Launching Of Surface Plasmons Research Articles

Efficient and Unidirectional Launching of Surface Plasmons from a Hyperbolic Meta‐Antenna

AbstractTunnel nanojunctions associated with inelastic electron tunneling have demonstrated crucial applications in on‐chip photonic and plasmonic circuitries due to their high photon modulation speed, large‐scale integration capability, and working‐wavelengths range tunability. However, because most electrons tunnel through a junction elastically, the external quantum efficiency of a nanojunction‐based plasmonic source tends to be around 10−4, severely limiting their applications to date. In this work, an integrated high‐efficiency unidirectional plasmonic source composed of an edge‐to‐edge thickness gradient hyperbolic meta‐antenna is proposed. By engineering the extra wavevector dimension, this study demonstrates a theoretical external quantum efficiency of up to 23% for this system. This is attributed to the large local density of optical states from hyperbolic dispersion and wavevector‐match conditions provided by the optical antennas. Furthermore, this study also demonstrates the tunability of this system across a range of wavelengths from 1300 to 1700 nm. The implementations of these metamaterial‐based tunneling structures enable fast and tunable on‐chip high‐efficiency sources for applications in high‐performance plasmonic circuitries.

Near-field launching and mapping unidirectional surface plasmon polaritons using an automated dual-tip scanning near-field optical microscope

The spatial distribution of electromagnetic fields emitted from the aperture tip of a scanning near-field optical microscope (SNOM), which is called the emission pattern, depends on the geometry of the apex and the material composition of the tip’s coating. In previous works, experimental measurements of the emission pattern from the aperture tip were performed mostly in the far field. Moreover, the corresponding theoretical models were also developed based on these far-field measurements. Here, we have used the automated dual-tip SNOM to systematically characterize the emission from the aperture tip in the near field. In this regard, we have considered three different pairs of excitation and detection tips with distinct geometries. The emission patterns of the excitation tips were mapped using detection tips. Unidirectional surface plasmon polaritons (SPPs) at the surface of a gold platelet were launched by an excitation tip and measured in the near field by the detection tip. The experimental results were numerically reproduced by means of the Bethe–Bouwkamp model. This work puts into evidence the applicability of the automated dual-tip SNOM as the only available characterization technique to measure the emission from aperture tips in the near field. The reported asymmetric SPP radiation patterns can find applications in photonic integrated circuits or in biological and chemical sensing.

Polarization-tunable compact plasmonic wavelength demultiplexers based on Fano nanoantennas

A compact plasmonic wavelength demultiplexer is an essential prerequisite for practical applications, including the next-generation on-chip devices, near-field optical trapping, and micromanipulation. However, alleviating the polarization dependence of the wavelength demultiplexers remains one of the most challenging issues in realizing such a plasmonic device. Here we propose a gold disk-slit-based compact Fano-type nanoantenna capable of launching surface plasmon polaritons (SPPs) directionally when irradiated under different wavelengths of light, realizing a wavelength demultiplexing function. More importantly, our simulation results show that the shortage of requirement of specific light polarization direction excitation in such wavelength demultiplexers can be circumvented. Furthermore, it is found that the output channel of the SPP can be switched in multiple directions under different polarization excitations. In addition, the results show that the applicable wave band of the wavelength demultiplexing device can be flexibly adjusted by changing the size of the nanoantenna. The technique of introducing the polarization-tunable function into the wavelength demultiplexers in the plasmonic interconnect application enhances the freedom of information transmission and offers a promising building block for future high-speed and high-bandwidth on-chip optical communication.

Diffraction of bessel beams on 2D amplitude gratings—a new branch in the talbot effect study

In this paper, an analytical theory for the diffraction of a Bessel beam of arbitrary order Jl(κr) on a 2D amplitude grating is presented. The diffraction pattern behind the grating turned out to be more complicated in comparison with the classical Talbot effect observed under illumination by a plane wave. In particular, the patterns in the main and fractional Talbot planes under certain conditions that are found in the article, are lattices of ring microbeams, the diameters of which depend on the period of the grating, the diameter of the illuminating beam, the number of the Talbot plane, and the topological charge l. For the rings near the optical axis, the latter reproduces l of the illuminating beam. The diffraction patterns observed in experiments on the Novosibirsk free electron laser behind gratings with periods of 1 to 6 mm, illuminated by zero- to second-order Bessel beams at a radiation wavelength λ = 141 µm, are in good agreement with both theoretical predictions and numerical calculations. We emphasize that an analytical theory based on the scalar theory of diffraction perfectly describes resulting diffraction patterns with hole diameters down to 0.25 mm, which are less than 2λ, when, formally speaking, the scalar theory becomes incorrect. Since the Laguerre–Gaussian beams can be represented as a superposition of Bessel beams, results of this paper can be applied to analysis of the Talbot effect with the Laguerre–Gaussian beams. A regular lattice of ring-like vortex microbeams, which can be formed in the Talbot planes, can be applied, for example, to creation of a lattice of optical traps, transmission of orbital angular momentum to elements of micromechanical devices, and to launching surface plasmon polaritons on an array of metal wires by ‘the end-fire coupling technique’.

Polarization controlled couplings of surface plasmon with asymmetrical nanoslit pairs

Light-driven surface plasmons offer an opportunity to ultrafast information processing combining the compactness of electric circuits with the bandwidth of photonic networks. For practical applications, the efficient and controllable conversion from signal light to surface plasmons is essential. This leads to the recent developments in the polarization controlled couplings of surface plasmons. Currently, most works only tailor the orientation and arrangement of nanoslits to control the launching of surface plasmons. In this paper, we consider both the orientation and size of each slit in a one-dimensional array of nanoslit dimers. We first realize the unidirectional propagation of surface plasmons with designed wavefronts. Next, the unidirectional coupling and bi-directional coupling of surface plasmons are realized for a pair of orthogonal polarizations, respectively. This is quite different from the conventional opposite propagating surface plasmons excited by two orthogonal polarizations. The manipulation of both orientation and size of nanoslits allows additional freedom in the photon-plasmon conversions.

Surface Plasmon Launching by Polariton Superradiance

The condition of phase matching prohibits the transfer of excitation from free-space photons to surface plasmon polaritons (SPP). We propose and analyze a scheme that excites an ensemble of emitters in a collective state, which is phase matched with the SPP by the optical pulses used for its preparation. By a collective enhancement the ensemble, hence, emits an SPP in a well defined direction. We demonstrate the scheme by analyzing the launching of near-infrared graphene SPP. Our theory incorporates the dispersive and dissipative properties of the plasmon modes to evaluate the non-Markovian emission by the ensembles and will also be applicable for other types of surface polaritons.

Launching low-energy surface plasmons in purple gold (AuAl2)

We confirm that the unusual purple color of the intermetallic compound AuAl2 is of a plasmonic origin by launching surface plasmons (SPs) in thin AuAl2 films. We measure the SP dispersion relation and also use the films to measure the index of refraction of sucrose solutions using standard SP resonance sensing. We find that the SP energy in planar AuAl2 is approximately 2.1 eV, about 0.4 eV lower than in gold, and the material is highly resistant to oxidation. This is close to what is expected from previously reported measurements of the dielectric function of AuAl2. On this basis, we predict that AuAl2 nanoparticles will a have very strong, spectrally nearly uniform light absorbance about an order of magnitude greater than standard carbon black. Such particles may therefore find applications as obscurants or as an alternative to more complex light-absorbing gold structures in areas such as photothermal therapy or solar steam generation, or in plasmonic catalysis.

Propagating surface plasmons with an interference envelope and a vision for time crystals

The influence of the film thickness and the substrate's refractive index on the surface mode at the superstrate is an important study step that may help clearing some of the misunderstandings surrounding their propagation mechanism. A single sub-wavelength slit perforating a thin metallic film is among the simplest nanostructure capable of launching Surface Plasmon Polaritons on its surrounding surface when excited by an incident field. Here, the impact of the substrate and the film thickness on surface waves is investigated. When the thickness of the film is comparable to its skin depth, SPP waves from the substrate penetrate the film and emerge from the superstrate, creating a superposition of two SPP waves, that leads to a beat interference envelope with well-defined loci which are the function of both the drive frequency and the dielectric constant of the substrate/superstrate. As the film thickness is reduced to the SPP's penetration depth, surface waves from optically denser dielectric/metal interface would dominate, leading to volume plasmons that propagate inside the film at optical frequencies. Interference of periodic volume charge density with the incident field over the film creates charge bundles that are periodic in space and time.

All-optical control of surface plasmons by second-harmonic generation

Light with light control of surface plasmon polaritons is theoretically demonstrated. A barely simple and compact source of these waves consists in a finite number of slits (evenly spaced) perforating a metal film. The system scatters electromagnetic fields in one side of the metal film when it is illuminated from the opposite side by a polarized light source. High intensity light sources moreover efficiently generate light at second harmonic and higher frequencies in the metal led by optical nonlinearities. It is shown how the mixing of fields scattered by the slits from a weak beam at $\lambda$ wavelength, with the second harmonic fields generated by a high intensity $2 \lambda$ beam, creates a destructive interference of surface plasmons in one of the two possible directions of emission from the slits, while these are enhanced along the opposite direction. The unidirectional launching of surface plasmons is due to the different properties of symmetry at $\lambda$ whether they are linearly or nonlinearly generated. It is envisaged a nanodevice which might allow sending digital information codified in the surface plasmon field or be used to build ultra-narrow bandwidth surface plasmon frequency combs.

Launching phase-controlled surface plasmons on Babinet metasurfaces.

Surface plasmons are electromagnetic modes bounded at the metal-dielectric interfaces, which have numerous applications due to the strong field enhancement effect. Therefore, controlling surface plasmons is in high demand for modern photonics and plasmonics. In this Letter, we propose a kind of metasurface for launching surface plasmons with a controlled phase based on the Babinet principle. The phase control is achieved by arranging C-shaped apertures on a metal surface that exhibits highly efficient electric dipole resonance. By properly designing the spatial separation and orientation of the aperture resonators, an arbitrary phase profile is achieved that allows anomalous (directional and focusing/diverging) surface plasmon launching and manipulation of the wavefront.

Unidirectional excitation of plasmonic waves via a multilayered metal-dielectric-metal Huygens' nanoantenna.

Huygens' nanoantennas maintain orthogonal electric and magnetic dipole resonances satisfying the Kerker condition and can generate directional radiation in both the near-field and far-field regimes. Here we study a multilayered metal-dielectric-metal (MDM) Huygens' type nanoantenna which is capable of launching surface plasmon polaritons (SPPs) unidirectionally when excited by a dipole source. We show that the radiative decay rates of the dipole source are strongly enhanced by the antenna, and the generated SPP waves propagate in opposite directions at two different wavelengths. The directionality of the excited SPPs can be switched by changing the geometry and the material composition. We further demonstrated that the beam width of the SPP waves can be narrowed by arranging the MDM antennas in a chain.

Tunable plasmonic nanocavity with Ge2Sb2Te5 film for directional launching of surface plasmons

A tunable plasmonic nanocavity which consists of a metallic groove with submerged ultra-thin Ge2Sb2Te5 film is proposed for controlling the on/off characteristics of directional surface plasmon polaritions (SPPs) launching. Different mechanisms of launching SPPs using two orthogonal incident polarizations are investigated to reveal the SPP generation characteristics from the proposed nanocavity. By choosing the appropriate position of Ge2Sb2Te5 film, we report that the directional launching characteristics of SPPs can be controlled by changing the phase state of extremely small volume of Ge2Sb2Te5 film, which shows up to 37 dB of extinction ratio changing characteristics.

Multiple-band directional excitation of surface plasmons based on electromagnetically induced transparency.

We propose a structure of two metallic slits, one of which is accompanied by two non-equal length cavities. Our simulation results, conducted by finite difference time-domain method, show that there are four bands that can achieve launching surface plasmons (SPs) unidirectionally in the communication region, and each band is very narrow (between 10nm and 30nm). Our design method is based on SPs interference, and the phase-shift variation of SPs in the electromagnetically induced transparency wavelength region. Our design method provides a new way to manipulate SPs and control SPs' propagating direction in photonic circuits.

Leaky Modes in Ag Nanowire over Substrate Configuration**Supported by the Ministry of Science and Technology of China under Grant No 2016YFA0301300, the National Natural Science Foundation of China under Grant Nos 11374041, 11574035 and 11404030, and the State Key Laboratory of Information Photonics and Optical Communications.

By launching surface plasmons propagating along the Ag nanowire deposited on a substrate, we clearly observe three leaky modes using the Fourier imaging method. The effective refractive indexes, propagation lengths and electric field distributions of the modes are investigated, which indicate that the energy of the mode with a lowest effective refractive index is mainly distributed in the air, while for the other two modes, it is mainly distributed in the substrate and in the gap between the Ag nanowire and the substrate. These modes enable such a configuration to be used as a multichannel waveguide or highly directional optical antenna, which is of fundamental importance for optical device miniaturizations and photonic circuit integrations.

Efficient Unidirectional Launching of Surface Plasmons by Multi-Groove Structures

Efficiency is an important criterion in developing a practical surface-plasmon-polariton (SPP) unidirectional launcher. In this paper, we show that multi-groove structures can efficiently launch SPPs by numerically optimizing structural parameters and normal incident light. Experimentally, a high efficiency of 58.4 % is demonstrated in a six-groove structure with a lateral dimension of 3.9 μm. For a three-groove structure with even smaller lateral dimension of 1.35 μm, the efficiency presents a broadband response, which remains higher than 42 % from 720 to 860 nm. The proposed multi-groove structures with high SPP launching efficiency and small size exhibit potential in highly integrated plasmonic circuits.

Spin-Controlled Directional Launching of Surface Plasmons Under Oblique Illumination

We proposed a scheme to tune the propagation direction of surface plasmon polaritons (SPPs) by external control of the polarization and/or the inclination angle of the incident light. The phase of the SPPs generated by the laser beam through slits can be divided into two parts: the position-related phase which is affected by the position of slits and spin-related phase which is affected by the orientation of slits and spin of photons. Using theoretical analysis and numerical simulation, we studied the position-related phase and spin-related phase of the SPPs excited by an inclined and circularly polarized light through a column of slits and then designed symmetric V-type slit array. We found when the incident light is in the symmetry plane of symmetric V-type slit array, spin of the incident light would give directions of surface plasmon propagation and the inclination angle of the incident light would give the inclination angle of surface plasmon propagation. The result shows that we may tune the propagation of the SPPs with significant flexibility, by changing the polarization of the incident light and the inclination angle of the incident light. Our designed launcher, as a spin plasmonic device under control, is expected to be of interest for future applications in photonic integrated circuits.

Spin-controlled directional launching of surface plasmons at the subwavelength scale**Project supported by the National Natural Science Foundation of China (Grant Nos. 61176120, 61378059, 60977015, 61422501, and 11374023), the National Basic Research Program of China (Grant Nos. 2012CB933004 and 2015CB932403), and Beijing Natural Science Foundation (Grant No. L140007).

In this paper, we demonstrate a spin-controlled directional launching of surface plasmons at the subwavelength scale. Based on the principle of optical spin’s effect for the geometric phase of light, the nanostructures were designed. The inclination of the structures decides the spin-related geometric phase and their relative positions decide the distance-related phase. Hence, the propagation direction of the generated surface plasmon polaritons (SPPs) can be controlled by the spin of photons. Numerical simulations by the finite difference time domain (FDTD) method have verified our theoretical prediction. Our structure is fabricated on the Au film by using a focused ion beam etching technique. The total size of the surface plasmon polariton (SPP) launcher is 320 nm by 180 nm. The observation of the SPP launching by using scanning near-field optical microscopy is in agreement with our theory and simulations. This result may provide a new way of spin-controlled directional launching of SPP.

Launching surface plasmon waves via vanishingly small periodic gratings.

The scattering of electromagnetic waves by periodic layered media plays a crucial role in many applications in optics and photonics, in particular in nanoplasmonics for topics as diverse as extraordinary optical transmission, photonic crystals, metamaterials, and surface plasmon resonance biosensing. With these applications in mind, we focus on surface plasmon resonances excited in the context of insulator-metal structures with a periodic, corrugated interface. The object of this contribution is to study the geometric limits required to generate these fundamentally important phenomena. For this we use the robust, rapid, and highly accurate field expansions method to investigate these delicate phenomena and demonstrate how very small perturbations (e.g., a 5 nm deviation on a 530 nm period grating) can generate strong (in this instance 20%) plasmonic absorption, and vanishingly small perturbations (e.g., a 1 nm deviation on a 530 nm period grating) can generate nontrivial (in this instance 1%) plasmonic absorption.

Dielectric-Grating-Coupled Surface Plasmon Resonance From the Back Side of the Metal Film for Ultrasensitive Sensing

We propose a dielectric grating that can launch surface plasmon resonance (SPR) modes efficiently on the other side of flat metal films, which is similar to the conventional prism coupling mechanism. Importantly, this structure can excite SPR under the normal incident light, which is particularly suitable for the integration with optical fiber tips. By launching the SPR mode near the wavelength of 1.55 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\mu\text{m}$</tex-math></inline-formula> with a very narrow resonance line width (∼4 nm), this structure is promising for the development of high-performance portable, flexible, and real-time refractometric sensing applications.

Efficient unidirectional launching of surface plasmons by a cascade asymmetric-groove structure.

Increasing the unidirectional launching efficiency of surface plasmon polaritons (SPPs) is crucial in plasmonics. Here, we demonstrate that this efficiency may be improved by cascading subwavelength unidirectional SPP launching units. A unidirectional SPP launching efficiency of at least 46% and an extinction ratio of 40 are experimentally demonstrated using a cascade asymmetric-groove structure. Meanwhile, the device is ultracompact, and has a lateral dimension of only 1.1 μm. The proposed structure also presents a broadband response and is easy to fabricate. This high-performance wavelength-scale unidirectional SPP launcher represents an important development in practical SPP sources.