Gap Plasmon Resonators Research Articles

Statistical and Correlative Characterization of Individual Nanoparticle in Gap Plasmon Resonator Sensors

AbstractPlasmonic nanocavities are receving increased attention from the nanophotonics, nanoelectronics, and quantum optics community due to their ability to confine light in extremely small volumes. Coupling colloidal plasmonic nanocrystals to metal films is an inexpensive approach to fabricate virtually unlimited individual resonators that can be tuned by adjusting nanoparticle size, gap thickness, and refractive index. The focus is on silver nanocubes separated from a gold mirror by thin amorphous dielectric layers (Al2O3, TiO2). The optical response is measured and correlative electron microscopy is performed on a large number (>800) of individual resonators to unveil the statistical distribution of gap plasmon modes and assess systematically their sensitivity. A sensitivity as large as 8 nm/nm to nanocube size variation, 50 nm/nm to Al2O3 thickness variation, and 130 nm/RIU for index change of the spacer layer is found. With the help of numerical simulations, this approach enables to infer a quantitative statistical distribution of molecular coatings present on the nanocubes surface, such as polyvinylpyrrolidone, which can affect the performance of nanoelectronic devices and assess the strategy to remove it. Finally, polarization‐resolved measurements enable to unveil a birefringent behavior when nanocubes are supported on annealed TiO2 layers (2–4 nm).

Adaptive Gap-Tunable Surface-Enhanced Raman Spectroscopy.

Gap plasmon (GP) resonance in static surface-enhanced Raman spectroscopy (SERS) structures is generally too narrow and not tunable. Here, we present an adaptive gap-tunable SERS device to selectively enhance and modulate different vibrational modes via active flexible Au nanogaps, with adaptive optical control. The tunability of GP resonance is up to ∼1200 cm-1 by engineering gap width, facilitated by mechanical bending of a polyethylene terephthalate substrate. We confirm that the tuned GP resonance selectively enhances different Raman spectral regions of the molecules. Additionally, we dynamically control the SERS intensity through the wavefront shaping of excitation beams. Furthermore, we demonstrate simulation results, exhibiting the mechanical and optical properties of a one-dimensional flexible nanogap and their advantage in high-speed biomedical sensing. Our work provides a unique approach for observing and controlling the enhanced chemical responses with dynamic tunability.

A visible light driven photocatalyst Ag/AgBr/SrSb2O6 for removal of model organic contaminants and hexavalent chromium

A single plausible solution to multi-environmental issues is an efficient photocatalyst. The SrSb2O6 with a band gap of 2.72 eV was successfully tailored to 1.99 eV by deposition of Ag/AgBr. The synergistic effect of reduced band gap and surface plasmon resonance (SPR) effect of Ag0 could enhance the photocatalytic efficiency to a greater extent. This work attempts to explore a composite photocatalyst Ag/AgBr/SrSb2O6 for the photodegradation of organic wastes such as dyes, organic solvents, drugs, and antibiotics. In addition, the photo remediation of heavy metal waste such as Cr6+ is also investigated. The propitious results promise Ag/AgBr/SrSb2O6 to be a wide-range photocatalyst that can eliminate both organic and inorganic waste from water. The photodegradation of RhB dye, phenol, and tetracycline reached up to 80.8%, 76.8%, and 64.5%, respectively. Notably, the Ag/AgBr/SrSb2O6 catalyst effectively converted 97.3% of lethal Cr6+ to benign Cr3+. The better electron hole pair separation of the composite was evidenced by PL spectra.

Regulation of band gap and localized surface plasmon resonance by loading Au nanorods on violet phosphene nanosheets for photodynamic/photothermal synergistic anti-infective therapy.

In the face of the serious threat to human health and the economic burden caused by bacterial antibiotic resistance, 2D phosphorus nanomaterials have been widely used as antibacterial agents. Violet phosphorus nanosheets (VPNSs) are an exciting bandgap-adjustable 2D nanomaterial due to their good physicochemical properties, yet the study of VPNS-based antibiotics is still in its infancy. Here, a composite of gold nanorods (AuNRs) loaded onto VPNS platforms (VPNS/AuNR) is constructed to maximize the potential of VPNSs for antimicrobial applications. The loading with AuNRs not only enhances the photothermal performance via a localized surface plasmon resonance (LSPR) effect, but also enhances the light absorption capacity due to the narrowing of the band gap of the VPNSs, thus increasing the ROS generation capacity. The results demonstrate that VPNS/AuNR exhibits outstanding antibacterial properties and good biocompatibility. Attractively, VPNS/AuNR is then extensively tested for treating skin wound infections, suggesting promising in vivo antibacterial and wound-healing features. Our findings may open a novel direction to develop a versatile VPNS-based treatment platform, which can significantly boost the progress of VPNS exploration.

Multifunctional DNA scaffold mediated gap plasmon resonance: Application to sensitive PD-L1 sensor

The introduction of noble metal nanoparticles with good LSPR characteristics can greatly improve the sensitivity of SPR through resonance coupling effect. The plasma resonance response and optical properties of film coupling nanoparticle systems largely depends on the ingenious design of gap structures. Nucleic acid nanostructures have good stability, flexibility, and high biocompatibility, making them ideal materials for gap construction. 2D MOF (Cu-Tcpp) has a large conjugated surface similar to graphene, which can provide a stable substrate for the directional fixation of nucleic acid nanostructures. However, research on gap coupling plasmon based on nucleic acid nanostructures and 2D MOF is still rarely reported. By integrating the advantages of Cu-Tcpp assembled film and DNA tetrahedron immobilization, a nano gap with porous scaffold structure between the gold film and gold nanorod was build. The rigidity of DNA tetrahedron can precisely control the gap size, and its unique programmability allows us to give the coupling structure greater flexibility through the design of nucleic acid chain. The experimental results and FDTD simulation show that the film coupling nanoparticle systems constructed with DNA tetrahedrons greatly enhance the electric field strength near the chip surface and effectively improve the sensitivity of SPR. This research shows the huge potential of nucleic acid nanomaterials in the construction of SPR chip surface microstructures.

Polarization- and Angle-Insensitive Tunable Metasurface for Electro-Optic Modulation

Tunable metasurfaces provide enormous degrees-of-freedom for dynamic control of light–matter interactions at sub-wavelength scales. However, the realization of polarization- and angle-insensitive active nanophotonic devices with low energy-consumption and high modulation depth remains a challenge. This letter reports gap plasmon resonance based electro-tunable metasurface for optical intensity modulation. The proposed metasurface features a 2D array of Au-nanograting on top of indium-tin-oxide (ITO)–Al <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> –Au stack. The external electrical biasing accumulates the free-carriers of ITO at ITO–Al <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> interface, which results in an electro-tunable resonance response in the reflectance spectrum. With a 2V electrical biasing, we observe the resonance dip shifts to the vicinity of 1.55 μm wavelength. Thus, a modulation depth of ~25 dB at 1.55 μm can be achieved for random polarization-angle of incident light. For the first time, the proposed modulator can operate over a wide range of incidence angle (up to 50 degrees) for both x- and y-polarized light. With ~303 fJ/bit energy-consumption and ~900 Mbps modulation speed, the proposed electro-tunable metasurface could help to realize polarization- and angle-independent active flat optics for nanophotonic systems.

Electro-tunable metasurface for tri-state dynamic polarization switching at near-infrared wavelengths

Control of polarization states of light is crucial for any photonic system. However, conventional polarization-controlling elements are typically static and bulky. Metasurfaces open a new paradigm to realize flat optical components by engineering meta-atoms at sub-wavelength scale. Tunable metasurfaces can provide enormous degrees-of-freedom to tailor electromagnetic properties of light and thus have the potential to realize dynamic polarization control in nanoscale. In this study, we propose a novel electro-tunable metasurface to enable dynamic control of polarization states of reflected light. The proposed metasurface comprises a two-dimensional array of elliptical Ag-nanopillars deposited on indium-tin-oxide (ITO)–Al2O3–Ag stack. In unbiased condition, excitation of gap-plasmon resonance in the metasurface leads to rotation of x-polarized incident light to orthogonally polarized reflected light (i.e., y-polarized) at 1.55 μm. On the other hand, by applying bias-voltage, we can alter the amplitude and phase of the electric field components of the reflected light. With 2 V applied bias, we achieved a linearly polarized reflected light with a polarization angle of −45°. Furthermore, we can tune the epsilon-near-zero wavelength of ITO at the vicinity of 1.55 μm wavelength by increasing the bias to 5 V, which reduces y-component of the electric field to a negligible amplitude, thus, resulting in an x-polarized reflected light. Thus, with an x-polarized incident wave, we can dynamically switch among the three linear polarization states of the reflected wave, allowing a tri-state polarization switching (viz. y-polarization at 0 V, −45° linear polarization at 2 V, and x-polarization at 5 V). The Stokes parameters are also calculated to show a real-time control over light polarization. Thus, the proposed device paves the way toward the realization of dynamic polarization switching in nanophotonic applications.

Highly sensitive and polarization-insensitive terahertz microfluidic biosensor based on gap plasmon model

The gap plasmon resonance can generate a local enhanced electric field and make the electromagnetic field energy mostly confined in the microfluidic, thereby increasing the interaction between the detection sample and terahertz wave. In this paper, a five-layer microfluidic terahertz biosensor based on gap plasmon resonance is presented. The simulation results show that the designed biosensor can realize a resonant absorption peak with near-perfect absorption at the frequency of 0.655 THz and high refractive index sensitivity of 191.3 GHz/RIU. In addition, the biosensor possesses the characteristics of polarization insensitivity and 50° incidence angle insensitivity due to the symmetry of the metal metasurface, which make the biosensor can be widely used in the biological label-free detection, medical trace element detection.

Electrically driven nanogap antennas and quantum tunneling regime

Abstract The optical and electrical characteristics of electrically-driven nanogap antennas are extremely sensitive to the nanogap region where the fields are tightly confined and electrons and photons can interplay. Upon injecting electrons in the nanogap, a conductance channel opens between the metal surfaces modifying the plasmon charge distribution and therefore inducing an electrical tuning of the gap plasmon resonance. Electron tunneling across the nanogap can be harnessed to induce broadband photon emission with boosted quantum efficiency. Under certain conditions, the energy of the emitted photons exceeds the energy of electrons, and this overbias light emission is due to spontaneous emission of the hot electron distribution in the electrode. We conclude with the potential of electrically controlled nanogap antennas for faster on-chip communication.

Thermosensitive Plasmonic Color Enabled by Sodium Metasurface

AbstractActive plasmonic nanostructures have attracted tremendous interest in nanophotonics and metamaterials owing to the dynamically switchable capabilities of plasmonic resonances. In this study, tunable hybrid plasmon resonances (HPR) of sodium metasurfaces through heat‐initiated structural transformation is experimentally demonstrated. A HPR is formed by coupling surface plasmon polaritons (SPP) and gap plasmon resonances (GPR), whose resonant wavelengths are highly sensitive to gaseous nanogaps. By carefully manipulating the thermo‐assisted spin‐coating process and post‐thermal treatment, tuning of the HPR is achieved because of the phase transition between the antidome and nanodome structural profiles of liquid sodium inside the patterned fused silica substrates. Furthermore, the figure of merit of the heat initiated variable structure‐spectrum is demonstrated that is highly dependent on the size of the substrate patterns, based on which temperature‐sensitive plasmonic color and “invisible ink” of sodium metasurfaces are demonstrated. These findings can lead to new solutions for manipulating low‐cost and high‐performance active plasmonic devices.

Preparation of Au/ZnO/Fe3O4 Composite for Degradation of Tartrazine under Visible Light

Zinc oxide has been shown to be a potential photocatalyst under UV light but its catalytic activity is limited under visible light due to its wide bandgap energy and rapid recombination of electrons and holes. Besides the catalytic recovery is a challenging issue because of its dispersion in solution. Previous work has shown that the interaction of gold nanoparticles with ZnO can reduce the band gap energy (Eg) and plasmon resonance (SPR) as well as the formation of the Schottky barrier in Au/ZnO composite can reduce the recombination of electrons and holes. In this study, Au/ZnO/Fe3O4 (AZF) composites were prepared by a simple mixing method using polyvinyl alcohol (PVA) as a binder. As-prepared composites were characterized by Scanning Electron Microscope (SEM), Energy Dispersive X-ray Spectroscopy (EDS), X-ray Diffraction (XRD), UV-Vis Diffuse Reflectance (UV-Vis-DR), and Fourier Transform Infra Red (FT-IR). The catalytic efficiency of as-prepared samples was evaluated through the decomposition of tartrazine (TA), a colorant that is difficult to decompose in wastewater and has harmful effects on human health. The effects of reaction parameters such as the content of PVA, solution pH, and oxidizing agents (O2 and H2O2) on the catalytic efficiency were studied. The AZF at PVA of 0.0125 g showed the highest performance among as-prepared samples. With the presence of 12 mM H2O2 in the catalyst system, the degradation efficiency and reaction rate of TA in composite increased to 81.5% and 0.020 min−1, respectively. At this condition, photocatalysis and Fenton system catalysis occurred together. The catalytic mechanism of Tartrazine (TA) on composite was proposed and the reaction of TA was studied by the first-order kinetic model. Copyright © 2023 by Authors, Published by BCREC Group. This is an open access article under the CC BY-SA License (https://creativecommons.org/licenses/by-sa/4.0).

Dual-band electro-optic modulator based on tunable broadband metamaterial absorber

Tunable metamaterials unfolded new scopes for active control of light absorption by engineering meta-atoms. In this study, for the first time, we propose a broadband electrically-tunable metamaterial absorber that can simultaneously modulate reflected light amplitude in both the key telecommunication wavelength bands, i.e., O-band and C-band. We leverage gap plasmon resonance (GPR) of a metal–insulator–metal (MIM) structure and epsilon-near-zero (ENZ) phenomenon of indium-tin-oxide (ITO) to enable real-time control of reflected light amplitude. The proposed metamaterial consists of an array of Au-nanograting on top of Al2O3–ITO–SiO2–Au stack. The coupling of GPR-mode with ENZ-mode is observed when an external bias-voltage is applied to accumulate free-carriers at Al2O3–ITO interface. This results in a broadening of resonance with more than 80 % absorption over a 370 nm spectral window ranging from 1.25 μm to 1.62 μm. Such dynamic control of absorption properties is then utilized for amplitude modulation of the reflected light. We achieved amplitude modulation with an extinction ratio as high as 20.5 dB and 16.7 dB at 1.55 μm and 1.31 μm of wavelength, respectively, while switching between 0 V and 5 V biasing conditions. Thus, the tunable metamaterial absorber could help realizing optical modulators operating at different wavelength bands for integrated nanophotonic systems.

Enhanced plasmonic waveguide sensing performance with a semicircular-ring resonator

In this study, a plasmonic refractive index sensor composed of a semicircular ring resonator embedded with silver nanorods (AgNDs) and a metal-insulator-metal (MIM) bus waveguide was developed. The designed system has the advantage of a shorter optical path than a regular circular resonator. Consequently, its size is half that of a typical one. In addition, the basement of the resonator can offer a more extended coupling interface facing the bus waveguide, increasing the gap plasmon resonance between the incident light and AgNDs. The finite element method (FEM) comprehensively simulated the transmittance and electromagnetic wave characteristics. The size of the AgNDs was found to be the main factor influencing the gap plasmon mode in the resonator and contributed to the increased sensitivity of the structure. Compared with its standard counterpart, the designed structure can increase the sensitivity of the device by 223.07%. The calculated maximum sensitivity was 2900 nm/RIU. This work paves the way for designing sensitive plasmonic sensors and could be rival candidates for sensing applications.

Magnetic dipole emission in resonant metal-dielectric-metal structures.

We explore a possibility to control magnetic dipole emission with plasmonic cavities, placing Eu3+ emitters inside profile-modulated metal-dielectric-metal structures. Significant variations in the branching ratio of the magnetic and electric dipole transitions are observed as the function of the thickness of the intermediate layer. The experimental results are confirmed with numerical simulations which account for cavity and gap plasmon resonances and predict modifications in the spontaneous emission spectrum as the function of the gap size and a strong directionality of the emission for small thicknesses of the intermediate layer. The implications of having a competition between electric and magnetic dipole relaxation channels in Eu3+ are discussed.

Efficient photocatalytic oxidation of VOCs using ZnO@Au nanoparticles

The emission of toxic VOCs (Volatile Organic Compounds) in the environment causes serious problems and, if from one side there are directives to reduce this threat, from the other side there are stringent necessities for their removal. Several methods have been proposed to remove toxic VOCs, but little attention has been paid towards their photooxidation using materials with improved photodetection. For these reasons, we account here on the good photocatalytic performance of semiconducting ZnO@Au nanostructures that revealed to be excellent for both ultraviolet and visible light detection, thanks to the wide ZnO band gap and visible Au surface plasmon resonance, towards the photooxidation of some toxic Volatile Organic Compounds. In particular, we tested these peculiar materials for the removal of some common VOCs as toluene, ethanol and formaldehyde under solar light irradiation. The ZnO@Au showed excellent performance in the total oxidation of toluene (95% of conversion) and formaldehyde (85%) with the consequent formation of only CO2 and water as by-products. This good activity was also confirmed in the photooxidation of ethanol that allowed to obtain the 72% of CO2 selectivity. Furthermore, in all tests the catalysts showed a good stability after several consecutive runs. The efficient electronic communication between the gold core and the zinc oxide shell permitted to enhance the overall photodetection of the solar irradiation, improving also the charge carriers separation. The ZnO@Au nanostructures can be considering promising candidates as photocatalysts for air purification.

Ultrawide bandstop filter with high sensitivity using semi-circular-like resonators

This paper proposes a plasmonic ultrawide bandstop filter using a metal-insulator-metal waveguide cooperating with five symmetrical semi-circular-like resonators, including the silver nanorods (AgNDs). We calculated transmittance spectrum and electromagnetic field distribution employing the finite element method. The advantages of the design reveal the surface, cavity, and gap plasmon resonance effects that can simultaneously exist in the proposed device. Our design can achieve a tunable wide-bandgap by adding the AgNDs in resonators and attain an extensive stop wavelength range of 2104 nm, which is the highest bandgap width to our knowledge. Simulations results show that the AgNDs and coupling distance are pivotal factors influencing the bandgap formation, changing the resonators' resonance condition and effective refractive index. Besides, the designed filter can also serve as a nanosensor with an ultrahigh sensitivity of 7980 nm/RIU. Thus, the designed plasmonic filter can be a promising alternative candidate for optical trapping and tunable devices working in visible, near-infrared, and mid-infrared spectra.

Characterization of gap-plasmon-based metasurfaces with scanning white-light interferometry

Subject of study. The possibility of using the well-established method of scanning white-light interferometry (SWLI), implemented as a commercially available device, is studied for characterizing promising metasurfaces based on third-order plasmon resonance. The characteristics of the samples were determined for two types of experimental metasurfaces, namely, for a binary grating with a period of 12.6 µm and a phase gradient grating with a period of 6.75 µm. Objective. The main goals of this study were to test the fundamental possibility of using a commercially available white-light interferometer to determine the phase characteristics of plasmonic metasurfaces and to estimate experimentally the lateral optical resolution in the phase image of metasurfaces. Main results. Although the method of SWLI was originally intended for measuring a geometric microrelief, it was found that it can be used to determine the phase increment of the optical radiation reflected from the studied metasurfaces in the range of 10°–100° with a relative accuracy of approximately 50% without any modification of the signal processing algorithm. The possibility of obtaining a phase mapping of the surface with such accuracy in this configuration is not obvious because the metasurface is a matrix of nanoelements with the same height of 50 nm, and the phase shift in the reflected optical radiation is created due to third-order gap plasmon resonance. On the obtained optical phase images of metasurfaces, unit (single) cells of the metasurface are reliably resolved owing to a sufficiently high lateral resolution (approximately 450 nm). The results of the characterization of metasurfaces obtained using SWLI are compared with those obtained in a laboratory setup using scanning differential heterodyne microscopy (SDHM). Although the SWLI method demonstrates better lateral resolution compared with the SDHM method, it is less accurate in restoring the phase characteristics of metasurfaces with a phase gradient. Practical significance. The results obtained allow us to conclude that the evaluation of the phase optical characteristics of nanostructured plasmonic metasurfaces by the method of SWLI is possible and requires further experimental and theoretical studies.

Near-infrared narrow plasmonic resonances for high-performance optical sensing in a sodium-based nanograting

Plasmonic resonances are attractive and promising for biosensing applications benefiting from their strong nanoscale light confinement and sensitivity to the refractive index of surrounding medium. Here, we numerically and theoretically investigate a metal–insulator-metal (MIM) nanograting structure, where gold nanostrip arrays are placed on a thin dielectric layer with a sodium film underlay. The structure is obliquely excited by a transverse magnetic (TM) wave from a prism supporting the whole MIM structure. We found that the plasmonic surface lattice resonance (SLR) mode and gap plasmon resonance (GPR) mode can be efficiently excited in our structure at the near infrared wavelength. Remarkably, the SLR mode can achieve ultranarrow linewidth less than 3 nm (quality factor > 500), sensitivity over 1000 nm/RIU and the corresponding figure of merit (FOM) larger than 300 RIU−1. Our study may pay a way to achieve high-performance sensing in a simple plasmonic nanostructure that could be vastly used in biological and chemical circumstances.

Enhanced photoluminescence and shortened lifetime of DCJTB by photoinduced metal deposition on a ferroelectric lithography substrate

The photodeposition of metallic nanostructures onto ferroelectric surfaces could enable new applications based on the assembly of molecules and patterning local surface reactivity by enhancing surface field intensity. DCJTB (4-(dicyanomethylene)-2-t-butyl-6-(1,1,7,7-tetramethyljulolidyl-9-enyl)-4H-pyran) is an excellent fluorescent dye and dopant material with a high quantum efficiency used for OLED displays on the market. However, how to raise the photoluminescence (PL) and reduce the lifetime of DCJTB in a substrate remain extraordinary challenges for its application. Here, we demonstrate a tunable ferroelectric lithography plasmon-enhanced substrate to generate photo-reduced silver nanoparticles (AgNPs) and achieve enhanced PL with a shortened lifetime depending on the substrate’s annealing time. The enhanced PL with shortened lifetimes can attribute to the localized electromagnetic (EM) wave produced by the nanotextured AgNPs layers' surface and gap plasmon resonances. The simulation is based on the three-dimensional finite element method to explain the mechanism of experimental results. Since the absorption increases, the remarkable enhanced PL of DCJTB can attain in the fabricated periodically proton exchanged (PPE) lithium niobate (LiNbO3) substrate. Furthermore, the proposed fabrication method demonstrates to help tune the surface EM wave distribution in the substrate, which can simultaneously achieve the significantly shortened lifetime and high PL intensity of DCJTB in the substrate. Compared with the un-annealed substrate, the PL intensity of DCJTB in the assembly metallic nanostructures is enhanced 13.70 times, and the PL’s lifetime is reduced by 12.50%, respectively. Thus, the fabricated substrate can be a promising candidate, verifying chemically patterned ferroelectrics' satisfaction as a PL-active substrate.

Ultra-broadband long-wave infrared metasurface absorber based on Peano fractal curve

Fractals have emerged as a promising platform for realizing broadband absorption metasurfaces. In this paper, we propose an ultra-broadband absorber with high absorption operational in the long-wave infrared regime by means of a second-order Peano fractal curve (PFC) metasurface with a four-layer insulator–metal–insulator–metal (Ge/Cr/Ge/Cr) waveguide structure. The unique space-filling shape of the PFC enables the generation of multiple gap plasmon resonance modes, which expands the bandwidth of the absorber. In addition, the hybridization of these modes with cavity mode resonances and localized surface plasmon resonances enhances the average absorption of the device in the spectrum of operation, with an average absorption of 94.3% obtained for TM-polarized light with wavelengths between 12.3 µm and 32 µm in our simulations. A second-order quasi-PFC is designed for polarization-insensitive broadband absorption. Our findings establish general and systematic strategies for guiding the design of metamaterial absorbers with excellent broadband absorption.