Plasmon Propagation Research Articles

High order plasmonic vortex generation based on spiral nanoslits

Highly localized plasmonic vortices carrying orbital angular momentum are of importance for many applications. Yet, it is a challenge to generating plasmonic vortex with a high topological charge because of no available technique. Here, a novel plasmonic vortex generator is proposed based on spiral nanoslits etched in a metal film, which can produce a high order plasmonic vortex. The consecutive spiral nanoslit can generate plasmonic vortex with high intensity and the segmented nanoslits enhances the controllability of the plasmonic vortex, which are demonstrated numerically and experimentally. High order plasmonic vortex generation will broaden the prospects for plasmonic vortices in practical applications for nanomanipulation and nanofabrication.

Nanoscale All-Solid-State Plasmochromic Waveguide Nonresonant Modulator.

Plasmochromics, the interaction of plasmons with an electrochromic material, have spawned a new class of active plasmonic devices. By introducing electrochromic materials into the plasmon's dielectric environment, plasmons can be actively manipulated. We introduce inorganic WO3 and ion conducting LiNbO3 layers as the core materials in a solid-state plasmochromic waveguide (PCWG) to demonstrate light modulation in a nanoplasmonic waveguide. The PCWG takes advantage of the high plasmonic loss at the high field located at the WO3/Au interface, where the Li+ ions are intercalated into a thin WO3 plasmon modulating layer. Through careful PCWG design, the direction for ion diffusion and plasmon propagation are decoupled, leading to enhanced modulation depth and fast EC switching times. We show that at a bias voltage of 2.5 V, the fabricated PCWG modulator achieves modulation depths as high as 20 and 38 dB for 10 and 20 μm long devices, respectively.

Frequency-Tunable Terahertz Plasmonic Structure Based on the Solid Immersed Method for Sensing.

Terahertz waves are located in the frequency band between radio waves and light, and they are being considered for various applications as a light source. Generally, the use of light requires focusing; however, when a terahertz wave is irradiated onto a small detector or a small measurement sample, its wavelength, which is much longer than that of visible light, causes problems. The diffraction limit may make it impossible to focus the terahertz light down to the desired range by using common lenses. The Bull’s Eye structure, which is a plasmonic structure, is a promising tool for focusing the terahertz light beyond the diffraction limit and into the sub-wavelength region. By utilizing the surface plasmon propagation, the electric field intensity and transmission coefficient can be enhanced. In this study, we improved the electric field intensity and light focusing in a small region by adapting the solid immersion method (SIM) from our previous study, which had a frequency-tunable nonconcentric Bull’s Eye structure. Through electromagnetic field analysis, the electric field intensity was confirmed to be approximately 20 times higher than that of the case without the SIM, and the transmission measurements confirmed that the transmission through an aperture had a gap of 1/20 that of the wavelength. This fabricated device can be used in imaging and sensing applications because of the close contact between the transmission aperture and the measurement sample.

Tkwant: a software package for time-dependent quantum transport

Tkwant is a Python package for the simulation of quantum nanoelectronics devices to which external time-dependent perturbations are applied. Tkwant is an extension of the kwant package (https://kwant-project.org/) and can handle the same types of systems: discrete tight-binding-like models that consist of an arbitrary central region connected to semi-infinite electrodes. The problem is genuinely many-body even in the absence of interactions and is treated within the non-equilibrium Keldysh formalism. Examples of Tkwant applications include the propagation of plasmons generated by voltage pulses, propagation of excitations in the quantum Hall regime, spectroscopy of Majorana fermions in semiconducting nanowires, current-induced skyrmion motion in spintronic devices, multiple Andreev reflection, Floquet topological insulators, thermoelectric effects, and more. The code has been designed to be easy to use and modular. Tkwant is free software distributed under a BSD license and can be found at https://tkwant.kwant-project.org/.

Enhancing plasmonic hot-carrier generation by strong coupling of multiple resonant modes.

Plasmon-induced hot carriers have recently attracted considerable interest, but the energy efficiency in visible light is often low due to the short lifetime of hot carriers and the limited optical absorption of plasmonic architectures. To increase the generation of hot carriers, we propose to exert multiple plasmonic resonant modes and their strong coupling using a metal-dielectric-metal (MDM) nanocavity that comprises an Au nanohole array (AuNHA), a TiO2 thin film and an Au reflector. Unlike common MDM structures, in addition to the Fabry-Pérot mode in the dielectric layer, AuNHA as the top layer is special because it excites the localized surface plasmon resonance (LSPR) mode in the Au nanoholes and launches the gap surface plasmon polariton (GSPP) mode in the Au reflector surface. The spatial field overlapping of the three resonance modes enables strong mode coupling by optimizing the TiO2 thickness, which leads to notably enhanced average IPCE (∼1.5%) and broadband photocurrent (170 μA·cm-2). This MDM structure would be useful for photochemistry and photovoltaics using sunlight.

Unravelling the coupling of surface plasmons in carbon nanotubes by near-field nanoscopy.

We experimentally and theoretically investigated the propagation and subwavelength coupling of surface plasmons in carbon nanotube pairs (CNTPs). We found two plasmon modes in the Fourier transformed near-field images of CNTPs, while only one propagating plasmon mode in the single carbon nanotube (CNT). Using the finite element method (FEM), we calculated plasmon coupling between two carbon nanotubes. We attributed the observed two plasmon modes to the symmetric and asymmetric coupling of the plasmons between the two CNTs. Using numerical simulation, we further investigated the plasmon coupling strength and found that it is sensitive to the separating distance of CNTs. When the separation is larger than 5 nm, the plasmon coupling attenuates. We show that near-field optical imaging is a feasible way to characterize the plasmon coupling of CNTs, even if the separation of the CNTs is only a few nanometers.

Atomistic description of plasmonic generation in alloys and core shell nanoparticles.

Using the extended discrete interaction model we investigate the tunabilty of surface plasmon resonances in alloys and core-shell nanoparticles made from silver and gold in the small (1-15 nm) nanoscale regime where classical models based on the bulk dielectric constant may not apply. We show that the surface plasmon resonance of these alloys and core-shell particles to a large extent follow Vegard's law irrespective of the geometry of the nanoparticle. The evolution of the polarizability with size demonstrates a highly non-linear behaviour of the polarizability with the ratio of the constituents and geometry in alloys and core-shell nanoparticles, with the exception of the longitudinal surface plasmon resonance in nanorods and, partly, nanodisc alloys. We here show that the non-linear behaviour can be explained in terms of the difference in polarizability of the mixing constituents and local effects causing a quenching of the dipoles for geometries with a low aspect ratio. A thorough statistical investigation reveals that there is only a small dependence of the surface plasmon resonance on atomic arrangement and exact distribution in a nanoparticle and that the standard deviation decreases rapidly with the size of the nanoparticles. The physical ground for the random distribution algorithm for alloys in discrete interaction models is explained in detail and verified by a statistical analysis. For nanoparticles below 4 nm a sampling strategy is recommended.

Generation of plasmon modes in a supernarrow nanoslit formed by silver surfaces

We report a theoretical study of plasmon generation of a giant electromagnetic field in a supernarrow nanoslit formed by a silver cylinder and a flat mirror surface. It is shown that as the silver surfaces approach each other, gap plasmons are excited in the gap between them, which results in a resonant amplification of the field. It is demonstrated for the first time that the electric field amplification increases with decreasing distance between the cylindrical and flat surfaces and reaches saturation, at which the field intensity becomes record high, exceeding the incident wave intensity by ten orders of magnitude. The found gap plasmon modes will increase the sensitivity to the detection of small concentrations of molecules, down to single molecules, by the methods of giant Raman scattering of light and plasmon-enhanced IR spectroscopy.

Gold Nanoparticle-Based Nanoengines for Light-Induced Plasmonic Bubble Generation

This paper explores the rate and severity of plasmonic bubble generation by gold nanoparticles (AuNPs) of different sizes and concentrations under sunlight illumination. An AuNP nanoengine has been built and optimized through related phenomena, and the size of AuNPs has been controlled through the automatic experiment robot. The bubble generation occurred within about 2–3 min before bursting within 1 s. It occurred 4–5 times and could propel an improved Stirling steam engine. This nanoengine can continuously output work at 40 W/m2. With increased illumination time, the pressure could exceed 10 atm, causing an explosion, indicating the superstrong energy capacity in the AuNP solution.

Compound plasmonic vortex generation based on spiral nanoslits

In view of wide applications of structured light fields and plasmonic vortices, we propose the concept of compound plasmonic vortex and design several structured plasmonic vortex generators. This kind of structured plasmonic vortex generators consists of multiple spiral nanoslits and they can generate two or more concentric plasmonic vortices. Different from Laguerre-Gaussian beam, the topological charge of the plasmonic vortex in different region is different. Theoretical analysis lays the basis for the design of radially structured plasmonic vortex generators and numerical simulations for several examples confirm the effectiveness of the design principle. The discussions about the interference of vortex fields definite the generation condition for the structured vortex. This work provides a design methodology for generating new vortices using spiral nanoslits and the advanced radially structured plasmonic vortices is helpful for broadening the applications of vortex fields.

Polarized plasmon resonance spectra of electrochemically modified titanium surfaces with gold nanoparticles

In this paper, the plasmon resonance on electrochemically modified titanium surfaces synthesized by anodic dissolution method has been studied in the presence with gold ablative nanoparticles. The permittivity functions and reflection coefficients of p- and s-polarized light spectra on the titanium oxide surface of various modification (roughness) have been analyzed. Spectral features of the negative refractive index in the area of surface plasmon generation on the rough titanium-oxide film interface have been also presented in this paper.

Increase in CO2 reduction rate via optical near-field effect

To reduce the effects of global warming, visible and near-infrared light must be used more efficiently. Deep ultraviolet light (8 eV) is required for the direct dissociation of CO2 by light; however, the introduction of a metal complex has made it possible to realize CO2 reduction with visible light. We demonstrate that the optical near field (ONF) can increase the CO2 reduction rate. For this, we used gold clusters, because they can be a suitable source for ONFs, as their size and density can be controlled by the number of gold atoms. By attaching a metal complex near gold clusters with diameters of 1.0 to 1.3 nm, we confirm that the reduction rate of CO2 to CO increased by 1.5 to 2.1 times. The gold clusters were sufficiently small; therefore, there was no plasmonic resonant peak or heat generation. Because the near-field effect is based on a photochemical reaction, it can be applied to other metal complexes used in CO2 reduction, and it has other applications such as water splitting and water purification.

Self-Assembly of Plasmonic Nanoantenna-Waveguide Structures for Subdiffractional Chiral Sensing.

Spin-momentum locking is a peculiar effect in the near-field of guided optical or plasmonic modes. It can be utilized to map the spinning or handedness of electromagnetic fields onto the propagation direction. This motivates a method to probe the circular dichroism of an illuminated chiral object. In this work, we demonstrate local, subdiffraction limited chiral coupling of light and propagating surface plasmon polaritons in a self-assembled system of a gold nanoantenna and a silver nanowire. A thin silica shell around the nanowire provides precise distance control and also serves as a host for fluorescent molecules, which indicate the direction of plasmon propagation. We characterize our nanoantenna-nanowire systems comprehensively through correlated electron microscopy, energy-dispersive X-ray spectroscopy, dark-field, and fluorescence imaging. Three-dimensional numerical simulations support the experimental findings. Besides our measurement of far-field polarization, we estimate sensing capabilities and derive not only a sensitivity of 1 mdeg for the ellipticity of the light field, but also find 103 deg cm2/dmol for the circular dichroism of an analyte locally introduced in the hot spot of the antenna-wire system. Thorough modeling of a prototypical design predicts on-chip sensing of chiral analytes. This introduces our system as an ultracompact sensor for chiral response far below the diffraction limit.

Plasmon-Enhanced Near-Field Chirality in Twisted van der Waals Heterostructures.

It is shown that chiral plasmons, characterized by a longitudinal magnetic moment accompanying the longitudinal charge plasmon, lead to electromagnetic near-fields that are also chiral. For twisted bilayer graphene, we estimate that the near-field chirality of screened plasmons can be several orders of magnitude larger than that of the related circularly polarized light. The chirality also manifests itself in a deflection angle that is formed between the direction of the plasmon propagation and its Poynting vector. Twisted van der Waals heterostructures might thus provide a novel platform to promote enantiomer-selective physio-chemical processes in chiral molecules without the application of a magnetic field or external nanopatterning that break time-reversal, mirror plane, or inversion symmetry, respectively.

Long-Range Surface Plasmon Enhanced Raman Spectroscopy at Catalytic Platinum Electrodes

Surface enhanced Raman scattering (SERS) is recognized as a powerful method for in-situ observation of electrode/electrolyte interfaces, due to the high chemical sensitivity and surface selectivity. However, the applicability of this technique is limited by the nature of surface plasmon resonances at the interface. Although platinum group metals (PGMs) are one of the most interesting materials for electrocatalysis, surface plasmon polaritons are largely damped at these metal surfaces by their localized d-electrons near the Fermi level. Thus, the plasmon enhanced Raman spectroscopy is normally useless for studying such catalytic surfaces. So far, the signal enhancement factors reported at PGM surfaces are in the order of 102 while those at coinage metal surfaces can reach the order of 106 to 1010. In order to gain more intensity at highly damping PGM surfaces, the intensity-borrowing method is a common method; for example, when a large number of Au nanoparticles are deposited on a PGM surface, Raman scattering signals are largely enhanced by the sphere-plane type coupled plasmon modes [1,2]. However, such a nanostructure may influence mass transport, selectivity, and reactivity of electrochemical reactions. For spectro-electrochemistry, it is necessary to reduce the plasmon damping without suffering from such a nanoscale effect. In this work, the attenuation of surface plasmons at highly damping Pt surface is reduced using a symmetric slab mode, which is a strongly coupled surface plasmons excited at both interfaces of a thin metal film [3]. The prolonged propagation of surface plasmons in this double-interface configuration leads to a substantial enhancement of local fields even at lossy Pt surface. The theoretical calculation indicates that the enhancement factor of Raman scattering intensity can reach the order of 104 at Pt/solution interfaces in this coupled plasmon mode. We experimentally demonstrate that Raman scattering signals are significantly enhanced at Pt surface under electrochemical conditions. This strategy provides us a novel surface-specific spectroscopic method for the molecular-scale investigation of electrocatalytic reactions.[1] J. Hu, N. Hoshi, K. Uosaki, K. Ikeda, Nano Lett., 15, 7982-7986 (2015).[2] J. Hu, M. Tanabe, J. Sato, K. Uosaki, K. Ikeda, J. Am. Chem. Soc.,136, 10299-10307 (2014).[3] S. Ikegaya, K. Motobayashi, K. Ikeda, J. Raman Spectrosc. DOI: 10.1002/jrs.5925 (2020)

Terahertz surface plasmon propagation in a near field coupled asymmetric resonators based planar waveguide

In this article, we investigate terahertz surface plasmons propagation in a planar waveguide comprising asymmetric resonators placed in close proximity. The waveguide is designed to support surface plasmon polaritons at two distinct terahertz frequencies, ω 1 and ω 2, which are near to each other. This is accomplished by carefully designing the unit cell comprising of two resonators with slightly different sizes. The resonators in the form of rectangular apertures are placed along the transverse direction in the near field regime. We observe an absorption window surrounded by two transmission resonances which are coupled to each other. As the resonators are moved apart, we observe a switching from the coupled to an uncoupled resonance state. The absorption window can be tuned by changing the resonance frequency of the resonators, which depends upon its structural parameters. We have employed a theoretical model to understand the coupling mechanism between the resonators and the dispersive behavior resulting in the absorption window. The proposed study is significant in the construction of planar terahertz components such as slow lights systems, buffers, etc, where strongly dispersive mediums play a vital role.

Plasmonic hybridization generation in self-aligned disk/hole nanocavities for multi-resonance sensing.

Plasmonic nanostructures have proven an extensive practical prospect in ultra-sensitive label-free biomolecule sensing due to their nanoscale localization and large near-field enhancement. Here, we demonstrate a photonic plasmonic hybridization in the self-aligned disk/hole nanocavity array under two specific cases of nanogap and nanooverlap achieved by adjusting pillar height embedded into hole. The proposed disk/hole arrays in above two cases exhibit three hybridized modes with extremely high absorption, mainly arising from the in-phase (bonding) and out-of-phase (antibonding) coupling of dipolar modes of their parent disk and hole. Surprisingly, when the nanogap feature of the disk/hole array is transformed to the nanooverlap, crossing the quantum effect region, the bonding mode in the disk/hole array has an enormous transition in the resonant frequency. In comparison with the counterpart in the nanogap structure, the bonding mode in the nanooverlap structure supports strongest near-field localization (i.e., the decay length down to merely 3.8 nm), although charge transfer channel provided by the geometry connect between disk and hole quenches partial field enhancement. Furthermore, we systematically investigate the sensing performances of multiple hybridized modes in above two cases by considering two crucial evaluating parameters, bulk refractive index sensitivity and surface sensitivity. It is demonstrated that, in the nanogap structure, the bonding mode possesses both high bulk refractive index sensitivity and surface sensitivity. Dissimilarly, for the nanooverlap structure, the bonding and antibonding modes show different surface sensitivities in different regions away from the surface, which can be used to monitoring different bio-molecular sizes and achieve the most optimum sensitivity. Due to its unique sensing features, this disk/hole array mechanism is very valuable and promising for developing of high sensitivity sensing platform.

Inverse design of graphene-assisted metallodielectric grating and its applications in the perfect absorber and plasmonic third harmonic generation.

In this article, we propose a graphene metamaterial coupled with metallodielectric grating (GMCMG) structures to achieve plasmon induced reflection effects in the reflection spectrums. In order to enhance the light-matter interaction in the graphene, the micro-genetic algorithm is applied in the performance optimization for the GMCMG. Due to the absorption enhancement of graphene and the inverse design of photonic structures, a perfect absorber and an efficient third harmonic generator are obtained by employing optimized GMCMG structures. Compared with previous works, our design scheme provides a simple and efficient method for the optimization of photonic devices and has significant applications in optical modulators, absorbers and sensors.

Plasmon recombination in narrowgap HgTe quantum wells

The dispersion laws of two-dimensional plasmons in narrow-gap HgTe/CdHgTe quantum wells are calculated taking into account the spatial dispersion of the electron susceptibility. At the energy scale of the band gap the dependence of plasmon frequencies on the wave vector is shown to be close to linear that changes significantly the critical concentration of noneqilibrium electron-hole gas corresponding to ‘switching-on’ the carrier recombination with plasmon emission. The recombination rates with the plasmon emission have been calculated. The ‘plasmon’ recombination is shown to dominate at the carrier concentration over (1.2–2) 1011 cm−2 in a 5-nm-wide HgTe quantum well (band gap of 35 meV) that makes plasmon generation (spasing) in THz frequency range feasible.

Room-Temperature Lasing in a Low-Loss Tamm Plasmon Cavity

Optical Tamm structures offer great possibilities for the development of new optoelectronic devices ranging from polarized laser sources to plasmon generators. Indeed, thanks to the patterning of a...