# Resonant properties of composite structures consisting of several resonant diffraction gratings

## TL;DR

This paper investigates the resonant optical properties of composite structures with multiple diffraction gratings, revealing multiple transmittance zeros, bound states in the continuum, and applications in optical filtering and sensing.

## Contribution

It introduces a theoretical and numerical analysis of multiple zeros and BICs in composite gratings, enabling precise control of optical resonances for advanced photonic devices.

## Key findings

- Multiple transmittance zeros of order N in N-grating structures
- Formation of (N-1)-degenerate bound states in the continuum
- Ability to create nearly rectangular, subnanometer width transmittance peaks

## Abstract

We theoretically and numerically investigate resonant optical properties of composite structures consisting of several subwavelength resonant diffraction gratings separated by homogeneous layers. Using the scattering matrix formalism, we demonstrate that the composite structure comprising N gratings has a multiple transmittance zero of the order N. We show that at the distance between the gratings satisfying the Fabry-P\'erot resonance condition, an (N-1)-degenerate bound state in the continuum (BIC) is formed. The results of rigorous numerical simulations fully confirm the theoretically predicted formation of multiple zeros and BICs in the composite structures. Near the BICs, an effect very similar to the electromagnetically induced transparency is observed. We show that by proper choice of the thicknesses of the layers separating the gratings, nearly rectangular reflectance or transmittance peaks with steep slopes and virtually no sidelobes can be obtained. In particular, one of the presented examples demonstrates the possibility of obtaining an approximately rectangular transmittance peak having a significantly subnanometer width. The presented results may find application in the design of optical filters, sensors and devices for optical differentiation and transformation of optical signals.

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Source: https://tomesphere.com/paper/1907.02315