Mirror-symmetry breaking mitigates finite-size related performance degradation in guided mode resonance filters

Guided mode resonances in subwavelength patterned thin-films endow them with narrow-linewidth near-unity reflectance peaks. Their ultrathin profile is particularly attractive when mated with image sensor arrays that enables compact field-deployable spectral filtering and sensing systems. While this approach enjoys several advantages over other approaches, a well known limitation is the trade-off between the lateral footprint and spectral linewidth. Mirroring strategies involving metallic or distributed Bragg reflectors have been explored in the past to improve lateral confinement at the expense of increased fabrication complexity, footprint, and insertion loss. Here, we numerically study mirrorless grating modification strategies and predict the mitigation of finite-size related performance degradation. Specifically, we consider mirror symmetry broken miniaturized medium refractive index contrast (silicon nitride) gratings, which exhibit quasi bound states in the continuum (QBIC) resonances. For the same lateral footprint, a nearly 2 fold improvement in quality factor is predicted for the proposed design in comparison to a simple grating surrounded by aluminium mirrors. Numerical study of the design and operational performance of visible-wavelength arrayed filters and multiplexed refractive index sensors is presented. For a typical lateral device footprint of 8 μm, the gratings span wavelengths ranging from 560 nm-800 nm with a coupling efficiency of 43-60%, and a full width half maximum (FWHM) of 4 nm-12 nm. Besides this, the proposed geometry gives a four times better figure of merit (FOM) than the unperturbed medium contrast grating in surface refractometric sensing.