Propagating Surface Plasmon Research Articles

Terahertz surface plasmon generation on a magnetized metal column by a rotating electron-beam

Terahertz surface plasmon generation on a magnetized metal column by a rotating electron-beam

High-efficiency terahertz surface plasmon metacoupler empowered by bilayer bright–dark mode coupling

Conversion from free-space waves to surface plasmons has been well studied as a key aspect of plasmonics. In particular, efficient coupling and propagation of surface plasmons via phase gradient metasurfaces are of great current research interest. Hereby, we demonstrate a terahertz metacoupler based on a bilayer bright–dark mode coupling structure attaining near-perfect conversion efficiency (exceeding 95%) without considering absorption loss of the materials and maintaining a high conversion level even when the area of the excitation region changes. To validate our design, a fabricated metacoupler was assessed by scanning near-field terahertz microscopy. Our findings could pave the way for developing high-performance plasmonic devices encompassing ultra-thin and compact functional devices for a diverse range of applications, especially within the realm of high-speed terahertz communications.

Plasmonic Coupling for High‐Sensitivity Detection of Low Molecular Weight Molecules

This article presents a novel plasmonic sensing platform designed for the detection of low molecular weight molecules, offering significant advancements in diagnostic applications. The platform features a periodic array of gold nanodisks on a 20 nm thin silica layer, supported by a 100 nm thick gold substrate. By leveraging the coupling between localized and propagating surface plasmon resonances, this design significantly enhances the sensitivity and specificity of molecular detection. Finite element method simulations are conducted to characterize the optical properties and reflectance response of the nanodisks array in the visible to near‐infrared range. Ellipsometric analysis is performed to measure the reflectance of the sample at various angles. Additionally, scanning near‐field optical microscopy in reflectance mode validates the design by revealing well‐defined plasmonic hot spots and interference patterns consistent with the simulated results. The findings demonstrate the platform's effectiveness in amplifying optical signals, achieving a limit of detection of 50 μM for molecules with a molecular weight of less than 1 KDa. This high sensitivity and specificity highlight the potential of the proposed plasmonic platform to advance the development of highly sensitive sensors for low molecular weight molecules, making it a valuable tool for diagnostics and precise molecular detection.

From localized to propagating surface plasmon resonances in Au nanoparticle-coated optical fiber sensors and its implications in biosensing

Nanoparticle-based plasmonic optical fiber sensors can exhibit high sensing performance, in terms of refractive index sensitivities (RISs). However, a comprehensive understanding of the factors governing the RIS in this type of sensor remains limited, with existing reports often overlooking the presence of surface plasmon resonance (SPR) phenomena in nanoparticle (NP) assemblies and attributing high RIS to plasmonic coupling or waveguiding effects. Herein, using plasmonic optical fiber sensors based on spherical Au nanoparticles, we investigate the basis of their enhanced RIS, both experimentally and theoretically. The bulk behavior of assembled Au NPs on the optical fiber was investigated using an effective medium approximation (EMA), specifically the gradient effective medium approximation (GEMA). Our findings demonstrate that the Au-coated optical fibers can support the localized surface plasmon resonance (LSPR) as well as SPR in particular scenarios. Interestingly, we found that the nanoparticle sizes and surface coverage dictate which effect takes precedence in determining the RIS of the fiber. Experimental data, in line with numerical simulations, revealed that increasing the Au NP diameter from 20 to 90 nm (15% surface coverage) led to an RIS increase from 135 to 6998 nm/RIU due to a transition from LSPR to SPR behavior. Likewise, increasing the surface coverage of the fiber from 9% to 15% with 90 nm Au nanoparticles resulted in an increase in RIS from 1297 (LSPR) to 6998 nm/RIU (SPR). Hence, we ascribe the exceptional performance of these plasmonic optical fibers primary to SPR effects, as evidenced by the nonlinear RIS behavior. The outstanding RIS of these plasmonic optical fibers was further demonstrated in the detection of thrombin protein, achieving very low limits of detection. These findings support broader applications of high-performance NP-based plasmonic optical fiber sensors in areas such as biomedical diagnostics, environmental monitoring, and chemical analysis.

Surface plasmon polariton excitation and propagation in metal tripod systems

Through the activation of a planar gaseous medium consisting of four-level tripod atoms, our approach provides a mechanism to generate the Surface Plasmon Polariton (SPP). We investigate a three-layer structure in which the bottom layer is composed of atoms arranged in a tripod configuration, a metal film is layered in the middle, and a transparent layer of either air or vacuum resides on top. The bottom layer facilitates SPP excitation via amplification in the atom arrangement, caused by three laser beams: control, weak probe, and signal. The electromagnetically induced transparency effect, which can be seen in atoms, compensates for the momentum difference between light and SPP. We can coherently change SPP propagation length by altering control field strength, resonance, and off-resonant detuning. SPP optical gain power, propagation length, penetration depth, and metal thickness effects are also examined. Our method to generate SPPs may find use in lithography, sensors, polarizers, along with photodetectors.

A Study of Degenerate Four‐Wave Mixing and Phase Conjugation in Metallic Nanohybrids

AbstractA theory of degenerate four‐wave mixing (DFWM) and phase conjugation is developed for metallic nanohybrids, which consist of an ensemble of interacting metallic nanoshells and noninteracting quantum dots (QDs). It is considered that three incident waves are applied to the metallic nanohybrid, and they produce a fourth output mixed wave. These waves induce dipoles in metallic nanoshells, generating surface plasmon polaritons, and interact with each other via the dipole–dipole interactions (DDI). In DFWM, the input and output waves travel in opposite directions, and this retroreflective nature is responsible for the phenomenon of phase conjugation. The analytical expressions for the input transmitted and output reflected wave intensities are calculated in the presence of the surface plasmon polaritons (SPPs) and the DDI polaritons. It is demonstrated that the phase conjugation, the phase coherent phenomena, and the intensities of both input transmitted and output reflected waves are enhanced due to SPPs and DDI polaritons. These findings indicate that the nanohybrid acts as a phase conjugate device and a phase coherent optical amplifier, which can be applied to fabricate optical nanoamplifiers, phase conjugate mirrors, and nanosensors by measuring the intensity of the output reflected wave.

Graphene-assisted metamaterial absorber for refractive index sensor at lower optical frequencies

Numerical studies on a graphene-assisted metamaterial absorber operating at lower optical frequencies are presented. The metamaterial absorber comprises one-dimensional gold grating with a gold disk array in grooves on a monolayer graphene supported by a glass substrate. Numerical simulations are conducted using Computer Simulation Technology Microwave Studio to investigate the underlying absorption mechanisms in the system. The absorber shows a very sharp absorption band with almost unity absorptance and nearly 20 nm full width at half maximum at a wavelength of 766 nm. The simulated absorption spectrum of the absorber is theoretically validated using coupled mode theory, showing close agreement with the theoretical predictions. The absorption occurs due to the simultaneous excitation of localized and propagating surface plasmons under the transverse electric mode (where the electric field is aligned parallel to the length of the grating ridge) excitation in the system. The sensing performance of the absorber has been evaluated, demonstrating a sensitivity as high as 667 nm/RIU. Due to the robustness of the design, this absorber can be used to sense biological, chemical, gaseous, and even acidic samples since it is not comprised of any photoresist. The absorber holds potential for diverse applications, including graphene-based devices, narrowband filters, thermal emitters, solar cells, imaging apparatus, etc.

Nanoscale photoemission in a plasmon focusing lens with a nanohole explored by time-of-flight photoemission electron microscopy

Propagating surface plasmon polaritons (SPPs) provide an important platform for the design of various photoelectric devices such as nanometer-scale ultrafast electron sources. Here, a high-brightness nanoscale photoelectron source in a plasmon focusing lens with a nanohole is investigated using time-of-flight photoemission electron microscopy (TOF-PEEM). By exploiting the high spatial resolution of TOF-PEEM, it is found that the photoemission was localized at the nanoscale in both x and y directions. In addition, a large multiphoton photoemission enhancement was achieved and a strong-field effect was observed due to the interplay between the SPP and the nanohole. This paper provides a new idea for the development of the high-brightness nanoscale photoelectron sources and a deep understanding of the interplay between SPP and LSP.

Research on multi-resonance mechanism to achieve ultra-wideband high absorption of a metamaterial absorber in the UV to MIR range

Wideband metamaterial perfect absorbers have been extensively researched due to their excellent properties. A four-layer ultra-broadband metamaterial perfect absorber engraved with a square cavity is proposed. A clever combination of grating and square cavity allows the absorber to manifests an overall absorption of over 92 % and an average absorption rate of 97.27 % within the operational band from 280 to 5400 nm. Through analysis, it can be concluded that the coupling effects of Rayleigh anomaly (RA), magnetic resonance (MR), cavity resonance (CR), propagating surface plasmon resonance (PSPR) and local surface plasmon resonance (LSPR) result in perfect absorption. The absorber is insensitive to polarization and large angle incidence. It is worth mentioning that the average absorption rate of our structure reaches 93.32 % at a large incidence angle of 60° in TE mode, which is much higher than that of other absorbers. At 1000 K, the total photothermal conversion efficiency of our absorber is still above 90 %. The designed structure has the following advantages: high absorption, ultra-wide bandwidth, high tolerance to fabrication errors and insensitivity to large angle incidence. Thus, the absorber can be used in the solar energy harvesting and perfect stealth. Additionally, it also can be served as a reference in thermophotovoltaics, thermal imaging and infrared detection.

Electrically driven cavity plasmons in Au nanowire over Au film

Light emission via inelastic tunneling electrons is appealing for integrated optoelectronic devices due to its femtosecond time scale that can in principle allow terahertz modulation bandwidth. It has gained renewed interest since 2015 due to the improved quantum efficiency, highly tunable emission wavelength, linewidth, or directionality once the electrodes are designed as a plasmonic nanocavity. However, efficient construction of stable tunnel junctions with desired plasmonic resonances is still technically challenging because of the subnanometer precision required in the electrical and optical design. Here, we demonstrate an easily accessible electrically driven cavity plasmon in metal-insulator-metal (MIM) tunnel junctions, comprised by a Au nanowire (NW) across two separate ultrasmooth Au electrodes. Two layers of self-assembled thiol molecule defines a reliable tunneling barrier. The contribution from the localized cavity plasmons to the total light emission is found to be dominant over that from the propagating surface plasmon polariton in the MIM waveguide, different from the traditional explanations. This work introduces a simplified method for constructing electrically driven cavity plasmons using crystalline metals, which holds promise for applications in in situ chemical or biosensing and the development of flexible light-emitting metasurfaces.

Influence of Aluminum Pillar Nanostructures on Thin‐Film Organic Solar Cells

This study explores the application of pillar nanostructures in organic solar cells (OSCs). The aluminum pillar nanostructures (AlPNSs) are fabricated on an active layer surface comprising of a blend poly(3‐hexylthiophene‐2,5‐diyl) and [6,6]‐phenyl C61 butyric acid methyl ester using nanoimprinting. Aluminum back electrodes are formed, resulting in AlPNSs with an imprinted pattern height of 60 ± 6 nm and a pitch of 212 ± 49 nm. Atomic force microscope images and current density versus voltage curves are obtained for the fabricated devices, both with and without AlPNSs. The results indicate a solar cell efficiency increase of 15.16% in the AlPNS OSCs compared to the reference cells. To investigate the role of AlPNSs in the enhancement, impedance spectroscopy, incident photon‐to‐current efficiency, UV–Vis reflection spectroscopy, and finite‐difference time‐domain simulations are performed for the both devices. The results demonstrate that the combination of propagating surface plasmon resonance and light‐trapping properties due to AlPNSs significantly enhances the overall optical performance. This research provides new insights into the potential of imprinted nanostructures for enhancing OSC performance, including their plasmonic and optical characteristics.

Magnet-free electromagnetic nonreciprocity in two-dimensional materials

In this Tutorial, we overview recent developments to break and manipulate electromagnetic nonreciprocity in two-dimensional (2D) materials without relying on magnetic fields. To this purpose, we provide a general conductivity model to describe gyrotropic metasurfaces that exhibit nonreciprocity through different physical mechanisms enabled by 2D materials, including optical pumping, drifting electrons, ferromagnetic monolayers, mechanical strain, and spatiotemporal modulation. We describe in detail the resulting systems, focusing on near-field phenomena, associated to nonreciprocal bulk and edge surface plasmon propagation, and on far-field responses, related to Faraday/Kerr rotation and optical dichroism of waves propagating in free-space. Additionally, we review and contextualize recent advancements in magnet-free nonreciprocal devices based on 2D materials, ranging from Faraday rotators and optical dichrosim, to plasmonic and photonic isolators, hyperlenses, and tunable optical traps. We conclude by providing our outlook for the future development of this technology and its potential applications in areas such as communications, sensing, wave generation, and spectroscopy, among others.

An ultra-wide-angle metasurface absorber operating in the ultraviolet to visible range

Here, we propose an ultra-wide-angle metasurface absorber. It consists of a one-dimensional titanium grating periodic array with dislocation distribution. The average absorptivity of the absorber exceeds 94% in the ultraviolet to visible light band (200–760 nm) under normal TM-polarized incident wave. This benefits from the coupling of propagating surface plasmon resonance, local surface plasmon resonance, and hybrid mode resonance. For normal TE-polarized incident wave, the absorber achieves perfect narrowband absorption (>99.22%) in the ultraviolet light region because of the excitation of cavity-mode resonance. The absorber exhibits broadband absorption with an average absorptivity exceeding 85% in the visible light band because of the excitation of slot mode. These excellent absorption properties are well retained at 70° oblique incidence. The structural symmetry of the absorber dramatically expands the flexibility of device applications. It can achieve great absorption performance in an ultra-wide-angle range of 280° along the incident light direction. In addition, the absorber is sensitive to the polarization angle and incident angle of ultraviolet light, which can be used for ultraviolet detection and sensing. The proposed ultra-wide-angle metasurface absorber has potential application in the field of optical communication, photoelectric detection, and solar energy harvesting.

High sensitivity strain and refractive index MZI-SPR sensing based on dual fiber winding structure.

Fiber strain and refractive index dual parameter sensor have enormous potential in the field of underwater structural health monitoring. This article proposes a strain and refractive index highly sensitive MZI-SPR composite sensor based on a dual fiber winding structure. By twisting single-mode fiber and multi-mode fiber, the two fibers are wound together to produce regular multiple bends. Among them, single-mode fiber achieves beam splitting and coupling of light during the first bending and the end of bending, forming a double beam interference MZI. The multi bending structure excites high-order cladding mode to achieve highly sensitive strain sensing. Meanwhile, multi-mode fiber excites cladding mode at the bending point and generate surface plasmon resonance effect, forming a cladding type SPR sensor. By controlling the winding parameters, the bending curvature is controlled, further controlling the SPR incidence angle and achieving highly sensitive refractive index sensing. The experiment indicates that the sensor can achieve simultaneous detection of strain and refractive index, and its sensitivity can reach 33pm/ µε and 4386.262 nm/RIU, respectively. The proposed MZI-SPR composite dual parameter sensor has a stable structure and high sensitivity, providing a reliable solution for simultaneously measuring strain and refractive index in the field.

High-performance microbolometers with metal-insulator-metal plasmonic absorbers in CMOS technology.

The practical applications of microbolometers in CMOS technology face challenges since the thermometer layers in microbolometers are typically of low IR absorption coefficients. In this paper, we demonstrate the integration of IR metal-insulator-metal (MIM) plasmonic absorber on the CMOS resistive-type microbolometers to maximize the optical detectivity. The MIM absorbers utilize Al metals and SiO2 dielectric films in the standard CMOS process and the configuration is numerically simulated and analyzed to assess the effectiveness of high IR absorption through propagating surface plasmon (PSP) resonance and localized surface plasmon (LSP) resonance. Experimental results show that the microbolometer integrated with the MIM plasmonic absorber has a 64% improvement in detectivity (D*) compared to the one without MIM absorber in the 7-13 µm wavelength range. The detector yields a maximum D* of 2.46 × 109 cm Hz1/2/W at 9.5 µm optimally under a working current of 30 μA. This performance-enhanced microbolometer provides a pathway for achieving ultra-sensitive imaging using a simple, cost-effective manufacturing process.

Terahertz Surface Plasmon Generation from Laser Interaction with a Magnetized Metallic Surface

Terahertz Surface Plasmon Generation from Laser Interaction with a Magnetized Metallic Surface

Novel in-situ Ag-decorated trithiocyanuric acid polymer with superior visible light photocatalytic activity

A novel Ag-decorated trithiocyanuric acid polymer (Ag-TTCA) was fabricated via a photocatalytic reduction method based on the strong affinity between Ag ion and trithiocyanic acid (TTCA). The as-prepared photocatalysts were characterized, and the paralleled experiments were conducted on the photocatalytic oxidation of bisphenol A (BPA) to evaluate the photocatalytic activity. Compared with TTCA, Ag-TTCA has a much higher photocatalytic activity. The 0.08Ag-TTCA catalyst can remove BPA (20 mg L−1) in 240 min with a degradation efficiency as high as 96.4 %. The improved photocatalytic performance of Ag-TTCA can be attributed to the dispersed nanoparticle morphology and unique electronic structure. The dispersed nanoparticle morphology of Ag-TTCA can provide more active sites for the adsorption and degradation of pollutants. After silver modification, the light absorption threshold of TTCA is redshifted from 466 nm to 522 nm, and the corresponding band gap is reduced from 2.74 eV to 2.54 eV, which broadens the light absorption range of the photocatalyst and significantly enhances the light absorption capacity. Moreover, metal silver can trap photogenerated electrons and generate surface plasmon resonance (SPR) when illuminated by sunlight, which can effectively promote the separation of electron-hole pairs. Trapping experiment indicates that photogenerated holes, singlet oxygen and superoxide free radicals are the main active species in photocatalytic system.

Strong coupling between WS2 monolayer excitons and a hybrid plasmon polariton at room temperature

Abstract Light–matter interactions between plasmonic and excitonic modes have attracted considerable interest in recent years. A major challenge in achieving strong coupling is the identification of suitable metallic nanostructures that combine tight field confinement with sufficiently low losses. Here, we report on a room-temperature study on the interaction of tungsten disulfide (WS2) monolayer excitons with a hybrid plasmon polariton (HPP) mode supported by nanogroove grating structures milled into single-crystalline silver flakes. By engineering the depth of the nanogroove grating, we can change the character of the HPP mode from propagating surface plasmon polariton-like (SPP-like) to localized surface plasmon resonance-like (LSPR-like). Using reflection spectroscopy, we demonstrate strong coupling with a Rabi splitting of 68 meV between the WS2 monolayer excitons and the lower HPP branch for an optimized nanograting configuration with 60 nm deep nanogrooves. In contrast, only weak coupling between the constituents is observed for shallower and deeper nanogratings since either the field confinement provided by the HPP is not sufficient or the damping is too large. The possibility to balance the field confinement and losses render nanogroove grating structures an attractive platform for future applications.

Vertical coupling to photonic crystal waveguide using chiral plasmonic lenses

Manipulating surface plasmon polariton waves for the development of micro-nano devices has been widely studied in recent years. Two-dimensional artificial photonic crystals have bandstructure characteristics like semiconductors. However, the requirement for light to be incident along the structural periodic direction poses a challenge in coupling light into the photonic crystal, thereby impeding its integrations and applications. In this work, we proposed coupling vertically incident left-circularly polarized light into a photonic crystal waveguide using a chiral plasmonic lens. Linearly-polarized light can also generate surface plasmon polariton waves and couple them into photonic crystal waveguides, but the intensity is lower. In contrast, right-circularly polarized light propagates in the opposite direction and exhibits minimal propagation into the photonic crystal waveguide. The results indicate that the proposed structure can operate broadband within the wavelength range of 620–670 nm. This method provides a simple and easily integrated coupling method for photonic crystal devices.

Far-Field Petahertz Sampling of Plasmonic Fields.

The response of metal nanostructures to optical excitation leads to localized surface plasmon (LSP) generation with nanoscale field confinement driving applications in, for example, quantum optics and nanophotonics. Field sampling in the terahertz domain has had a tremendous impact on the ability to trace such collective excitations. Here, we extend such capabilities and introduce direct sampling of LSPs in a more relevant petahertz domain. The method allows to measure the LSP field in arbitrary nanostructures with subcycle precision. We demonstrate the technique for colloidal nanoparticles and compare the results to finite-difference time-domain calculations, which show that the build-up and dephasing of the plasmonic excitation can be resolved. Furthermore, we observe a reshaping of the spectral phase of the few-cycle pulse, and we demonstrate ad-hoc pulse shaping by tailoring the plasmonic sample. The methodology can be extended to single nanosystems and applied in exploring subcycle, attosecond phenomena.