# Reconfigurable Topological Photonic Crystal

## TL;DR

This paper demonstrates a reconfigurable topological photonic crystal where liquid crystals are used to dynamically control edge states without losing topological protection, enabling robust and tunable photonic devices.

## Contribution

It introduces a method to dynamically reconfigure topological photonic edge states using liquid crystals, maintaining topological order while allowing spectral control.

## Key findings

- Refractive index tuning shifts the spectral position of edge states.
- Topological protection is preserved during reconfiguration.
- Potential for integration into silicon-based photonic systems.

## Abstract

Topological insulators are materials that conduct on the surface and insulate in their interior due to non-trivial topological order. The edge states on the interface between topological (non-trivial) and conventional (trivial) insulators are topologically protected from scattering due to structural defects and disorders. Recently, it was shown that photonic crystals can serve as a platform for realizing a scatter-free propagation of light waves. In conventional photonic crystals, imperfections, structural disorders, and surface roughness lead to significant losses. The breakthrough in overcoming these problems is likely to come from the synergy of the topological photonic crystals and silicon-based photonics technology that enables high integration density, lossless propagation, and immunity to fabrication imperfections. For many applications, reconfigurability and capability to control the propagation of these non-trivial photonic edge states is essential. One way to facilitate such dynamic control is to use liquid crystals, which allow to modify the refractive index with external electric field. Here, we demonstrate dynamic control of topological edge states by modifying the refractive index of a liquid crystal background medium. Background index is changed depending on the orientation of a liquid crystal, while preserving the topological order of the system. This results in a change of the spectral position of the photonic bandgap and the topologically protected edge states. The proposed concept might be implemented using conventional semiconductor technology, and can be used for robust energy transport in integrated photonic devices, all-optical circuity, and optical communication systems.