What is the Brillouin Zone of an Anisotropic Photonic Crystal?

TL;DR

This paper investigates how the Brillouin zone concept applies to anisotropic photonic crystals, revealing that traditional definitions do not hold and proposing an alternative based on Bragg planes, supported by experimental and theoretical analysis.

Contribution

It introduces a new way to define the Brillouin zone in anisotropic photonic crystals using Bragg planes, enhancing understanding of their dispersion properties.

Findings

The lowest bandgap forms inside the BZ in lithium niobate PhC.

The traditional BZ boundary does not align with Bragg planes in anisotropic materials.

Curved Bragg surfaces replace planar Bragg planes in dispersive media.

Abstract

The concept of the Brillouin zone (BZ) in relation to a photonic crystal fabricated in an optically anisotropic material is explored both experimentally and theoretically. In experiment, we used femtosecond laser pulses to excite THz polaritons and image their propagation in lithium niobate and lithium tantalate photonic crystal (PhC) slabs. We directly measured the dispersion relation inside PhCs and observed that the lowest bandgap expected to form at the BZ boundary forms inside the BZ in the anisotropic lithium niobate PhC. Our analysis shows that in an anisotropic material the BZ - defined as the Wigner-Seitz cell in the reciprocal lattice - is no longer bounded by Bragg planes and thus does not conform to the original definition of the BZ by Brillouin. We construct an alternative Brillouin zone defined by Bragg planes and show its utility in identifying features of the dispersion bands. We show that for an anisotropic 2D PhC without dispersion, the Bragg plane BZ can be constructed by applying the Wigner-Seitz method to a stretched or compressed reciprocal lattice. We also show that in the presence of the dispersion in the underlying material or in a slab waveguide, the Bragg planes are generally represented by curved surfaces rather than planes. The concept of constructing a BZ with Bragg planes should prove useful in understanding the formation of dispersion bands in anisotropic PhCs and in selectively tailoring their optical properties.