Shuhao Wu

Metasurfaces supporting high quality factor resonances have shown the ability to enhance spontaneous parametric down-conversion (SPDC) process and are therefore seen as a promising platform for entangled quantum photon pair generation. We propose a high quality (Q) factor flat-band optical metasurface for efficient quantum light generation via SPDC. The structure consisting of periodic GaAs nano-resonators on SiO ₂ was numerically optimised to realise a metasurface that is dispersionless along the Γ–Y axis near the telecommunications band (∼1550 nm). Our simulations show that the metasurface hosts two quasi–bound-states-in-the-continuum (q-BIC) modes with quality factors exceeding 10 ⁹ ; the intentional merging of the BICs results in high-Q (>10 ⁵ ) across a finite in-plane wave-vector range. This combination of angle-independent resonance frequency with persistently high Q mitigates the off-Γ degradation typical of symmetry-broken q-BICs. The design increases the optical density of states and allows for efficient collection of the generated photons using a high numerical aperture. It is anticipated that the resulting devices could be used for both quantum photon pair generation and for classical nonlinear second-harmonic generation. Our results demonstrate flat-band, high-Q metasurfaces that can be fabricated as a compact route to on-chip nonlinear and quantum photonics.

Metasurfaces supporting high‐quality‐factor quasi‐bound states in the continuum (q‐BIC) modes typically operate in reflection mode. By using the chiral circular polarization conversion property, q‐BIC can be applied to transmission‐mode wavefront control. However, in applications involving broadband mid‐infrared spectral filtering, it is challenging to generate broadband circularly polarized light owing to the absence of achromatic waveplates in MIR. In this work, a metasurface that supports linear‐to‐linear cross‐polarization conversion using dual symmetry‐breaking perturbations is experimentally demonstrated. The new device is experimentally shown to have a polarization conversion efficiency of 30% that exceeds the theoretical limit of 25% for a single‐layer, single‐resonance, thin metasurface. The improved efficiency is achieved via inter‐mode coupling that is supported by the dual perturbation metasurface. A Q‐factor of ≈60 is obtained, enabling narrowband spectral filtering using q‐BICs in transmission mode. The narrowband filtering and low angular dispersion of the incident light of the new metasurface are exploited to demonstrate imaging‐based spectroscopy, including the reconstruction and real‐time monitoring of butane gas absorption spectra and polarization‐encrypted spectrally selective imaging.

Complex color and polarization selective technologies are of increasing importance in scientific, security, and commercial imaging applications. A new dimeric plasmonic filter structure based on periodic aperture arrays is reported to provide an effective method for making planar color‐selective structures by exploiting the properties of extraordinary optical transmission in thin metal films. The visible band transmission‐mode polarization‐dependent color filters reported in this work exploit only a single layer of aluminum patterned using a hexagonally periodic dimer‐ellipse aperture structure. It is shown experimentally that the structure exhibits a minimum extinction ratio of over 20, 100, and 150 for red, green, and blue channels respectively, and a peak transmission of over 30%. It is demonstrated that dual images can be encoded using polarization selectivity into a single structure. The fidelity of the method is demonstrated with micro‐scale reproductions of complex artworks showing the ability to reproduce 76% of the sRGB color gamut with polarization selectivity. The structure can be readily fabricated with only a single‐step lithography and etching process, so that the technique may be widely used.