Plasmonic Nanohole Research Articles

Dealing with plasmonic crystal biosensors: Sensitivity assessment of nanodisks/nanoholes arrayed plasmonic system for label-free DNA detection

Label-free optical deoxyribonucleic acid (DNA) sensing with arrayed plasmonic nanostructures (plasmonic crystals) is a promising technology for biomedical diagnosis and bioanalytical science. Plasmonic biosensors can detect target biomolecules by utilizing the shift in plasmonic resonance caused by changes in the surrounding refractive index (RI) attributed to the capture of target biomolecules using a recognizer. Conventional explanations for the sensitivity of plasmonic crystals are based on bulk (BRIS) and surface RI sensitivities (SRIS) for basic plasmonic nanoparticles despite their unique properties such as surface lattice resonances (SLRs), wherein localized surface plasmons (LSPs) cooperatively oscillate with their pitch. Therefore, investigating the sensitivity of SLRs is imperative for improving sensing performance. In this study, the sensitivity of adenomatous polyposis coli (APC) gene-related DNA hybridization detection of complementary plasmonic crystals composed of nanodisks (PNDs) on or under plasmonic nanoholes (PNHs) was investigated considering the SLR properties. The BRIS was measured using the conventional definition of the peak wavelength shift per unit RI increment (nm/RIU) followed by the SRIS measurement using the layer-by-layer method. The BRIS and SRIS measurements reflect the practical sensitivity for DNA detection. PNHs had higher sensitivity than PNDs, with a limit of detection of 0.30 nM. Further, only the SLR-based mode responded to localized RI changes because of DNA hybridization, whereas both the LSPs- and SLR-based modes responded to uniform RI changes caused by layer-by-layer coating. Our investigation will open up possibilities and opportunities for plasmonic crystal biosensors.

Rational Design and Optimization of Plasmonic Nanohole Arrays for Sensing Applications

Rational Design and Optimization of Plasmonic Nanohole Arrays for Sensing Applications

Demonstration of extrinsic chirality in self-assembled asymmetric plasmonic metasurfaces and nanohole arrays

Chirality, the lack of mirror symmetry, can be mimicked in nanophotonics and plasmonics by breaking the symmetry in light-nanostructure interaction. Here we report on versatile use of nanosphere lithography for the fabrication of low-cost metasurfaces, which exhibit broadband handedness- and angle-dependent extinction in the near-infrared range, thus offering extrinsic chiro-optical behavior. We measure wavelength and angle dependence of the extinction for four samples. Two samples are made of polystyrene nanospheres asymmetrically covered by silver and gold in one case and silver only in the other case, with a nanohole array at the bottom. The other two samples are nanohole arrays, obtained after the nanosphere removal from the first two samples. Rich extrinsic chiral features are governed by different chiro-optical mechanisms in the three-dimensional plasmonic semi-shells and planar nanohole arrays. We also measure Stokes parameters in the same wavelength and incidence angle range and show that the transmitted fields follow the extrinsic chirality features of the extinction dissymmetry. We further study the influences of the nanostructured shapes and in-plane orientations on the intrinsic vs extrinsic chirality. The nanoholes are modelled as oval shapes in metal, showing good agreement with the experiments. We thus confirm that nanosphere lithography can provide different geometries for chiral light manipulation at the nanoscale, with the possibility to extend functionalities with optimized oval shapes and combination of constituent metals.

Angle-Resolved Fluorescence of a Dye Coupled to a Plasmonic Nanohole Array

Gold nanohole arrays are periodic metasurfaces that are gathering huge interest in biosensing applications. The bi-dimensional grating-like structure defines their plasmonic response, together with the corresponding mode of angular dispersion. These properties can be used to investigate the interaction processes with the fluorescence features of a properly chosen emitting molecule. By employing a custom gold nanohole array alongside a commercial organic dye, we conducted an accurate angle-resolved optical characterization resorting to fluorescence, reflectance, and transmittance spectra. The coupling between the plasmonic modes and the fluorescence features was then identified as a modification of the dye fluorescence signal in terms of both spectral redistribution and enhancement. By carefully analyzing the results, different measurement efficiencies can be identified, depending on the set-up configuration, to be properly engineered for sensitivity maximization in plasmon-enhanced fluorescence-based 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.

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.

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.

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.

High-speed nanoscale optical trapping with plasmonic double nanohole aperture.

Optical trapping with plasmonic double nanohole (DNH) apertures has proven to be an efficient method for trapping sub-50 nm particles due to their suppressed plasmonic heating effect and very high electric field enhancement in the gap region of the aperture. However, plasmonic tweezers are generally diffusion-limited, requiring particles to diffuse down to a few tens of nanometres from the high field enhancement regions before they can be trapped. The loading of target particles to the plasmonic hotspots can take several minutes for diluted samples. In this work, rapid particle transport and trapping of a 25 nm polystyrene sphere is demonstrated, leveraging an electrothermoplasmonic flow induced upon application of an AC field in the presence of a laser-induced temperature gradient. Using this approach, we demonstrate the rapid transport of a 25 nm polystyrene particle across a distance of 63 μm and trapping at the DNH under 16 s. This platform shows great potential for applications involving simultaneous trapping and plasmon-enhanced spectroscopies, such as Raman enhancement via the intense electric field enhancement in the DNH gap.

Photothermomechanical Nanopump: A Flow-Through Plasmonic Sensor at the Fiber Tip.

Optical fibers equipped with plasmonic flow sensors at their tips are fabricated and investigated as photothermomechanical nanopumps for the active transport of target analytes to the sensor surface. The nanopumps are prepared using a bottom-up strategy: i.e., by sequentially stacking a monolayer of a thermoresponsive polymer and a plasmonic nanohole array on an optical fiber tip. The temperature-dependent collapse and swelling of the polymer is used to create a flow-through pumping mechanism. The heat required for pumping is generated by exploiting the photothermal effect in the plasmonic nanohole array upon irradiation with laser light (405 nm). Simultaneous detection of analytes by the plasmonic sensor is achieved by monitoring changes in its optical response at longer wavelengths (∼500-800 nm). Active mass transport by pumping through the holes of the plasmonic nanohole array is visualized by particle imaging velocimetry. Finally, the performance of the photothermomechanical nanopumps is investigated for two types of analytes, namely nanoscale objects (gold nanoparticles) and molecules (11-mercaptoundecanoic acid). In the presence of the pumping mechanism, a 4-fold increase in sensitivity was observed compared to the purely photothermal effect, demonstrating the potential of the presented photothermomechanical nanopumps for sensing applications.

Freestanding Metal Nanomembranes and Nanowires by Template Transfer with a Soluble Adhesive.

The fabrication of nanostructures usually involves chemical processes that have in certain steps. Especially, it is necessary to use the chemical etching method to release the as-patterned structures from the substrate in most of the transfer techniques. Here, a novel scheme of template transfer as developed for the fabrication of freestanding Au nanomembranes and nanowires by using a soluble PVP adhesive. The nanomembranes feature the periodic nanohole arrays with high uniformity. Without the substrates, these plasmonic nanohole arrays show symmetric and antisymmetric resonance modes with bright and dark spectral features, respectively, in transmission. Through the spectral analysis for reflection, we have disclosed that the usual dark mode in transmission is not really dark, but it reveals a distinct feature in reflection. Two coupling modes present distinct spectral features in transmission and reflection due to their different loss channels. To show their versatility, the freestanding nanomembranes were also employed as secondary templates to form Si nanowire arrays by the metal-assisted chemical etching method.

Optimization of Fully Integrated Al Nanohole Array-Based Refractive Index Sensors for Use With a LED Light Source

Plasmonic nanohole arrays that are directly structured into the top metallization of vertical Ge-on-Si PIN photodiodes can be used to sense refractive index changes. This concept can pave the way to fully integrated biosensors with small footprint and high sensitivities. Here, we show that by optimizing the layer structure of the Ge-on-Si PIN photodiode as well as the geometry of the nanohole arrays, we can achieve high and narrow optical responsivity peaks with Fano-resonance lineshapes. We used an iterative, simulation-based approach in order to optimize the photodiode layer structure as well as the nanohole geometry and compared the predictions to measurement results obtained from fabricated devices. Our fabricated device shows large peak shifts upon changes in the superstrate refractive index. Finally, we argue that the improved device setup can have potential applications in a sensor system that uses an LED light source and estimate the limit of detection to be 2.2·10^−5 RIU.

Bottom, Top, or in Between: Combining Plasmonic Nanohole Arrays and Hydrogel Microgels for Optical Fiber Sensor Applications

AbstractAttractive label‐free plasmonic optical fiber sensors can be developed by cleverly choosing the arrangement of plasmonic nanostructures and other building blocks. Here, the final response depends very much on the alignment and position (stacking) of the individual elements. In this work, three different types of fiber optic sensing geometries fabricated by simple layer‐by‐layer stacking are presented, consisting of stimulus‐sensitive poly‐N‐isopropylacrylamide (polyNIPAM) microgel arrays and plasmonic nanohole arrays (NHAs), namely NHA/polyNIPAM, polyNIPAM/NHA, polyNIPAM/NHA/polyNIPAM. Their optical response to a representative stimulus, namely temperature, is investigated. NHA/polyNIPAM monitors the volume phase transition of polyNIPAM microgels through changes in the spectral position and the amplitude of the reflection minimum of plasmonic NHA. In contrast, polyNIPAM/NHA shows a more complex response to the swelling and collapse of polyNIPAM microgels in their reflectance spectra. The most pronounced changes in optical response are observed by monitoring the amplitude of the reflectance minimum of this sensor during heating/cooling cycles. Finally, the triple stack of polyNIPAM/NHA/polyNIPAM at the end of a optical fiber tip combines the advantages of the NHA/polyNIPAM, polyNIPAM/NHA double stacks for optical sensing. The unique layer‐by‐layer stacking of microgel and nanostructure is customizable and can be easily adopted for other applications.

Extrinsic Chirality and Circular Dichroism at Visible Frequencies Enabled by Birefringent α-MoO3Nanoscale-Thick Films: Implications for Chiro-Optical Control

Nanophotonics community has shown great interest in 2D materials because of their unique properties of electromagnetic field manipulation. Many of these materials exhibit strong natural anisotropy, which further opens possibilities of polarization manipulation. Here, we show that α-MoO3, an emerging natural hyperbolic 2D material, can be combined with plasmonic nanostructures to provide strong extrinsic chirality in the visible range. A combination of biaxial anisotropy in α-MoO3 and Fabry–Perot cavities with nanoscale features leads to different absorption of left and right circularly polarized photons, hence exhibiting circular dichroism (CD). Our simulation results predict that multilayer nanoscale-thick films including α-MoO3 are potential candidates for achieving extrinsic chirality across the visible range. Furthermore, we show a significant CD increase when the α-MoO3 layer is coupled with plasmonic nanohole arrays or plasmonic nanocubes. Such designs are achiral in geometry and therefore easier to fabricate. Moreover, we optimize the CD dissymmetry factor gCD for the nanocube-based design at 780 nm, obtaining 84%. We believe that utilizing biaxially anisotropic α-MoO3 films to control and engineer chiro-optical properties in the visible frequency range will open research directions and enable enhanced functionalities in chiro-optical control at the nanoscale, further leading to applications in chiral sensing and CD.

Uncovering surface plasmon optical resonances in nanohole arrays through interferometric photoemission electron microscopy

The role of surface plasmon polaritons (SPPs) in nanohole array optical extinction spectra is explored using a time-resolved technique capable of isolating the air/metal interfacial SPP contribution to the typical Fano profile in optical transmission curves. A pair of interferometrically locked broad-band femtosecond pulses is used to launch SPPs from lithographically patterned plasmonic nanohole arrays. SPPs launched in the co- and counter-propagating directions are probed using a third probe pulse in a photoemission electron microscope. Using this approach, we record interferometric SPP–SPP linear autocorrelations that selectively report on the resonances of SPPs launched from arrays of varying pitches and hole diameters. Aside from advancing an approach to selective SPP spectroscopy, we illustrate that resonant coupling in the counter-propagating direction may be exploited to control the spatial, temporal, and spectral characteristics of SPPs. For the counter-propagating direction, we show that tuning the array pitch near the fundamental plasmon resonance generates color-tuned (∼770–820 nm), narrow bandwidth SPPs, and the bandwidth may be controlled by changing the ratio of pitch to hole diameter. The SPP resonances we recover through Fourier transforms of the interferometric autocorrelations shed light on the classical problem of Fano interference in nanohole array extinction spectra.

A theoretical study of broadband extraordinary optical transmission in gold plasmonic square nanohole arrays and its application on refractive index sensor

A theoretical study of broadband extraordinary optical transmission in gold plasmonic square nanohole arrays and its application on refractive index sensor

Modelling conformational dynamics of enzymes trapped in a plasmonc hotspot

Recently we demonstrated that trapping single proteins from solution in a plasmonic double nanohole (DNH) and monitoring the intensity changes of the transmitted light makes it possible to resolve their conformational changes. The power of this technique lies in the possibility of studying unmodified enzymes at work for several minutes up to hours with a temporal resolution of at least 40 μs. Here, we show that for active enzymes that undergo conformational changes, a combination of automated signal analysis and kinetic modelling allows to take a step further: reconstructing the reaction cycle from the detected sequence of conformations.Two llustrative examples will be discussed: adenylate kinase (AdK) and citrate synthase (CS).

Plasmonic Nanohole Arrays with Enhanced Visible Light Photoelectrocatalytic Activity

Plasmonic Nanohole Arrays with Enhanced Visible Light Photoelectrocatalytic Activity

Exploring near-field sensing efficiency of complementary plasmonic metasurfaces for immunodetection of tumor markers

Plasmonic metasurface biosensors have great potential on label-free high-throughput clinical detection of human tumor markers. In the past decades, nanopillar and nanohole metasurfaces have become the common choices for plasmonic biosensing, because they typically enable universal simple large-area nanopatterns via a low-cost reproducible fabrication manner. The two kinds of metasurfaces have the complementary shapes and are used to be assumed as the same type of two-dimensional plasmonic nanograting for biosensing. Up to date, there is still a lack of comparison study on their biosensing performance, which is critical to guide their better applications on tumor marker detection. In this study, we compare the bulk/surface refractive index and sensitivity of plasmonic nanopillar (PNP) and plasmonic nanohole (PNH) metasurfaces in order to evaluate their biosensing capabilities. The sensing physics about their space near-field utilization is systematically revealed. The PNH metasurface demonstrates a higher biomolecule sensitivity versus the complementary PNP metasurface, and its limit of detection for bovine serum albumin reaches ∼0.078 ng/mL, which implies a greater potential of detecting cancer biomarkers. We further adopt the PNH metasurfaces for immunoassay of three typical tumor markers by testing clinical human serum samples. The results imply that the immunodetection of alpha-fetoprotein has the most optimal sensing efficiency with the lowest detection concentration (<5 IU/mL), which is much lower than its clinical diagnosis threshold of ∼16.5 IU/mL for medical examination. Our work has not only illuminated the distinct biosensing properties of complementary metasurfaces, but also provided a promising way to boost plasmonic biosensing for point-of-care testing.

Switchable polarization manipulation at the telecom wavelength based on L-shaped hybrid Au-VO2 nanoholes.

We propose to achieve switchable polarization manipulation at the telecom wavelength at nanoscale based on L-shaped plasmonic nanoholes in an Au-VO2 film. The L-shaped nanohole acts as a quarter-wave plate or a half-wave plate owing to the phase differences between different plasmon resonant modes, which is controlled by the insulator or metallic phases of VO2. In addition, by changing the structure and removing the bottom Au layer, a switchable full-/quarter-wave plate can be achieved when VO2 transits from the insulating state to the metallic state. Furthermore, we vary the geometrical parameters of the L-shaped hole to tune its resonant spectra and achieve a switchable full-wave plate/polarizer. The multifunctional switchable polarization manipulation abilities together with large bandwidths enable the proposed structures promising applications in nanophotonics and integrated optics.