Non-local resonant metasurface enhanced spectral filtering

A metasurface is a quasi-two-dimensional micro-nano structure with subwavelength scale thickness. Owing to their compactness and the ability to integrate multiple functions, metasurfaces are seen as an excellent platform for optical field manipulation and photonrelated experiments. In this thesis the focus of the research is the emerging concept of nonlocal resonant metasurfaces. Appearing in array form, they demonstrate narrowband wavelength-selective control of the optical field through collective non-local resonant modes. Two important parameters are spectral and polarisation control, and these are the main topics of study in the thesis. The ability to manipulate the spectral and polarisation selective properties of metasurfaces has applications in sensing and several novel devices in the visible and infrared wavelength range are proposed and demonstrated. In the visible spectral range, a novel plasmonic metasurface filter with a periodic dimericaperture hole structure on a metal film is proposed. It integrates colour-dependent filtering and polarisation-dependent transmission, enabling two functions simultaneously. Simulations show that the structure has improved transmittance compared to a conventional monomeric-aperture structure, exceeding 30% in the blue, green, and red channels, and achieves an orthogonal polarisation extinction ratio greater than 1000. Fabricated devices made by electron-beam lithography and dry etching exhibited approximately 30% transmittance in red, green and blue channels, a filtering linewidth of about 100 nm, and an average polarisation extinction ratio exceeding 20. A polarisation–colour palette was made to observe simultaneous modulation of colour and polarisation, and a colour-gamut diagram was plotted to show vivid colours with more than 76% sRGB coverage. The simultaneous colour–polarisation modulation was further applied to polarisation-dependent encrypted displays and the reproduction of micro-structured artwork. In the infrared spectral range, a novel narrowband filter based on quasi–bound states in the continuum (q-BIC) is proposed. The device is composed of periodic nano-disks with offcentre holes along orthogonal axes to realise dual symmetry-breaking perturbations. The device therefore enables resonance-related linear-to-linear cross-polarisation coupling. Unidirectional transmission enhancement is achieved by tuning the coupling between eigenmodes through adjusting the size of the holes, showing a transmission exceeding the theoretical limit value of 25% derived from temporal coupled mode theory for single-layer, single-excitation metasurface. By extracting the cross-polarisation transmission channel, narrowband filtering is realised in transmission mode. Multipolar decomposition and mode- coupling related simulations explained the operating mechanism of the device. Devices fabricated using electron-beam lithography and dry-etching processes showed 30 % crosspolarisation conversion efficiency in Fourier-transform infrared microscope measurements. The experiments also exhibited a resonance linewidth of less than 60 nm at centre wavelength around 3500nm, corresponding to a quality-factor (Q) of approximately 60, which is an order of magnitude higher than previous experimental reports of free-space infrared filters based on plasmonic structures. The fabricated device was further applied to the reconstruction of infrared gas spectra. Experiments demonstrated real-time, in situ, spectral reconstruction of the semi-quantitative infrared absorption spectrum of butane gas, showing good agreement with the NIST ground-truth dataset. A novel flat-band q-BIC metasurface working in the infrared spectral range is also proposed. High Q resonances are often accompanied by high sensitivity to the incident angle that is a limitation of non-local metasurfaces. In this study, band-structure engineering has been demonstrated through simulations. Using Brillouin-zone folding and symmetry breaking, strong coupling can be achieved between two modes that are not previously coupled because of a symmetry mismatch. The anti-crossing of the bands, because of the strong-coupling effect, adjusts the line shape of the band on which the mode lies in momentum space, thereby realising band-structure control hence exhibits a flat-band characteristic within an incidentangle range of approximately 15°. This new result presents opportunities for future experimental studies.