Thickness-Based Dispersion in Opal Photonic Crystals
Authors: Grant Alex , O'Dwyer Colm 
Published: November 2022

Photonic crystals (PhCs) are types of colloidal crystals which demonstrate a host of exciting optical phenomena, the nature of which can lend insight into their structure and enable a range of potential applications in fields such as optics and optoelectronics,1 energy storage,2-6 and sensors for environmental monitoring, biomedicine and food preservation.7 Artificial opals8 are PhCs which can be formed easily using low-cost self-assembly methods and their structures are composed of highly ordered, close-packed, polymeric sphere arrays. While the optical behaviour of opals has received significant attention over the last number of decades, there is limited information on the effect of crystal thickness on the optical properties they display. Most studies focus their attention on the lattice parameters and refractive index contrast within the materials.9,10

Here, the relationship between volume fraction and crystal thickness using an evaporation-induced self-assembly (EISA) method of formation is established.11-13 The extent to which thickness can be used to manipulate the optical properties of the crystals is explored, focusing on the change in the photonic band gap (PBG). Specifically, the thickness-induced changes in the properties which define the PBG for a range of volume fractions. Through meticulous structural and optical characterization, primarily in the form of scanning electron microscopy (SEM) images and angle-resolved ultraviolet-visible (UV-Vis) spectroscopy, the quality of the crystals formed is examined, with thicknesses exceeding 40 layers grown from volume fractions below 0.15 %, for the first time, to our knowledge. Crystal structure is correlated to optical fingerprint, shown in the form of transmission spectra in the UV-Vis region.
We expect this work to provide our colleagues in the scientific community with a direct correlation between crystal thickness and the optical fingerprint of opal PhCs. Our analysis also stands as a manual to aid any future attempts at the growth of ultra-thick colloidal crystals. Such research may act as a precursor to further developments in materials employed in the photonics, sensing, energy storage and related communities.

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Figure 1. Optical and structural analysis for 350 nm PS opals formed by EISA. Analysis presented in the form of transmission spectra obtained for samples formed from a range of five volume fractions, labelled by their dilution factors (a,d,g), sample plan view SEM images (b,e,f), and sample cross-sectional SEM images (c,f,i). (a-c) correspond to regions of maximum opal thickness, (d-f) to regions of lesser thickness, (g-i) to regions of minimal thickness.

Figure 1