Nanoporous Photonic Crystals Research Articles

Refractive Index-Adaptive Nanoporous Chiral Photonic Crystal Film for Chemical Detection Visualized via Selective Reflection.

Photonic crystals (PC) are of great importance in technology, especially in optics and photonics. In general, the structural color of PCs responds to external stimuli primarily by changing their periodicity. Herein, the authors report on refractive index (RI) adaptive PCs. Cross-linked cholesteric films with interconnected nanopores exhibit a very low RI without light scattering. Transparent PC films with maximum reflectance in the ultravoilet(UV)region respond to various chemicals by changing the reflective color of the PC. The authors demonstrate its unique colorimetric chemical detections of hazardous organic liquids. Loading various chemicals into nanopores significantly shifts the structural color into the visible range depending on the chemical's RI. These results are unique in that the structural color of photonic films is mediated by RI changes rather than periodicity changes. In principle, nanoporous photonic crystal films can detect the RI of a chemical substance by its unique color. In contrast to volumetric changes, this sensing mechanism offers several advantages, including durability, excellent sensitivity, fast response time, and wide detection range. These results provide useful insight into stimulus-responsive PCs. The structural color of PC films can be effectively tuned by adjusting average RIs instead of changing periodicity.

Harnessing Slow Light in Optoelectronically Engineered Nanoporous Photonic Crystals for Visible Light-Enhanced Photocatalysis

Spectrally tunable nanoporous anodic alumina distributed Bragg reflectors (NAA-DBRs) are modified with titanium dioxide (TiO2) coatings via atomic layer deposition and used as model optoelectronic platforms to harness slow light for photocatalysis under visible–NIR illumination. Photocatalytic breakdown of methylene blue (MB) with a visible absorbance band is used as a benchmark reaction to unveil the mechanism of slow light-enhanced photocatalysis in TiO2–NAA-DBRs with a tunable photonic stop band (PSB) and thickness of TiO2. Assessment of the optical arrangement between MB’s absorbance band and the PSB of TiO2–NAA-DBRs is used to identify and quantify slow light contributions in driving this model photocatalytic breakdown reaction. Our findings reveal that photodegradation rates rely on both the spectral position of PSB and thickness of the semiconductor. The performance of these photocatalysts is the maximum when the red edge of the PSB is spectrally close to the red or blue boundary of the MB’s absorbance band and to dramatically decrease within the absorbance maximum of MB due to light screening by dye molecules. It is also demonstrated that TiO2–NAA-DBRs featuring thicker photoactive TiO2 layers can harvest more efficiently incident slow light by generating extra pairs of charge carriers on the semiconductor coating’s surface. The crystallographic phase of TiO2 in the functional coatings is found to be critical in determining the performance of these model photocatalyst platforms, where the anatase phase provides ∼69% higher performance over its amorphous TiO2 form. This study provides opportunities toward the development of energy-efficient photocatalysts for environmental remediation and energy generation and other optoelectronic applications.

Role of Spectral Resonance Features and Surface Chemistry in the Optical Sensitivity of Light-Confining Nanoporous Photonic Crystals

Nanoporous anodic alumina optical microcavities (NAA-μQVs) with spectrally tunable resonance band and surface chemistry are used as model light-confining photonic crystal (PC) platforms to elucidate the combined effect of spectral light confinement features and surface chemistry on optical sensitivity. These model nanoporous PCs show well-resolved, spectrally tunable resonance bands (RBs), the central wavelength of which is engineered from ∼400 to 800 nm by the period of the input anodization profile. The optical sensitivity of the as-produced (hydrophilic) and dichlorodimethylsilane-functionalized (hydrophobic) NAA-μQVs is studied by monitoring dynamic spectral shifts of their RB upon infiltration with organic- and aqueous-based analytical solutions of equally varying refractive index, from 1.333 to 1.345 RIU. Our findings demonstrate that hydrophilic NAA-μQVs show ∼81 and 35% superior sensitivity to their hydrophobic counterparts for organic- and aqueous-based analytical solutions, respectively. Interestingly, the sensitivity of hydrophilic NAA-μQVs per unit of spectral shift is more than 3-fold higher in organic than in aqueous matrices upon equal change of refractive index, with values of 0.347 ± 0.002 and 0.109 ± 0.001 (nm RIU-1) nm-1, respectively. Conversely, hydrophobic NAA-μQVs are found to be slightly more sensitive toward changes of refractive index in aqueous medium, with sensitivities of 0.072 ± 0.002 and 0.066 ± 0.006 (nm RIU-1) nm-1 in water- and organic-based analytical solutions, respectively. Our advances provide insights into critical factors determining optical sensitivity in light-confining nanoporous PC structures, with implications across optical sensing applications, and other photonic technologies.

Precursor Film Spreading during Liquid Imbibition in Nanoporous Photonic Crystals

When a macroscopic droplet spreads, a thin precursor film of liquid moves ahead of the advancing liquid-solid-vapor contact line. Whereas this phenomenon has been explored extensively for planar solid substrates, its presence in nanostructured geometries has barely been studied so far, despite its importance for many natural and technological fluid transport processes. Here we use porous photonic crystals in silicon to resolve by light interferometry capillarity-driven spreading of liquid fronts in pores of few nanometers in radius. Upon spatiotemporal rescaling the fluid profiles collapse on master curves indicating that all imbibition fronts follow a square-root-of-time broadening dynamics. For the simple liquid (glycerol) a sharp front with a widening typical of Lucas-Washburn capillary-rise dynamics in a medium with pore-size distribution occurs. By contrast, for a polymer (PDMS) a precursor film moving ahead of the main menisci entirely alters the nature of the nanoscale transport, in agreement with predictions of computer simulations.

Optical engineering of nanoporous photonic crystals by Gaussian-Like pulse anodization

Abstract A new class of nanoporous anodic alumina photonic crystals (NAA–PCs) are optically engineered by four distinct forms of Gaussian-like pulse anodization (GLPA) – Gaussian, Lorentzian, logarithmic normal, and Laplacian. NAA–PCs produced by GLPA under current density control conditions show well-resolved, spectrally tunable photonic stopbands (PSBs), the features of which – central wavelength, full width at half maximum, intensity, and quality factor – can be precisely tuned across the UV–visible spectrum by the input Gaussian-like current density pulses. Effects of type, width, and period of Gaussian-like current density pulses on the structural and optical properties of NAA–PCs are systematically assessed. Comprehensive electrochemical, structural, and optical characterizations allow us to elucidate the interplay of input GLPA profile and structural features in determining forbidden light propagation within these NAA–PCs. Sensitivity of these PC structures toward changes in their effective medium are determined by quantifying spectral shifts in their characteristic PSB upon infiltration of their nanoporous structure with analytical solutions of alcohol mixtures with varying refractive index, from 1.362 to 1.383 RIU. It is found that, at fixed anodization period, NAA–PCs produced by Laplacian current density pulses achieve the highest sensitivity (105 ± 4 nm RIU−1) and, for a given type of GLPA, NAA–PCs produced with longer anodization period achieve higher sensitivity (123 ± 10 nm RIU−1 for NAA–PCs fabricated by Gaussian pulses with a 1400 s-period). Our advances provide exciting new opportunities for future developments of high-quality NAA–PCs with broad applicability across various photonic technologies, including optical sensing, photocatalysis, lasing, and optical encoding.

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.

Nanoporous photonic crystals with tailored surface chemistry for ionic copper sensing

We present a study on optical and surface chemistry engineering of nanoporous photonic crystals as sensing platforms for detection of ionic copper.

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

Optical sensors are a class of devices that enable the identification and/or quantification of analyte molecules across multiple fields and disciplines such as environmental protection, medical diagnosis, security, food technology, biotechnology, and animal welfare. Nanoporous photonic crystal (PC) structures provide excellent platforms to develop such systems for a plethora of applications since these engineered materials enable precise and versatile control of light–matter interactions at the nanoscale. Nanoporous PCs provide both high sensitivity to monitor in real-time molecular binding events and a nanoporous matrix for selective immobilization of molecules of interest over increased surface areas. Nanoporous anodic alumina (NAA), a nanomaterial long envisaged as a PC, is an outstanding platform material to develop optical sensing systems in combination with multiple photonic technologies. Nanoporous anodic alumina photonic crystals (NAA-PCs) provide a versatile nanoporous structure that can be engineered in a multidimensional fashion to create unique PC sensing platforms such as Fabry–Pérot interferometers, distributed Bragg reflectors, gradient-index filters, optical microcavities, and others. The effective medium of NAA-PCs undergoes changes upon interactions with analyte molecules. These changes modify the NAA-PCs’ spectral fingerprints, which can be readily quantified to develop different sensing systems. This review introduces the fundamental development of NAA-PCs, compiling the most significant advances in the use of these optical materials for chemo- and biosensing applications, with a final prospective outlook about this exciting and dynamic field.

Engineering the Slow Photon Effect in Photoactive Nanoporous Anodic Alumina Gradient-Index Filters for Photocatalysis.

In this study, we explore for the first time the capabilities of nanoporous anodic alumina gradient-index filters (NAA-GIFs) functionalized with titanium dioxide (TiO2) photoactive layers to enhance photon-to-electron conversion rates and improve the efficiency of photocatalytic reactions by "slow photon" effect. A set of NAA-GIFs was fabricated by sinusoidal pulse anodization, in which a systematic modification of various anodization parameters (i.e., pore widening time, anodization period, and anodization time) enables the fine-tuning of the photonic stopband (PSB) of these nanoporous photonic crystals (PCs) across the spectral regions. The surface of NAA-GIFs was chemically modified with photoactive layers of TiO2 to create a composite photoactive material with precisely engineered optical properties. The photocatalytic performance of TiO2-functionalized NAA-GIFs was assessed by studying the photodegradation of three model organic dyes (i.e., methyl orange, Rhodamine B, and methylene blue) with well-defined absorption bands across different spectral regions under simulated irradiation conditions. Our study demonstrates that when the edges of characteristic PSB of TiO2-modified NAA-GIFs are completely or partially aligned with the absorption band of the organic dyes, the photodegradation rate is enhanced due to "slow photon" effect. A rational design of the photocatalyst material with respect to the organic dye is demonstrated to be optimal to speed up photocatalytic reactions by an efficient management of photons from high-irradiance spectral regions. This provides new opportunities to develop high-performing photocatalytic materials for efficient photocatalysis with broad applicability.

Direct label-free detection of Rotavirus using a hydrogel based nanoporous photonic crystal

A direct label-free biosensor based on 3D photonic crystal structures for Rotavirus has been demonstrated. This proposed method will be useful for developing a direct and easy-to-use virus detection kit in the form of POCT in the near future.

Peptide-Modified Optical Filters for Detecting Protease Activity

The organic derivatization of silicon-based nanoporous photonic crystals is presented as a method to immobilize peptides for the detection of protease enzymes in solution. A narrow-line-width rugate filter, a one-dimensional photonic crystal, is fabricated that exhibits a high-reflectivity optical resonance that is sensitive to small changes in the refractive index at the pore walls. To immobilize peptide in the pore of the photonic crystal, the hydrogen-terminated silicon surface was first modified with the alkene 10-succinimidyl undecenoate via hydrosilylation. The monolayer with the succinimide ester moiety at the distal end served the dual function of protecting the underlying silicon from oxidation as well as providing a surface suitable for subsequent derivatization with amines. The surface was further modified with 1-aminohexa(ethylene glycol) (EG(6)) to resist nonspecific adsorption of proteins common in complex biological samples. The distal hydroxyl of the EG(6) is activated using the solid-phase coupling reagent disuccinimidyl carbonate for selective immobilization of peptides as protease recognition elements. X-ray photoelectron spectroscopy analysis reveals high activation and coupling efficiency at each stage of the functionalization. Exposure of the peptide-modified crystals to the protease subtilisin in solution causes a change in the refractive index, resulting in a shift of the resonance to shorter wavelengths, indicating cleavage of organic material within the pores. The lowest detected concentration of enzyme was 37 nM (7.4 pmol in 200 microL).