Surface Plasmon Polariton Beam Research Articles

Polarization-based dynamic manipulation of Bessel-like surface plasmon polaritons beam.

Taking advantage of the phase modulation of surface plasmon polaritons (SPPs) generated by subwavelength slits, we elaborately designed an axicon-shaped slit array to dynamically manipulate the profiles of Bessel-like SPPs beams by changing the polarization of the incident light. As the polarization direction of linearly polarized light changes from horizontal to vertical, the profile of SPPs beam will evolve from the zeroth-order Bessel function to the first-order Bessel function. For circularly polarized light, the geometry Pancharatnam-Berry (PB) phase is introduced and can lead to the transverse shift of the main lobe of SPPs beam. The evolution of the beam profile and the distance of the transverse shift are theoretically illustrated by analyzing the phase distribution of SPPs. The proposed versatile approach suggests charming applications including on-chip communications, polarization-controlled particle manipulation and the design of polarization-based dynamic SPPs devices.

Flat top surface plasmon polariton beams.

Surface plasmon polaritons (SPPs) have emerged as powerful tools for guiding and manipulating light below the diffraction limit. In this context, the availability of flat top SPP beams displaying a constant transversal profile can allow for uniform excitation and coupling scenarios, thus opening the door to developing novel applications that cannot be achieved using conventional Gaussian SPP beams. Here, we present a rigorous theoretical description of flat top SPP beams propagating along flat metal-dielectric interfaces. This is accomplished through the use of Hermite-Gaussian SPP modes that constitute a complete basis set for the solutions of Maxwell's equations for a metal-dielectric interface in the paraxial approximation. We provide a comprehensive analysis of the evolution of the transversal profiles of these beams as they propagate, which is complemented with the study of the width and kurtosis parameters. Our results serve to enlarge the capabilities of surface plasmon polaritons to control and manipulate light below the diffraction limit.

Diffraction properties of binary graphene sheet arrays**Project supported by the National Natural Science Foundation of China (Grant Nos. 51109215 and 11604388) and the Natural Science Foundation of Hubei Province, China (Grant No. 2015CFC869).

We theoretically and numerically investigate the diffraction properties of surface plasmon polariton (SPP) in binary graphene sheet arrays. The single SPP band splits into two minibands by alternatively arranging the graphene waveguides with two different chemical potentials. Numerical simulations show that SPP beams in the array split into two different paths due to the different diffraction relation.

Surface plasmon polariton beams from an electrically excited plasmonic crystal.

Surface plasmon polariton (SPP) beams with an in-plane angular spread of 8° are produced by electrically exciting a 2D plasmonic crystal using a scanning tunneling microscope (STM). The plasmonic crystal consists of a gold nanoparticle (NP) array on a thin gold film on a glass substrate and it is the inelastic tunnel electrons (IET) from the STM that provide a localized and spectrally broadband SPP source. Surface waves on the gold film are shown to be essential for the coupling of the local, electrical excitation to the extended NP array, thus leading to the creation of SPP beams. A simple model of the scattering of SPPs by the array is used to explain the origin and direction of the generated SPP beams under certain conditions. In order to take into account the broadband spectrum of the source, calculations realized using finite-difference time-domain (FDTD) methods are obtained, showing that bandgaps for SPP propagation exist for certain wavelengths and indicating how changing the pitch of the NP array may enhance the SPP beaming effect.

Enhanced and unusual angle-dependent optical forces exerted on Mie particles by Airy surface plasmon wave

In this paper, using an angular spectrum method, we develop an analytical theory for Airy surface plasmon wave excited in a classical Kretschmann setup. It is found that the center of an Airy surface plasmon polariton (SPP) wave has a giant positive lateral shift, and the sidelobes move forward along the surface. The intensity of the Airy SPP wave is greatly enhanced, the corresponding optical forces can be enhanced by more than one order of magnitude. Importantly, we show that the sidelobes of the Airy SPP beam can lead to the splitting of optical force spectra with the variation of incident angle, which is accompanied by strong oscillations emerging at the optimal metal layer thickness. Moreover, the effects of multiple scatterings of the Airy SPP wave between the particle and the metal layer are also discussed. The theoretical analysis could open up new perspectives for the applications of Airy beam in optical manipulation and surface-enhanced Raman scattering.

Surface plasmon polaritons mode conversion via a coupled plasmonic system

A coupled plasmonic system for effective mode conversion between single interface surface plasmon polaritons (SPP) in a metal-dielectric waveguide and gap SPP in a metal-dielectric-metal waveguide is proposed. With the modal analysis, it is shown that the interference of the two plasmonic modes in a metal-dielectric–metal-dielectric coupled structure plays the key role in the mode conversion. With typical parameters, the conversion efficiency is as high as 61% (equivalent to 87% of the output total energy flow) at 1μm wavelength, and 1 dB bandwidth is as broad as 300 nm. The proposed structure can be used to implement an SPP mode convertor, router and beam splitter, which enables the interconnection between two important waveguides in plasmonics. The method presented here is fully-analytical, and is tested against fully-vectorial numerical results.

A simple method for generating unidirectional surface plasmon polariton beams with arbitrary profiles.

The efficient steering of surface plasmon polariton (SPP) fields is a vital issue in various plasmonic applications, such as plasmonic circuitry. We present a straightforward and efficient method for generating unidirectionally propagating SPP beams with arbitrary amplitude and phase profiles by manipulating Δ-shaped nanoantennas. As an example, a second-order Hermite-Gauss SPP beam is generated with this method. The near-field distribution of the generated SPP beam is experimentally characterized to validate the effectiveness of the method.

Efficient excitation and control of arbitrary surface plasmon polariton beams using one-dimensional metallic gratings

We derive a general approach for efficient excitation and control of arbitrarily shaped surface plasmon polariton waves, using one-dimensional metallic gratings by tailoring the exciting beam. We present a complete and rigorous theory that relates the electromagnetic fields of the free-space beam impinging on the grating to the resulting surface plasmon. We deduce an optimal grating and efficient polarization schemes which facilitate the excitation of surface plasmons propagating at angles up to almost 90 deg with respect to the grating’s axis with negligible polarization losses. The theoretical predictions are verified both numerically and experimentally by exciting two-dimensional surface plasmons through one-dimensional metallic gratings. We show that this method can be readily applied for generating arbitrary plasmonic wave fronts, and the general design formalism is given. Finally, we show how the developed design rules can be applied for the excitation of optically controlled surface plasmon hot spots.

Plasmon-negative refraction at the heterointerface of graphene sheet arrays.

We demonstrate negative refraction of surface plasmon polaritons (SPPs) at the heterointerface of two monolayer graphene sheet arrays (MGSAs) with different periods. The refraction angle is specifically related to the period ratio of the two MGSAs. By varying the incident Bloch momentum, the SPPs might be refracted in the direction normal to the heterointerface. Moreover, both positive and negative refraction could appear simultaneously. Because of the linear diffraction relation, the incident and refracted SPP beams experience diffraction-free propagation. The heterostructures composed of the MGSAs may find great applications in deep-subwavelength spatial light modulators, optical splitters, and switches.

Plasmonic routing in aperiodic graphene sheet arrays.

We discover deep-subwavelength, low-loss, and diffraction-free surface plasmon polariton (SPP) beam routing effects in aperiodic graphene sheet arrays (a-GSAs). The a-GSAs are constructed by varying either the interlayer spaces between graphene or the individual graphene chemical potentials. The SPP beams can be accelerated or decelerated in the a-GSAs, resulting in beam routing in different paths. The wave fronts of the beams are always parallel to the propagation direction, allowing the generation of transverse radiation pressure. All of these behaviors of SPPs are demonstrated by fully vectorial simulation and Hamilton optics analysis.

Cascaded second-order surface plasmon solitons due to intrinsic metal nonlinearity

We theoretically show the existence of cascaded second-order surface plasmon solitons propagating at the interface between a metal and a linear dielectric. Non-local multipole nonlinearities originating from the free conduction electron plasma of the metal lead to strong interaction between co-propagating surface plasmon polariton beams at the fundamental and second-harmonic frequencies. Finite element numerical modelling for an effective two-dimensional medium explicitly demonstrates soliton formation, confirming the theoretical results. The non-diffractive regime of propagation has been demonstrated at a silica/silver interface for 5λ-wide surface plasmon polariton beams with the loss-limited propagation distance of the order of 100 μm for the 750/1550 nm wavelength pair. Plasmon–soliton formation in phase-matched conditions has been shown to be beneficial for non-diffractive surface plasmon polariton propagation.

Two-dimensional Bessel-like surface plasmon-polariton beams

We present experimental evidence of non-diffracting two-dimensional Bessel-like surface plasmon-polariton (SPP) beams using the simultaneous excitation of two grating couplers forming an angle. The Bessel-like SPP beam properties were verified experimentally using the plasmon-coupled leakage radiation microscopy technique. The good agreement between simulations and measured intensity beam profiles confirms the effectiveness of the grating-coupler approach. Our results revealed that the spreading and propagation length characteristics of the Bessel-like SPP beams are primarily influenced by the angle between the grating couplers.

Electrically generated unidirectional surface plasmon source

We experimentally demonstrated an electrically excited surface plasmon source, which was fulfilled in a silver coated light emitting diode (LED) with well-designed gratings. With a DC current supply, surface plasmon polariton (SPP) waves were generated directly from the illuminations of the LED via the grating coupler. By adjusting the grating to a tilted one, a unidirectional SPP beam was successfully attained with a high extinction ratio (ER~10) and an improved launching efficiency. Detailed analyses show that this electrically generated unidirectional SPP has a considerable long propagation distance (~14 μm), allowing for further manipulations in plasmonic integrations and sensors.

Broad Band Focusing and Demultiplexing of In-Plane Propagating Surface Plasmons

On the basis of a novel phase modulation method by in-plane diffraction processes, a well-designed nanoarray on metal surface is proposed to realize a broad band focusing (bandwidth ∼100 nm) and a demultiplexing element (resolution ∼12 nm) of surface plasmon polariton (SPP) waves. Moreover, sublattice arrays are developed to achieve an improved demultiplexer and confocal SPP beams. The proposed scheme with implemented functionalities is designed totally in planar dimension, which is free of the SPP coupling process and indicates more practical application in photonic integrations.

The Goos-Hänchen effect for surface plasmon polaritons

By means of an impedance boundary condition and numerical solution of integral equations for the scattering amplitudes to which its use gives rise, we study as a function of its angle of incidence the reflection of a surface plasmon polariton beam propagating on a metal surface whose dielectric function is ɛ1(ω) when it is incident on a planar interface with a coplanar metal surface whose dielectric function is ɛ2(ω). When the surface of incidence is optically more dense than the surface of scattering, i.e. when |ɛ2(ω)|≫|ɛ1(ω)|, the reflected beam undergoes a lateral displacement whose magnitude is several times the wavelength of the incident beam. This displacement is the surface plasmon polariton analogue of the Goos-Hänchen effect. Since this displacement is sensitive to the dielectric properties of the surface, this effect can be exploited to sense modifications of the dielectric environment of a metal surface, e.g. due to adsorption of atomic or molecular layers on it.

Study of interference in a double-slit without walls by plasmon tomography techniques

We investigate the free propagation of two parallel surface plasmon polaritons (SPP) beams using plasmon tomography. In the Fourier-plane images, we observed interference features that are not in correspondence with the images of SPPs on the sample's surface. We clearly observed that the interference maxima and minima are distributed over an arc of a circle. We explain the characteristics of the observed interference patterns assuming that each SPP beam can be considered as a “slit without walls”. We discuss important implications of this work for SPP tomography and interferometric plasmonic sensors.

Some consequences of experiments with a plasmonic quantum eraser for plasmon tomography

We discuss two important consequences of recent experiments using surface plasmon polariton (SPP) tomography in a quantum eraser arrangement. In these experiments surface-emission images were modified by manipulating the polarization state of the leakage radiation. We show that SPP tomography does have the potential to produce images that mirror with high fidelity the propagation and interference of SPP beams at the metal–air interface of a sample. We reveal the physical mechanism behind this capability of SPP tomography. In addition, we show how SPP tomography can be used to detect photons passing through the dark fringes of an interference pattern and why photons propagate in such a way that looks like a photon can propagate across a region where it is never observed.

Asymmetric and symmetric local surface-plasmon-polariton excitation on chains of nanoparticles

We theoretically study the features of the surface-plasmon-polariton (SPP) excitation on single or chains of spherical metal nanoparticles located near a metal surface with an inclined incident light beam. It is found that by tuning the incident angle of an external light beam and the parameters of the surface nanoparticle structures one could obtain symmetric or asymmetric excitation of SPP beams propagating into certain directions. The reasons and conditions for this behavior and the efficiency of SPP excitation as a function of the incident angle are discussed.

Scattering of a surface plasmon polariton beam by chains of dipole nanoparticles

Scattering and splitting of surface plasmon polaritons (SPPs) by a chain of strongly interacting nanoparticles located near a metal surface are numerically studied. The applied numerical model is based on the Green’s function formalism and point–dipole approximation for scattering by nanoparticles. Dependencies of the splitting efficiency on the inter-particle distance in the chain and on the angle of incidence of the SPP Gaussian beam are considered. It is found that the splitting efficiency depends on the inter-particle distances especially when the angle between the SPP beam and the chain is relatively small. The role of multiple scattering in the SPP splitting by the chains of nanoparticles is also discussed.

Surface plasmon polariton beam focusing with parabolic nanoparticle chains

We report on the focusing of surface plasmon polariton (SPP) beams with parabolic chains of gold nanoparticles fabricated on thin gold films. SPP focusing with different parabolic chains is investigated in the wavelength range of 700-860 nm, both experimentally and theoretically. Mapping of SPP fields is accomplished by making use of leakage radiation microscopy, demonstrating robust and efficient SPP focusing into submicron spots. Numerical simulations based on the Green's tensor formalism show very good agreement with the experimental results, suggesting the usage of elliptical corrections for parabolic structures to improve their focusing of slightly divergent SPP beams.