Stacked nanoporous anodic alumina gradient-index filters with tunable multispectral photonic stopbands as sensing platforms

This study presents the development and optical engineering of stacked nanoporous anodic alumina gradient-index (NAA-GIFs) filters with tunable multispectral photonic stopbands for sensing applications. The structure of these photonic crystals (PC) is formed by stacked layers of NAA produced with sinusoidally modified effective medium. The progressive modification of the sinusoidal period during the anodization process enables the generation and precise tuning of the characteristic photonic stopbands (PSB) (i.e., one per sinusoidal period in the anodization profile) of these PC structures. Four types of NAA-GIFs featuring three distinctive PSBs positioned within the visible spectral region are developed. The sensitivity of the effective medium of these NAA-GIFs is systematically assessed by measuring spectral shifts in the characteristic PSBs upon infiltration of their nanoporous structure with analytical solutions of d-glucose with several concentrations (0.025–1 M). This study provides new insights into the intrinsic relationship between the nanoporous architecture of these PCs and their optical properties, generating opportunities to fabricate advanced optical sensing systems for high-throughput and multiplexed detection of analytes in a single sensing platform.Laura K. Acosta, Francesc Bertó-Roselló, Elisabet Xifre-Perez, Abel Santos, Josep Ferré-Borrull, and Lluis F. Marsa

3D-FDTD modelling of optical biosensing based on gold-coated nanoporous anodic alumina

The suitability of using gold-coated nanoporous anodic alumina structures as a platform for reflectometry-based plasmonic biosensors is investigated by numerical simulation. Reflectance spectra of such structures has been obtained using 3D-FDTD while the sensing capabilities have been evaluated as the change in spectra upon the adsorption of a layer of a biological-related molecule (biolayer) on the gold coating and inner pore surface. Results show that the gold-coated nanoporous structure enables the coupling of normally incident light to a localized surface plasmon resonance, and that such resonance shifts upon the adsorption of the biolayer. A sensitivity can be defined as the resonance wavelength shift with the biolayer refractive index. It is demonstrated that smaller gold coating thicknesses result in an increase in sensitivity, but at the cost of a decrease in the resonance sharpness, what suggests the existence of an optimal gold coating thickness. Keywords: FDTD simulation, Gold-coated nanoporous anodic alumina, Reflectometry-based plasmonic biosensor, Biolaye

Tunable nanoporous anodic alumina photonic crystals by Gaussian pulse anodization

This study presents a Gaussian pulse anodization approach to generate nanoporous photonic crystals with highly tunable and controllable optical properties across the visible-NIR spectrum. Nanoporous anodic alumina Gaussian photonic crystals (NAA-GPCs) are fabricated in oxalic acid electrolyte by Gaussian pulse anodization, a novel form of pulse-like anodization. The effect of the Gaussian pulse width in the anodization profile on the optical properties of these photonic crystals is assessed by systematically varying this fabrication parameter from 5 to 60 s. The optical features of the characteristic photonic stopband (PSB) of NAA-GPCs-the position of the central wavelength, full width at half-maximum, and intensity-are found to be highly dependent on the Gaussian pulse width, the angle of incidence of incoming photons, and the nanopore diameter of NAA-GPCs. The effective medium of NAA-GPCs is assessed by monitoring spectral shifts in their characteristic PSB upon infiltration of their nanoporous structure with analytical solutions of d-glucose of varying concentration (0.0125-1 M). Experimental results are validated and mechanistically described by theoretical simulations, using the Looyenga-Landau-Lifshitz effective medium approximation model. Our findings demonstrate that Gaussian pulse anodization is an effective nanofabrication approach to producing highly sensitive NAA-based PC structures with versatile and tunable PSBs across the spectral regions. The findings provide new exiting opportunities to integrate these unique PC structures into photonic sensors and other platform materials for light-based technologies.Laura K. Acosta, Francesc Bertó-Roselló, Elisabet Xifre-Perez, Cheryl Suwen Law, Abel Santos, Josep Ferré-Borrull, and Lluis F. Marsa

Tailor-engineered plasmonic single-lattices: harnessing localized surface plasmon resonances for visible-NIR light-enhanced photocatalysis

A platform material composed of 2D gold (Au) nanodot plasmonic single-lattices (Au-nD-PSLs) featuring tailor-engineered geometric features for visible-NIR light-driven enhanced photocatalysis is presented. Au-nD-PSLs efficiently harness incident visible-NIR electromagnetic waves to accelerate photo-chemical reactions by localized surface plasmon resonance (LSPR) effects. Au-nD-PSLs are fabricated by a straightforward, industrially scalable template-assisted approach, using nanopatterned aluminum substrates as templates. The method overcomes the constraints of direct writing lithography and allows Au-nD-PSLs to be transferred to arbitrary functional flexible substrates. Triangular lattice Au-nD-PSLs feature tunable and controllable characteristic LSPR bands across the visible spectrum. Strongly localized electromagnetic fields around Au-nD-PSLs are responsible for the outstanding photocatalytic performance of these plasmonic nanostructures, as demonstrated by finite-difference time-domain simulations and experimental observations. Our approach of rational engineering of LSPR effects in Au-nD-PSLs provides exciting opportunities to develop high-performing and reusable photocatalysts that harvest the visible-NIR spectrum for a broad range of optoelectronic and plasmonic applications.Siew Yee Lim, Cheryl Suwen Law, Francesc Bertó-Roselló, Lina Liu, Marijana Markovic, Josep Ferré-Borrull, Andrew D. Abell, Nicolas H. Voelcker, Lluís F. Marsal and Abel Santo