Fabrication of self-ordered nanoporous alumina for optical and structural characterization

Presentem la fabricació, caracterització estructural i òptica de membranes d'alúmina nanoporosa autoordenada. Les mostres han estat fabricades per el procés de doble anodització. La morfologia de les membranes obtinguda mitjançant les diferents condicions d'anodització ha estat examinada amb un microscopi electrònic d'escombrat per tal d'estimar les seves propietats geomètriques. Les propietats estructurals estudiades mitjançant difracció de raigs X mostren que durant el procés de recuit termic, des de 600 ºC fins a 1200 ºC, l'alúmina nanoporosa ha cristal·litzat des de la fase amorfa fins a les formes gamma i alfa (cristal·lines). L'espectre de la transmissió òptica de les membranes d'alúmina porosa autoordenada va ser mesurat mitjançant un espectròmetre UV-VIS en el rang 300 - 1000 nmWe present the fabrication and optical and structural characterization of self-ordered nanoporous alumina membranes. The samples were fabricated using a two-step anodization process. The morphology of the membranes produced using different anodizing conditions was examined with a scanning electron microscope in order to estimate their geometrical properties. The structural properties studied by X-ray diffraction showed that nanoporous alumina is crystallized from amorphous phase to gamma and alpha (crystalline) forms during annealing at 600 ºC to 1200 ºC. The optical transmission spectra of the membranes were measured using a UV-VIS spectrometer in the range of 300 - 1000 nm

Photonic stop bands in quasi-random nanoporous anodic alumina structures

The existence of photonic stop bands in the self-assembled arrangement of pores in porous anodic alumina structures is investigated by means of rigorous 2D finite- difference time-domain calculations. Self-assembled porous anodic alumina shows a random distribution of domains, each of them with a very definite triangular pattern, constituting a quasi-random structure. The observed stop bands are similar to those of photonic quasicrystals or random structures. As the pores of nanoporous anodic alumina can be infiltrated with noble metals, nonlinear or active media, it makes this material very attractive and cost-effective for applications including inhibition of spontaneous emission, random lasing, LEDs and biosensors

Generation of Tamm Plasmon Resonances for Light Confinement Applications in Narrowband Gradient-Index Filters Based on Nanoporous Anodic Alumina

Gold-coated gradient-index filters based on nanoporous anodic alumina (Au-coated NAA−GIFs) were used as model platforms to elucidate how Tamm plasmons can be tailored by engineering the geometric features of the plasmonic and photonic components of these hybrid structures. NAA−GIFs with well-resolved, intense photonic stopbands at two positions of the visible spectrum were fabricated through sinusoidal pulse anodization. These model photonic crystals were used to assess how the quality of Tamm plasmon resonances can be enhanced by tuning the features of the dielectric mirror and the thickness of the porous gold coating layer. It is found that the highest value of the quality factor of Tamm resonance (QTamm = 237) is obtained for 11 nm of gold on a dielectric mirror with low porosity corresponding to the resonant spectral position of λTamm of ∼698 nm. Our analysis indicates that Tamm resonances in asproduced Au-coated NAA−GIFs are weak due to the constrained range of wavelengths (narrow bands) at which these photonic crystal structures reflect light. However, after broadening of their photonic stopband upon pore widening, Tamm resonances become better resolved, with higher intensity. It is also observed that the quality of light confinement worsens progressively with the thickness of the porous gold coating layer after a critical value. In contrast to conventional surface plasmon resonance systems, this hybrid Tamm porous system does not require complex coupling systems and provides a nanoporous structure that can be readily tailored for a range of photonic technologies such as sensing and lasing.Alejandro Rojas Gómez, Laura K. Acosta, Josep Ferré-Borrull, Abel Santos, and Lluis F. Marsa

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

Tunable Fabry-Perot interferometer based on nanoporous anodic alumina for optical biosensing purposes

Here, we present a systematic study about the effect of the pore length and its diameter on the specular reflection in nanoporous anodic alumina. As we demonstrate, the specular reflection can be controlled at will by structural tuning (i.e., by designing the pore geometry). This makes it possible to produce a wide range of Fabry-Pérot interferometers based on nanoporous anodic alumina, which are envisaged for developing smart and accurate optical sensors in such research fields as biotechnology and medicine. Additionally, to systematize the responsiveness to external changes in optical sensors based on nanoporous anodic alumina, we put forward a barcode system based on the oscillations in the specular reflection.Abel Santos, Victor S Balderrama, María Alba, Pilar Formentín, Josep Ferré-Borrull, Josep Pallarès, Lluís F Marsa

Structural tuning of photoluminescence in nanoporous anodic alumina by hard anodization in oxalic and malonic acids

We report on an exhaustive and systematic study about the photoluminescent properties of nanoporous anodic alumina membranes fabricated by the one-step anodization process under hard conditions in oxalic and malonic acids. This optical property is analysed as a function of several parameters (i.e. hard anodization voltage, pore diameter, membrane thickness, annealing temperature and acid electrolyte). This analysis makes it possible to tune the photoluminescent behaviour at will simply by modifying the structural characteristics of these membranes. This structural tuning ability is of special interest in such fields as optoelectronics, in which an accurate design of the basic nanostructures (e.g. microcavities, resonators, filters, supports, etc.) yields the control over their optical properties and, thus, upon the performance of the nanodevices derived from them (biosensors, interferometers, selective filters, etc.).Abel Santos, Maria Alba, Mahbubur M Rahman, Pilar Formentín, Josep Ferré-Borrull, Josep Pallarès, Lluis F Marsa