Integrated flat-top reflection filters operating near bound states in the continuum

TL;DR

This paper introduces integrated flat-top reflection filters based on bound states in the continuum (BICs) on dielectric waveguides, achieving narrowband, high-quality spectral filtering with customizable profiles and narrow bandwidths.

Contribution

It presents a novel design of integrated filters using dielectric ridges near BICs, enabling flat-top reflectance profiles and high-order Butterworth filter approximations without increasing device size.

Findings

Flat-top reflectance profiles with steep slopes achieved with few ridges.

Two ridges can implement second-order Butterworth filter.

Narrower filtering bands possible due to BICs without size increase.

Abstract

We propose and theoretically and numerically investigate narrowband integrated filters consisting of identical resonant dielectric ridges on the surface of a single-mode dielectric slab waveguide. The proposed composite structures operate near a bound state in the continuum (BIC) and enable spectral filtering of transverse-electric-polarized guided modes propagating in the waveguide. We demonstrate that by proper choice of the distances between the ridges, flat-top reflectance profiles with steep slopes and virtually no sidelobes can be obtained using just a few ridges. In particular, the structure consisting of two ridges can optically implement the second-order Butterworth filter, whereas at a larger number of ridges, excellent approximations to higher-order Butterworth filters can be achieved. Owing to the BIC supported by the ridges constituting the composite structure, the flat-top reflection band can be made arbitrarily narrow without increasing the structure size. In addition to the filtering properties, the investigated structures support another type of BICs - Fabry-P\'erot BICs arising when the distances between the adjacent ridges meet the Fabry-P\'erot resonance condition. In the vicinity of the Fabry-P\'erot BICs, an effect similar to the electromagnetically induced transparency is observed, namely, sharp transmittance peaks against the background of a wide transmittance dip.