Launching Of Surface Plasmon Polaritons Research Articles

Active control of surface plasmon polaritons with phase change materials

Active control of surface plasmon polaritons (SPPs) is highly desired for nanophotonics. Here we employ a phase change material Ge2Sb2Te5 (GST) to actively manipulate the propagating direction of SPPs at the telecom wavelength. By utilizing the phase transition-induced refractive index change of GST, coupled with interference effects, a nanoantenna pair containing GST is designed to realize switchable one-way launching of SPPs. Devices based on the nanoantenna pairs are proposed to manipulate SPPs, including the direction tuning of SPP beams, switchable SPP focusing, and switchable cosine–Gauss SPP beam generating. Our design can be employed in compact optical circuits and photonics integration.

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.

Efficient unidirectional SPP launcher: coupling the SPP to a smooth surface for propagation.

We propose a unidirectional surface plasmon polariton (SPP) launcher with high coupling efficiency and long propagation length. The structure consists of a metallic slanted grating and a metal film that are separated by a SiO2 layer on a quartz substrate. By inserting the SiO2 layer, strong interaction between the metal grating and metal film can significantly increase the conversion of incident light into SPPs. Meanwhile, due to the characteristics of the smooth interface between metal film and substrate, the dissipation of SPP originating from the propagation process will be remarkably reduced. Numerical simulations show that this structure with 11 grating periods will obtain high contrast and efficiency of launching. Compared with the rough metal-quartz interface, the smooth one can improve the efficiency of light conversion to SPPs by more than 22.6% and extend the propagating distance by approximately 158%.

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.

Directional launching of surface plasmon polaritons by electrically driven aperiodic groove array reflectors

Abstract Access to surface plasmon polaritons (SPPs) with directional control excited by electrical means is important for applications in (on-chip) nano-optoelectronic devices and to circumvent limitations inherent to approaches where SPPs are excited by optical means (e.g., diffraction limit). This paper describes directional excitation of surface plasmon polaritons propagating along a plasmonic strip waveguide integrated with an aperiodic groove array electrically driven by an Al–Al2O3–Au tunnel junction. The aperiodic groove array consists of six grooves and is optimized to specifically reflect the SPPs by 180° in the desired direction (+x or −x) along the plasmonic strip waveguide. We used constrained nonlinear optimization of the groove array based on the sequential quadratic programming algorithms coupled with finite-difference time-domain (FDTD) simulations to achieve the optimal structures. Leakage radiation microscopy (Fourier and real plane imaging) shows that the propagation direction of selectively only one SPP mode (propagating along the metal–substrate interface) is controlled. In our experiments, we achieved a directionality (i.e., +x/−x ratio) of close to 8, and all of our experimental findings are supported by detailed theoretical simulations.

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’.

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.

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.

Surface‐Plasmon‐Polariton Diode by Asymmetric Plano‐Concave Nanocavities

AbstractHigh‐efficiency and direction‐controllable launching of surface plasmon polaritons (SPPs) is highly desired in plasmonic devices and photonic circuits. Here, a heteroplasmonic device is reported consisting of an array of asymmetric nanocavities with each nanocavity constructed by an asymmetric Ag nanoshell (AgNS) and a planar gold film (AuFilm). A photoresist grating is used both as a template for fabricating the AgNS array and as the dielectric filling medium of the nanocavities. Interaction between localized surface plasmon resonance in the asymmetric AgNSs and SPPs in the AuFilm has enhanced the propagating surface modes along the gold film. A phase modulation induced by the asymmetry and interference between the SPP modes have been the main physics supporting the directional propagation of the surface waves or the in‐plane optical diode effect at normal incidence. The SPP diode works in different directions at different wavelengths, which can be controlled by adjusting the asymmetry of the AgNSs.

Spin‐Controlled Integrated Near‐ and Far‐Field Optical Launcher

AbstractWith the evergrowing demand for miniaturization of photonic devices, the integration of different functionalities in a single module is highly desired for the next generation of ultracompact photonic devices. Optical modules based on the near field and scattered far field are both key elements in the construction of nanophotonic devices. However, integrating the near‐ and far‐field functionalities into a single module is a great challenge, which hinders the integration and minimization of optical devices. Here, a bifunctional integrated near‐ and far‐field optical launcher with a single metasurface structure is theoretically proposed and experimentally demonstrated, where the unidirectional launching of surface plasmon polaritons (SPPs) and the focusing of scattered far fields can be simultaneously achieved. Moreover, the SPP propagating direction and the real/virtual focus of the far‐field scattering can be actively controlled by the spin state of the incident light. With the additional degree of freedom provided by the positions of the metasurface elements, the optical performances of this bifunctional structure can be compared to the one with single functionality. The work provides a new platform for the integration and control of different optical components at subwavelength scale, and opens a way to design multifunctional optical devices for the future.

Chirality Driven by Magnetic Dipole Response for Demultiplexing of Surface Waves

AbstractSurface electromagnetic waves are characterized by the intrinsic spin‐orbit interaction which results in the fascinating spin‐momentum locking. Therefore, directional coupling of light to surface waves can be achieved through chiral nanoantennas. Here, we show that dielectric nanoantenna provides chiral response with strong spectral dependence due to the interference of electric and magnetic dipole momenta when placed in the vicinity of the metal‐air interface. Remarkably, chiral behaviour in the proposed scheme does not require elliptical polarization of the pump beam or the geometric chirality of the nanoantenna. We show that the proposed ultracompact and simple dielectric nanoantenna allows for both directional launching of surface plasmon polaritons on a thin gold film and their demultiplexing with a high spectral resolution.

Launching of surface plasmon polaritons with tunable directions and intensity ratios by phase control of dual fundamental Gaussian beams.

This study proposes a method to achieve excitation of surface plasmon polaritons (SPPs) with tunable directions and intensity ratios on a designed two thin slit structure by the phase control of dual fundamental Gaussian beams. Simply modulating the phase difference between two incident fundamental Gaussian beams (i.e. TEM0,0 mode laser beam) controls the propagating direction of the resulting SPP wave between two opposite linear directions and also the value of the intensity ratio between propagating SPP waves in two opposite directions. The proposed method achieves a wide dynamic SPP intensity ratio adjusting range from -20 dB to 20 dB. This easy method of changing the direction of SPPs makes the dynamic control of the direction of SPP waves practicable, which shows great potential in plasmonic applications.

Tunable unidirectional surface plasmon polariton launcher utilizing a graphene-based single asymmetric nanoantenna

We design and numerically investigate a graphene-based asymmetric nanoantenna microstructure that can be used to realize electrically controllable, unidirectionally propagating broadband surface plasmon polaritons. The device geometry facilitates the simultaneous excitation of two localized surface plasmons resonances in the whole structure, and consequently, the asymmetric nanoantenna can be considered as being composed of two oscillating magnetic dipoles, wherein the interference of the radiated electromagnetic waves leads to a unidirectional propagation effect. Our results indicate that our proposed active device is promising for realizing compactable, tunable, terahertz plasmonic light sources.

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.

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.

Plasmonic Directional Beam Switching With Tilted Nanoslit Array Surrounded By Gratings

We propose a scheme for directional beam switching from line-envelope tilted nanoslit array surrounded by gratings. The line-envelope nanoslit array composed of double arrays of tilted nanoslits can be utilized for directional launching of surface plasmon polaritons (SPPs). On the left and right side of the nanoslit array, metallic gratings are asymmetrically carved in order to design the diffracted lights from the gratings in different beaming angles according to the incident polarization states. Both numerical and experimental results verify that the beaming angles from the proposed structure follow the diffraction theory of surface gratings, and the extinction ratio between two beams can be switched by tuning of the incident polarization. We expect that the proposed beam switching mechanism can be applied to the basic platform of tunable SPP-based radiation or nanoantenna manufacturing.

Near-field observation of surface plasmon polaritons launched by V-shaped nanorods on a gold surface.

By scanning near-field optical microscopy, we study the propagation of surface waves created by V-shaped nanorods deposited on a gold thin film. The nanorods launch surface plasmon polaritons that interfere with the incident light, producing interference patterns. The angle of the V-shaped rods varies from 110° to 180° (straight rod). We observe that the near-field distribution strongly depends on the angle of the V. For angles close to straight rods, a hot spot is visible, whereas for a narrower angle, the surface plasmon waves are launched in specific directions. The experimental results are in good qualitative agreement with numerical simulations performed with a simple analytical model that considers the rods as a sum of isolated surface plasmon sources.

Active Control of Graphene-Based Unidirectional Surface Plasmon Launcher

The manipulation of surface plasmon polariton (SPP) propagation is a basic subject for the realization of novel optical devices. Although the unidirectional SPP launcher has been demonstrated, it has been difficult to realize active control suitable for device applications. Here, we propose a graphene device with electrically controllable propagation of SPPs. The source/drain electrode of this device is made of a reflective antenna pair structure, with its optical resonance controlled by the corresponding applied voltage. With appropriate source and drain bias, this device can serve as a good unidirectional SPP launcher, with a high extinction ratio over 2000 and generation efficiency of 0.38. The electrical control allows us to tune the propagation of SPPs with significant flexibility, which may be used for future novel plasmonic devices and photonic circuits.

Manipulating surface-plasmon-polariton launching with quasi-cylindrical waves

Launching the free-space light to the surface plasmon polaritons (SPPs) in a broad bandwidth is of importance for the future plasmonic circuits. Based on the interference of the pure SPP component, the bandwidths of the unidirectional SPP launching is difficult to be further broadened. By greatly manipulating the SPP intensities with the quasi-cylindrical waves (Quasi-CWs), an ultra-broadband unidirectional SPP launcher is experimentally realized in a submicron asymmetric slit. In the nano-groove of the asymmetric slit, the excited Quasi-CWs are not totally damped, and they can be scattered into the SPPs along the metal surface. This brings additional interference and thus greatly manipulates the SPP launching. Consequently, a broadband unidirectional SPP launcher is realized in the asymmetric slit. More importantly, it is found that this principle can be extended to the three-dimensional subwavelength plasmonic waveguide, in which the excited Quasi-CWs in the aperture could be effectively converted to the tightly guided SPP mode along the subwavelength plasmonic waveguide. In the large wavelength range from about 600 nm to 1300 nm, the SPP mode mainly propagates to one direction along the plasmonic waveguide, revealing an ultra-broad (about 700 nm) operation bandwidth of the unidirectional SPP launching.

Surface Plasmon Polariton Unidirectional Nano-Launcher Based on the Strong Coupling Effects in an Asymmetric Optical Slot Nanoantenna Pair

We investigate the strong coupling effects in asymmetric optical slot nanoantenna pairs (AOSNPs) consisting of two parallelly positioned optical slot nanoantennas with different lengths and propose a surface plasmon polariton (SPP) unidirectional nano-launcher based on the strongly coupled AOSNPs (SCAOSNPs). Two subradiant modes can be excited in the SCAOSNPs, which originate from the hybridizations of the dipolar plasmon resonant modes in the two slot antennas. The subradiant modes give rise to large phase difference and nearly equal near-field electric field amplitudes between the two slot antennas, which can be used for unidirectional SPP launching and enable an ultra-compact nano-launcher. An extinction ratio up to 246 is achieved in the calculation via an SCAOSNP with a footprint of 170 nm by 260 nm on the surface of the gold film. The ultra-small footprint and tunable working wavelength enable the SCAOSNP important applications in future optical circuits and on-chip plasmonic devices.