Observation of sub-Bragg diffraction of waves in crystals

TL;DR

This study reveals a new diffraction phenomenon called sub-Bragg diffraction in crystals, showing it occurs at frequencies below the standard Bragg condition and affects wave propagation in various lattice types.

Contribution

The paper identifies and demonstrates sub-Bragg diffraction as a new diffraction condition in crystals, expanding understanding of wave behavior in different lattice symmetries.

Findings

Sub-Bragg diffraction occurs at frequencies below the first-order Bragg condition.

Reflectance spectroscopy confirms the existence of sub-Bragg diffraction in 2D photonic crystals.

Implications for wave propagation in multiple 2D and 3D Bravais lattices.

Abstract

We investigate the diffraction conditions and associated formation of stopgaps for waves in crystals with different Bravais lattices. We identify a prominent stopgap in high-symmetry directions that occurs at a frequency below the ubiquitous first-order Bragg condition. This sub-Bragg diffraction condition is demonstrated by reflectance spectroscopy on two-dimensional photonic crystals with a centred rectangular lattice, revealing prominent diffraction peaks for both the sub-Bragg and first-order Bragg condition. These results have implications for wave propagation in 2 of the 5 two-dimensional Bravais lattices and 7 out of 14 three-dimensional Bravais lattices, such as centred rectangular, triangular, hexagonal and body-centred cubic.