surfaces often exhibit colorful fringes along the lines of equal thickness because of the interference of light.This simple phenomenon allows one to observe subwavelength corrugations on a reflective surface by simply placing on it a flat reference dielectric surface,a so-called optical flat,and inspecting the resultant interference pattern.In this work,we extend this principle from dielectric surfaces to two-dimensional plasmonic nanostructures.Optical couplings between an Au nanodisk array and an Au thin film were measured quantitatively using two different techniques,namely,the classical Newton’s rings method and a closed-loop nano-positioning system.Extremely high spectral sensitivity to the inter-surface distance was observed in the near-field coupling regime,where a 1-nm change in distance could alter the resonance wavelength by almost 10 nm,440 times greater than the variation in the case without near-field coupling.With the help of a numerical fitting technique,the resonance wavelength could be determined with a precision of 0.03 nm,corresponding to a distance precision as high as 0.003 nm.Utilizing this effect,we demonstrated that a plasmonic nanodisk array can be utilized as a plasmonic optical flat,with which nanometer-deep grooves can be directly visualized using a low-cost microscope.