Structural tailoring of nanoporous anodic alumina optical microcavities for enhanced resonant recirculation of light

Abstract

A comprehensive study about the structural engineering of high quality nanoporous anodic alumina optical microcavities (NAA-μCVs) fabricated by rationally designed anodisation strategies to enhance the light-confining capabilities of these photonic crystal (PC) structures is presented. Two types of NAA-μCV architectures are produced: (i) GIF-NAA-μCVs composed of a cavity layer featuring straight nanopores that is sandwiched between two gradient-index filters (GIFs) with sinusoidally modulated porosity in depth, and (ii) DBR-NAA-μCVs formed by sandwiching a cavity layer with straight nanopores between two distributed Bragg reflectors (DBRs), in which the porosity is engineered in a stepwise fashion. The geometric features of GIF-NAA-μCVs and DBR-NAA-μCVs are engineered and optimised through a systematic modification of the anodisation parameters (i.e. cavity anodisation time, cavity anodisation current density, anodisation period and number of anodisation pulses, and pore widening time). This methodology enables fine-tuning of the optical properties of GIF-NAA-μCVs and DBR-NAA-μCVs, such as quality factor and position and width of resonance band, to generate NAA-μCVs with unprecedented quality factors (i.e. 170 ± 8 and 206 ± 10 for the first and second order resonance bands - threefold and fourfold quality enhancement as compared to previous studies). Our results demonstrate that an optimal design of the geometric features and the nanoporous architecture of NAA-μCVs can significantly enhance resonant recirculation of light within these PC structures, creating new opportunities to develop ultrasensitive optical platforms, highly selective optical filters, and other photonic devices.

Revisiting anodic alumina templates: from fabrication to applications

“…Both types of microcavities offer great versatility in the way that resonant peaks and stop bands are tuned across the UV-visible and near-infrared spectra. Moreover, the specifically added resonant peaks are narrower and the energy transmission of such peaks is enhanced compared to the actual photonic stop bands of the distributed Bragg reflectors, thus the performance (higher sensitivity and quality factor Q ref) of such photonic structures is improved 16,363,364 . Combining the improved photonic response of the aforementioned, type II microcavities, along with the intrinsic dependence of Bragg reflectors to the effective medium authors developed a humidity sensor 363 .…”

“…To the right, illustration establishing the correlation between the profile of the applied current curve and the final pore geometry of the AAO membrane. Reprinted with permission from reference 364 Such gradient-index filters have proved to be useful for sensing applications 16,368,369 . Several studies show that engineering of the porous alumina structure can improve the sensibility to certain molecules (i.e detention of different concentration levels of Dglucose, as the sensibility of the GIF structure towards changes of the refractive index, were enhanced, 4.93 nm/M, the detention limit was lower down to 0.01 M 368 .…”

Realization of high-quality optical nanoporous gradient-index filters by optimal combination of anodization conditions

“…Recent developments in pulse anodization technology have aimed at improving light control quality of NAA-PCs. Optimal structural engineering of NAA-µCVs has shown that these NAA-PCs can achieve outstanding light-confining capabilities, with resonance bands achieving unprecedented quality factors as high as 113 49 and 170 50 . A variety of sinusoidal pulse anodization strategies have been explored to improve the quality of light control in NAA-GIFs, including implementation of apodization functions into anodization profiles 51 and application of anodizing voltage as a function of optical path length 52 .…”

Rational Management of Photons for Enhanced Photocatalysis in Structurally-Colored Nanoporous Anodic Alumina Photonic Crystals

“…4 NAA-PCs have distinctive PSBs, the features of which can be readily tuned across the spectral regions by engineering the effective medium of NAA in a multi-dimensional fashion, through various anodization approaches. 45,[47][48][49][50][51][52] These NAA-PC platforms include optical microcavities 47,48 , distributed Bragg reflectors 5 (DBR) 45,49 , linear variable bandpass filters 50 , gradient-index filters 51 , and hybrid PCs 52 . These PC structures provide new opportunities to rationally utilize the "slow photon" effect to attain an efficient management of photons at the nanoscale for photocatalytic applications.…”