Hybrid Plasmonic Modes Research Articles

Hybrid plasmonic optical modulator based on multi-layer graphene

Hybrid plasmonic waveguide provided high confinement of the propagating light and it is compatible with silicon on insulator technology. Graphene with unique optical properties has been used in optical modulator to give a high extinction ratio, broadband operation, and small footprint. In this paper, we used graphene as an absorber layer in a hybrid plasmonic waveguide structure to get sub-wavelength field confinement and better interaction between the graphene and the propagating light to reach maximum absorption of the graphene. We studied the effects of a different number (2–6) of the graphene layers on the extinction ratio, bandwidth, and energy consumption. The modulation extinction ratio is as high as 36 dB/μm and it has a flat response in a wide range of wavelength. The power consumption is about 14 fJ/bit. This modulator has a good response in all communication bands.

Hybrid plasmonic graphene modulator with buried silicon waveguide

We propose a low-loss broadband modulator by combining plasmonic effect with graphene to realize highly efficient modulation on the SOI (Silicon-On-Insulator) platform. Due to a hybrid plasmonic waveguide including buried silicon waveguide and silver waveguides, the fundamental mode is mainly confined in two single-layer graphene sheets, and the electric field vector is parallel to the graphene sheets ensuring the high efficient light absorption of graphene. The specific hybrid structure makes it easy for graphene transfer. Also, non-coplanar structure of silicon waveguide and silver waveguides provides simplified fabrication process and fabrication tolerance. The simulation results demonstrated that modulation bandwidth of 346 GHz is obtained, and the 3 dB modulation length is only 9.493μm. In addition, competitively low propagation loss of 0.85 dB was realized of the hybrid modulator.

Hybridized plasmon modes in a system of metal thin film–nanodisk array

Controlling the hybridization is a very powerful tool to manipulate the modes in a single nanostructure. We investigate the hybridization between localized and propagating surface plasmons in a nanostructure system where a thin metal layer strongly interacts with a nanodisk array. Hybrid plasmon resonances are observed in the reflection spectra obtained from finite-difference time domain simulations and experimental measurements in the visible-near-infrared region. We demonstrate how the geometrical parameters of the nanostructure can be utilized to bring these plasmon modes in the strong coupling regime. The hybrid plasmon modes exhibit anticrossing with a Rabi splitting of ∼0.1eV, which is the signature of strong coupling. Near-field profiles of the hybrid modes exhibit a mixture of localized and propagating plasmon characteristics, with propagating modes excited on both sides of the metal film. Our design promises richer implementations in light manipulation towards novel photonic applications compared to the systems with thick metal films.

Tip-Enhanced Raman Excitation Spectroscopy (TERES): Direct Spectral Characterization of the Gap-Mode Plasmon.

The plasmonic properties of tip-substrate composite systems are of vital importance to near-field optical spectroscopy, in particular tip-enhanced Raman spectroscopy (TERS), which enables operando studies of nanoscale chemistry at a single molecule level. The nanocavities formed in the tip-substrate junction also offer a highly tunable platform for studying field-matter interactions at the nanoscale. While the coupled nanoparticle dimer model offers a correct qualitative description of gap-mode plasmon effects, it ignores the full spectrum of multipolar tip plasmon modes and their interaction with surface plasmon polariton (SPP) excitation in the substrate. Herein, we perform the first tip-enhanced Raman excitation spectroscopy (TERES) experiment and use the results, both in ambient and aqueous media, in combination with electrodynamics simulations, to explore the plasmonic response of a Au tip-Au substrate composite system. The gap-mode plasmon features a wide spectral window corresponding to a host of tip plasmon modes interacting with the plasmonic substrate. Simulations of the electric field confinement demonstrate that optimal spatial resolution is achieved when a hybrid plasmon mode that combines a multipolar tip plasmon and a substrate SPP is excited. Nevertheless, a wide spectral window over 1000 nm is available for exciting the tip plasmon with high spatial resolution, which enables the simultaneous resonant detection of different molecular species. This window is robust as a function of tip-substrate distance and tip radius of curvature, indicating that many choices of tips will work, but it is restricted to wavelengths longer than ∼600 nm for the Au tip-Au substrate combination. Other combinations, such as Ag tip-Ag substrate, can access wavelengths as low as 350 nm.

Sub-Femtojoule Hybrid Plasmonic Optical Modulator

A highly compact optical modulator is proposed and analyzed. The modulator is based on the hybrid plasmonic waveguide platform. The structure of the modulator is built from silicon, silver, and electro-optical polymer layers. Hybrid plasmonic modes of symmetric and asymmetric natures excited in the structure were investigated. The symmetric mode is characterized by its lower propagation loss, so it was selected for the excitation of the device. The modulator's operation mechanism is based on the racetrack resonator configuration. Full three-dimensional electromagnetic analysis was performed around the 1.55-μm telecommunication wavelength. Different parameters such as the power coupling to the bus waveguide, the bend loss, the propagation loss, and the Q-factor of the resonances were studied in order to optimize the racetrack resonator design. The modulator is characterized by its extremely low energy consumption of 0.5 fJ/bit, high modulation depth of 10.5 dB, while the insertion losses were limited to 0.5 dB.

Active Far-Field Control of the Thermal Near-Field via Plasmon Hybridization.

The ability to control and manipulate temperature at nanoscale dimensions has the potential to impact applications including heat-assisted magnetic recording, photothermal therapies, and temperature-driven reactivity. One challenge with controlling temperature at nanometer dimensions is the need to mitigate heat diffusion, such that the temperature only changes in well-defined nanoscopic regions of the sample. Here we demonstrate the ability to use far-field laser excitation to actively shape the thermal near-field in individual gold nanorod heterodimers by resonantly pumping either the in-phase or out-of-phase hybridized dipole plasmon modes. Using single-particle photothermal heterodyne imaging, we demonstrate localization bias in the photothermal intensity due to preferential heating of one of the nanorods within the pair. Theoretical modeling and numerical simulation make explicit how the resulting photothermal images encode wavelength-dependent temperature biases between each nanorod within a heterodimer, demonstrating the ability to actively manage the thermal near-field by simply tuning the color of incident light.

Nanophotonic on-chip hybrid plasmonic electro-optic modulator with phase change materials

A vertical hybrid plasmonic waveguide (HPWG) modulator incorporating Au nano-rings and operating in the near-infrared (NIR) band is proposed using phase change chalcogenide thin film (Ge2Sb2Te5, commonly known as GST). It is shown that with amorphous to crystalline phase transition of GST, significant modulation depths can be achieved in the optical C-band. Further, localized surface plasmon resonance (LSPR) excitations from the Au nano-ring embedded in the low-indexed slot region is used to enhance the overall performance of the reported GST based electro-optic modulator. In addition, significant wavelength shifts, and improved extinction ratio (ER) is obtained by varying Au nano-ring radius and other geometrical parameters. Our finite element method based 3D-COMSOL numerical simulations reveal that 12.24dB/μm (9.3dB/μm) extinction ratio per unit length can be achieved with (without) Au nano-rings considering r=80 nm; L=500 nm; and λ=1.56 um, respectively.

Excitation of Hybrid Plasmonic Waveguide Modes by Colloidal Quantum Dots

Quantum information technologies will greatly benefit from efficiently coupled quantum emitter–waveguide systems. Hybrid plasmonic waveguide modes are relatively low-loss confined modes, which prov...

Experimental demonstration of a graphene-based hybrid plasmonic modulator.

In this Letter, we report a graphene-based hybrid plasmonic modulator (GHPM) realized by employing the electro-absorption effect of graphene. The simulation results show that the modulation efficiency of GHPM, i.e., extinction ratio per length, can be as large as 0.417dB/μm, which is more than twice as much as that of recently presented graphene-on-silicon modulator. It was found that the improvement in modulation efficiency is mainly due to the enhancement of the overlap between graphene and the mode field in GHPM. A prototype of GHPM was fabricated. The measurement results showed that the GHPM can work in a broadband from 1530 to 1570nm and an improved modulation efficiency of 1.08dB (at 30μm). Finally, we have discussed the factors that influence the modulation efficiency. Our proof-of-concept design may promote the development of on-chip graphene-based plasmonic devices.

Guiding and controlling light at nanoscale in field effect transistor

Metal–oxide–semiconductor field-effect transistor (MOSFET) and tunnel field-effect transistor (TFET) are proposed to guide and control the light at nanoscale utilizing the hybridization of plasmonic and optical modes. The hybrid plasmonic (HP) mode is confined in the dielectric sandwiched between the metal gate and semiconductor channel, which results from the coupling of surface plasmonic polariton mode at the metal–dielectric interface with the optical mode in the dielectric. Conventional conductivity modulations in the channel through gate and drain–source voltages are utilized to control the guided light. A long propagation length of 74 μm and a very small mode area of $$\lambda ^2/96$$ are reported for field-effect transistor at an operating wavelength of 1550 nm which are useful to realize low loss and compact optoelectronic devices. The charge-carrier dynamics along with the plasma dispersion effect in the silicon channel, through voltages applied on the gate and source–drain, result in the optical phase modulation in MOSFET and TFET. Phase shift of $$\pi$$ radian at a length of 1.2 mm and 0.21 mm is obtained in MOSFET and TFET, respectively. The proposed concept has the potential to enable multifunctionality of the mature field effect transistors.

Enhanced plasmonic rectification of terahertz radiation in spatially periodic graphene structures towards the charge neutrality point

The rectification of terahertz radiation in a periodically almost-depleted graphene structures with dual grating gate is studied. Plasmonic electron-hole ratchet based rectified current grows with decreasing the Fermi energy of charge carriers in the gated parts of graphene. It is shown that the charge carriers accumulated at boundaries of depleted parts of graphene enhance the magnitude and inhomogeneity of oscillating electric field of the plasmon, which leads to significant growth in the rectification efficiency. Interaction of plasmon modes in different gated parts of graphene can increase the rectification efficiency due to excitation of the hybrid plasmon modes.

Tunable hybridization of graphene plasmons and dielectric modes for highly confined light transmit at terahertz wavelength.

We theoretically report a novel graphene-based hybrid plasmonic waveguide (GHPW) by integrating a GaAs micro-tube on a silica spacer that is supported by a graphene-coated substrate. In comprehensive numerical simulations on guiding properties of the GHPW, it was found that the size of hybrid plasmonic mode (TM) can be reduced significantly to ~10-4(λ2/4), in conjunction with long propagation distances up to tens of micrometers by tuning the the waveguide's key structure parameters and graphene's chemical potential. Moreover, crosstalk between two adjacent GHPWs that are placed on the same substrate has been analyzed and ultralow crosstalk can be realized. The proposed scheme potentially enables realization of the various high performance nanophotonic components-based subwavelength plasmonic waveguides in terahertz domain.

Electromagnetic analysis of the lasing thresholds of hybrid plasmon modes of a silver tube nanolaser with active core and active shell.

Results from the electromagnetic modeling of the threshold conditions of hybrid plasmon modes of a laser based on a silver nanotube with an active core and covered with an active shell are presented. We study the modes of such a nanolaser that have their emission wavelengths in the visible-light range. Our analysis uses the mathematically grounded approach called the lasing eigenvalue problem (LEP) for the set of the Maxwell equations and the boundary and radiation conditions. As we study the modes exactly at the threshold, there is no need to invoke nonlinear and quantum models of lasing. Instead, we consider a laser as an open plasmonic resonator equipped with an active region. This allows us to assume that at threshold the natural-mode frequency is real-valued, according to the situation where the losses, in the metal and for the radiation, are exactly balanced with the gain in the active region. Then the emission wavelength and the associated threshold gain can be viewed as parts of two-component eigenvalues, each corresponding to a certain mode. In the configuration considered, potentially there are three types of modes that can lase: the hybrid localized surface plasmon (HLSP) modes of the metal tube, the core modes, and the shell modes. The latter two types can be kept off the visible range in thin enough configurations. Keeping this in mind, we focus on the HLSP modes and study how their threshold gain values change with variations in the geometrical parameters of the nanotube, the core, and the shell. It is found that essentially a single-mode laser can be designed on the difference-type HLSP mode of the azimuth order m = 1, shining in the orange or red spectral region. Furthermore, the threshold values of gain for similar HLSP modes of order m = 2 and 3 can be several times lower, with emission in the violet or blue parts of the spectrum.

Plasmonic Optical and Chiroptical Response of Self-Assembled Au Nanorod Equilateral Trimers.

Assembling metamolecules from anisotropic, shape-engineered nanocrystals provides the opportunity to orchestrate distinct optical responses one nanocrystal at a time. The Au nanorod has long been a structural archetype in plasmonics, but nanorod assemblies have largely been limited to end-to-end or side-to-side arrangements, accessing only a subset of potential metamolecule structures. Here, we employ triangular templates to direct the assembly of Au nanorods along the edges of an equilateral triangle. Using spatially resolved, dark-field scattering spectroscopy in concert with numerical simulation of individual metamolecules, we map the evolution in surface plasmon resonances as we add one, two, and three nanorods to construct triangular nanorod assemblies. The assemblies exhibit rotation- and polarization-dependent hybridized plasmon modes, which are sensitive to variations in nanorod size, position, and orientation that lead to geometrical symmetry breaking. The triangular arrangement of nanorods supports magnetic plasmon modes where electric fields are directed along the perimeter of the triangle, and the magnetic field intensity within the triangle's open interior is enhanced. Circumferential displacements of the nanorods within the templates impart either a left- or right-handed sense of rotation to the structure, which generates a chiroptical response under unidirectional oblique illumination. Our results represent an important step in realizing and characterizing metamaterial assemblies with "open" structures utilizing anisotropic plasmonic building blocks, with implications for optical magnetic field enhancement and chiral plasmonics.

Nano-Architecture Driven Plasmonic Field Enhancement in 3D Graphene Structures

The limited spatial coverage of the plasmon enhanced near-field in 2D graphene ribbons presents a major hurdle in practical applications. In this study, diverse self-assembled 3D graphene architectures are explored that induce hybridized plasmon modes by simultaneous in-plane and out-of-plane coupling to overcome the limited coverage in 2D ribbons. While 2D graphene can only demonstrate in-plane, bidirectional coupling through the edges, 3D architectures benefit from fully symmetric 360° coupling at the apex of pyramidal graphene, orthogonal four-directional coupling in cubic graphene, and uniform cross-sectional radial coupling in tubular graphene. The 3D coupled vertices, edges, surfaces, and volume induce corresponding enhancement modes that are highly dependent on the shape and dimensions comprising the 3D geometries. The hybridized modes introduced through the 3D coupling amplify the limited plasmon response in 2D ribbons to deliver nondiffusion limited sensors, high efficiency fuel cells, and extreme propagation length optical interconnects.

Selective embedment of silver nanocrystals into spatially segregated domains in thin polymer films for controlled fabrication of functional nanocomposites.

Fabrication of polymer-nanoparticle nanocomposites typically relies on mixing nanoparticle and polymer solutions, which renders little control over nanoparticle incorporation, and homogeneity of the resulting composite material. This work focuses on the thermally induced embedment of monocrystalline silver nanocubes (AgNCs) into polymer surfaces. The AgNCs are initially deposited through a Langmuir approach onto films of immiscible blended polymer films, which allows fine control over nanoparticle density and aggregation state. This nanoparticle/polymer composite is then heated above the glass transition temperature (Tg) of a polymer, which initiates the irreversible embedding of the AgNCs. The immiscible ternary polymer films featured discrete domains (with different Tgs), which were altered by changing the amount of polystyrene, poly(2-vinylpyridine) and poly(methyl methacrylate) within the polymer solution. The Tg dependence of the embedding process allowed the selective embedment of AgNCs into discrete polymer domains. The process was monitored in real time by using spatially separated hybrid plasmon modes, through peak shifts observed in a UV-vis spectrum. Enhanced surface confinement was observed for certain tripolymer films when compared to polystyrene–AgNC nanocomposites, due to changes in the surface energy within the blend. This work brings interesting insight on nanoparticle-blended polymer interactions and provides a fairly universal approach for the fabrication of these polymer–metal nanoparticle nanocomposites, which is of particular interest in fields that require fine control over nanoparticle incorporation within segregated polymer domains.

Plasmonic particle-on-film nanocavities: a versatile platform for plasmon-enhanced spectroscopy and photochemistry

Abstract Metallic nanostructures with nanometer gaps support hybrid plasmonic modes with an extremely small mode volume and strong local field intensity, which constitutes an attractive plasmonic platform for exploring novel light-matter interaction phenomena at the nanoscale. Particularly, the plasmonic nanocavity formed by a metal nanoparticle closely separated from a thin metal film has received intensive attention in the nanophotonics community, largely attributed to its ease of fabrication, tunable optical properties over a wide spectral range, and the ultrastrong confinement of light at the small gap region scaled down to sub-nanometer. In this article, we review the recent exciting progress in exploring the plasmonic properties of such metal particle-on-film nanocavities (MPoFNs), as well as their fascinating applications in the area of plasmon-enhanced imaging and spectroscopies. We focus our discussion on the experimental fabrication and optical characterization of MPoFNs and the theoretical interpretation of their hybridized plasmon modes, with particular interest on the nanocavity-enhanced photoluminescence and Raman spectroscopies, as well as photocatalysis and molecular nanochemistry.

Low loss photonic nanocavity via dark magnetic dipole resonant mode near metal

The dielectric-semiconductor-dielectric-metal 4 layered structure is a well-established configuration to support TM hybrid plasmonic modes, which have demonstrated significant advantages over pure photonic modes in structures without metal to achieve low loss resonant cavities at sub-diffraction limited volumes. The photonic modes with TE characteristics supported by the same 4 layered structure, on the other hand, are less studied. Here we show that a low loss photonic mode with TE01 characteristics exists in the dielectric-semiconductor-dielectric-metal 4 layered structure if a truncated cylindrical disk is chosen as the semiconductor core. This mode exhibits the lowest cavity loss among all resonant modes, including both pure photonic and hybrid plasmonic modes, at cavity radius <150 nm and within the wavelength range 620 nm to 685 nm, with a footprint ~0.83 (λ/2neff)2, physical size ~0.47 (λ/2neff)3 and a mode volume down to 0.3 (λ/2neff)3. The low cavity loss of this TE01 mode is attributed to its substantially reduced radiation loss to the far field by the creation of image charges through the metal response. Because of the low mode penetration in the metal, this photonic mode show equally low cavity loss near industry relevant metals such as Cu. Our study demonstrates an alternative to hybrid plamonic modes and metallo-dielectric modes to achieve low loss cavities with extremely small footprints.

Surface enhanced perfect absorption in metamaterials with periodic dielectric nanostrips on silver film.

Integrated dielectric metamaterials with plasmonic structures can cause drastic optical resonances and strengthen the capacity of light absorption. Here, we describe the optical properties of silicon nanoarrays on a thin silver film for extreme light confinement at subwavelength nanoscales. We attain the nearly total absorption in silicon nanostrips, which support magnetic quadruple Mie-type resonances in the visible regions. The Mie resonant field of the dielectric nanostrip engages the screening response of the silver film, resulting in plasmon resonance configuration and thus achieving perfect light absorption in the dielectric nanostrip. Moreover, we can attain similar results in other nanostructures, such as silicon cylinder and rhombus column arrays. Because it can sustain hybridized plasmon modes and magnetic modes, the combined system will benefit the application of solar energy accumulation.

Optical Interaction Effects in Nanoplasmonic Antennas at Visible Wavelengths

In this article we study the effects of nanoelements’ optical interactions on the properties and response of the nanoplasmonic antennas. Here, we report on some simple, ultra directive, and compact optical antenna designs working based on a hybridized plasmon mode. By enhancing the electromagnetic (EM) coupling between the nanoparticles, we achieve new optimum designs. Using two, three, or four closed nanoelements, the nano-antenna shows high directionality and gives us the opportunity to have a compact, color-switched, and spectrally-tunable optical nanoantenna. Such design are experimentally realized by HCL – a bottom-up nanofabrication based on colloidal self-assembly. Finally, in order to design a highly directive nanoantenna we present a new highly directive and packed Yagi-Uda (P-YU) nano-antenna.