Excitation Of Surface Plasmons Research Articles

Mechano-Optical Resonant Emission by Edge Angle Modulation of Wrinkled Graphene on Plasmonic Metal Gratings

Graphene coated on top of photonic-plasmonic metasurfaces can produce resonant radiative emission in the mid-infrared region. Narrowband emission peaks are observed through folding graphene into “origami” ridges over metal grating grooves, creating a complementary cavity mode above the trench. This geometrically tuned phenomenon of graphene surface plasmon excitation along the folded sheet enhances the emission when added to magnetic polariton (MP) resonance induced within the plasmonic grating groove. Our analytical models describe how this graphene surface plasmon polariton (SPPG) is a function of folded graphene geometric parameters, and most importantly, the graphene edge angle that distends from the grating surface. The frequency-dependent phase shift of SPPG is fitted to grating parameters, and a modified inductor-capacitor circuit model was developed for predicting MP resonance mode with graphene influence. It was found that the edge angle of wrinkled graphene blue-shifts the groove MP resonance and SPPG resonant emission peak in both wrinkled graphene alone and with the grating substrate. The understanding of geometrically modulated graphene adhered on plasmonic gratings impacts the design and capability of narrowband cavity emitters and contributes toward the development of mechanical-optical environmental sensors.

Graphene-based metasurface absorber for the active and broadband manipulation of terahertz radiation

Graphene-based metasurface nearly perfect absorbers (MPAs) can be used as an efficient tool for the active control and manipulation of waves in the terahertz (THz) gap. Here, we propose a novel, to the best of our knowledge, graphene-based MPA that is designed based on a simple configuration and is capable of absorbing THz radiation within a broad bandwidth of almost 3 THz with polarization-insensitive and omnidirectional characteristics. The MPA comprises a periodic array of graphene patches with two different dimensions that are separated from a gold bottom reflector with an S i O 2 spacer layer. The broadband spectral response of the MPA, which is also verified by analytical calculations, is due to the support of propagating surface plasmon excitations and can be either actively tuned via changes in the chemical potential of graphene or passively adjusted by the modification of the geometrical parameters of the patches and thickness of the spacer layer. As a complement to the previous studies in the literature, due to the simplicity of its design and broad spectral response, it is believed that the suggested graphene-based MPA will find potential applications in THz spectroscopy and communications.

Surface Plasmon-Enhanced Photomagnetic Excitation of Spin Dynamics in Au/YIG:Co Magneto-Plasmonic Crystals.

We report strong amplification of the photomagnetic spin precession in Co-doped YIG employing a surface plasmon excitation in a metal-dielectric magneto-plasmonic crystal. Plasmonic enhancement is accompanied by the localization of the excitation within the 300 nm thick layer inside the transparent dielectric garnet. Experimental results are nicely reproduced by numerical simulations of the photomagnetic excitation. Our findings demonstrate the magneto-plasmonic concept of subwavelength localization and amplification of the photomagnetic excitation in dielectric YIG:Co, which can potentially be employed for all-optical magnetization switching below the diffraction limit, with energy efficiency approaching the fundamental limit for magnetic memories.

Investigation of the Possibility to Use Ge p-i-n Photodiodes in Infrared SPR Sensors

Considered in this work are the possibility and advantages of applying a Ge photodiode to measure the surface plasmon resonance (SPR) in the near infrared range (IR). The investigations were performed using the prototype of IR SPR refractometer that operates with two fixed wavelengths (1310 and 1552 nm) for excitation of surface plasmons in a thin gold film in the Kretschmann optical scheme. The obtained results of experiments enabled us to draw the conclusion that application of the Ge photodiode in the SPR sensor at the wavelength 1.5 µm allows increasing the sensitivity and response magnitude as well as widening the dynamic range of this sensor device.

A narrowband dual-polarization filter based on photonic crystal fiber with Au-Ag double-layer films

A narrowband dual-polarization filter based on photonic crystal fiber (PCF) with Au-Ag double-layer films is proposed. The cladding of PCF is composed of three layers of air holes, and Au-Ag double-layer films are coated on two symmetrical air holes to excite surface plasmon resonance (SPR) effect. The special mechanism of the Au-Ag double-layer films takes advantage of the narrower full width at half maximum (FWHM) of silver. On the other hand, gold is used to protect the silver from oxidation and improve the stability of the filter, and can also enhance SPR effect. The simulation results show that the dual-polarization filter with double-layer metal films has higher resonance absorption and better spectra than that with the single-layer metal film. When the thickness of the double-layer metal films is 22.5 nm (the thickness of silver film is 18 nm), the confinement loss of dual-polarization filter at 1430 nm is achieved to 64.08 dB/cm (y-pol core mode) and at 1530 nm is achieved to 37.26 dB/cm (x-pol core mode). The wavelength range of crosstalk (CT) greater than 20 dB and less than −20 dB are 1410–1450 nm and 1510–1550 nm for the device length of 10 mm, respectively. The CT is achieved 555.07 dB and −321.54 dB at the wavelength of 1430 nm and 1530 nm, respectively. These characteristics show that the filter has excellent performance, and will become a suitable candidate for a narrowband polarization filter.

PfHRP2 detection using plasmonic optrodes: performance analysis

BackgroundEarly malaria diagnosis and its profiling require the development of new sensing platforms enabling rapid and early analysis of parasites in blood or saliva, aside the widespread rapid diagnostic tests (RDTs).MethodsThis study shows the performance of a cost-effective optical fiber-based solution to target the presence of Plasmodium falciparum histidine-rich protein 2 (PfHRP2). Unclad multimode optical fiber probes are coated with a thin gold film to excite Surface Plasmon Resonance (SPR) yielding high sensitivity to bio-interactions between targets and bioreceptors grafted on the metal surface.ResultsTheir performances are presented in laboratory conditions using PBS spiked with growing concentrations of purified target proteins and within in vitro cultures. Two probe configurations are studied through label-free detection and amplification using secondary antibodies to show the possibility to lower the intrisic limit of detection.ConclusionsAs malaria hits millions of people worldwide, the improvement and multiplexing of this optical fiber technique can be of great interest, especially for a future purpose of using multiple receptors on the fiber surface or several coated-nanoparticles as amplifiers.

Locally excited surface plasmon resonance for refractive index sensing with high sensitivity and high resolution.

An angle-interrogated surface plasmon resonance (SPR) sensor based on a prism-coupled configuration has been extensively applied in biomedicine, environment monitoring, and food safety. Yet, the low sensitivity and low spatial resolution impede its further development. In this Letter, we investigated objective-coupled locally excited SPR for refractive index (RI) sensing with high sensitivity and high resolution. Through theoretical analysis, the SPR angle was retrieved from back focal plane imaging, which was highly correlated to the RI of the surrounding medium. Experimentally, a RI sensitivity of 77.41° refractive index unit (RIU)-1 was achieved with a detection range of 0.068 RIU when using glucose solutions for the demonstration. Furthermore, we acquired the spatial resolution of the configuration being 290 nm, and the RI measurement to a polydimethylsiloxane droplet with high spatial resolution was implemented. As a result, compared with the classical prism-coupled configuration, the locally excited SPR provides a method to achieve RI sensing with high sensitivity and high resolution.

Gap mode induced photo-oxidation of p-methyl thiophenol and relating phenomena

Surface plasmon excitation at a nanogap between adjacent metal nanostructures generates extremely enhanced electric field, which enables us to detect and characterize individual molecules by surface enhanced Raman scattering (SERS). However, versatile nanogap fabrication methods have not been established, making difficult to efficiently exploit a gap mode plasmon in practical applications such as biosensing, biomedical, and environmental analyses. Here, we describe reproducible nanogap assemblies using adjacent metal nanoparticles (MNPs) and MNPs/adsorbates/metal substrates based on the Derjaguin − Landau − Verway − Overbeek theory and irreversible optical trapping of MNPs on metal substrates. The assembled nanogaps between adjacent MNPs, here, M is Ag or Au, enabled us to characterize adsorbates such as solvent shared ion pairs of hydrated metal ions, and three distinct adsorbed states of adenine depending on pH in aqueous solutions. Also we found plasmon-induced oxidation of p-alkyl thiophenol (p-AlkTP) at the nanogap on AgNP/p-AlkTP/Ag films. This oxidation has a specific reactivity at the p-position in the p-AlkTP phenyl ring, while providing intermediate species under nitrogen atmosphere. This specific oxidation is not driven by a thermal process but by a hot carrier transfer process. Thus, the nanogap fabrication methods using adjacent metal nanostructures we developed are promising to elucidate static and dynamical nature of target molecules using SERS spectroscopy.

Noble Metal Thin Film Thickness Optimization for Sharp Surface Plasmon Resonance Reflectance Curve

The production of a high-sensitivity surface Plasmon resonance (SPR) sensor depends on a few main factors, such as metal thin film types and thicknesses, light coupling techniques and acceptable EM wave polarization modes. This work is carried out to investigate the impact on the SPR characteristics of noble metal thicknesses, namely gold and silver, for optical sensor applications. To excite surface Plasmon polaritons (SPP), a Kretschmann prism coupling was used. The thicknesses of noble metal thin films were varied between t=30nm and 60nm. The characteristics of SPR peaks such as Q-factor and FWHM were studied in SPR signal analyses. In comparison with silver, Q-factor results showed an outstanding optical property of gold at t= 40 nm due to its high Q-factor values, which confirms its ability to produce excellent SPP quantities. In conclusion, gold noble metal at t= 40nm is capable of producing optimum SPR. In the development of the highly sensitive SPR sensor, this excellent criterion put gold as a spectacular candidate for optical sensor applications.

Size-dependent plasmon effects in periodic W-Si- based mirrors, investigated by X-ray photoelectron spectroscopy

Thickness-dependent shift of energy position and fine structure of the collective oscillation of electron gas (plasmons) in the upper layer of silicon (Si) and tungsten (W) in the W/Si and Si/W multilayer mirrors was studied by X-ray photoelectron spectroscopy, respectively. In the spectra, doublet energy components of plasmon excitation were observed, which belong from the Si/vacuum or W/vacuum, and W/Si or Si/W interfaces. The plasmon excitation energy depended on the thickness of the Si and W layers in the multilayers. The intensity of the plasmon component along with the Si/vacuum or W/vacuum interfaces was increased with increasing the thicknesses of Si (or W). However, the intensity was declined along with the W/Si (or Si/W) interfaces. The fine structure of the plasmon was explained by proposing the framework of the electron scattering model in a thin film with the consideration of a semi-infinite solid. The contribution of surface and volume plasmons excitation to the reflectivity of the multilayer mirrors was described. The reflectivity of mirrors was higher for the lower value of plasmons excitation intensity.

Molecular Monolayer Sensing Using Surface Plasmon Resonance and Angular Goos-Hänchen Shift.

We demonstrate potential molecular monolayer detection using measurements of surface plasmon resonance (SPR) and angular Goos-Hänchen (GH) shift. Here, the molecular monolayer of interest is a benzenethiol self-assembled monolayer (BT-SAM) adsorbed on a gold (Au) substrate. Excitation of surface plasmons enhanced the GH shift which was dominated by angular GH shift because we focused the incident beam to a small beam waist making spatial GH shift negligible. For measurements in ambient, the presence of BT-SAM on a Au substrate induces hydrophobicity which decreases the likelihood of contamination on the surface allowing for molecular monolayer sensing. This is in contrast to the hydrophilic nature of a clean Au surface that is highly susceptible to contamination. Since our measurements were made in ambient, larger SPR angle than the expected value was measured due to the contamination in the Au substrate. In contrast, the SPR angle was smaller when BT-SAM coated the Au substrate due to the minimization of contaminants brought about by Au surface modification. Detection of the molecular monolayer acounts for the small change in the SPR angle from the expected value.

High-Efficiency Directional Excitation of Spoof Surface Plasmon Under Arbitrary Illumination

Recent advances in spoof surface plasmon (SSP) have offered enormous opportunities for engineering electromagnetic waves at the subwavelength scale. However, the fundamental limitation of the extremely finite space channels on exciting SSP challenges their practical applications. Here, to the best of our knowledge, a new method that can realize high-efficiency infinite-channel SSP excitation based on hyperbolic plasmonic metasurfaces is proposed. Free-space propagating waves impinging from arbitrary directions can be coupled to SSP owing to the theoretical perfect wave vector match between the modulated incident waves and the hyperbolic plasmonic metasurfaces. A proof-of-method metacoupler working at 8 GHz is designed and fabricated with the novel functionality numerically and experimentally verified by the consistent guide wavelength with theory. Further experimental investigation reveals a total coupling efficiency exceeding 50% for the impinging waves with incidence angles ranging from -44° to 44°. The proposed methodology provides a new route to design multichannel antennas and detectors.

Dual-band absorbers based on multiplexed gratings at mid-infrared frequencies

We design a dual-band absorber based on metal multiplexed gratings at mid-infrared (mid-IR) frequencies. The metal multiplexed gratings are realized with two constitutive gratings and separated from a metallic ground plate by a thick dielectric spacer in order to construct the metal–insulator–metal architecture. Simulations indicate that two different absorption frequencies from the excitation of surface plasmon–polariton modes can be achieved under normal incidence, due to the different field distributions of each resonance node numbers in supercell. The doublet resonance positions can be independently and arbitrarily adjusted by varying the constitutive grating periods. We also confirmed that the absorption performance of the resonance peak is related to the non-uniform distribution level of the corresponding constitutive grating ridge. The designed absorber can be easily fabricated with planar processing technology and used in many applications where multispectral control of mid-IR signals is required.

Passive tunable and polarization-insensitive fan-like metamaterial absorber in the visible spectrum

We numerically study a fan-like metamaterial absorber (FMMA) composed of an A u - T i O 2 - A u multilayer with a 3-D finite-difference-time-domain method (FDTD) and verify it experimentally. Our proposed design shows high absorbance, > --> 98 % numerically, in the visible spectrum, which is due to the excitation of both localized surface plasmons (LSPs) and propagating surface plasmons (PSPs). In particular, the T i O 2 dielectric spacer sandwiched between the top fan-like nanopatterned structure and the bottom planar Au-film is identified as playing a crucial role in the spectral response of the FMMA. Its introduction excites the PSP mode, which first enhances the absorbance of the FMMA and subsequently redshifts the spectral absorbance with a further increase in the thickness. Moreover, the proposed FMMA, numerically and experimentally, also shows polarization and angle insensitivity in the visible spectrum under normal and selected oblique incidence. These results offer unique potential in sensing applications and devices requiring selective absorption in the visible spectral region.

Demonstration of microwave plasmonic-like vortices with tunable topological charges by a single metaparticle

Light beams with helical wave fronts, also called optical vortices, have attracted great interest in the community of optics and photonics. They provide an additional degree of freedom for light manipulation, leading to wide-ranging potential applications in micro-particle trapping, optical microscopy, and even quantum information processing. Recently, metallic microstructures are introduced to confine the plasmonic vortices into deep subwavelength dimension, which benefits photonic integration on chip. In this Letter, exploiting the excitation of spoof surface plasmon, we experimentally demonstrate the near-field optical vortices with tunable topological charges supported by a single metaparticle in the microwave regime. These microwave plasmonic-like vortices are excited by surface waves with a spatial asymmetric distribution of electromagnetic field, which are launched by a metallic comb-shaped waveguide. Experimental characterization of highly localized and controllable near-field vortices with the nature of deep subwavelength confirms the numerical simulation. In addition, an equivalent physical model based on the coupled mode theory is proposed to understand the generation mechanism of these spoof plasmonic vortices. Our approach offers an efficient way to generate deterministic subwavelength optical vortices, which provides the potential for critical vortex elements on photonic integrated chip.

Sub-100 nm high spatial frequency periodic structures driven by femtosecond laser induced desorption in GaAs

This paper presents a previously unreported mechanism for the formation of High Spatial Frequency Laser Induced Periodic Surface Structures (HSFL) in GaAs upon irradiation by femtosecond laser pulses (repetition rate = 1 kHz, τ = 150 fs, λ = 390 nm) that is driven by point defect diffusion, desorption of surface atoms, and roughening of the surface. The HSFL have trenches that are 100 nm deep, an average spatial period of 65 nm, and are completely below the original surface. Sub-100 nm periodicity with high depth to period aspect ratio has not been previously observed in GaAs. In the proposed mechanism, laser irradiation generates point defects that diffuse to the surface. Interstitials that reach the surface can be easily desorbed and the remaining vacancies coalesce into vacancy islands. This results in a rough surface, which can excite surface plasmon polaritons. Despite our observations of periodicity corresponding to SPPs, calculations done using the excited dielectric function indicate that SPPs should not be supported. This points to the need to incorporate the cumulative effects of laser irradiation in existing models. This paper also presents evidence that desorption can also occur during HSFL formation in GaAs when irradiated with 780 nm in vacuum. The HSFL have the similar spatial period as GaAs irradiated in air with 780 nm, but completely below the original surface.

Electromagnetic resonances observed in small, charged particles

When a particle is charged, electrons can move freely along its surface and influence its optical properties in the same way as a thin, nonuniform metallic layer. These electrons contribute to scattering phenomena, including resonances. We model the light scattering from charged particles and demonstrate that these resonances result from excitation of an anti-symmetric surface plasmon at the layer interfaces. The modeling explains suppression of absorption when the size of the charged particle decreases, as well as differences in the light-scattering efficiencies of as much as a factor of 2 that occur before and after the resonance. These light-scattering properties must be taken into consideration when performing remote-sensing studies of charged particles, like those in the interstellar medium and dust storms.

Silicon-Based Quantum Mechanical Tunnel Junction for Plasmon Excitation from Low-Energy Electron Tunneling

Light emission from metal–insulator–semiconductor junctions (MISJs) has been explored for decades as a possible on-chip light source; however it is not clear whether the mechanism of light emission is plasmonic in nature or is dominated by electroluminescence. Previous studies only investigated silicon with low doping levels, but here we show that only highly doped silicon allows us to excite surface plasmon polaritons (SPPs) in MISJs via inelastic tunneling. This paper describes the mechanism of charge transport and light emission from silicon-based Au-SiO2-nSi MISJs as a function of the doping level Nd varying from 1.6 × 1015 cm–3 to 1.0 × 1020 cm–3. At low doping levels (Nd ∼ 1015 cm–3), the MISJs behave as Schottky diodes, and the mechanism of light emission involves a radiative recombination of electrons and holes from minority carrier injection under high applied bias (>5.5 V). With increasing doping levels, the current–voltage characteristics of the MISJs change, resulting in symmetrical current–voltage curves with parabolic conductance behavior characteristic of quantum mechanical tunneling. MISJs with the highest doping level (Nd ∼ 1020 cm–3) are dominated by quantum mechanical tunneling, and light emission originates from radiative decay of surface plasmon polaritons (SPPs) via scattering at threshold voltages as low as 1.5 V. Our simulations indicate that tunneling over the thin SiO2 barrier between the Au and highly doped nSi excites a hybrid-SPP mode localized to the Au whose dispersion depends on the effective index induced by the SiO2–nSi interface. Our studies show that Si needs to be sufficiently doped to be conductive enough to enable SPP excitation via inelastic tunneling.

Invisible Gateway by Superscattering Effect of Metamaterials

Illusion devices, such as superscatterer and invisible gateway, have been theoretically studied under the theory of transformation optics and folded geometry transformations. The realization of these devices needs building blocks of metamaterials with negative permittivities and permeabilities. However, superscattering effects, such as stopping wave propagation in an air channel, have not been verified from illusion devices physically because of the challenge of metamaterial design, fabrication, and material loss. In this Letter, we implement a big metamaterial superscatterer, and experimentally demonstrate its superscattering effect at microwave frequencies by field-mapping technology. We confirm that superscattering is originated from the excitation of surface plasmons. Integrated with superscatterer, we experimentally display that an invisible gateway could stop electromagnetic waves in an air channel with a width much larger than the cutoff width of the corresponding rectangular waveguide. Our results provide a first direct observation of superscattering effect of double negative metamaterials and invisible gateway for electromagnetic waves. It builds up an ideal platform for future designs of other illusion devices.

Spectroscopic study of gap-surface plasmons in a metallic convex groove array and their applications in nanofocusing and plasmonic sensing

Plasmonic tapered grooves have been proven to be good candidates for the excitation of gap surface plasmons (GSPs), surface plasmons trapped vertically inside a metallic tapered groove or slit. GSPs have attracted tremendous interest due to their unique properties of concentrating light in nanosized gaps with significant field enhancement, thus offering potential applications such as ultracompact nanocircuits, broadband light absorbers, and plasmonic sensors. In this paper, we focus on GSPs supported by periodic arrays of narrow convex grooves and study in detail their properties by using visible-near-infrared (VIS-NIR) spectroscopy. We identify strong second- and third-order GSP modes excited in ultrasharp convex grooves. The dependence of GSP resonances on the groove profile is analyzed with the help of detailed full-wave simulations, revealing the fact that an ultrasharp, but finite, gap exists at the groove bottom and plays a crucial role in determining both the GSP resonance positions and the nanofocusing capability with much improved field enhancement inside the grooves. Spectral shifts of the observed GSP resonances relative to the simulation results are found in a shorter wavelength range and are qualitatively explained as nonlocal effects originating from the nonclassical microscopic behavior of local currents and charges at imperfect interfaces. Utilizing such strong and distinguishable GSP resonance line shapes in an otherwise flat reflectivity spectral baseline, we experimentally demonstrate the capability of convex groove arrays to perform dual-band refractive index sensing in the VIS-NIR range.