Rational engineering of nanoporous anodic alumina optical bandpass filters

Abstract

Herein, we present a rationally designed advanced nanofabrication approach aiming at producing a new type of optical bandpass filters based on nanoporous anodic alumina photonic crystals. The photonic stop band of nanoporous anodic alumina (NAA) is engineered in depth by means of a pseudo-stepwise pulse anodisation (PSPA) approach consisting in pseudostepwise asymmetric current density pulses. This nanofabrication method makes it possible to tune the transmission bands of NAA at specific wavelengths and bandwidths, which can be broadly modified across the UV-visible-NIR spectrum through the anodisation period (i.e. time between consecutive pulses). First, we establish the effect of the anodisation period as a means of tuning the position and width of the transmission bands of NAA across the UV -visible-NIR spectrum. To this end, a set of nanoporous anodic alumina bandpass filters (NAA-BPFs) are produced with different anodisation periods, ranging from 500 to 1200 s, and their optical properties (i.e. characteristic transmission bands and interferometric colours) are systematically assessed. Then, we demonstrate that the rational combination of stacked NAABPFs consisting of layers of NAA produced with different PSPA periods can be readily used to create a set of unique and highly selective optical bandpass filters with characteristic transmission bands, the position, width and number of which can be precisely engineered by this rational anodisation approach. Finally, as a proof-of-concept, we demonstrate that the superposition of stacked NAA-BPFs produced with slight modifications of the anodisation period enables the fabrication of NAA-BPFs with unprecedented broad transmission bands across the UV-visible-NIR spectrum. The results obtained from our study constitute the first comprehensive rationale towards advanced NAA-BPFs with fully controllable photonic properties. These photonic crystal structures could become a promising alternative to traditional optical bandpass filters based on glass and plastic.

On the Precise Tuning of Optical Filtering Features in Nanoporous Anodic Alumina Distributed Bragg Reflectors

“…This top-down nanofabrication approach offers industrial scalability (from mm 2 to m 2 ), cost-competitiveness, and versatile control over the features of nanopores, which can be modulated with precision by means of the anodization parameters to generate unique multi-dimensional PC structures able to guide, reflect, modulate, confine, transmit, emit, and enhance incident light selectively across the spectral regions 17 , 18 . Recent studies have demonstrated that rationally designed pulse-like anodization profiles under suitable conditions enable the precise engineering of the photonic stopband (PSB) of NAA-PCs by creating structures such as gradient index filters 19 – 22 , optical microcavities 23 , distributed Bragg reflectors 24 – 28 , bandpass and linear variable bandpass filters 29 , 30 . These PCs can be used as optical filters for a plethora of applications due to the flexibility and selectively to engineer their light-filtering features across the spectral regions 31 , 32 .…”

Nanoporous Anodic Alumina Photonic Crystals for Optical Chemo- and Biosensing: Fundamentals, Advances, and Perspectives

“…The optical spectrum of these NAA-PCs has a characteristic PSB when light flows transversally through the NAA-FPIs’ structure, which is established by the interpore distance (i.e., lattice constant) and porosity (i.e., nanopore diameter) [ 46 , 47 , 48 , 49 ]. NAA-BPFs are PC structures that allow the transmission of a specific portion of the light spectrum in a selective manner while impeding the pass of light of all other wavelengths [ 79 , 86 ]. NAA-BPFs can be classified into three categories according to the range of allowed wavelengths: (i) longpass filters, which allow the transmission of light of long wavelengths, (ii) shortpass filters, which allow the pass of light of short wavelengths, and (iii) bandpass filters, which allow the transmission of a band of wavelengths while blocking the pass of light of shorter and longer wavelengths.…”

Light-Confining Nanoporous Anodic Alumina Microcavities by Apodized Stepwise Pulse Anodization

“…As demonstrated in previous studies, this blue shift is associated with the reduction of the anodization period, which results in a shorter period length (L TP ) within the nanoporous structure of NAA-µCVs. [43][44][45][46][47][48][49][50][51][52][53][54][55][56][57][58][59] The transmission spectra of these NAA-µCVs displays a PSB that increases its intensity with t pw and a well-defined resonance band at the center of the PSB. The quality factor (Q C ), defined as the ratio of the resonance band wavelength (λ R ) to its full width at half maximum (FWHM R ) (Equation 5), is an important criteria in assessing the strength of photon confinement within optical microcavities.…”