Filling in the gaps: The nature of light transmission through solvent-filled inverse opal photonic crystals

Abstract

Understanding the nature of light transmission and the photonic bandgap in inverse opal photonic crystals is essential for linking their optical characteristics to any application. This is especially important when these structures are examined in liquids or solvents. Knowledge of the true correlation between the nature of the inverse opal (IO) photonic bandgap, their structure, and the theories that describe their optical spectra is surprisingly limited compared to colloidal opals or more classical photonic crystal structures. We examined TiO2 and SnO2 IOs in a range of common solvents to solve the conflict between Bragg-Snell theory, optical and physical measurements by a comprehensive angle-resolved light transmission study coupled to microscopy examination of the IO structure. Tuning the position of the photonic bandgap and index contrast by solvent infiltration of each inverse opal requires a modification to the Bragg-Snell theory and the photonic crystal unit cell definition. We also demonstrate experimentally and theroetically that low fill factors are caused by less desne material infilling all interstitial vancancies in the opal template to form an IO. By also including an optical interference condition for inverse opals with an effective refractive index greater than its substrate, and an alternative internal refraction angle in the substrate, angle-resolved transmission spectra for inverse opals are now consistent with physical measurements. This work now allows an accurate correlation between the true response of an IO to the index contrast with a solvent, how an IO is infilled, and the directionality and bandwidth of the photonic bandgap. As control in functional photonic materials becomes more prevalent outside of optics and photonics, such as biosensing and energy storage, for example, a comprehensive and consistent correlation between photonic crystals structures and their primary optical signatures is a fundamental requirement for application.

Many Facets of Photonic Crystals: From Optics and Sensors to Energy Storage and Photocatalysis

“…The wavelength position of these spectral features is strongly linked to the refractive index of the surrounding medium, through evanescent waves of the optical cavity interacting with the neighboring material of the fiber. [125] and TiO 2 [137] IO red-shift continually with the addition of higher refractive index solvents. Adapted with permission.…”

“…The photonic stopbands of SiO 2 , ZrO 2 , TiO 2 , SnO 2 , CeO 2, and Al 2 O 3 IOs have exhibited a predictable and reversible shift of the principle diffraction peak upon the addition of various solvents to occupy voids in the structures. [ 80,124,125,137,138 ] The refractive index of the solvent and the Bragg‐Snell relation proved to function as an excellent predictor of the wavelength shift observed for the photonic stopband. The transmission dips associated with the stopband red‐shifted to longer wavelengths with increasing solvent refractive index.…”

“…Adapted with permission. [ 137 ] Copyright 2020, American Physical Society. b) Design of a TiO 2 IO ethanol sensor where the shift in stopband can be related to ethanol concentration in ethanol/water mixtures.…”

“…Several of these sensing applications have been highlighted throughout this review. PhC sensors have been developed for identification of solvents, [125,137] vapors, [153] oils, [148] ions, [163] biomolecules [162,166] and nerve agents [165] to name just a few. The photonic bandgap of the PhC material defines the sensing response of the structure with systems calibrated to interpret changes to the photonic bandgap position to changes in concentration or structure.…”

“…Applications of the photonic bandgap in solvent and vapor sensing. a) The optical stopbands for a γ-Al 2 O 3[125] and TiO 2[137] IO red-shift continually with the addition of higher refractive index solvents. Adapted with permission [125].…”