Randomness in self-assembled colloidal crystals can widen photonic band gaps through particle shape and internal structure

TL;DR

This study uses simulations to show that randomness in particle arrangement and shape in self-assembled colloidal crystals can enhance photonic band gaps, challenging the idea that order always maximizes these gaps.

Contribution

It demonstrates that particle shape and internal structure can be engineered to improve photonic band gaps in self-assembled systems despite inherent randomness.

Findings

Largest band gaps do not always occur at maximum order.

Square cross section rods can have larger TM band gaps than perfect lattices.

Hollow rods can significantly increase TE mode band gaps.

Abstract

Using computer simulations, we explore how thermal noise-induced randomness in a self-assembled photonic crystal affects its photonic band gaps (PBGs). We consider a two-dimensional photonic crystal comprised of a self-assembled array of parallel dielectric hard rods of infinite length with circular or square cross section. We find the PBGs can exist over a large range of intermediate packing densities. Counterintuitively, the largest band gap does not always appear at the packing density where the crystal is most ordered, despite the randomness inherent in any self-assembled structure. For rods with square cross section at intermediate packing densities, we find that the transverse magnetic (TM) band gap of the self-assembled (i.e. thermal) system can be larger than that of identical rods arranged in a perfect square lattice. By considering hollow rods, we find the band gap of transverse electric (TE) modes can be substantially increased while that of TM modes show no obvious improvement over solid rods. Our study suggests that particle shape and internal structure can be used to engineer the PBG of a self-assembled system despite the positional and orientational randomness arising from thermal noise.