Focusing Of Surface Plasmon Polaritons Research Articles

Multiplexing near- and far-field functionalities with high-efficiency bi-channel metasurfaces

Propagating waves and surface waves are two distinct types of light-transporting modes, the free control of which are both highly desired in integration photonics. However, previously realized devices are bulky in sizes, inefficient, and/or can only achieve one type of light-manipulation functionality with a single device. Here, we propose a generic approach to design bi-channel meta-devices, constructed by carefully selected meta-atoms possessing reflection phases of both structural-resonance and geometric origins, which can exhibit two distinct light-manipulation functionalities in near-field (NF) and far-field (FF) channels, respectively. After characterizing the scattering properties of basic meta-atoms and briefly stating the theoretical strategy, we design/fabricate three different meta-devices and experimentally characterize their bi-channel wave-control functionalities in the telecom regime. Our experiments show that the first two devices can multiplex the generations of NF and FF optical vortices with different topological charges, while the third one exhibits anomalous surface plasmon polariton focusing in the NF and hologram formation in the FF simultaneously. Our results expand the wave-control functionalities of metasurfaces to all wave-transporting channels, which may inspire many exciting applications in integration optics.

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.

High‐Efficiency Metasurface‐Based Surface‐Plasmon Lenses

AbstractA device that can couple propagating light into surface plasmon polaritons (SPPs) focused into a small region is highly desired for on‐chip photonics applications (e.g., energy‐harvesting, sensing, etc.). However, current technologies suffer from large device footprint, low working efficiency, and insufficient light‐manipulation freedom. Here, a generic approach for designing plasmonic lenses to generate predesigned vector SPP vortices with high efficiencies is established. Constructed with a set of meta‐atoms exhibiting tailored reflection phases and polarization‐conversion capabilities, the devices can convert normally incident circularly polarized light into predesigned vector SPP vortices with high efficiencies, due to both phase and polarization matching. As the illustrations, this study experimentally demonstrates directional SPP conversion (coupling efficiency: 35%; utilization efficiency: 98%) and SPP focusing effect at the wavelength of 1064 nm, with two meta‐couplers in stripe and arc shapes, respectively. Finally, a ring‐shaped meta‐coupler is designed/fabricated, and the generation of a vector SPP vortex with significantly enhanced efficiency as compared to previous schemes is experimentally demonstrated. The results pave the way for realizing on‐chip plasmonic devices to efficiently utilize SPPs with minimal footprints.

Focusing of surface plasmon polaritons propagating at the SiO2/Ag interface with 2-level and 4-level Fresnel phase zone pad structures.

We propose and demonstrate dielectric Fresnel phase zone pad (FPZP) structures for focusing surface plasmon polaritons (SPPs) propagating at the SiO2/Ag interfaces. We exploited up-conversion fluorescence microscopy to characterize the SPP focusing. We first report on the SPP focusing with 2-level FPZP structures that introduced a π-phase shift in the SPP wavefront between adjacent zones. We optimized the SPP focusing by fine-tuning the longitudinal width of the FPZP structure. This led to the enhancement of the peak intensity of the SPP focal spot and the reduction of the focal spot size in both the longitudinal and transverse directions. Such focusing was also demonstrated with different focal lengths. To further improve the SPP focusing, we developed a 4-level FPZP structure, which introduced a π/2-phase shift in the SPP wavefront between adjacent zones. With the optimized 4-level FPZP structure, the SPP focal spot peak intensity is further improved, and the spot size is reduced. To assist the design of the FPZP structures, we carried out theoretical analysis and numerical calculations to determine the SPP wavelengths at various oxide/Ag interfaces. We also carried out finite difference time domain (FDTD) calculations to simulate the SPP focusing with the FPZP structures. The results of the FDTD simulation agree with the experimental results qualitatively.

Multifunctional Plasmonic Grating Based on the Phase Modulation of Excitation Light.

Multifunctional optical devices are desirable at all times due to their features of flexibility and high efficiency. Based on the principle that the phase of excitation light can be transferred to the generated surface plasmon polaritons (SPPs), a plasmonic grating with three functions is proposed and numerically demonstrated. The Cherenkov SPPs wake or nondiffracting SPPs Bessel beam or focusing SPPs field can be correspondingly excited for the excitation light, which is modulated by a linear gradient phase or a symmetrical phase or a spherical phase, respectively. Moreover, the features of these functions such as the propagation direction of SPPs wake, the size and direction of the SPPs Bessel beam, and the position of SPPs focus can be dynamically manipulated. In consideration of the fact that no extra fabrication is required to obtain the different SPPs fields, the proposed approach can effectively reduce the cost in practical applications.

Efficient plasmonic functional lens constructed via a nano-dichroic element

The plasmonic functional lens can realize an efficient and functional focusing of surface plasmon polaritons (SPPs), making it have great potential in applications including nano-electron point sources, smart pixels, and particle manipulation. Here, we report for the first time a novel plasmonic functional lens constructed via a nano-dichroic element. The results show that the wavelength-selective nano-focusing of SPPs in the plasmonic functional lens can be achieved. Different from the conventional plasmonic lens that equally splits SPP power to propagate inward and outward from the boundaries, the plasmonic functional lens directs all of the SPP power of the matched wavelength inward for the focusing, resulting in higher collecting efficiency and SPP focusing intensity. Furthermore, we have demonstrated that a much higher focal spot intensity can be obtained even under the condition of a smaller inner radius. These findings provide a new way for the design of plasmonic functional lenses and can be facilitated to develop high-efficiency miniaturized focusing devices.

Spatiotemporal manipulation on focusing and propagation of surface plasmon polariton pulses.

Surface plasmon polariton (SPP) provides an important platform for the design of various nanophotonic devices. However, it is still a big challenge to achieve spatiotemporal manipulation of SPP under both spatially nanoscale and temporally ultrafast conditions. Here, we propose a method of spatiotemporal manipulation of SPP pulse in a plasmonic focusing structure illuminated by a dispersed femtosecond light. Based on dispersion effect of SPP pulse, we achieve the functions of dynamically controlled wavefront rotation in SPP focusing and redirection in SPP propagation within femtosecond range. The influences of structural parameters on the spatiotemporal properties of SPP pulse are numerically studied, and an analytical model is built to explain the results. The spatiotemporal coupling of modulated SPP pulses to dielectric waveguides is also investigated, demonstrating an ultrafast turning of propagation direction. This work has great potential in applications such as on-chip ultrafast photonic information processing, ultrafast beam shaping and attosecond pulse generation.

Dynamically Modulating Plasmonic Field by Tuning the Spatial Frequency of Excitation Light.

Based on the Fourier transform (FT) of surface plasmon polaritons (SPPs), the relation between the displacement of the plasmonic field and the spatial frequency of the excitation light is theoretically established. The SPPs’ field shifts transversally or longitudinally when the spatial frequency components or are correspondingly changed. The SPPs’ focus and vortex field can be precisely located at the desired position by choosing the appropriate spatial frequency. Simulation results are in good agreement with the theoretical analyses. Dynamically tailoring the plasmonic field based on the spatial frequency modulation can find potential applications in microparticle manipulation and angular multiplexed SPP focusing and propagation.

Dynamical Manipulation of Surface Plasmon Polaritons

As the fundamental and promising branch of nanophotonics, surface plasmon polaritons (SPP) with the ability of manipulating the electromagnetic field on the subwavelength scale are of interest to a wide spectrum of scientists. Composed of metallic or dielectric structures whose shape and position are carefully engineered on the metal surface, traditional SPP devices are generally static and lack tunability. Dynamical manipulation of SPP is meaningful in both fundamental research and practical applications. In this article, the achievements in dynamical SPP excitation, SPP focusing, SPP vortex, and SPP nondiffracting beams are presented. The mechanisms of dynamical SPP devices are revealed and compared, and future perspectives are discussed.

Plasmonic Spin-Hall Effect in Surface Plasmon Polariton Focusing

Surface plasmon polaritons (SPPs) are fundamental collective charge density excitations at surfaces of solid-state plasmas. They can energize physical and chemical processes at the nanoscale, but control of the spatiotemporal evolution of their energy, momentum, and spin is necessary to effectively transfer their quantum properties to molecules and other nanoscopic objects. Thus, to design the coupling of SPPs into other modes of an electronic system, it is important to describe and control their nanofemto spatiotemporal distributions. We investigate the spatial SPP field and spin distributions when launched by illuminating a converging lens coupling structure in an Ag film with obliquely incident linearly or circularly polarized femtosecond light pulses. The propagation and focusing of SPPs are recorded as movies by scanning the pump–probe pulse delay and recording the evolving interference patterns between the SPP and optical fields by ultrafast interferometric time-resolved two-photon photoemission ele...

Spatiotemporal Analysis of an Efficient Fresnel Grating Coupler for Focusing Surface Plasmon Polaritons

Time-resolved photoemission microscopy is used to investigate the spatiotemporal properties of surface plasmon polaritons launched at a Fresnel-type grating coupler. By milling a multiline structure of segmented grooves with dimensions derived from Fresnel zones into a plasmonic material, efficient focusing of surface plasmon polaritons can be accomplished. We demonstrate the presence of pulse broadening at the focus associated with propagation delays in the device. Moreover, our experimental data implies an enhancement of the plasmonic field energy density at the focus in excess of a factor of 10, with a focal spot size at the Abbe limit for our lens.

Broadband Spin‐Driven Anomalous Surface Plasmon Polariton Steering via V‐Shaped Aperture Metasurfaces

AbstractNumerically and theoretically the realization of broadband spin‐driven anomalous surface plasmon polaritons (SPP) steering via a V‐shaped aperture metasurface is demonstrated. With various orientation angles, asymmetrical phase profiles can be achieved on the left and right sides of the V‐shaped aperture, which also exhibit spin sensitive character. Spin‐driven anomalous SPP steering such as SPP launching and focusing are numerically demonstrated by properly arranging the V‐shaped apertures, and enabling extra degrees of freedom in controlling the propagation of SPPs. It is worth noting that spin‐driven anomalous SPP launching can be achieved within a broad bandwidth (800–1100 nm), being more conducive to practical applications. Moreover, an improved theoretical model is established as a physical interpretation of anomalous SPP wavefront steering and the theoretical results agree well with numerical results. These results will largely enhance the flexibility and tunability to steer the wavefront of SPPs, giving rise to broad applications in plasmonic circuits, polarization sensitive beam splitting, and near‐field imaging.

Sub-one-third wavelength focusing of surface plasmon polaritons excited by linearly polarized light.

We report the generation of a subwavelength focal spot for surface plasmon polaritons (SPPs) by increasing the proportion of high-spatial-frequency components in the plasmonic focusing field. We have derived an analytical expression for the angular-dependent contribution of an arbitrary-shaped SPP line source to the focal field and have found that the proportion for high-spatial-frequency components can be significantly increased by launching SPPs from a horizontal line source. Accordingly, we propose a rectangular-groove plasmonic lens (PL) consisting of horizontally-arrayed central grooves and slantingly-arrayed flanking grooves on gold film. We demonstrate both numerically and experimentally that, under linearly polarized illumination, such a PL generates a focal spot of full width half maximum 274 nm at an operating wavelength of 830 nm. The method we describe provides guidance to the further structure design and optimization for plasmonic focusing devices.

Creation of multiple on-axis foci and ultra-long focal depth for SPPs.

We present the design of a plasmonic lens (PL) which is composed of pixelated nano-grooves on a gold film for the coupling and focusing of surface plasmon polaritons (SPPs) into multiple focal spots on the optical axis. The pixelated grooves are arranged along the y-axis and the x-position of each groove is optimized by the simulated annealing algorithm. PLs that implement two and three on-axis foci are presented and the designed structures have been validated with FDTD simulations. We also successfully constructed a long-focal-depth PL with a longitudinal FWHM of the focus that reached 25 plasmonic wavelengths, while its transverse field profile is maintained over 15 µm distance. The presented design method constitutes a new basis for plasmonic beam engineering, and the proposed particular SPP focal fields have potential applications in multiple imaging, particle manipulating, and plasmonic on-chip signal transmission.

Metasurface Lens for both Surface Plasmon Polaritons and Transmitted Wave

Metasurface lenses which could simultaneously focus both surface plasmon polaritons (SPPs) and transmitted wave are designed. This kind of device is composed of slit antennas and is optimized with the simulated annealing algorithm to realize a single-focus or double-focus lens. Interestingly, the focusing of SPPs is polarization dependent while the focusing of the transmitted wave is immune from the polarization of incident light. The proposed methodology may inspire more designs of device steering both surface wave and transmitted wave.

Centrally Symmetric Focusing of Surface Plasmon Polaritons with a Rectangular Holes Arrayed Plasmonic Lens

Nano rectangular holes etched in a gold thin film are optimized and arrayed to form a plasmonic lens. We demonstrate both theoretically and numerically that the proposed plasmonic lens can focus surface plasmon polaritons to a centrally symmetric field with spatially homogeneous arbitrary polarization incidence, including linear, elliptical, and circular polarization incidence. Centrally symmetric field distributions with highly confined spots whose size beyond the diffraction limit are achieved. Particularly, the intensity distribution of the focused field is insensitive to the polarization direction of linearly polarized incident light and the inclination angle of elliptically polarized incident light. Moreover, the intensity of the focal spot can be modulated by the ellipticity angle of elliptically polarized incident light. The intensity is maximized for right-handed circular polarization incidence, and is twice that for linear polarization incidence, while that is null for left-handed circular polarization incidence.

Circular polarization analyzer with polarization tunable focusing of surface plasmon polaritons

A practical circular polarization analyzer (CPA) that can selectively focus surface plasmon polaritons (SPPs) at two separate locations, according to the helicity of the circularly polarized light, is designed and experimentally verified in the terahertz frequency range. The CPA consists of fishbone-slit units and is designed using the simulated annealing algorithm. By differentially detecting the intensities of the two SPPs focuses, the helicity of the incident circularly polarized light can be obtained and the CPA is less vulnerable to the noise of incident light. The proposed device may also have wide potential applications in chiral SPPs photonics and the analysis of chiral molecules in biology.

Tunable Multi-Port Surface Plasmon Polariton Excitation with Nanostructures

Surface plasmon polaritons (SPPs) have appealing features such as tighter spatial confinement and higher local field intensity. Manipulation of surface plasmon polaritons on metal/dielectric interface is an important aspect in the achievement of integrated plasmonic circuit beyond the diffraction limit. Here, we introduce a design of pin cushion structure and a holographic groove pattern structure for tunable multi-port SPPs excitation and focusing. Free space light is coupled into SPPs through momentum matching conditions. Both nanostructures are capable of tunably controlling of SPPs depending on the incident polarizations, while the holographic method provides more flexibility of wavelength-dependent excitations. Furthermore, a quantitative method is applied to calculate the efficiencies of excitation for both nanostructures under different conditions, including radially polarized incident beams. These results can work as a guidance and be helpful to further choice of the suitable design strategies for variable plasmonic applications such as beam splitter, on-chip spectroscopy, and plasmonic detectors.

Real-time phase error compensation in phase sensitive scanning near-field optical microscopy.

Phase measurements are critical for investigations on the optical properties of surface plasmon polariton (SPP) nanostructures. In this paper, a real-time phase error compensation method based on a phase sensitive scanning near-field optical microscopy (SNOM) measurement system is proposed. The method adopts the common optical path configuration and CMR (common-mode rejection) principle. It can be seen that the phase error compensation is real-time and mainly relies on optical devices, therefore neither post processing nor previous knowledge of environmental effects is required. The causes of the phase drift errors are discussed. We demonstrate experimentally the effectiveness of this method by measuring a SPP focusing device. Regardless of the drift velocity, degree of linearity, or phase accuracy, the compensation method shows great improvement compared to the previous phase sensitive SNOMs. All the measured distributions are in good agreement with theoretical simulations obtained by the finite-different time-domain (FDTD) method.

Observation and explanation of polarization-controlled focusing of terahertz surface plasmon polaritons

Polarization-controlled focusing of surface plasmon polaritons (SPPs) is experimentally and theoretically investigated in the terahertz (THz) frequency range. The spiral phase distribution and the phase singularities, which, respectively, lead to the transverse shift of the focal spot for the circularly polarized THz radiation and the split of the focal spot for the linearly polarized THz radiation, are experimentally observed. Furthermore, an alternative theoretical model based on the Huygens-Fresnel principle and Fourier transformation of THz SPPs is proposed to analyze the transverse shift of the focal spot, which provides a simple way to understand the polarization-related SPP phenomena. This research is instructive for the design of polarization-based plasmonic devices.