Bifunctional gap-plasmon metasurfaces for visible light: polarization-controlled unidirectional surface plasmon excitation and beam steering at normal incidence

TLDR: The design, fabricate and experimentally demonstrate bifunctional gap-plasmon metasurfaces for visible light, allowing for simultaneous polarization-controlled unidirectional surface plasmon polariton (SPP) excitation and beam steering at normal incidence.

Abstract: Integration of multiple diversified functionalities into a single, planar and ultra-compact device has become an emerging research area with fascinating possibilities for realization of very dense integration and miniaturization in photonics that requires addressing formidable challenges, particularly for operation in the visible range. Here we design, fabricate and experimentally demonstrate bifunctional gap-plasmon metasurfaces for visible light, allowing for simultaneous polarization-controlled unidirectional surface plasmon polariton (SPP) excitation and beam steering at normal incidence. The designed bifunctional metasurfaces, consisting of anisotropic gap-plasmon resonator arrays, produce two different linear phase gradients along the same direction for respective linear polarizations of incident light, resulting in distinctly different functionalities realized by the same metasurface. The proof-of-concept fabricated metasurfaces exhibit efficient (>25% on average) unidirectional (extinction ratio >20 dB) SPP excitation within the wavelength range of 600–650 nm when illuminated with normally incident light polarized in the direction of the phase gradient. At the same time, broadband (580–700 nm) beam steering (30.6°–37.9°) is realized when normally incident light is polarized perpendicularly to the phase gradient direction. The bifunctional metasurfaces developed in this study can enable advanced research and applications related to other distinct functionalities for photonics integration. Arrays that convert visible light into surface plasmon polaritons and provide broadband beam steering can help miniaturize optical circuits. Optical metasurfaces, thin-layer devices with subwavelength-scale patterns, offer extraordinary control over light in ultra-compact packages. Now, Fei Ding and co-workers from the University of Southern Denmark have built and tested a metasurface that can perform two distinct functions simply by switching light polarization. The proof-ofconcept device uses customized arrays of tiny silver ‘nanobricks’ to create linear phase gradients that propagate along a single direction, but with different characteristics for orthogonal polarizations. The team reports an average coupling efficiency of more than 25% for unidirectional surface plasmon polariton excitation with an x-polarized beam. Concurrent beam steering of light wavelengths between 580 to 700 nanometers was achieved with y-polarization, with low crosstalk between functionalities.