Realisation and optical engineering of linear variable bandpass filters in nanoporous anodic alumina photonic crystals

Abstract

We present the first realisation of linear variable bandpass filters in nanoporous anodic alumina (NAA-LVBPFs) photonic crystal structures. NAA gradient-index filters (NAA-GIFs) are produced by sinusoidal pulse anodisation and used as photonic crystal platforms to generate NAA-LVBPFs. The anodisation period of NAA-GIFs is modified from 650 to 850 s to systematically tune the characteristic photonic stopband of these photonic crystals across the UV-visible-NIR spectrum. Then, the nanoporous structure of NAA-GIFs is gradually widened along the surface under controlled conditions by wet chemical etching using a dip coating approach aiming to create NAA-LVBPFs with finely engineered optical properties. We demonstrate that the characteristic photonic stopband and the iridescent interferometric colour displayed by these photonic crystals can be tuned with precision across the surface of NAA-LVBPFs by adjusting the fabrication and etching conditions. Here, we envisage for the first time the combination of the anodisation period and etching conditions as a cost-competitive, facile, and versatile nanofabrication approach that enables the generation of a broad range of unique LVBPFs covering the spectral regions. These photonic crystal structures open new opportunities for multiple applications, including adaptive optics, hyperspectral imaging, fluorescence diagnostics, spectroscopy, and sensing.

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.…”

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

“…NAA-LVBPFs have a PSB with variable central wavelength, the position of which is shifted across the surface of the filter in a linear fashion. This effect is achieved by a selective etching of the nanoporous structure of NAA-LVBPFs to engineer their effective medium in the perpendicular direction to the nanopores’ growth [ 87 ].…”

“…Different forms of NAA-PCs (e.g., distributed Bragg reflectors, grading-index filters, optical microcavities, Fabry–Pérot interferometers, etc.) can be integrated with various optical techniques such as reflectometric interference spectroscopy (RIfS), reflection and transmission spectroscopy, and photoluminescence spectroscopy (PL) ( Table 2 ) [ 29 , 87 , 230 ]. This section summarizes the most representative examples of chemo- and biosensing systems combining NAA-PC structures with optical techniques.…”