# Optical pump-rejection filter based on silicon sub-wavelength engineered photonic structures

## TL;DR

This paper introduces a novel silicon-based Bragg filter using sub-wavelength index engineering, enabling narrow-band optical rejection with simplified fabrication and improved performance over traditional designs.

## Contribution

It presents the first implementation of SOI Bragg filters with sub-wavelength index engineering and double periodicity, simplifying fabrication and enhancing rejection bandwidth.

## Key findings

- Achieved a 1.1 nm rejection bandwidth with over 40 dB extinction ratio.
- Demonstrated a ten-fold increase in width compared to conventional filters.
- Implemented a single-etch process for fabrication.

## Abstract

The high index contrast of the silicon-on-insulator (SOI) platform allows the realization of ultra-compact photonic circuits. However, this high contrast hinders the implementation of narrow-band Bragg filters. These typically require corrugations widths of a few nanometers or double-etch geometries, hampering device fabrication. Here we report, for the first time, on the realization of SOI Bragg filters based on sub-wavelength index engineering in a differential corrugation width configuration. The proposed double periodicity structure allows narrow-band rejection with a single etch step and relaxed width constraints. Based on this concept, we experimentally demonstrate a single-etch, $\mathbf{220\,nm}$ thick, Si Bragg filter featuring a corrugation width of $\mathbf{150\,nm}$, a rejection bandwidth of $\mathbf{1.1\,nm}$ and an extinction ratio exceeding $\mathbf{40\,dB}$. This represents a ten-fold width increase compared to conventional single-periodicity, single-etch counterparts with similar bandwidths.