Nanodisk Arrays Research Articles

Plasmonic gold disk nanoarrays-based surface lattice resonance sensor with high figure of merit and its near-field and far-field coupling characteristics

In the field of localized surface plasmon resonance sensing, narrow linewidth, high-quality factor, and high absorptivity have always been the goals that researchers are committed to achieving. The primary methods for achieving this goal are near-field coupling and far-field coupling. Due to the limitations of nanomanufacturing processes, current research often only focuses on single coupling methods such as far-field coupling or near-field coupling, with few relevant reports combining the two coupling methods. Based on this, this paper proposes a single-layer gold nano-disk array structure, which is simple and easy to manufacture. Through modal field analysis and structural parameter adjustment, some significant results have been achieved. On the one hand, the suggested gold nano-disk array structure yields a resonant peak based on far-field coupling with a full width at half maximum (FWHM) of less than 0.05 nm and a figure of merit of more than 15 000 (the quality factor is more than 20 000). On the other hand, it has been successfully accomplished to combine near-field coupling and far-field coupling, leading to a composite coupling peak that incorporates the benefits of both types of coupling. Moreover, this coupling peak has excellent characteristics such as narrow FWHM (less than 3 nm) and high absorptivity (up to 80.5%), which is superior to most reported sensing structures and more in line with practical sensing application requirements. In this paper, the potential of the gold disk array structure as a high-performance sensing structure is further explored, providing reference and new ideas for the design of easy-to-prepare and high-performance electromagnetic resonance devices.

Optical absorption regulation of Ag nanodisc arrays in a monolayer MoS2 hybrid system

Optical absorption regulation of Ag nanodisc arrays in a monolayer MoS2 hybrid system

The anapole state excited by an oblique incidence

Anapole states supported by high-refractive-index dielectric nanoparticles have mostly been studied under normal incidence, but this work explores the oblique incidence excitation. For a single silicon nanodisk, as the incident angle (θ) increases, the anapole wavelength undergoes a gradual blueshift, while the wavelength of maximum near-field enhancement remains almost unchanged with increasing E-field enhancement factor (|E/E 0 |) due to phase retardation effect caused by oblique incidence, and some unique features in field distributions differed from normal excitation are exhibited. In the case of a silicon nanodisk array, the anapole state and near-field enhancement are affected by near-field coupling and the phase retardation effect is weakened. With increasing θ, the coupling between the units is enhanced, and the anapole wavelength and maximum field enhancement wavelength both blue shift. The field distributions in anapole wavelength and maximum enhancement wavelength have obvious near-field coupling characteristics. Oblique incident excitation gives us a deeper understanding of anapole state and may have potential applications in nanophotonics.

Label-Free Detection of Ultra-Low Weight Molecules Based on Fiber Optic Sensors With Low Loss Dielectric Nanostructures

Fiber optic (FO) sensors have great potential for low-cost, microsample testing and compact medical diagnostics. In this work, a FO sensor based on dielectric nanostructure is proposed and demonstrated based on the unique characteristics of dielectric materials, such as low ohmic loss and good biocompatibility. Specifically, we fabricate a SPR-FO sensor by inserting a sleeve with a large-scale, highly ordered dielectric nanodisk array at the end of the fiber probe. And we systematically investigate the sensing performance of the FO sensor including bulk and surface sensitivities. It is found that due to very low absorption losses, the FO sensor based on dielectric nanodisk exhibits a narrow resonance, thus leading to a sensing figure-of-merit almost twice larger than that of based on the metal plasmonic nanodisk. Also, the FO sensor shows a linear response to adsorbate layer thickness changes, which is clearly advantageous for quantitative analysis. Due to the highly localized electromagnetic fields, it can detect small molecules with a concentration of 1 μM, in which a molecular layer with thickness less than 1 nm can produce resonance changes as large as 6 nm. Our work could open a new opportunity for future bio-compatible, robust and low-loss medical diagnostic devices.

Enhancing emission in a QD–nanodisk system via the alignment of the orientation of excitons with the polarization of Mie modes

A Ge quantum dot (QD)–resonator system is an attractive solution for the Si-based emitter applied in monolithic optical–electronic integrated circuits, in which efficient coupling of QDs and cavity modes is crucial for achieving significant emission enhancement. Here, we present unique emission characteristics of a Ge QD–nanodisk array system based on the spatial matching of the two components. In this system, the two-order-of-magnitude emission enhancement is derived from self-assembled QDs, which is related to the orientation of excitons in QDs coinciding with a vertical-polarized Mie mode. In addition, the emission efficiency is also strongly dependent on the position of the QDs in nanodisks. As the orientation of the emitter is matched with the polarized mode, and the QD is located at the maximum of the field, optimal spatial matching can be achieved. These experimental and simulated results demonstrate the importance of spatial matching between emitters with proper polarization and cavity modes, which promises the realization of efficient on-chip light sources.

Photosensitizing Metasurface Empowered by Enhanced Magnetic Field of Toroidal Dipole Resonance.

Photochemical reaction exploiting an excited triplet state (T1 ) of a molecule requires two steps for the excitation, i.e., electronic transition from the ground (S0 ) to singlet excited (S1 ) states and intersystem crossing to the T1 state. A dielectric metasurface coupled with photosensitizer that enables energy efficient photochemical reaction via the enhanced S0 →T1 magnetic dipole transition is developed. In the direct S0 →T1 transition, the photon energy of several hundreds of meV is saved compared to the conventional S0 → S1 →T1 transition. To maximize the magnetic field intensity on the surface, a silicon (Si) nanodisk array metasurface with toroidal dipole resonances is designed. The surface of the metasurface is functionalized with ruthenium (Ru(II)) complexes that work as a photosensitizer for singlet oxygen generation. In the coupled system, the rate of the direct S0 →T1 transition of Ru(II) complexes is 41-fold enhanced at the toroidal dipole resonance of a Si nanodisk array. The enhancement of a singlet oxygen generation rate is observed when the toroidal dipole resonance of a Si nanodisk array is matched with the direct S0 →T1 transition wavelength of Ru(II) complexes.

Mode-Coupling Generation Using ITO Nanodisk Arrays with Au Substrate Enabling Narrow-Band Biosensing.

Plasmonic metal nanostructures have promising applications in biosensing due to their ability to facilitate light-matter interaction. However, the damping of noble metal leads to a wide full width at half maximum (FWHM) spectrum which restricts sensing capabilities. Herein, we present a novel non-full-metal nanostructure sensor, namely indium tin oxide (ITO)-Au nanodisk arrays consisting of periodic arrays of ITO nanodisk arrays and a continuous gold substrate. A narrow-band spectral feature under normal incidence emerges in the visible region, corresponding to the mode-coupling of surface plasmon modes, which are excited by lattice resonance at metal interfaces with magnetic resonance mode. The FWHM of our proposed nanostructure is barely 14 nm, which is one fifth of that of full-metal nanodisk arrays, and effectively improves the sensing performance. Furthermore, the thickness variation of nanodisks hardly affects the sensing performance of this ITO-based nanostructure, ensuring excellent tolerance during preparation. We fabricate the sensor ship using template transfer and vacuum deposition techniques to achieve large-area and low-cost nanostructure preparation. The sensing performance is used to detect immunoglobulin G (IgG) protein molecules, promoting the widespread application of plasmonic nanostructures in label-free biomedical studies and point-of-care diagnostics. The introduction of dielectric materials effectively reduces FWHM, but sacrifices sensitivity. Therefore, utilizing structural configurations or introducing other materials to generate mode-coupling and hybridization is an effective way to provide local field enhancement and effective regulation.

Dynamic Tunable Deflection of Radiation Based on Epsilon-Near-Zero Material

Epsilon-near-zero nanoantennas can be used to tune the far-field radiation pattern due to their exceptionally large intensity-dependent refractive index. In this study, we propose a hybrid optical antenna based on indium tin oxide (ITO) to enable optical tuning of the deflection of radiation, specifically a hybrid structure antenna of ITO and dielectric material, which makes the deflection angle changes 17∘ as incident intensities increase. Moreover, by employing an array of ITO or hybrid nanodisks, we can enhance the unidirectionality of the radiation pattern, resulting in a needle-like shape with an angular beam width α< 8∘ of the main lobe. The deflection angle of the radiation pattern response with the needle-like lobe paves the way for further studies and applications in beam steering and optical modulation where dynamic control of the nanoantennas is highly desirable.

Anapole-assisted ultra-narrow-band lattice resonance in slotted silicon nanodisk arrays

Anapole modes supported by well-designed dielectric nanostructures have attracted extensive attention in the field of nanophotonic applications owing to their unique strong near-field enhancement and non-radiative far-field scattering characteristics, yet it is still difficult to achieve high Q-factor resonance features with a narrow linewidth. In this work, a periodic slotted silicon nanodisk array is theoretically proposed to realize narrow linewidth and high Q-factor resonance in the near-infrared wavelength range. Through introducing the coupling between the anapole modes in the single dielectric nanostructure and the diffractive wave mode arising from the periodic array, the as-designed dielectric nanostructure synchronously manifests excellent spectral features with a bandwidth as narrow as about 2.0 nm, a large Q-factor of 599, an almost-perfect transmission amplitude of 96% and a relatively high electric field intensity (>2809 times) in the middle of the slotted silicon nanodisk. The as-designed nanostructure possessing these outstanding optical features can work as a high-efficiency refractive index sensor, whose sensitivity can reach 161.5 nm RIU−1 with its figure of merit attaining 80.8 RIU−1, efficiently distinguishing an index change of less than 0.01. The proposed slotted silicon nanodisk array exhibits tremendous potential for expanding applications such as label-free biochemical sensing, plasmonic refractive index sensing and surface enhancement spectroscopy.

Numerical simulation of efficient solar absorbers and thermal emitters based on multilayer nanodisk arrays

In this work, we design a solar absorber and thermal emitter with ultra-broadband perfect absorption and high thermal radiation efficiency. The solar absorber has a high absorption efficiency of 91.5 % in the full wavelength range (280–4000 nm), and the weighted average absorption efficiency (AM1.5) is as high as 99 % by the finite difference time domain method (FDTD) simulation calculation. And the absorption bandwidth with absorption efficiency greater than 90 % reaches 2929 nm (280–3209 nm), and the average absorption efficiency in this band is as high as 97.4 %. Such strong absorption is due to the plasmon resonance and near-field coupling of the multilayer nanodisks. In addition to being used in solar absorbers, the structure also has potential applications in thermal emitters. The novelty of this work is to explore the application of the structure in the field of thermal radiation, and the optimal working temperature of the structure for thermal radiation is obtained. Through calculation, we approximately consider that the optimal working temperature of the structure as a thermal emitter is 2000 K, and the thermal radiation efficiency at this temperature is 94.8 %. The structure proposed by us is polarization-independent, insensitive to incident angle changes, the absorption spectra of transverse electric (TE) mode and transverse magnetic (TM) mode are the same, and the absorption efficiency remains 80 % even when the incident angle is increased to 60°. The excellent performance makes the structure widely used in the fields of solar energy absorption and emission.

Nanoscale Thermometry of Plasmonic Structures via Raman Shifts in Copper Phthalocyanine.

Temperature measurements at the nanoscale are vital for the application of plasmonic structures in medical photothermal therapy and materials science but very challenging to realize in practice. In this work, we exploit a combination of surface-enhanced Raman spectroscopy together with the characteristic temperature dependence of the Raman peak maxima observed in β-phase copper phthalocyanine (β-CuPc) to measure the surface temperature of plasmonic gold nanoparticles under laser irradiation. We begin by measuring the temperature-dependent Raman shifts of the three most prominent modes of β-CuPc films coated on an array of Au nanodisks over a temperature range of 100-500 K. We then use these calibration curves to determine the temperature of an array of Au nanodisks irradiated with varying laser powers. The extracted temperatures agree quantitatively with the ones obtained via numerical modeling of electromagnetic and thermodynamic properties of the irradiated array. Thin films of β-CuPc display low extinction coefficients in the blue-green region of the visible spectrum as well as exceptional thermal stability, allowing a wide temperature range of operation of our Raman thermometer, with minimal optical distortion of the underlying structures. Thanks to the strong thermal response of the Raman shifts in β-CuPc, our work opens the opportunity to investigate photothermal effects at the nanoscale in real time.

Dynamic selection of visible wavelengths using resonant TiO2 nanostructures

Abstract All-dielectric nanoarrays have strong electromagnetic resonances with various interesting applications and are tuned by adjusting their geometrical parameters. However, their optical properties are permanently encoded during fabrication. This study presents robust dynamically tunable all-dielectric nanoresonators for controllable, reversible, and reproducible color filtering. Our design uses an array of TiO2 nanodiscs embedded in a transparent, stretchable polydimethylsiloxane (PDMS) membrane and exhibits a narrow spectral response due to Mie magnetic and electric dipole resonances hybridized with the TiO2 nanodiscs lattice modes. By mechanically stretching the PDMS membrane, the pitch of the TiO2 nanodiscs was increased and the spectral location of the resonances was altered. Additionally, an optically asymmetric structure was fabricated by partially embedding TiO2 nanodiscs in PDMS. Thus, the magnitude of the Rayleigh anomaly diffraction, which could interrupt the dipole resonances, was reduced. Our design has sharp, frequency-tunable resonances in the visible spectrum, and we demonstrated dynamic tunability by stretching the metasurfaces.

Bound states in the continuum (BIC) in silicon nanodisk array on mirror structure: Perfect absorption associated with quasi-BIC below the bandgap

A structure composed of a hexagonal array of Si nanodisks having toroidal dipole resonances and a reflecting mirror separated by a SiO2 spacer is proposed as a platform that exhibits narrow-band perfect absorption in the Si sub-bandgap wavelength range for a CMOS compatible Si based photodetector operating below the bandgap range. The numerical simulation reveals that the structure possesses Fabry–Pérot bound states in the continuum at proper spacer thicknesses due to the interference between the toroidal dipole and its image dipole. By slightly detuning the spacer thickness to meet the critical coupling condition, narrow-band perfect absorption appears despite assumption of a very small extinction coefficient (5 × 10−4). The wavelength of the perfect absorption is controlled in a wide range by the structural parameters of a Si nanodisk hexagonal array and is insensitive to the fluctuation of the extinction coefficient and the choice of a metallic mirror. In the structure, over 90% of incident power can be absorbed in the Si region. This suggests that the structure can be used as a narrow-band photodetector operating in the Si sub-bandgap wavelength range. We also evaluate the sensing performance of the proposed structure as an intensity based refractive index sensor operating in the near-infrared range.

Multiphysics Modeling of Plasmon-Enhanced All-Optical Helicity-Dependent Switching.

In this work, we propose a multiphysics approach to simulate all-optical helicity-dependent switching induced by the local hot spots of plasmonic nanostructures. Due to the plasmonic resonance of an array of gold nanodisks, strong electromagnetic fields are generated within the magnetic recording media underneath the gold nanodisks. We construct a multiphysics framework considering the opto-magnetic and opto-thermal effects, and then model the magnetization switching using the Monte Carlo method. Our approach bridges the gap between plasmonic nanostructure design and magnetization switching modeling, allowing for the simulation of helicity-dependent, nanoscale magnetization switching in the presence of localized surface plasmons.

Bandwidth-tunable absorption enhancement of visible and near-infrared light in monolayer graphene by localized plasmon resonances and their diffraction coupling

Bandwidth-tunable light absorption enhancement in monolayer graphene is practically important for graphene-based photoelectric devices. Especially, it is still a huge challenge to achieve high-efficiency graphene absorption with extremely narrow sub-nanometer bandwidth much smaller than one nanometer. In this work, both broadband and narrowband absorption peaks of monolayer graphene are numerically achieved in visible and near-infrared wavelength ranges. The broadband absorption peak is ascribed to localized dipolar plasmon resonances in individual silver nanodisks, and the narrowband absorption peak arises from collective first-order diffraction coupling effect in periodic array of silver nanodisks. By changing the array period, the full width at half maximum (FWHM) of the broadband absorption peak can vary from 100 nm to 50 nm. Correspondingly, the FWHM of the narrowband absorption peak is tuned from about 6.4 nm to only 0.25 nm, thus realizing an ultra-narrow sub-nanometer bandwidth.

Plasmonic Resonance Coupling of Nanodisk Array/Thin Film on the Optical Fiber Tip for Integrated and Miniaturized Sensing Detection.

Fiber-optic surface plasmon resonance (FOSPR) sensing technology has become an appealing candidate in biochemical sensing applications due to its distinguished capability of remote and point-of-care detection. However, FOSPR sensing devices with a flat plasmonic film on the optical fiber tip are seldom proposed with most reports concentrating on fiber sidewalls. In this paper, we propose and experimentally demonstrate the plasmonic coupled structure of a gold (Au) nanodisk array and a thin film integrated into the fiber facet, enabling the excitation of the plasmon mode on the planar gold film by strong coupling. This plasmonic fiber sensor is fabricated by the ultraviolet (UV) curing adhesive transferring technology from a planar substrate to a fiber facet. The experimental results demonstrate that the fabricated sensing probe has a bulk refractive index sensitivity of 137.28 nm/RIU and exhibits moderate surface sensitivity by measuring the spatial localization of its excited plasmon mode on Au film by layer-by-layer self-assembly technology. Furthermore, the fabricated plasmonic sensing probe enables the detection of bovine serum albumin (BSA) biomolecule with a detection limit of 19.35 μM. The demonstrated fiber probe here provides a potential strategy to integrate plasmonic nanostructure on the fiber facet with excellent sensing performance, which has a unique application prospect in the detection of remote, in situ, and in vivo invasion.

Tuning Smith-Purcell radiation by rotating a metallic nanodisk array.

Smith-Purcell radiation (SPR) refers to the far-field, strong, spike radiation generated by the interaction of the evanescent Coulomb field of the moving charged particles and the surrounding medium. In applying SPR for particle detection and nanoscale on-chip light sources, wavelength tunability is desired. Here we report on tunable SPR achieved by moving an electron beam parallel to a two-dimensional (2D) metallic nanodisk array. By in-plane rotating the nanodisk array, the emission spectrum of the SPR splits into two peaks, with the shorter-wavelength peak blueshifted and the longer-wavelength one redshifted by increasing the tuning angle. This effect originates from the fact that the electrons fly effectively over a one-dimensional (1D) quasicrystal projected from the surrounding 2D lattice, and the wavelength of SPR is modulated by quasiperiodic characteristic lengths. The experimental data are in agreement with the simulated ones. We suggest that this tunable radiation provides free-electron-driven tunable multiple photon sources at the nanoscale.

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.

Plasmon-Enhanced Raman Scattering by Multilayered Graphene at the Micro- and Nanoscale: SERS and TERS Analysis

This work is devoted to the study of plasmon-enhanced Raman scattering by the fundamental vibrational modes of multilayered graphene films. The film thickness was ∼3.5 nm, which corresponds to ∼10 monolayers. Multilayered graphene films were placed on a plasmonic substrate consisting of arrays of gold nanodisks (50–250 nm in diameter). Surface-enhanced Raman scattering by the main vibrational modes of multilayered graphene film placed on an array of Au nanodisks of various sizes was implemented. The measurements were performed at excitation wavelengths of 532, 638, and 785 nm. A resonant SERS enhancement of the main vibrational modes of multilayered graphene by a factor of 25 was achieved for nanodisks with a diameter of 103 nm upon excitation at 638 nm. A stronger local enhancement of Raman scattering in multilayered graphene (by a factor of 50) placed on Au nanodisk array is achieved using gap-mode tip-enhanced Raman scattering (gap-mode TERS). Nanofolds in the graphene film appeared due to the corrugated surface of the plasmonic substrate were visualized with nanometer spatial resolution. It is shown that the frequency positions of G and 2D modes of nanofolds decrease with respect to the corresponding values in flat multilayered graphene manifesting mechanical stresses in the nanofolds up to 0.7%. The results obtained shed light on the effects of the interaction of multilayered graphene with metal nanostructures and are important in creating hybrid metal/graphene plasmonic substrates.

Numerical Analysis of Three-dimensional Nanodisk Array-based Surface Plasmon Resonance Biosensors for SARS-CoV-2 Detection.

The online version contains supplementary material available at 10.1007/s11468-023-01802-3.