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

Abstract

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.

Enhancing light emission near low-refractive-index contrast nanostructures via mirror-symmetry breaking

“…Additionally, with geometrical parameter variation in resonant gratings based on transparent materials such as silicon nitride (unlike in silicon), the resonance can be suitably positioned across the visible and near IR wavelengths to suit applications. This contribution builds on our previous reports using a mirror-symmetry broken grating geometry for refractometric sensing application 38 , 39 . Compared with simple 1D gratings, our study predicts a considerable improvement in radiative emission enhancement.…”

“…Both these operations preserve the mirror symmetry of the grating (the mirror-symmetry planes are at the centers of ribs and/or gap between the ribs), which precludes the normally-incident plane wave excitation of specific grating modes having odd parity. Previously, 38,39 we have reported a dimerization process that simultaneously perturbs the gap and width, resulting in the w þ g− DLCG. This process breaks the mirror symmetry and allows the excitation of odd-parity modes of the grating with normal incident light.…”

“…Broadly, all these cases are seen to exhibit transmittance dips which correspond to the resonant excitation of modes propagating along the grating (as predicted for the case of infinite-sized gratings in Refs. 38 and 39).…”

“…This contribution builds on our previous reports using a mirrorsymmetry broken grating geometry for refractometric sensing application. 38,39 Compared with simple 1D gratings, our study predicts a considerable improvement in radiative emission enhancement. Furthermore, the enhancement factors are seen over a larger fraction of the surrounding volume.…”

“…We can do incoherent source modeling by using a single emitter source coupled with a finite-sized grating (applying open boundary conditions in all directions) 22 . In our previous work, 39 we studied the optical response of finite mirror-symmetry broken gratings by applying absorbing boundary conditions on all axes. It was concluded that the resonances approached the periodic case when the grating width was >20 periods for finite structures.…”