Propagation Of Surface Plasmon Polaritons Research Articles

Ultrathin tunable UWB metasurface absorber design by exciting plasmonic modes in bilayer graphene stack

This work presents a graphene-based ultra-wideband (UWB) tunable metasurface absorber (MSA) consisting of top patterned graphene (Gpat) and bottom continuous graphene (Gcont) separated by a 2 μm thick silicon dioxide (SiO2) substrate. An absorptivity (Af) ≥ 90 % from 0.1 to 14 THz (fractional bandwidth = 197.2 %) has been achieved in three steps: (i) Fabry-Perot cavity mode has been generated near 0.1 THz using bilayer graphene stack, (ii) multiple plasmonic modes have been excited by engraving resonant slots on Gpat and (iii) plasmonic coupling strength has been enhanced between Gpat and Gcont by adjusting SiO2 substrate's thickness. The excited plasmonic modes show high absorption due to the generation of localized surface plasmon polariton (LSPP) waves. The fourfold symmetry in design nullifies the effect of the polarization angle (ϕ). Interestingly, the orthogonal modes' excitation helps to achieve similar Af response under both transverse electric and transverse magnetic polarizations for incidence angle (θ) up to 60°. Further, an effective DC biasing mechanism has been devised to tune the chemical potential (μc) of the interconnected Gcont, which ensures common DC bias between unit cells. A DC voltage from 0 to 10.2 V can vary μc (Bottom) from 0 to 1 eV to provide frequency tunability.

Exploiting graded triangular gratings for optimal nano-focusing: A novel approach to enhance SERS efficiency

Plasmonic graded nano-gratings enable rainbow trapping of multiple resonant modes over a wide wavelength spectrum, useful for multi-channel Surface Enhanced Raman Spectroscopy (SERS) of molecular species. However, rectangular nano-gratings have limitations in achieving efficient rainbow trapping and localizing a wide spectrum of plasmonic modes due to their stepwise geometry, which induces high dissipation of surface plasmon polaritons into the substrate. An alternative platform of graded triangular nano-gratings enables increased localization and more efficient adiabatic transformation between neighboring grooves. Varying groove angles, depths, and periods in the tapered geometry allow for smooth adjustment of the surface plasmon polariton propagation constant, reducing losses and maximizing nano-focusing inside the groove tips. To overcome the limitation of low aspect ratio in wet-etching silicon, we employed a multi-step process of reactive ion etching of a SiO2 barrier layer to generate aperture width, followed by anisotropic wet-etching. The resulting graded triangular nano-gratings showed excellent SERS enhancement along three laser wavelength excitations. The enhancement factors of 638 and 785 nm wavelengths are 8.5 × 109 and 9 × 108, respectively, for the detection of 1 µM Rhodamine 6G. In addition, graded triangular nano-gratings show similar enhancement factors for other species, specifically the lipid DPEE-PEG, at the 532 nm laser excitation wavelength with an excellent SERS enhancement factor of 1.5 × 109. Owing to the ability of the graded triangular gratings to elicit pronounced SERS responses across three distinct laser excitations, they unequivocally qualify as “rainbow trapping” structures. Wider apertures, lower ohmic losses, and the ability to tune the groove angle beyond conventional etching methods bode well for graded triangular gratings as a superior platform for miniature sensors.

Theoretical Investigation of Surface Plasmon Polariton Propagation in Grating Nanostructure with Perturbation Approach

Theoretical Investigation of Surface Plasmon Polariton Propagation in Grating Nanostructure with Perturbation Approach

Propagating surface plasmon polaritons in graphene under applied uniform strain

In this work, we theoretically investigate the propagation length of plasmon waves in graphene layer under uniform strain surrounded by two dielectric media of dielectric constants ε1 and ε2, respectively. The plasmon losses (plasmon damping), plasmon propagation length and the penetration depth of the electric field associates with the charge fluctuations can be controlled by varying the direction and the strength of the applied strain and the direction of the plasmon wave propagation with respect to the direction of the applied strain. Because strain induces anisotropy in graphene optical conductivity, the strain-dependent orientation plays an important role to manipulate the direction and variations of the graphene plasmon energy, which may be useful to tune graphene properties in plasmonic devices to enhance light-matter interaction.

Microscopy of terahertz spoof surface plasmons propagating on planar metamaterial waveguides

Surface plasmon polaritons (SPPs) are electromagnetic waves that have attracted significant interest owing to their subwavelength confinement and the strong field enhancement that they provide. Yet in the terahertz (THz) frequency region of the spectrum, which is well below the plasma frequency of metals, these surface waves are characterized by extremely weak confinement that has severely limited their exploitation for information processing and sensing. One means to circumvent this limitation is through subwavelength structuring of a metallic surface, which can thereby be engineered to support the propagation of spoof surface plasmon polaritons (SSPPs) that closely mimic the properties of SPPs. In this work, we report the design and experimental characterization of an ultra-thin metamaterial planar waveguide that supports SSPPs at THz frequencies. Finite-element method simulations are shown to predict the excitation of SSPPs on the surface of our devices under free-space illumination at 3.45 THz. We investigate these structures experimentally using THz scattering-type scanning near-field microscopy (THz-s-SNOM) to map directly the out-of-plane electric field associated with the propagation of SSPPs on the surface of the waveguides. Our work paves the way for the future development of plasmonic integrated circuit technologies and components operating in the THz frequency band.

Reconstruction Filters Improving the Spatial Resolution and Signal-to-Noise Ratio of Surface Plasmon Resonance Microscopy.

Benefitting from high sensitivity, real-time, and label-free imaging, surface plasmon resonance microscopy (SPRM) has become a powerful tool for dynamic detection of nanoparticles. However, the evanescent propagation of surface plasmon polaritons (SPPs) induces interference between scattered and launched SPPs, which deteriorates the spatial resolution and signal-to-noise ratio (SNR). Due to the simplicity and fast processing, image reconstruction based on deconvolution has shown the feasibility of improving the spatial resolution of SPRM imaging. Retrieving the particle scattering from SPRM interference imaging by filters is crucial for reconstruction. In this work, we illustrate the effect of filters extracting SPP scattering of nanoparticles with different sizes and shapes for reconstruction. The results indicate that the optimum filters are determined by the material of nanoparticles instead of particle sizes. The reconstruction of single Au and PS nanospheres as well as Ag nanowires with optimum filters is achieved. The reconstructed spatial resolution is improved to 254 nm, and the SNR is increased by 8.1 times. Our research improves the quality of SPRM imaging and provides a reliable method for fast detection of particles with diverse sizes and shapes.

Suppression of the plasmon-quenching effect on light amplification in 20-μm-diameter plasmonic whispering gallery mode resonators fabricated from bowl-shaped organic/metal thin films.

Bowl-shaped plasmonic whispering gallery mode (WGM) resonators were fabricated from a 10-nm-thick metal (Al, Ag, or Au) plasmonic layer that was covered with a 100-nm-thick 4,4'-bis(N-carbazolyl)-1,1'-biphenyl spacer layer and a 250-nm-thick 2,7-bis[9,9-di(4-methylphenyl)-fluoren-2-yl]-9,9-di(4-methylphenyl)fluorene light-emitting layer; the layer structure was grown on a 20-μm-diameter silica microsphere. When compared with a reference structure without the plasmonic layer, the resonators, which included either Al or Ag, showed almost the same threshold excitation intensities for generation of amplified spontaneous emission (ASE). This result indicates that the ease of light amplification in the plasmonic resonators was comparable to that in the reference structure. Excitons that exist in the vicinity of metal thin films are generally easy to quench because propagating surface plasmon polaritons (SPPs) absorb the exciton energy. Therefore, the observed comparability demonstrates that the plasmonic WGM resonators overcome this quenching effect on ASE via localization of the SPPs in the vicinity of the excitons.

Effect of Anderson localization on surface plasmon polariton propagation and outward leakage when scattered by a randomly corrugated section of the interface

Effect of Anderson localization on surface plasmon polariton propagation and outward leakage when scattered by a randomly corrugated section of the interface

Versatile optical manipulation of trions, dark excitons and biexcitons through contrasting exciton-photon coupling

Various exciton species in transition metal dichalcogenides (TMDs), such as neutral excitons, trions (charged excitons), dark excitons, and biexcitons, have been individually discovered with distinct light-matter interactions. In terms of valley-spin locked band structures and electron-hole configurations, these exciton species demonstrate flexible control of emission light with degrees of freedom (DOFs) such as intensity, polarization, frequency, and dynamics. However, it remains elusive to fully manipulate different exciton species on demand for practical photonic applications. Here, we investigate the contrasting light-matter interactions to control multiple DOFs of emission light in a hybrid monolayer WSe2-Ag nanowire (NW) structure by taking advantage of various exciton species. These excitons, including trions, dark excitons, and biexcitons, are found to couple independently with propagating surface plasmon polaritons (SPPs) of Ag NW in quite different ways, thanks to the orientations of transition dipoles. Consistent with the simulations, the dark excitons and dark trions show extremely high coupling efficiency with SPPs, while the trions demonstrate directional chiral-coupling features. This study presents a crucial step towards the ultimate goal of exploiting the comprehensive spectrum of TMD excitons for optical information processing and quantum optics.

Distinctive Electric Properties of Group 14 Oxides: SiO2, SiO, and SnO2

The oxides of group 14 have been widely used in numerous applications in glass, ceramics, optics, pharmaceuticals, and food industries and semiconductors, photovoltaics, thermoelectrics, sensors, and energy storage, namely, batteries. Herein, we simulate and experimentally determine by scanning kelvin probe (SKP) the work functions of three oxides, SiO2, SiO, and SnO2, which were found to be very similar. Electrical properties such as electronic band structure, electron localization function, and carrier mobility were also simulated for the three crystalline oxides, amorphous SiO, and surfaces. The most exciting results were obtained for SiO and seem to show Poole–Frankel emissions or trap-assisted tunneling and propagation of surface plasmon polariton (SPP) with nucleation of solitons on the surface of the Aluminum. These phenomena and proposed models may also describe other oxide-metal heterojunctions and plasmonic and metamaterials devices. The SiO2 was demonstrated to be a stable insulator interacting less with the metals composing the cell than SnO2 and much less than SiO, configuring a typical Cu/SiO2/Al cell potential well. Its surface charge carrier mobility is small, as expected for an insulator. The highest charge carrier mobility at the lowest conduction band energy is the SnO2’s and the most symmetrical the SiO’s with a similar number of electron holes at the conduction and valence bands, respectively. The SnO2 shows it may perform as an n-type semiconductor.

Anisotropic Generation and Detection of Surface Plasmon Polaritons Using Near-Field Apertured Probes

Anisotropic Generation and Detection of Surface Plasmon Polaritons Using Near-Field Apertured Probes

Plasmonic Nanostructure Biosensors: A Review.

Plasmonic nanostructure biosensors based on metal are a powerful tool in the biosensing field. Surface plasmon resonance (SPR) can be classified into localized surface plasmon resonance (LSPR) and propagating surface plasmon polariton (PSPP), based on the transmission mode. Initially, the physical principles of LSPR and PSPP are elaborated. In what follows, the recent development of the biosensors related to SPR principle is summarized. For clarity, they are categorized into three groups according to the sensing principle: (i) inherent resonance-based biosensors, which are sensitive to the refractive index changes of the surroundings; (ii) plasmon nanoruler biosensors in which the distances of the nanostructure can be changed by biomolecules at the nanoscale; and (iii) surface-enhanced Raman scattering biosensors in which the nanostructure serves as an amplifier for Raman scattering signals. Moreover, the advanced application of single-molecule detection is discussed in terms of metal nanoparticle and nanopore structures. The review concludes by providing perspectives on the future development of plasmonic nanostructure biosensors.

Fine periodic nanostructure formation on stainless steel and gallium arsenide with few-cycle 7-fs laser pulses

We report the fine periodic nanostructure formation process on metal and semiconductor surfaces in air with few-cycle 7-fs laser pulses and its physical mechanism. Using appropriate peak power densities and scanning speeds for the laser pulses, nanostructures could be formed on stainless steel and gallium arsenide (GaAs) with periods of 60–110 nm and 130–165 nm, respectively, which are 1/5–1/4 of the period of nanostructures formed with 100-fs laser pulses. The periodicity can be explained as arising from the excitation of short-range propagating surface plasmon polaritons, and the observed periods are in good agreement with the model calculation results.

(Invited) Dynamics of Light-Matter Interactions in Plasmonic Optical Cavities for Characterizing Nanostructures Relevant to Energy Conversion and Photocatalysis

Optical cavities are an established means to increase light-matter interactions with a wide range molecules, nanostructures and bulk materials. The use of an optical cavity to increase the likelihood of photon absorption by a material has clear potential for being of value for optical energy conversion and photocatalysis. Furthermore, optical cavities can significantly alter excited state dynamics due to the potential for Purcell effects that increase radiative rates of emission of a given material, molecule, or nanostructure. These dynamic changes can serve as a sensing mechanism of cavity impact to photoprocesses, making characterization tools such as transient absorption and time-resolved emission measurements an effective means to probe the degree of light-matter coupling. Taken a step further, these characterization tools can help establish the degree of light-matter coupling necessary to control excited state lifetimes for a particular purpose or application. Here, we explore the dynamics of nanostructured systems coupled to, or made out of, plasmonic materials. Importantly, plasmonic structures are well-known to serve as a type of optical cavity, and though plasmonic cavities are lossier than their dielectric counterparts, they can also confine light to a small mode volume which is very helpful for increasing photonic interactions with nanostructures. The focus of this talk is on materials and nanostructures of interest for solar energy conversion or photocatalysis, such plasmonic nanoparticles and semiconducting nanoparticles, which are interacting with or functioning as, a plasmonic cavity. The plasmonic cavity can be as simple as a thin metal film that supports a propagating surface plasmon polariton (SPP). We also explore refractory plasmonic systems due to their potential durability and reduced likelihood of melting under optical illumination as compared to noble metal nanostructures. The dynamics of nanostructure photoprocesses as a function of photon energy relative to the cavity resonance is explored in detail and impact on applications is described. Work performed at the Center for Nanoscale Materials, a U.S. Department of Energy Office of Science User Facility, was supported by the U.S. DOE, Office of Basic Energy Sciences, under Contract No. DE-AC02-06CH11357.

Electrically induced orientational instability of the director in a homeotropic nematic liquid crystal cell and its effect on surface plasmon oscillations

ABSTRACT This study examines the electric field-induced orientational instability of the director in a homeotropically oriented nematic liquid crystal (NLC) cell. Transitions from homogeneous homeotropic state to non-homogeneous state and from non-homogeneous to homogeneous planar state were investigated. These transitions have a threshold and may exhibit hysteresis. Threshold voltages for these transitions were calculated and regions of existence of hysteresis were determined depending on the cell parameters. Increasing anchoring energy narrows hysteresis range for homeotropic to non-homogeneous transition and expands it for planar to non-homogeneous transition. The impact of the external electric field on the conditions and parameters of surface plasmon polariton (SPP) propagation in the Au–polymer film–NLC system was theoretically investigated. The effective refractive index of the SPP was found to increase with increasing anchoring energy of the NLC with the polymer film and with decreasing wavelength and applied voltage.

On-chip generation of single-photon circularly polarized single-mode vortex beams.

Generation of single photons carrying spin and orbital angular momenta (SAM and OAM) opens enticing perspectives for exploiting multiple degrees of freedom for high-dimensional quantum systems. However, on-chip generation of single photons encoded with single-mode SAM-OAM states has been a major challenge. Here, by using carefully designed anisotropic nanodimers fabricated atop a substrate, supporting surface plasmon polariton (SPP) propagation, and accurately positioned around a quantum emitter (QE), we enable nonradiative QE-SPP coupling and the SPP outcoupling into free-space propagating radiation featuring the designed SAM and OAM. We demonstrate on-chip room-temperature generation of well-collimated (divergence < 7.5°) circularly polarized (chirality > 0.97) single-mode vortex beams with different topological charges (𝓁 = 0, 1, and 2) and high single-photon purity, g(2)(0) < 0.15. The developed approach can straightforwardly be extended to produce multiple, differently polarized, single-mode single-photon radiation channels and enable thereby realization of high-dimensional quantum sources for advanced quantum photonic technologies.

Evaluation of the Efficiency of Generation of Terahertz Surface Plasmon Polaritons by the End-Fire Coupling Technique

One of the most important problems in the plasmonics of the terahertz (THz) range, which is actively developing now, is the efficient generation of surface plasmon polaritons (SPPs). The simplest and most promising technological technique of photon excitation of THz SPPs is through diffraction of radiation on the edge of the conducting surface of the sample (the end-fire coupling technique). In this paper, we experimentally evaluated the efficiency of the generation of monochromatic THz SPPs (λ0 = 141 μm) by this method with a sample in the form of a cylindrical segment, the convex surface of which has a gold layer coated by zinc sulfide (ZnS) with thickness d = 0–2 µm. Such configuration of the surface supporting the SPPs not only shields the detector from parasitic bulk waves arising during diffraction but also enables one to change the distribution of the SPP field in the air by varying the coating layer thickness d. On an uncoated gold surface, the SPP generation efficiency was η ≈ 20%. In the presence of a ZnS layer on the gold, the SPP generation efficiency gradually increased with d, reached the maximum (ηmax ≈ 60%) at d ≈ 1 μm, and then gradually decreased. Theoretical analysis showed that the efficiency of the SPP generation can be raised up to 80% due to the selection of an optimal SPP field profile via variation of the thickness of the dielectric layer on the metal surface, as well as with optimal incidence of the focused radiation on the edge of the sample.

Spectrum enhancement of extraordinary terahertz transmission through metal-dielectric multilayer compound annular hole array metamaterial

This study presents, what we believe to be, a novel approach to enhance the spectrum of extraordinary terahertz transmission utilizing surface plasmon polaritons (SPPs) mode coupling in metal-dielectric compound annular hole array metamaterial. The transmitting properties of the structure are thoroughly assessed through a combination of theoretical analysis and numerical simulation, with a particular focus on understanding the surface plasmon mode coupling. Our investigation revealed the presence of propagating surface plasmon polaritons (PSPP), localized surface plasmon resonance (LSPR) and Fabry-Perot (FP) resonances within the compound layer. We find that the coupling of PSPP and LSPR modes plays a crucial role in determining the broadband nature of the structure. Interference theory is utilized to mitigate the FP resonance, while further optimization of the transmission bands is attained by implementing the impedance matching hypothesis. By adjusting the structure parameters, the effective coupling of the SPPs modes and numerous reflections in the dielectric cavity produced a remarkably smooth and transparent character, leading to a significant enhancement of transmission spectrum and a reduction in the incident loss of terahertz wave.

Soliton-like surface plasmon polaritons generated on the surface of a silver nanowire embedded in a Kerr nonlinear medium.

The behavior of surface plasmon polaritons (SPPs) generated on the surface of a silver nanowire by coaxial Gaussian beams in Kerr nonlinear mediums is studied numerically. Enhancement of the propagation of the SPPs is realized due to the introduction of the nonlinear effect. Further adjusting the nonlinearity or the beam's intensity results in a soliton-like propagation of SPPs. This can be explained by the nonlinear self-focusing effect transferring more light into SPP modes and counteracting the attenuation caused by the absorption of metal. This result may contribute to SPP-based applications where an enhanced propagation length is needed.

Amplified hybrid surface plasmon polaritons in partially reduced graphene oxide supported on gold

We experimentally investigated the surface plasmon polaritons' (SPPs) localisation, resonance, and propagation in three plasmonic waveguides: graphene oxide (GO), partially reduced graphene oxide (PRGO), and reduced graphene oxide (RGO) in two configurations; as stand-alone structures and when deposited on gold films. Our experiments measuring the nonlinear absorption/refraction and leakage radiation microscopy (LRM) reveal that the intense Lorenz/Fano resonance of surface plasmon polaritons can occur in the PRGO-based waveguide. We also observe an increasing surface plasmon polaritons generation rate in PRGO-supported Au waveguide, a procedure similar to the spaser (surface plasmon nanolaser) mechanism. This, in turn, leads to increasing the propagation length. Furthermore, our analytical simulation results also suggest the suitability of PRGO as a superior optical material for spaser application. Our density functional calculations confirm that oxidised graphene has a more robust absorption spectrum in infrared wavelengths than pure graphene. Additionally, through van der Waals interaction, PRGO forms a more consistent and uniform bond with gold substrate than pristine graphene, reinforcing the suitability of PRGO as plasmonic waveguides.