Nanohole Arrays Research Articles

Preparation of Ordered Nanostructures by Anodization of Ag Substrate with Depression Patterns

Nanostructures composed of Ag oxides are key materials with various potential applications such as sensors, catalysts, and batteries. We successfully fabricated nanohole array structures with an ordered arrangement of pores by anodizing a Ag substrate with a depression pattern in an ethylene glycol solution containing NH4F and KOH. Under these anodization conditions, each depression acts as a starting point for pore generation in the early stages of anodization. In addition, ordered nanopillar array structures were obtained by anodizing Ag substrates with depression patterns in ethylene glycol solution containing NH4F and CH3COONa. This is because the depressions formed by Ar ion beam irradiation act as masks during anodization, and the depressions remain during anodization, forming a nanopillar array structure. This is the first report on the fabrication of nanohole and nanopillar array structures by the anodization of a Ag substrate. In addition, this is the first report to show that ordered nanohole and nanopillar array structures can be fabricated by changing the electrolyte used for anodization. The Ag oxide nanostructures obtained by this process are expected to be applicable to various functional devices.

Fano resonance-enhanced Si/MoS2 photodetector

In this work, a Si/MoS2 heterojunction photodetector enhanced by hot electron injection through Fano resonance is developed. By preparing Au oligomers using capillary-assisted particle assembly (CAPA) on the silicon substrate with a nanohole array and covering few-layer MoS2 with Au electrodes on top of the oligomer structures, the Fano resonance couples with a Si/MoS2 heterojunction. With on-resonance excitation, Fano resonance generated many hot electrons on the surface of oligomers, and the hot electrons were injected into MoS2, providing an increased current in the photodetector under a bias voltage. The photodetectors exhibited a broadband photoresponse ranging from 450 to 1064 nm, and a large responsivity up to 52 A/W at a wavelength of 785 nm under a bias voltage of 3 V. The demonstrated Fano resonance-enhanced Si/MoS2 heterojunction photodetector provides a strategy to improve the photoresponsivity of two-dimensional materials-based photodetectors for optoelectronic applications in the field of visible and near-infrared detection.

Achieving high sensitivity by adding rings to a plasmonic metasurface with nano-holes

This report demonstrates a subwavelength plasmonic metasurface for a refractive index sensor at near-infrared (NIR) wavelengths. This sensor includes a two-dimensional periodic array of gold nano-rings and nano-holes in a gold film. The structure's potential for high-sensitivity sensing in reflection configuration has been demonstrated. We use a TiN layer in this structure. It prevents light transmission from the resonators, making the transmission spectrum near zero and causing a deeper reflection dip. This sensor can be suitable for biosensing applications like poliovirus sensing. Generally, high sensitivity is obtained for poliovirus of over 817 nm/RIU. It is shown that making ledges around nano-holes increases the resonance wavelength shift and, therefore, the sensitivity by more than 33 times.

Optical Kerr nonlinearity of dielectric nanohole array metasurfaces with different hole shapes near the anapole state

At present, optical anapole resonances in nanostructures have attracted increasing attention due to the strong field confinement and substantially suppressed scattering. This study provides the results of three-dimensional finite-difference time-domain simulations exhibiting the possibility of the anapole state in high refractive index dielectric nanohole array metasurfaces having different profiles of the holes (square, hexagon, and octagon). Behavior of the effective optical Kerr nonlinearity of the metasurfaces in the vicinity of the anapole state is investigated. Depending on the geometry, the absolute value of the effective nonlinear Kerr coefficient of the metasurface may be up to three orders of magnitude greater than that of the unstructured film. A square transverse section of the nanohole is preferable for the optical Kerr effect in the holey metasurfaces. The effect of the random rotation of the square holes representing the metasurface irregularity on the optical nonlinearity is examined. As a result, the dielectric nanohole array metasurfaces display a concrete possibility to have the anapole state with large enhancement of the optical nonlinearity.

Enhanced optical sensing with metal-insulator-metal nanohole arrays integrated with silver nanoparticles

The field of plasmonic sensing has witnessed rapid advancements in recent years. This work proposes an innovative methodology that employs metal–insulator-metal (MIM) nanohole arrays integrated with ring-shaped silver nanoparticles (AgNPs) to serve as an effective plasmonic refractive index sensor. The hybrid substrates are prepared using a combination of nanosphere lithography (NSL), reactive ion etching, and an enhanced plasmonic site-selective photocatalytic reaction, ensuring large-area fabrication, high throughput, and cost-effectiveness. The resonance peaks observed in the visible-NIR spectra are attributed to the coupling of surface plasmon resonance modes between MIM structures and AgNPs. The resonance wavelength exhibits sensitively to changes in the surrounding medium within the refractive index range of 1 to 1.5, with a remarkable sensitivity of 835 nm/RIU. Our findings reveal the significant potential of this technology for advancing optical sensing applications.

Multi-band all-silicon TM-pass polarizer based on one-dimensional photonic crystals nanohole array

We propose an on-chip transverse magnetic (TM)-pass polarizer utilizing one-dimensional photonic crystals for multi-band operation. The TE0 modes in the 1550/2000nm wave band are suppressed by carefully selecting the pitch lengths of the nanoholes, leveraging the bandgap of the nanohole array. Conversely, the TM0 modes remain almost unaffected. The TM-pass polarizer employs a single-etched design on a standard 220 nm SOI platform and has a compact length of ∼ 17.9 µm. The simulated bandwidths (BWs) for polarization extinction ratios (PERs) > 20 dB and > 25 dB are about 210 nm and 195 nm for the 1550 nm wave band, and 265 nm and 240 nm for the 2000nm wave band. Moreover, the insertion losses (ILs) are ∼ 0.5/0.3 dB at wavelengths of 1550/2000nm, respectively. For the fabricated device, the measured BWs for PER > 20 dB and > 25 dB are evaluated to be larger than 100 nm for both 1550/2000nm wave bands. The measured ILs are 1/0.8 dB at wavelengths of 1550/2000nm. This straightforward and compatible design opens possibilities for the development of practical multi-band silicon photonic integrated circuits.

Plasmonic Titanium Nitride Nanohole Arrays for Refractometric Sensing.

Group IVB metal nitrides have attracted great interest as alternative plasmonic materials. Among them, titanium nitride (TiN) stands out due to the ease of deposition and relative abundance of Ti compared to those of Zr and Hf metals. Even though they do not have Au or Ag-like plasmonic characteristics, they offer many advantages, from high mechanical stability to refractory behavior and complementary metal oxide semiconductor-compatible fabrication to tunable electrical/optical properties. In this study, we utilized reactive RF magnetron sputtering to deposit plasmonic TiN thin films. The flow rate and ratio of Ar/N2 and oxygen scavenging methods were optimized to improve the plasmonic performance of TiN thin films. The stoichiometry and structure of the TiN thin films were thoroughly investigated to assess the viability of the optimized operation procedures. To assess the plasmonic performance of TiN thin films, periodic nanohole arrays were perforated on TiN thin films by using electron beam lithography and reactive ion etching methods. The resulting TiN periodic nanohole array with varying periods was investigated by using a custom microspectroscopy setup for both reflection and transmission characteristics in various media to underline the efficacy of TiN for refractometric sensing.

Light blocking and phase modulation based on thermo-plasmonic hotspots in WS2 monolayer

Here we demonstrate light blocking and phase modulation in nano-plasmonic donate shape rings based on the thermo-plasmonic effect in these structures. For this purpose, we use a laser writing system to fabricate nanohole arrays and cover them with plasmonic gold thin film via a sputtering machine. The chemical vapor deposition method is also used to produce a WS2 layer, which is suitable for light blocking and phase modulation due to the nonlinearity of this two-dimensional material. After theoretically and experimentally evaluating the plasmonic donate-shaped substrate, we use the ellipsometric method to characterize the optical modes of the samples and record the switching manner and light-blocking phenomena under the probe laser excitation set to 980 nm. Our results show phase modulation based on the thermo-plasmonic effect of nano gap in donated double rings and light blocking by thermal expansion of the WS2 layer, which can open new insight into plasmon and two-dimensional material-based devices.

Focused ion beams from GaBiLi liquid metal alloy ion sources for nanofabrication and ion imaging

In this work, we present an overview of nanopatterning and imaging applications using newly developed workflows with focused ion beams (FIBs) produced with a GaBiLi liquid metal alloy ion source. The primary beam of this source type contains gallium, bismuth, and lithium as well as cluster ions which can be separated quickly using a Wien filter. Lithium ion milling has been applied to generate heptamer-arranged nanohole arrays in gold films with high resolution. Workflows for two-step bowtie nanofabrication using lithium and bismuth ions from the same source have been established. Furthermore, we present ion beam imaging results that were obtained with lithium ions on various sample materials. Combining the large sputter yield and high depth resolution of heavy bismuth ions with the high lateral imaging resolution of light lithium ions enables 3D nanoscale tomography using different ion species generated from the same source. Sample tilt is not required due to the top-down geometry of the FIB.

Ultra-narrowband circular dichroism in all-dielectric chiral nanohole arrays based on griddings and monolayer MoS2

The strong circular dichroism (CD) of metallic chiral nanostructures can be applied to polarization conversion, negative refraction materials, and chiral molecule detection. However, metallic chiral nanostructures with losses struggle to produce dynamically tunable ultra-narrow band circular dichroism signals, which greatly limits their applications in sensing. In this paper, the MoS2 and Sb2S3 with phase change characteristics are introduced into all-dielectric chiral nanopore arrays (ACNAs) based on gridding to generate dynamically tunable ultra-narrow band circular dichroism signals. Simulation results show that ACNAs/MoS2 produce two strong narrow-band CD signals in the visible range. The maximum CD intensity can reach 0.75, and the maximum half-width can reach 0.8 nm. The electric field distribution indicates that two CD signals are mainly caused by the guided mode resonance. The ultra-narrow band CD signals both lie on their geometric parameters and can realize the dynamic regulation by changing the incidence angle of lights and the crystalline state of Sb2S3. Furthermore, the sensing characteristics of this device were evaluated by analyzing the effects of different concentrations of SARS-CoV-2 solutions on the CD spectra. These findings will facilitate the design of all-dielectric chiral nanostructures with dynamically tunable ultra-narrow band CDs and facilitate their applications in biosensing.

Nanohole-Array Induced Metallic Molybdenum Selenide Nanozyme for Photoenhanced Tumor-Specific Therapy.

Deficient catalytic sensitivity to the tumor microenvironment is a major obstacle to nanozyme-mediated tumor therapy. Electron transfer is the intrinsic essence for a nanozyme-catalyzed redox reaction. Here, we developed a nanohole-array-induced metallic molybdenum selenide (n-MoSe2) that is enriched with Se vacancies and can serve as an electronic transfer station for cycling electrons between H2O2 decomposition and glutathione (GSH) depletion. In a MoSe2 nanohole array, the metallic phase reaches up to 84.5%, which has been experimentally and theoretically demonstrated to exhibit ultrasensitive H2O2 responses and enhanced peroxidase (POD)-like activities for H2O2 thermodynamic heterolysis. More intriguingly, plenty of delocalized electrons appear due to phase- and vacancy-facilitated band structure reconstruction. Combined with the limited characteristic sizes of nanoholes, the surface plasmon resonance effect can be excited, leading to the broad absorption spectrum spanning of n-MoSe2 from the visible to near-infrared region (NIR) for photothermal conversion. Under NIR laser irradiation, metallic MoSe2 is able to induce out-of-balance redox and metabolism homeostasis in the tumor region, thus significantly improving therapeutic effects. This study that takes advantage of phase and defect engineering offers inspiring insights into the development of high-efficiency photothermal nanozymes.

Investigation of perfect narrow-band absorber in silicon nano hole array

In this paper, we proposed a triple layer structure consisting of the bottom silver layer, thin silicon oxide space layer, and ultrathin semiconductor silicon film with nano hole array achieving three absorption peaks with narrow band. The absorption spectrum can be easily controlled by adjusting the structural parameters including the radius and period of the nano hole array, and the maximal absorption can reach 99.0% and the narrowest full width of half maximum can reach about 6.5 nm in theory. We also clarified the physical mechanism of the proposed structure in details by finite-difference time-domain simulation, in which the three narrow band perfect adsorption peaks can be attributed to electric dipole resonance, magnetic dipole resonance and plasmonic resonance respectively. At the same time, we used a low-cost nanosphere lithography method to fabricate the proposed nano hole array in large area. In experiment, the absorption peak of the proposed triple layer structure can reach up to 98.3% and the narrowest full width of half maximum can reach up to about 10.1 nm. The highest quality factor Q can reach up to 98.4. This work can open a new avenue for high-quality factor narrow band perfect absorption using ultrathin semiconductor film and benefit for many fields such as infrared sensors, plasmonic filters, and hyperspectral imaging.

High aspect-ratio sub-500 nm UV-PDMS bilayer stamps by means of hybrid thermal-ultraviolet curing for resonant nanopillars fabrication through soft UV-NIL

A novel protocol for the fabrication of bilayer stamps for soft UV-NIL is presented. The patterned layer is composed of a commercially available UV-curable PDMS, while a quartz backplane acts as support layer. The pattern reproduced in the stamp through direct casting comes from a hard template and is composed of nanopillar arrays with 250–300 nm of diameter and 300–400 nm in height. The master template with nanoholes is treated with a self-assembled monolayer before pattern reproduction for improved separation of UV-PDMS layer. The hybrid cross-linking protocol includes a thermal and a UV process, and parameters of the former are investigated for best results. The pattern was reproduced in a resist on a Si sample by means of soft UV-NIL, and the imprinted nanohole arrays show dimensions coherent with the stamp. Finally, its application in the fabrication of resonant nanopillars for sensing purposes is briefly described. This is, to the best of our knowledge, the first time a hybrid thermal-ultraviolet curing of UV-PDMS has been reported and that protruding nanostructures with aspect-ratio higher than 1 in soft UV-NIL stamps have been demonstrated.

Sol-Gel Barium Titanate Nanohole Array as a Nonlinear Metasurface and a Photonic Crystal.

The quest of a nonlinear optical material that can be easily nanostructured over a large surface area is still ongoing. Here,wedemonstrate a nanoimprinted nonlinear barium titanate2D nanohole array that shows the optical properties of a 2D photonic crystal and a metasurface, depending on the direction of the optical axis. The challenge of nanostructuring the inert metal-oxide is resolved by direct soft nanoimprint lithography with sol-gel derived barium titanate enabling critical dimensions of 120 nm with aspect ratios of five. The nanohole array exhibits a photonic bandgap in the infrared range when probed along the slab axis, while lattice resonant states are observed in out-of-plane transmission configuration. The enhanced light-matter interaction from the resonant structure enables to increase in the second-harmonic generation in the near-ultraviolet by a factor of 18 illustrating the potential in the flexible fabrication technique for barium titanate photonicdevices.

Multiple and simultaneous detection for cytokines based on the nanohole array by electrochemical sandwich immunoassay

Monitoring of cytokine storm markers is critical for determining the disease progression and treatment efficacy. In this study, we developed a novel fourplex electrochemical (EC) biosensor using nanohole array (NHA) structures containing several nanoholes on a single electrode for detecting cytokines with a low limit. A framework was developed for fabricating and optimizing multiplexed EC sandwich immunoassays using NHA electrodes because an NHA can enhance the EC detection of molecular binding by controlling the binding sites of captured biomolecules. For detecting multiple cytokines, the captured antibodies were immobilized on the surface-modified NHA and sandwich immunoassays with amperometric signal amplifications using poly-streptavidin horseradish peroxidase conjugates and 3,3′,5,5′-tetramethylbenzidine. Under the optimal conditions, the EC NHA immunosensors exhibited high sensitivity and specificity with a limit of detection of 1 pg/mL and a detection range of 1–300 pg/mL. The results revealed that the developed NHA sensor exhibits reliable EC characteristics and could be used to analyze multiple cytokines using small sample volume and subsequently developed for other applications.

Inverse Design of Plasmonic Nanohole Arrays by Combing Spectra and Structural Color in Deep Learning

Herein, deep learning (DL) is used to predict the structural parameters of Ag nanohole arrays (NAs) for spectrum‐driving and color‐driving plasmonic applications. A dataset of transmission spectra and structural parameters of NAs is generated using finite‐difference time‐domain (FDTD) calculations and is converted to vivid structural colors using the corresponding transmission spectrum. A bidirectional neural network is used to train the transmission spectrum and structural color together. The accuracy of predicting the structural parameters using a desired spectrum is tested and found to be up to 0.99, with a determination coefficient of reproducing the desired spectrum and color to be 0.97 and 0.96, respectively. These values are higher compared to those when only training for spectrum, but requiring less training time. This strategy is able to inverse design the NAs in less than 1 s to maximize surface‐enhanced Raman scattering (SERS) enhancement by matching transmission resonance and laser excitation wavelength, and accurately regenerate colored images in 7.5 s, allowing for nanoscale printing at a resolution of approximately 100 000 dots in−1. This work has important implications for the efficient design of nanostructures for various plasmonic applications, such as plasmonic sensors, optical filters, metal‐enhanced fluorescence, SERS, and super‐resolution displays.

Element Distribution in Porous Ga Oxide Obtained by Anodizing Ga in Phosphoric Acid

A STEM/EDS study of a porous Ga oxide film formed by an anodization process was conducted in this study to examine the crystalline structure of the film and the elemental distribution in the oxide film before and after heat treatment. The as-formed anodic film with a morphology resembling the well-known porous anodic Al oxide film was amorphous, crystallizing after heat treatment at 600 °C without changing the morphology and elemental distribution. The EDS elemental maps disclosed the duplex nature of the pore wall oxide; the phosphate anion was contaminated in the outer oxide layer next to the pores, and the inner layer consisted of relatively pure Ga oxide, practically free from phosphate. The similarity of morphology and elemental distributions between the porous anodic Al and Ga oxides suggests that the growth of both anodic oxide films proceeds under the same mechanism. In addition, crystallized porous Ga oxides are expected to be applied to fabricate various functional devices requiring geometrically controlled semiconductor nanohole arrays, such as devices for hydrogen formation.

Mid-infrared plasmonic filter arrays using nanoholes in gold and silver films.

Plasmonic filters based on subwavelength nanohole arrays are an attractive solution for creating arrays of filters with varying passbands in a single lithography step. In this work, we have developed a fabrication method which allows fabrication of nanohole arrays in silver by use of a thin layer of aluminum oxide, which serves the dual purpose of both capping layer and hardmask for metal patterning. We demonstrate arrays of gold and silver mid-infrared plasmonic filters, fabricated on silicon, intended for use in optical filter blocks or for future integration with infrared imagers. The filter arrays are designed for the wavelength range 2-7 µm, and exhibit peak filter transmission efficiencies around 70%.

Ultrathin Twisted Stacked Gap‐Plasmon Metasurface with Giant and Tunable Shortwave Infrared Chirality

AbstractUltrathin optical components with giant circular dichroism hold great promise for polarization‐sensitive nanophotonics. However, existing chiral metamaterials often rely on complex‐shaped nanoinclusions, and there remains a paucity of designs active in the shortwave infrared (SWIR) useful for telecommunication and night vision. Here a detailed numerical analysis of the chiroptical response of a simple twisted stacked variant of gap‐plasmon metasurface is presented based on periodic nanohole arrays. Impressively, the 100‐nm free‐standing plasmonic metasurface can attain a thickness‐normalized ellipticity of up to 74.2° µm−1 with tunable bands in the SWIR. Strong and robust optical activity is achieved through energy confinement by gap surface plasmons and effective inter‐nanohole coupling, resulting in an intense superchiral near field. The ultrathin plasmonic metasurface serves as an excellent platform for refractive index sensing with a sensitivity Sn of up to 648.1 nm RIU−1 and field‐enhanced enantiodiscrimination. The simple metasurface with strong SWIR chirality can enjoy a multitude of potential applications including polarization‐sensitive photodetection, optical telecommunication, machine vision, medical imaging, and spectroscopy. These findings also offer insights into some fundamental design principles for twisted stacked aperture‐based metasurfaces and should facilitate the development of new chiral metamaterials with exceptional performance.

Gold nanohole arrays with ring-shaped silver nanoparticles for highly efficient plasmon-enhanced fluorescence

To enhance the interaction between electromagnetic fields and reduce photoluminescence (PL) lifetime in plasmon-enhanced fluorescence, this study presents a novel approach involving the fabrication of gold (Au) nanohole arrays (ANA) decorated with ring-shaped silver nanoparticles (AgNPs) on a silicon-dioxide (SiO2)/silver (Ag) substrate. The surface plasmon resonance coupling in terms of the PL reactions of ANA substrates with and without ring-shaped AgNPs is investigated via experiments and numerical simulations. The remarkable enhancement of PL intensity of the proposed substrate is attributed to increased absorption, which enables the tuning of surface-enhanced electromagnetic fields. Specifically, the Raman signal of rhodamine 6G (R6G) dye and the PL intensity of 4-(dicyanomethylene)-2-t-butyl-6-(1,1,7,7-tetramethyljulolidyl-9-enyl)-4H-pyran (DCJTB) molecules are significantly enhanced 3.7 and 2.2 times, respectively, compared to those of the bare ANA substrate. Meanwhile, the PL lifetime is reduced by 46.15%. These results confirm that ANAs decorated with ring-shaped AgNPs can significantly improve plasmon-enhanced fluorescence. The findings presented herein demonstrate the potential to revolutionize biosensing, imaging, and photonics.