Single-Crystal Colloidal Multilayers of Controlled Thickness

TLDR

Materials with submicrometer spatial variations in dielectric constant exhibit diffractive optical properties useful for many applications. The authors fabricate planar, up to 50 µm thick, close‑packed silica‑air colloidal multilayers by vertical deposition of monodisperse silica spheres, and verify ordering and uniformity with SEM and optical measurements. Transmission spectra at normal incidence reveal distinct diffraction peaks in the visible and near‑infrared, confirming the expected optical behavior of the multilayers.

Abstract

Materials whose dielectric constant varies spatially with submicrometer periodicity exhibit diffractive optical properties which are potentially valuable in a number of existing and emerging applications. Here, such systems are fabricated by exploiting the spontaneous crystallization of monodisperse silica spheres into close-packed arrays. By reliance on a vertical deposition technique to pack the spherical colloids into close-packed silica−air arrays, high quality samples can be prepared with thicknesses up to 50 μm. These samples are planar and thus suitable for optical characterization. Scanning electron microscopy (SEM) of these materials illustrates the close-packed ordering of the spherical colloids in planes parallel to the substrate; cross-sectional SEM micrographs of the arrays as well as optical methods are used to measure sample thickness and uniformity. Normal-incidence transmission spectra in the visible and near-infrared regions show distinct peaks due to diffraction from the colloidal layer...