Plasmon Propagation Research Articles

Feasibility of surface plasmon lasing in HgTe quantum wells with population inversion

Surface plasmon lasing in semiconductor gain media at far-infrared frequencies requires simultaneously long non-radiative recombination times and large plasmon propagation length. In this paper, we show that these conditions are realized in mercury-telluride quantum wells (HgTe QWs) near the topological transition. We derive the conditions of surface plasmon amplification in HgTe QWs with interband population inversion. To this end, we calculate the spatially-dispersive high-frequency conductivity of pumped HgTe QWs taking into account their realistic band structure, and compare the interband gain with Drude absorption and collisionless Landau damping. An extra necessary condition of plasmon lasing is revealed, namely, the non-equilibrium carrier density should be high enough to make the plasmon spectrum overlap with the frequency domain of interband excitations. The latter condition limits the processes of both stimulated and spontaneous plasmon emission at low temperatures, and should have a strong impact on the recombination kinetics of HgTe QWs at low temperatures.

Quantitative analysis of plasmon excitations in hard x-ray photoelectron spectra of bulk black phosphorus

Black phosphorus (BPh) is a layered material with strong in-plane anisotropy of its structural and electronic properties; in spite of the great potential of BPh for conceptually new devices in optoelectronics and plasmonics, its fundamental electronic excitations have not yet been fully elucidated. In order to discriminate collective (plasmons) and single-particle (inter band transitions) excitations, we investigate the energy-loss distribution of P 1s photoelectrons in hard X-ray photoelectron spectra of BPh over a wide energy range. The energy-loss function (ELF), averaged over the principal directions of the BPh crystal, has been retrieved by using a Fourier Transform analysis to eliminate multiple inelastic scattering events. At low loss energies (1–8 eV), weak unresolved energy loss peaks are well described by DFT calculated inter band transitions, showing some anisotropy in the dielectric function ε(ω,q) tensor of BPh. At high loss energies, the ELF is dominated by the collective excitation of valence electrons with a peak energy at 20.1 ± 0.2 eV, and weak anisotropy is found in the DFT calculated Im(-1/ε) tensor. The anomalously small peak energy (9.0 ± 0.5 eV) of a weak surface plasmon resonance is attributed either to low surface electron density in the terminal phosphorene layer or to some anisotropic surface plasmon propagation.

Tunable On‐Chip Sources with Aperiodic Metasurface

AbstractSurface plasmons show tremendous capability in integrated communication, quantum computing and sensing. Excitations and manipulations of surface plasmons are essential in developing integrated photonic devices. Here, a systematic study of tunable emission of surface plasmons with an eightfold quasicrystal metasurface, which acts as an on‐chip source, is presented. It is shown that the quasicrystal structure can switch on or off the surface plasmons propagation channels in the desired direction. Meanwhile, such a quasicrystal structure can be polarization‐dependent or polarization‐independent based on different constituent slit pairs. The proposed quasicrystal design provides more freedom for steering surface plasmons in the launching process. Thus, it may significantly simplify the design and fabrication of integrated plasmonic devices.

LSPR based uric acid sensor using graphene oxide and gold nanoparticles functionalized tapered fiber

Abstract In this work, two different tapered optical fiber sensor probes were developed to detect the Uric acid (UA) presents in human bodies. In first sensor probe, a uniform layer of graphene oxide (GO) was coated over the tapered fiber structure. In second probe, a layer of gold nanoparticles (AuNPs) was sandwiched between GO layer and fiber surface. The AuNPs ensures the generation of surface plasmons (SPs) and results in localized surface plasmon resonance (LSPR) phenomenon. Thereafter, LSPR phenomenon was used to sense the change in refractive index around the sensor head. The GO layer was used due to its inherent properties of binding the biomolecules. The AuNPs and GO solutions were characterized by transmission electron microscopy (TEM). The coating of AuNPs and GO over the sensing region was confirmed by scanning electron microscopy (SEM). The linearity test was performed by detecting the different concentration of UA solutions in the range of 10 µM–800 µM. The selectivity of sensor was enhanced by functionalization of nanomaterials-coated fiber with uricase enzyme. To verify the selectivity, several biomolecules present in human serum/urine were detected.

A theoretical insight into the use of anti-reflective coatings for the upliftment of sensitivity of surface plasmon resonance sensors

Surface Plasmon Resonance (SPR) based biosensors have been around in the scientific community for more than two decades and these have facilitated enormous improvement in the detection of complex biomolecules. Since, sensitivity of these sensors directly affects the ease of detection, our work here focuses on using dielectric nanosheets of Titania and Silica to enhance the sensitivity. The anti-reflective property of these dielectric materials promotes greater interaction between the incident light and the plasmonic metal, resulting in generation of more surface plasmons that promotes greater sensitivity. With 40 nm of gold, 3 layers of Titania (9 nm) and a layer of Silica (3 nm), our simulation results predict a sensitivity of 214 deg/RIU at 532 nm, which is a major improvement when compared to other sensor structures.

Nanoscale aluminum plasmonic waveguide with monolithically integrated germanium detector

Surface plasmon polaritons have rapidly established themselves as a promising concept for molecular sensing, near-field nanoimaging, and transmission lines for emerging integrated ultracompact photonic circuits. In this letter, we demonstrate a highly compact surface plasmon polariton detector based on an axial metal-semiconductor-metal nanowire heterostructure device. Here, an in-coupled surface plasmon polariton propagates along an aluminum nanowire waveguide joined to a high index germanium segment, which effectively acts as a photoconductor at low bias. Based on this system, we experimentally verify surface plasmon propagation along monocrystalline Al nanowires as thin as 40 nm in diameters. Furthermore, the monolithic integration of plasmon generation, guiding, and detection enables us to examine the bending losses of kinked waveguides. These systematic investigations of ultrathin monocrystalline Al nanowires represent a general platform for the evaluation of nanoscale metal based waveguides for transmission lines of next generation high-speed ultracompact on-chip photonic circuits.

Nanoscale magnetization and third-order nonlinearity by the plasmon-induced inverse Faraday effect in graphene-covered semiconductors

In this paper we study an approach for nanoscale spatially inhomogeneous excitation of quasistatic magnetic fields by the plasmon-induced inverse Faraday effect (IFE) in graphene-covered semiconductors and we present analytical and numerical results for the induced magnetic field distribution. The effective magnetic field is predicted to reach about 1 T and the direction of the magnetic field can be switched by surface plasmon polaritons propagating into the opposite direction. By electrically controlling the chemical potential of the graphene sheet the spatial inhomogeneity of the magnetization near field can be broadly tuned. The response of the induced magnetization to the plasmon propagation is manifested by a nonlinear phase shift which is measurable in the far field. By using the Lorentz reciprocity theorem we analytically calculated the nonlinear susceptibility and the nonlinear absorption coefficient in dependence on the chemical potential, the frequency, and the other material parameters. The plasmon-induced IFE and the nonlinearity can be very strong by decreasing the chemical potential which is flexibly controllable by using the graphene's gate voltage. The studied system could pave the way for an alternative approach for nanometer spatial all-optical magnetization control.

\u201cHot\u201d electrons in metallic nanostructures\u2014non-thermal carriers or heating?

Understanding the interplay between illumination and the electron distribution in metallic nanostructures is a crucial step towards developing applications such as plasmonic photocatalysis for green fuels, nanoscale photodetection and more. Elucidating this interplay is challenging, as it requires taking into account all channels of energy flow in the electronic system. Here, we develop such a theory, which is based on a coupled Boltzmann-heat equations and requires only energy conservation and basic thermodynamics, where the electron distribution, and the electron and phonon (lattice) temperatures are determined uniquely. Applying this theory to realistic illuminated nanoparticle systems, we find that the electron and phonon temperatures are similar, thus justifying the (classical) single-temperature models. We show that while the fraction of high-energy “hot” carriers compared to thermalized carriers grows substantially with illumination intensity, it remains extremely small (on the order of 10−8). Importantly, most of the absorbed illumination power goes into heating rather than generating hot carriers, thus rendering plasmonic hot carrier generation extremely inefficient. Our formulation allows for the first time a unique quantitative comparison of theory and measurements of steady-state electron distributions in metallic nanostructures.

Theoretical Study of Unconventional Plasmon Generation by Solving Finite-Size 3D Hubbard Model within Mean Field Approximation

Recent experimental study on Strontium Niobate Oxide system has revealed unconventional plasmons generated due to confinement by oxygen planes. A phenomenological model accompanied the experimental data on that report has suggested that the confined electrons behave as harmonic oscillators. These motivate us to further study the effect of space confinement to the electrons on the formation of unconventional plasmons theoretically. For this purpose, we propose a method of modeling the generation of the unconventional plasmons in a finite system. The dynamics of correlated electrons in this confined system is described using finite-size 3D Hubbard model that is solved within mean field approximation. We study a hypothetical cubic system that consists of 5x5x5 single-orbital atoms. The calculation is done with and without incorporating the on-site Coulomb repulsion. We also consider both restricted and unrestricted mean field treatments to the calculation. Our dielectric function results show that for restricted mean field with U = 0 eV and unrestricted mean field with U ≤ 2 eV, the system is metal and only two and four unconventional plasmons found respectively. While, for U > 2 eV, it becomes insulator and shows more emergence of unconventional plasmons.

Plasmonic wave propagation mode analysis of single and multi-layer graphene-pec structures

Plasmonic graphene-PEC structure is presented for THz frequency range in detailed analysis. The number of graphene layer effect on Surface Plasmon Polariton (SPP) wave behaviour is analyzed. Moreover, the graphene characteristics and propagation mode are presented. In the following, the proposed structure, graphene-PEC, analyzes in each electromagnetic field component in details. The numerical solver COMSOL Multiphysics is utilized to demonstrate that the graphene-PEC structure support quasi-TEM mode which classical approach is still true in THz band.

Thermal Optofluidics: Principles and Applications

AbstractThermal optofluidics is an emerging field that promises to create numerous research and application opportunities in biophysics, biochemistry, and clinical biology. Innovation in plasmonic optics has led to the development of various invaluable tools in the fields of biosensing and microfluidic manipulation. The optothermal effect originates from light–matter interactions during photon–phonon conversion, which can lead to micro‐ or nanoscale inhomogeneities in the thermal distribution. This further induces a series of hydrodynamic phenomena such as natural convection, Marangoni convection, thermophoresis, the electrolyte Seebeck effect, depletion forces, and interfacial effects in colloidal particles. Light–matter interactions are particularly important for three aspects of microfluidics, namely the motion of colloidal particles, fluidic actuation, and biochemical reactions. This review first systematically elucidates the role of both nanoscale plasmonic thermal generation and heat‐induced fluidic motion in optofluidic microsystems. Then, recent state‐of‐the‐art thermal optofluidic applications of the above‐listed three aspects are presented. The paper aims to provide an insightful reference for future research in optofluidic biochemical systems.

Au Plasmonic Shofar Structures

We demonstrate a simple two-step etching method to fabricate Au plasmonic shofars (“shofar” is the Hebrew name for certain types of curved and tapered horns, such as from a ram). The structural, optical, and morphological properties were studied by using X-ray diffraction, UV-Vis spectrophotometry, and scanning electron microscopy (SEM). The etching mechanism and the light-guiding properties for this unique morphology are described. The leaky surface plasmon propagation (SPP) and guiding properties of shofars are investigated by using leakage radiation microscopy (LRM).

Propagation of toroidal localized spoof surface plasmons using conductive coupling.

Here, on a platform of two split-ring resonator (SRR) disks in the microwave regime, we have numerically and experimentally investigated the coupling of toroidal localized spoof surface plasmons (LSSPs). The coupling effect is investigated both theoretically and experimentally. We observe that magnetic dipole coupling exists in the toroidal LSSPs coupling and causes a rearrangement of the toroidal LSSPs, which suppresses the propagation of toroidal LSSPs. To realize the propagation of toroidal LSSPs, we introduce conductive coupling into the SRR disks. The conductive coupling can correct magnetic dipole coupling and enhance toroidal LSSPs coupling. Both numerical simulations and experiments are in good agreement. The toroidal LSSPs can be effectively propagated, even in the three right-angle-bent SRR disks. This study paves the way toward a better understanding of toroidal LSSPs coupling and finds many applications in the transfer of electromagnetic energy using toroidal moments.

A Theoretical Treatment of THz Resonances in Semiconductor GaAs p-n Junctions.

Semiconductor heterostructures are suitable for the design and fabrication of terahertz (THz) plasmonic devices, due to their matching carrier densities. The classical dispersion relations in the current literature are derived for metal plasmonic materials, such as gold and silver, for which a homogeneous dielectric function is valid. Penetration of the electric fields into semiconductors induces locally varying charge densities and a spatially varying dielectric function is expected. While such an occurrence renders tunable THz plasmonics a possibility, it is crucial to understand the conditions under which propagating resonant conditions for the carriers occur, upon incidence of an electromagnetic radiation. In this manuscript, we derive a dispersion relation for a p–n heterojunction and apply the methodology to a GaAs p–n junction, a material of interest for optoelectronic devices. Considering symmetrically doped p- and n-type regions with equal width, the effect of certain parameters (such as doping and voltage bias) on the dispersion curve of the p–n heterojunction were investigated. Keeping in sight the different effective masses and mobilities of the carriers, we were able to obtain the conditions that yield identical dielectric functions for the p- and n-regions. Our results indicated that the p–n GaAs system can sustain propagating resonances and can be used as a layered plasmonic waveguide. The conditions under which this is feasible fall in the frequency region between the transverse optical phonon resonance of GaAs and the traditional cut-off frequency of the diode waveguide. In addition, our results indicated when the excitation was slightly above the phonon resonance frequency, the plasmon propagation attained low-loss characteristics. We also showed that the existence or nonexistence of the depletion zone between the p- and n- interfaces allowed certain plasmon modes to propagate, while others decayed rapidly, pointing out the possibility for a design of selective filters.

Manipulating surface plasmon polaritons with M-shaped nanoslit array via polarized incident waves

Surface plasmonic bending beams (SPBs), which preserve their spatial shapes while propagating along the desired curved trajectories in a metal-dielectric interface, offer important applications in the fields of fiber sensors, optical trapping, and micro-nano fabrication. In this work, the straight-lined, single-curved and double mirror-symmetric curved distributions of M-shaped nanoslit arrays are designed on a metal film to generate SPBs. These SPBs include unidirectional coupling of SPPs, single arbitrary bending beams, and double arbitrary bending beams. The electric-field intensity and the propagation directions of these SPBs are controlled by the polarization angle of input light waves. Especially, the reverse propagation of unidirectional coupling of arbitrary SPBs is generated by the polarization angles 45° and 135°. These findings provide guidance in the design and optimization of plasmonic bending beam generators.

Propagation of surface plasmons on plasmonic Bragg gratings

We use coupled-mode theory to describe the scattering of a surface-plasmon polariton (SPP) from a square-wave grating (Bragg grating) of finite extension written on the surface of either a metal-dielectric interface or a dielectric-dielectric interface covered with a patterned graphene sheet. We find analytical solutions for the reflectance and transmittance of SPPs when only two modes (forward- and back-scattered) are considered. We show that in both cases, the reflectance spectrum presents stop-bands where the SPP is completely back-scattered, if the grating is not too shallow. In addition, the reflectance coefficient shows Fabry-Pérot oscillations when the frequency of the SPP is out of the stop-band region. For a single dielectric well, we show that there are frequencies of transmission equal to 1. We also provide a simple analytical expression for the different quantities in the electrostatic limit.

Photothermal Modulation of Propagating Surface Plasmons on Silver Nanowires

Nanoplasmonic devices have received much attention in recent years due to their ability to confine light beyond the diffraction limit. Manipulating the propagation of surface plasmons (SPs) is of v...

Plasmon-Enhanced Fluorescence of Carbon Nanodots in Gold Nanoslit Cavities.

Carbon nanodots (CNDs) are featured with a wide range of light absorption and excitation-dependent fluorescence. The emission enhancement of CNDs is of great interest for the development of nanophotonics. Although the phenomenon of plasmon-enhanced fluorescence for quantum dots and molecular dyes has been well investigated, rarely has it been reported for CNDs. In this work, a series of plasmonic nanoslit designs were fabricated and utilized for immobilization of CNDs in nanoslits and examination of the best match for plasmonic fluorescence enhancement of CNDs. In concert, to better understand the plasmonic effect on the enhancement, the surface optical field is measured with or without CND immobilization using a hyperspectral imaging system as a comparison, and a semianalytical model is conducted for a quantitative analysis of surface plasmon generation under the plane-wave illumination. Both the fluorescence and surface reflection light intensity enhancement are demonstrated as a function of nanoslit width and are maximized at the 100 nm nanoslit width. The analysis of surface plasmon-exciton coupling of CNDs in the nanoslit area suggests that the enhancement is primarily due to plasmonic light trapping for increased electromagnetic field and plasmon-induced resonance energy transfer. This study suggests that incorporating CNDs in the plasmonic nanoslits may provide a largely enhanced CND-based photoemission system for optical applications.

Tunable surface plasmon polaritons in a Weyl semimetal waveguide

Weyl semimetals have recently attracted extensive attention due to their anomalous band structure manifested by topological properties that lead to some unusual and unique physical properties. We investigate novel features of surface plasmon polaritons in a slot waveguide comprised from two semi-infinite Weyl semimetals. We consider symmetric Voigt–Voigt and Faraday–Faraday configurations for plasmon polaritons in two interfaces of waveguide and show that the resulting dispersion is symmetric and the propagation of surface plasmon polaritons is bidirectional. We introduce exotic and novel asymmetric structures making use of difference in magnitude or orientation of chiral anomalies in two Weyl semimetals in both Voigt and Faraday configurations. These structures show a tremendous nonreciprocal dispersion and unidirectional propagation of surface plasmon polaritons. Moreover, we study a hybrid configuration of Voigt–Faraday for surface plasmon polartions in two interfaces of the waveguide. We find that this structure possesses unique futures. It shows surface plasmon polariton modes with unidirectional propagation above the bulk plasmon frequency. Furthermore, we find a surface plasmon polariton band which admits the Voigt and Faraday features simultaneously. Also, we show that the waveguide thickness and the chemical potential of the Weyl semimetals can be used as a fine-tuning parameters in these structures. Our findings may be employed in optical devices which exploit the unidirectional surface plasmon propagation features.

Self-focusing and self-bending of surface plasmons in longitudinally modulated metasurfaces

We propose an effective method to manipulate the flow of surface plasmons propagating in a longitudinally modulated metasurface formed by periodic silver ridge gratings. We show that band structures with elliptical, flat and hyperbolic dispersion profiles can be achieved in surface ridge grating metasurface with different ridge height and width, therefore, the propagation behaviors of the surface plasmons in the metasurface can be actively molded in longitudinally modulated ridge grating metasurface. Both self-focusing and self-bending propagation of surface plasmons are demonstrated. Such longitudinally modulated metasurfaces provide a good platform to steer or mold the flow of surface plasmons, which may have many potential applications in nanoplasmonics such as on-chip directional routing of surface plasmons, subwavelength beam steering and on-chip signal processing.