Brilliant colors originating from weak reflections of an ill-defined inverse opal structure and its application as a humidity sensor

Abstract

Ethiopian opal exhibits higher transparency and more vivid structural colors as compared to other opal varieties; however, the underlying mechanisms of these unique optical properties remain largely unexplained. This study analyzed the microstructure of Ethiopian opal, revealing that its vivid coloration results from the superposition of extremely weak reflections originating from a subtly defined inverse opal structure with submicron-scale periodicity. Regions of relatively high refractive indices exist in the cement parts between the gaps of arrayed submicron colloidal particles, creating highly transmissive and weakly reflective interfaces unique to this opal architecture. Interference across thousands of these weakly reflecting layers produces a sharp reflection spectrum with a narrow half-width. The colloidal particles were found to be comprised of silica nanograins approximately 20 nm in size and contain pores of about 6 nm as gaps among the nanograins. Because the refractive index of these particles shifts upon the condensation of water in the nanopores, the structural color exhibits a sensitive response to variations in humidity, demonstrating the potential of the Ethiopian opal architecture as a humidity sensor.