Nanohole Arrays Research Articles

Tunable Multiple Surface Plasmonic Bending Beams into Single One by Changing Incident Light Wavelength

Controllable surface plasmonic bending beams (SPBs) with propagating along bending curves have a wide range of applications in the fields of fiber sensors, optical trapping, and micro-nano manipulations. In terms of designing and optimizing controllable SPB generators, there is great significance in realizing conversion between multiple SPBs and single SPB without rebuilding metasurface structures. In this study, a SPB generator, composed of an X-shaped nanohole array, is proposed to realize conversion between multiple SPBs and a single one by changing the incident light wavelength. The Fabry–Pérot (F–P) resonance effect of SPPs in nanoholes and localized surface plasmonic (LSP) resonance of the nanohole are utilized to explain this conversion. It turns out that the relationship between the electric field intensities of SPBs and the polarization angle of incident light satisfies the sine distribution, which is consistent with dipole radiation theory. In addition, we also find that the electric field intensities of SPBs rely on the width, length, and angle of the X-shaped nanohole. These findings could help in designing and optimizing controllable and multi-functions SPBs converters.

Multifunctional additive CdAc2 for efficient perovskite-based solar cells.

Polycrystalline perovskite films fabricated on flexible and textured substrates often are highly defective, leading to poor performance of perovskite devices. Finding substrate-tolerant perovskite fabrication strategies is therefore paramount. Herein, we show that adding a small amount of Cadmium Acetate (CdAc2 ) in the PbI2 precursor solution results in nano-hole array films and improves the diffusion of organic salts in PbI2 , and promotes favorable crystal orientation and suppresses non-radiative recombination. Polycrystalline perovskite films on the flexible substrate with ultra-long carrier lifetimes exceeding 6 μs are achieved. Eventually, a PCE of 22.78% is obtained for single-junction flexible perovskite solar cells (FPSCs). Furthermore, we find the strategy is also applicable for textured tandem solar cells. A champion PCE of 29.25% (0.5003 cm2 ) was demonstrated for perovskite/silicon tandem solar cells (TSCs) with CdAc2 . Moreover, the un-encapsulated TSCs maintains 109.78% of its initial efficiency after 300 hours operational at 45°C in N2 atmosphere. Our work provides a facile strategy for achieving high-efficiency perovskite-based solar cells. This article is protected by copyright. All rights reserved.

A Route toward High-Detectivity and Low-Cost Short-Wave Infrared Photodetection: GeSn/Ge Multiple-Quantum-Well Photodetectors with a Dielectric Nanohole Array Metasurface.

High-detectivity and low-cost short-wave infrared photodetectors with complementary metal-oxide-semiconductor (CMOS) compatibility are attractive for various applications such as next-generation optical communication, LiDAR, and molecular sensing. Here, GeSn/Ge multiple-quantum-well (MQW) photodetectors with a dielectric nanohole array metasurface were proposed to realize high-detectivity and low-cost SWIR photodetection. The Ge nanohole array metasurface was utilized to enhance the light absorption in the GeSn/Ge MQW active layer. Compared with metallic nanostructures, the dielectric nanohole structure has the advantages of low intrinsic loss and CMOS compatibility. The introduction of metasurface architecture facilitates a 10.5 times enhanced responsivity of 0.232 A/W at 2 μm wavelength while slightly sacrificing the dark current density. Besides, the metasurface GeSn/Ge MQW photodetectors benefit 35% improvement in the 3 dB bandwidth compared to control GeSn/Ge MQW photodetectors, which can be attributed to the reduced RC delay. Due to the high responsivity and low dark current density, the room temperature specific detectivity at 2 μm is as high as 5.34 × 109 cm·Hz1/2/W, which is the highest among GeSn photodetectors and is better than commercial InSb and PbSe photodetectors operating at the similar wavelength. This work offers a promising approach for achieving low-cost and effective photodetection at 2 μm, contributing to the development of the 2 μm communication band.

Multiple Features-Based Improved Nanohole Array Plasmonic Biosensor

In recent years, gold nanohole array-based biosensors have gained tremendous attention due to their high sensitivity, label-free biosensing, and real-time simultaneous multiple analyte detection capabilities. However, nanohole array-based biosensors using conventional sensing methods lack the resolution of traditional surface plasmon resonance (SPR) sensors. In this work, we present numerical methods utilizing multiple peaks of these biosensors’ transmission spectra to achieve higher sensitivity and lower detection limits. Using finite-difference time-domain (FDTD) simulations, we compared the sensing performance of the proposed numerical methods to traditional peak-shift and spectral integration methods. We added noise to the transmission spectra to simulate a realistic system and determined the change in the performance. We also optimized the geometrical parameters of the gold nanohole array for optimal sensing performance. Finally, we applied the proposed method to two critical real-life biosensing applications and reported the sensitivity and limit of detection (LOD). The proposed numerical methods show much higher sensitivity and lower detection limits than the traditional techniques. The observed detection limits are as low as <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\sim {2} \times {10}^{-{6}}$ </tex-math></inline-formula> refractive index unit changes near the surface. The proposed methods can be utilized for any sensing systems that employ transmission spectra and will outperform traditional methods if there are multiple peaks present in the transmission spectra.

The Sensitivity of a Hexagonal Au Nanohole Array under Different Incident Angles

Surface plasmon resonance sensors have been widely used in various fields for label-free and real-time detection of biochemical species due to their high sensitivity to the refractive index change of the surrounding environment. The common practices to achieve the improvement of sensitivity are to adjust the size and morphology of the sensor structure. This strategy is tedious and, to some extent, limits the applications of surface plasmon resonance sensors. Instead, the effect of the incident angle of excited light on the sensitivity of a hexagonal Au nanohole array sensor with a period of 630 nm and a hole diameter of 320 nm is theoretically investigated in this work. By exploring the peak shift of reflectance spectra of the sensor when facing a refractive index change in (1) the bulk environment and (2) the surface environment adjacent to the sensor, we can obtain the bulk sensitivity and surface sensitivity. The results show that the bulk sensitivity and surface sensitivity of the Au nanohole array sensor can be improved by 80% and 150%, respectively, by simply increasing the incident angle from 0° to 40°. The two sensitivities both remain nearly unchanged when the incident angle further changes from 40° to 50°. This work provides new understanding of the performance improvement and advanced sensing applications of surface plasmon resonance sensors.

Development of Nanopackaging for Storage and Transport of Loaded Lipid Nanoparticles.

Easily deploying new vaccines globally to combat disease outbreaks has been highlighted as a major necessity by the World Health Organization. RNA-based vaccines using lipid nanoparticles (LNPs) as a drug delivery system were employed to great effect during the recent COVID-19 pandemic. However, LNPs are still unstable at room temperature and agglomerate over time during storage, rendering them ineffective for intracellular delivery. We demonstrate the suitability of nanohole arrays (nanopackaging) as patterned surfaces to separate and store functionalized LNPs (fLNPs) in individual recesses, which can be expanded to other therapeutics. Encapsulating calcein as a model drug, we show through confocal microscopy the effective loading of fLNPs into our nanopackaging for both wet and dry systems. We prove quantifiably pH-mediated capture and subsequent unloading of over 30% of the fLNPs using QCM-D on alumina surfaces altering the pH from 5.5 to 7, displaying controllable storage at the nanoscale.

Investigation of Conditions for Preparation of Ordered Nanohole Arrays by Anodization of Iron Substrates with Depression Patterns

Ordered iron oxide nanohole arrays were fabricated by the anodization of iron substrates with depression patterns formed by Ar ion milling with alumina masks in an ethylene glycol electrolyte containing NH4F and H2O. It was found that the optimization of anodization voltage, NH4F and H2O concentrations in the electrolyte, and electrolyte temperature is necessary to achieve straight pore growth induced from the depression patterns in the depth direction. The optimization of the anodization conditions enabled the formation of ordered iron oxide nanohole arrays with aspect ratios exceeding 10. The resulting ordered iron oxide nanohole arrays with high aspect ratios are expected to be applied to various functional devices such as photocatalysts and solar cells.

Broadband, angle-dependent optical characterization of asymmetric self-assembled nanohole arrays in silver

Plasmonic nanostructured materials made of nanohole arrays in metal are significant plasmonic devices exhibiting resonances and strong electromagnetic confinement in the visible and near-infrared range. As such, they have been proposed for use in many applications such as biosensing and communications. In this work, we introduce the asymmetry in nanoholes, and investigate its influence on the electromagnetic response by means of broadband experimental characterization and numerical simulations. As a low-cost fabrication process, we use nanosphere lithography, combined with tilted silver evaporation, to obtain a 2D hexagonal array of asymmetric nanoholes in Ag. Our experimental set-up is based on a laser, widely tunable in the near-infrared range, with precise polarization control in the input and in the output. We next resolve the circular polarization degree of the transmitted light when the nanohole array is excited with linear polarization. We attribute the disbalance of left and right transmitted light to the asymmetry of the nanohole, which we support by numerical simulations. We believe that the optimization of such simple plasmonic geometry could lead to multifunctional flat-optic devices.

Strongly enhanced sensitivities of CMOS compatible plasmonic titanium nitride nanohole arrays for refractive index sensing under oblique incidence.

Titanium nitride (TiN) is a complementary metal-oxide-semiconductor (CMOS) compatible material with large potential for the fabrication of plasmonic structures suited for device integration. However, the comparatively large optical losses can be detrimental for application. This work reports a CMOS compatible TiN nanohole array (NHA) on top of a multilayer stack for potential use in integrated refractive index sensing with high sensitivities at wavelengths between 800 and 1500 nm. The stack, consisting of the TiN NHA on a silicon dioxide (SiO2) layer with Si as substrate (TiN NHA/SiO2/Si), is prepared using an industrial CMOS compatible process. The TiN NHA/SiO2/Si shows Fano resonances in reflectance spectra under oblique excitation, which are well reproduced by simulation using both finite difference time domain (FDTD) and rigorous coupled-wave analysis (RCWA) methods. The sensitivities derived from spectroscopic characterizations increase with the increasing incident angle and match well with the simulated sensitivities. Our systematic simulation-based investigation of the sensitivity of the TiN NHA/SiO2/Si stack under varied conditions reveals that very large sensitivities up to 2305 nm per refractive index unit (nm RIU-1) are predicted when the refractive index of superstrate is similar to that of the SiO2 layer. We analyze in detail how the interplay between plasmonic and photonic resonances such as surface plasmon polaritons (SPPs), localized surface plasmon resonances (LSPRs), Rayleigh Anomalies (RAs), and photonic microcavity modes (Fabry-Pérot resonances) contributes to this result. This work not only reveals the tunability of TiN nanostructures for plasmonic applications but also paves the way to explore efficient devices for sensing in broad conditions.

High-Performance Ultraviolet Photodetector Based on Single-Crystal Integrated Self-Supporting 4H-SiC Nanohole Arrays.

Currently, the photodetectors (PDs) assembled by vertically aligned nanostructured arrays have attracted intensive interest owing to their unique virtues of low light reflectivity and rapid charge transport. However, in terms of the inherent limitations caused by numerous interfaces often existed within the assembled arrays, the photogenerated carriers cannot be effectively separated, thus weakening the performance of target PDs. Aiming at resolving this critical point, a high-performance ultraviolet (UV) PD with a single-crystal integrated self-supporting 4H-SiC nanohole arrays is constructed, which are prepared via the anode oxidation approach. As a result, the PD delivers an excellent performance with a high switching ratio (∼250), remarkable detectivity (6 × 1010 Jones), fast response (0.5 s/0.88 s), and excellent stability under 375 nm light illumination with a bias voltage of 5 V. Moreover, it has a high responsivity (824 mA/W), superior to those of most reported ones based on 4H-SiC. The overall high performance of the PDs could be mainly attributed to the synergistic effect of the SiC nanohole arrays' geometry, a whole single-crystal integrated self-supporting film without interfaces, established reliable Schottky contact, and incorporated N dopants.

Large-Area Ultrathin Moiré Chiral Metamaterials by Thermal-Tape-Transfer Printing

Moiré chiral metamaterials (MCMs) consisting of stacked plasmonic nanohole arrays with twist angles exhibit strong chiroptical response, thereby opening doors to ultrathin chiroptical sensors, switches, and detectors. The existing fabrication of MCMs on the basis of colloidal lithography results in significant inhomogeneity within the structure, which impairs the responsiveness and tunability of its optical chirality. Here, we develop thermal-tape-transfer printing to enable the fabrication of large-scale and homogeneous MCMs with arbitrary twist angles and tunable optical chirality. As a demonstration, large-scale (100 × 100 μm2) gold MCMs with an ultrathin thickness (∼40 nm) were fabricated, which marked a 10-fold increase in single domain size over the colloidal lithographic method, while showing spatially uniform and strong chiroptical response. With the ultrathin thickness and high tunability, the MCMs developed by our fabrication method will advance a variety of biological, photonic, and optoelectronic applications.

Circular dichroism in a plasmonic array of elliptical nanoholes with square lattice.

Chiral properties of plasmonic metasurfaces, especially related to different absorption of left and right circularly polarized light leading to circular dichroism (CD), are a research hot topic in nanophotonics. There is often a need to understand the physical origin of CD for different chiral metasurfaces, and to get guidelines for the design of structures with optimized and robust CD. In this work, we numerically study CD at normal incidence in square arrays of elliptic nanoholes etched in thin metallic layers (Ag, Au, Al) on a glass substrate and tilted with respect to the symmetry axes. Strong CD arises in absorption spectra at the same wavelength region of extraordinary optical transmission, indicating highly resonant coupling between light and surface plasmon polaritons at the metal/glass and metal/air interfaces. We elucidate the physical origin of absorption CD by a careful comparison of optical spectra for different polarizations (linear and circular), with the aid of static and dynamic simulations of local enhancement of the electric field. Furthermore, we optimize the CD as a function of the ellipse parameters (diameters and tilt), the thickness of the metallic layer, and the lattice constant. We find that silver and gold metasurfaces are most useful for CD resonances above 600 nm, while aluminum metasurfaces are convenient for achieving strong CD resonances in the short-wavelength range of the visible regime and in the near UV. The results give a full picture of chiral optical effects at normal incidence in this simple nanohole array, and suggest interesting applications for chiral biomolecules sensing in such plasmonic geometries.

Fabrication of Au/AAO/Au nanopore arrays based on induced excitation SPPs for the fabrication of nanopores on silicon surfaces

In this paper, a new method for preparing a nanohole surface based on surface plasmon polaritons (SPPs) is proposed. In the proposed method, a layer of gold film is sputtered onto the upper and lower surfaces of porous anodic aluminum oxide (AAO) by ion sputtering to obtain the composite structure of Au/AAO/Au (Metal-Media-Metal, MIM) periodic hole array. The periodic plasma excitation conditions and coupling of the nanohole array are deduced using Maxwell's equations to obtain the locally enhanced surface plasma wave. Afterward, the composite structure is irradiated with a nanosecond laser with a working wavelength of 532 nm to excite the surface plasmon. Simulation results show that Au/AAO/Au (MIM) can excite the surface plasmon under laser induction, and the preparation of nanoholes is realized through experiments. The nanohole array produced by surface plasmon can be used to increase the specific surface area. For some biosensors with special properties, such as microcantilever, enzyme, and other biosensors, a large surface area is required without changing their chemical stability and mechanical strength to increase the detection sensitivity.

Selective-area growth of β-Ga2O3 nanowire films on nano-patterned Si(111) substrate by metal-organic chemical vapor deposition

β-Ga2O3 nanowire films were prepared on Si(111) substrate by selective area growth (SAG) using metal-organic chemical vapor deposition (MOCVD). A large-area periodic nano-hole array TiN mask was fabricated. The effects of MOCVD process parameters on SAG were discussed by calculating Ga supersaturation. It was found that the SAG is closely related to the selective area nucleation and inhibition of lateral growth. Transmission electron microscopy confirmed that β-Ga2O3 nanowires have a preferential orientation of the (002) crystal plane. The sample after in-situ O2 annealing showed an x-ray diffraction intensity increase of (400), (002), and (-111) crystal planes. On this basis, P–Si/β‐Ga2O3 nanowire heterojunctions were fabricated and presented high resistance of the films. At a bias voltage of 20 V, the current decreased from 6.14 × 10−6 to 2.25 × 10−6 A. The MOCVD-based SAG paves a novel way to fabricate β-Ga2O3 nanowire films with high surface-to-volume ratio and homogeneity.

Actively Tunable Spectral Filter for Compact Hyperspectral Camera Using Angle‐Sensitive Plasmonic Structures

AbstractHyperspectral imaging provides enhanced classification and identification of veiled features for diverse biomedical applications such as label‐free cancer detection or non‐invasive vascular disease diagnostics. However, hyperspectral cameras still have technical limitations in miniaturization due to the inherently complex and bulky configurations of conventional tunable filters. Herein, a compact hyperspectral camera using an active plasmonic tunable filter (APTF) with electrothermally driven spectral modulation for feature‐augmented imaging is reported. APTF consists of angle‐sensitive plasmonic structures (APS) over an electrothermal MEMS (Microelectromechanical systems) actuator, fabricated by combining nanoimprint lithography, and MEMS fabrication ona 6‐inch wafer. APS have a complementary configuration of Au nanohole and nanodisk arrays supported on asilicon nitride membrane. APTF shows a large angular motion at operational voltages of 5–9 VDC for continuous spectral modulation between 820 and 1000 nm (45 nm/V). The compact hyperspectral camera was fully packaged with a linear polarizer, APTF and amonochromatic camera, exhibiting asize of 16 mm (ϕ) × 9.5 mm (h). Feature‐augmented images of subcutaneous vein and a fresh fruit have been successfully demonstrated after the hyperspectral reconstruction and spectral feature extraction. This functional camera provides a new compact platform for point‐of‐care or in vivo hyperspectral imaging in biomedical applications.

How Colloidal Lithography Limits the Optical Quality of Plasmonic Nanohole Arrays.

Colloidal lithography utilizes self-assembled particle monolayers as lithographic masks to fabricate arrays of nanostructures by combination of directed evaporation and etching steps. This process provides complex nanostructures over macroscopic areas in a simple, convenient, and parallel fashion without requiring clean-room infrastructure and specialized equipment. The appeal of the method comes at the price of imperfections impairing the optical quality, especially for arrayed nanostructures relying on well-ordered lattices. Imperfections are often generically mentioned to rationalize the discrepancy between experimental and simulated resonances. Yet, little attention is given to detailed structure-property relationships connecting typical defects directly with the optical properties. Here, we use a correlative approach to connect nano- and microscopic defects occurring from the colloidal lithography process with the resulting local optical properties. We use nanohole arrays as a common plasmonic structure known to be sensitive to lattice imperfections. Correlative optical and electron microscopies reveal the individual role of packing order, organic impurities, and solid polymer bridges. Our findings show that simple cleaning processes with solvents and oxygen plasma already improve the optical quality but also highlight how well-controlled self-assembly processes are required for predictable optical properties of such nanostructures.

Multiscale models of plasmonic structural colors with nanoscale surface roughness.

Plasmonic coloration arises from resonant interaction between visible light and metallic nanostructures, which causes wavelength-selective absorption or scattering of light. This effect is sensitive to surface roughness that can perturb these resonant interactions and cause observed coloration to deviate from coloration predicted by simulations. We present a computational visualization approach that incorporates electrodynamic simulations and physically based rendering (PBR) to investigate the effect of nanoscale roughness on the structural coloration from thin, planar silver films decorated with nanohole arrays. Nanoscale roughness is modeled mathematically by a surface correlation function and parameterized in terms of roughness that is either out of or into the plane of the film. Our results provide photorealistic visualization of the influence of nanoscale roughness on the coloration from silver nanohole arrays in both reflectance and transmittance. Out-of-plane roughness has a significantly greater effect on coloration than in-plane roughness. The methodology introduced in this work is useful for modeling artificial coloration phenomena.

A Superconducting Micro-Magnetometer for Quantum Vortex in Superconducting Nanoflakes.

Superconducting quantum interferometer device (SQUID) plays a key role in understanding electromagnetic properties and emergent phenomena in quantum materials. The technological appeal of SQUID is that its detection accuracy for the electromagnetic signal can precisely reach the quantum level of a single magnetic flux. However, conventional SQUID techniques normally can only be applied to a bulky sample and do not have the capability to probe the magnetic properties of micro-scale samples with small magnetic signals. Herein, it is demonstrated that, based on a specially designed superconducting nano-hole array, the contactless detection of magnetic properties and quantized vortices in micro-sized superconducting nanoflakes is realized. An anomalous hysteresis loop and a suppression of Little-Parks oscillation are observed in the detected magnetoresistance signal, which originates from the disordered distribution of the pinned vortices in Bi2 Sr2 CaCu2 O8+δ . Therefore, the density of pinning centers of the quantized vortices on such micro-sized superconducting samples can be quantitatively evaluated, which is technically inaccessible for conventional SQUID detection. The superconducting micro-magnetometer provides a new approach to exploring mesoscopic electromagnetic phenomena of quantum materials.

Spin-momentum-locking–induced dynamic circular dichroism in extraordinary optical transmission

We present an experimental and theoretical study of circular dichroism observed in a symmetric nanohole array in a metal film---a nonchiral planar structure exhibiting extraordinary optical transmission. The effect is attributed to an extrinsic chirality dynamically induced in the excitation of surface plasmon polariton waves via spin-momentum locking.

Bloch-Surface Plasmon Polariton Enhanced Amplified and Directional Spontaneous Emission from Plasmonic Hexagonal Nanohole Array.

The light-matter interactions at nanoscale can be enhanced by Bloch-surface plasmon polariton (Bloch-SPP) on the plasmonic lattice. An Ag nanohole array in hexagonal arrangement served as an optical cavity to realize the directional and polarized amplified spontaneous emission (ASE) of R6G. A 100-fold enhanced ASE was observed at 15° emission angle under TM polarization when the pump power density exceeded the threshold of 198 W/cm2 based on the degenerated high state density modes. Moreover, a specific polarization dependence of ASE was modulated by the Bloch-SPP modes, and the degree of polarization was enhanced from 1.3 to 2.1 when the pump power density exceeded the threshold of ASE. This work clarifies the interaction between the gain media and plasmonic systems, which lays a foundation for the plasmonic device designing.