Cross conversion between surface plasmon polaritons and quasicylindrical waves

Quasicylindrical Waves for Ordered Nanostructuring

Laser-induced self-organization of periodic nanostructures on highly absorbing materials is widely understood to be due to interference between laser and surface plasmon polaritons (SPPs) that are excited by initial surface roughness. The structure order naturally emerges from the propagation phase of SPPs. Here, we reveal an unexplored mechanism that is predominantly induced by quasicylindrical waves (QCWs) with negligible contributions from SPPs. This mechanism features a new principle of order emergence in growth of periodic nanostructures through short-range electromagnetic interactions between QCWs and marginal nanofringes. In this scenario, the periodicity of nanostructures is not simply determined by the electromagnetic wavelength. With suppressed long-range interactions, the formation of nanostructures shows a domino-like growth process, thus significantly improving structure uniformity. An in situ microscopic observation is performed to characterize the temporal dynamics of structural growth and verify the new mechanism. Further, the QCWs are directly observed in experiments, which are theoretically supported by a scattering model.

Controlling surface-plasmon-polaritons launching with hot spot cylindrical waves in a metallic slit structure

Plasmonic nanostructures, which are used to generate surface plasmon polaritons (SPPs), always involve sharp corners where the charges can accumulate. This can result in strong localized electromagnetic fields at the metallic corners, forming the hot spots. The influence of the hot spots on the propagating SPPs are investigated theoretically and experimentally in a metallic slit structure. It is found that the electromagnetic fields radiated from the hot spots, termed as the hot spot cylindrical wave (HSCW), can greatly manipulate the SPP launching in the slit structure. The physical mechanism behind the manipulation of the SPP launching with the HSCW is explicated by a semi-analytic model. By using the HSCW, unidirectional SPP launching is experimentally realized in an ultra-small metallic step-slit structure. The HSCW bridges the localized surface plasmons and the propagating surface plasmons in an integrated platform and thus may pave a new route to the design of plasmonic devices and circuits.

Single-plasmon interferences

Surface plasmon polaritons are electromagnetic waves coupled to collective electron oscillations propagating along metal-dielectric interfaces, exhibiting a bosonic character. Recent experiments involving surface plasmons guided by wires or stripes allowed the reproduction of quantum optics effects, such as antibunching with a single surface plasmon state, coalescence with a two-plasmon state, conservation of squeezing, or entanglement through plasmonic channels. We report the first direct demonstration of the wave-particle duality for a single surface plasmon freely propagating along a planar metal-air interface. We develop a platform that enables two complementary experiments, one revealing the particle behavior of the single-plasmon state through antibunching, and the other one where the interferences prove its wave nature. This result opens up new ways to exploit quantum conversion effects between different bosonic species as shown here with photons and polaritons.

Phase change dispersion of plasmonic nano-objects

Phase is an inherent and important feature for coherent processes, which, unfortunately, has not been completely understood for surface plasmon polariton (SPP) and matter interactions. Here we propose a practical approach to extract the phase change dispersion during the interaction between free-space light, SPPs and nanogroove/slit based on far-field information only. Numerical simulation and experimental validation were both presented using nanoslit-groove plasmonic interferometers, agreeing well with theoretical near-field analysis. This approach is generally feasible to extract the intrinsic phase dispersion of other plasmonic nanostructures and can reveal more fundamental features of SPP-matter interactions.

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.

Plasmonic focusing of infrared SNOM tip patterned with asymmetric structures

Several scattering type metal tips patterned with asymmetric metal/dielectric bump gratings are studied and proved to be efficient in focusing light energy into nano 'hot spot'. The dielectric bump tip shows complex mechanisms including local geometric resonance, surface plasmon polariton (SPP) standing wave resonance and Fano effect in the near-field enhancement. Additionally, considering the practical situation, we also demonstrate that, for the case of bending tip surface, the grating coupling method for plasmonic nano-focusing is still applicable if the intervals between neighboring bumps are well designed according to the surface bending curvature. With practical realizations, our results could benefit not only infrared scanning near-field optical microscopes (SNOMs) but also many other applications in nanotechnology such as sensing and lithography.

Experimental solution for scattered imaging of the interference of plasmonic and photonic mode waves launched by metal nano-slits

Using an L-shaped metal nanoslit to generate waves of the pure photonic and plasmonic modes simultaneously, we perform an experimental solution for the scattered imaging of the interference of the two waves. From the fringe data of interference, the amplitudes and the wavevector components of the two waves are obtained. The initial phases of the two waves are obtained from the phase map reconstructed with the interference of the scattered image and the reference wave in the interferometer. The difference in the wavevector components gives rise to an additional phase delay. We introduce the scattering theory under Kirchhoff's approximation to metal slit regime and explain the wavevector difference reasonably. The solution of the quantities is a comprehensive reflection of excitation, scattering and interference of the two waves. By decomposing the polarized incident field with respect to the slit element, the scattered image produced by slit of arbitrary shape can be solved with the nanoscale Huygens-Fresnel principle. This is demonstrated by the experimental intensity pattern and phase map produced by a ring-slit and its consistency with the calculated results.

Theoretical study on the generation of a low-noise plasmonic hotspot by means of a trench-assisted circular nano-slit

We propose a novel trench-assisted circular metal nano-slit (CMNS) structure implementable on a fiber platform for the generation of a low-noise cylindrical surface plasmon (CSP) hotspot. We design trench structures based on a multi-pole cancellation method in order that a converging surface plasmon signal is well separated from co-propagating non-confined diffracted light (NCDL) at the hotspot location. In fact, the secondary radiation by the quasi-pole oscillation at the edge of the trench cancels the primary NCDL, thereby enhancing the signal-to-noise ratio (SNR) of the CSP hotspot. In particular, we investigate two types of trench structures: a rectangular-trench (RT) structure and an asymmetric-parabolic-trench (APT) structure, which are considered for the sake of the simplicity of fabrication and of the maximal enhancement of the SNR, respectively. In comparison with a conventional CMNS having no trenches, we highlight that the mean SNR of the CSP hotspot is enhanced by 6.97 and 11.89 dB in case of the optimized RT and APT CMNSs, respectively. The proposed schemes are expected to be useful for increasing the SNR of plasmonic devices that are interfered by NCDL, such as various types of nano-slits for generating high-resolution plasmonic signals, for example.

Independently analyzing different surface plasmon polariton modes on silver nanowire

In this paper, surface plasmon polariton (SPP) modes on silver nanowire (AgNW), with different field symmetric, are studied by different near field methods, respectively. In the experiment, the excitation and detection of SPPs are performed by two probes of near field scanning optical microscope (NSOM) simultaneously, which realizes the study of SPPs in complete near field. By controlling the experimental conditions, two of the fundamental SPP modes are detected separately and their intensity distributions on AgNW are given by the NSOM images. In the discussion, creeping wave (CW) is introduced to analyze the experimental results, which improves the coincidence of the experimental results and the theoretical calculations. A detailed characterization of SPPs modes in near field, which gives a further insight into optical properties of AgNW, will benefit integrated optical circuits.

Broadband and efficient plasmonic control in the near-infrared and visible via strong interference of surface plasmon polaritons

Broadband and tunable control of surface plasmon polaritons in the near-infrared and visible spectrum is demonstrated theoretically and numerically with a pair of phased nanoslits. We establish, with simulations supported by a coupled wave model, that by dividing the incident power equally between two input channels, the maximum plasmon intensity deliverable to either side of the nanoslit pair is twice that for an isolated slit. For a broadband source, a compact device with nanoslit separation of the order of a tenth of the wavelength is shown to steer nearly all the generated plasmons to one side for the same phase delay, thereby achieving a broadband unidirectional plasmon launcher. The reported effect can be applied to the design of ultra-broadband and efficient tunable plasmonic devices.

Compact aperiodic metallic groove arrays for unidirectional launching of surface plasmons

The ever-increasing power of computers and the development of new optimization methodologies have enabled the ability to design complex aperiodic devices, which can outperform periodic ones and offer new functionalities. Here, we describe the realization of an ultracompact aperiodic grating coupler capable of selectively launching surface plasmon polaritons (SPPs) in a desired direction. We use a transfer matrix model to facilitate the rapid optimization of such structures. We demonstrate both numerically and experimentally that a structure consisting of five subwavelength grooves patterned into silver can unidirectionally launch SPPs in the visible spectral range with a record right-to-left contrast ratio of 55. The general design principles behind this study can readily be extended to a great diversity of sophisticated aperiodic nanophotonic structures.

Tetrad phase vortex structure in scattered SPP field produced by silver nano-ring-slit under linearly polarized illumination

We report the tetrad phase vortex structure in the scattered surface plasmon polariton (SPP) field produced by a silver nano-ring-slit with linearly polarized illumination. In the experiment, Mach-Zehnder type interferometer is constructed in which a microscopic objective (MO) is used to collect and image the scattered SPP field, and the phase map is extracted by Fourier transform of the interference intensity. To explain the formation of the tetrad phase vortices in the central area of the ring, we propose an empirical model for the ring-slit-excited SPP source field by trial calculations with the Huygens-Fresnel principle for SPP propagations. It is shown that the azimuthal variation of the amplitude of the source SPP is roughly a half of a constant base, and the variation of the phase is a little greater than π/2. The intensity and the phase distributions of the SSP field calculated with the formulations of this model phenomenologically conform the experimental results.

General properties of the surface charge pattern of one-dimensional metallic gratings

Under light illumination, metallic gratings present unexpected and fascinating phenomena, which are due to the complex charge patterns generated on the grating surfaces. The moving electrons are due to the launching of surface plasmon polaritons (SPPs), but only in part. We derive analytical expressions quantifying the plasmonic character of the surface charge patterns, i.e. the contribution of SPPs to its formation. The expressions have a general significance, in the sense that they may be applied to a variety of geometries and spectral ranges, irrespective of whether the grating absorbs, transmits, reflects, or how strongly it resonates.

A submicron broadband surface-plasmon-polariton unidirectional coupler

The manipulation of light propagation is a basic subject in optics and has many important applications. With the development of nano-optics, this area has been downscaled to wavelength or even subwavelength scales. One of the most efficient ways to control light propagation is to exploit interference effects. Here, by manipulating the interference between two nanogrooves on a metal surface, we realize a submicron broadband surface-plasmon-polariton (SPP) unidirectional coupler. More importantly, we find an anomalous bandwidth shrinking behavior in the proposed SPP unidirectional coupler as the groove separation is down to a subwavelength scale of one-quarter of the SPP wavelength. This abnormal behavior is well explained by considering the contribution of the near-field quasi-cylindrical waves in addition to the interference of propagating SPPs and the dispersion effects of individual grooves. Such near-field effects provide new opportunities for the design of ultracompact optical devices.

Directional excitation of surface plasmon polaritons via nanoslits under varied incidence observed using leakage radiation microscopy

Surface Plasmon Polaritons (SPPs) are excited at the interface between a thin gold film and air via the illumination of nanoslits etched into the film. The coupling efficiency to the two propagation directions away from the slits is determined by leakage radiation microscopy, when the angle of incidence of the pump beam is changed from 0° to 20°. We find that preferential coupling of SPPs into one direction can be achieved for non-normal incidence in the case of single slits and slit pairs. The proportion of SPP excited into one direction can be in excess of 90%. We further provide a simple model of the process, and directly compare the performances of the two approaches.

Light funneling mechanism explained by magnetoelectric interference

We investigate the mechanisms involved in the funneling of optical energy into subwavelength grooves etched on a metallic surface. The key phenomenon is unveiled thanks to the decomposition of the electromagnetic field into its propagative and evanescent parts. We unambiguously show that the funneling is not due to plasmonic waves flowing toward the grooves, but rather to the magnetoelectric interference of the incident wave with the evanescent field, this field being mainly due to the resonant wave escaping from the groove.

Scattering of pulsed plane wave from a symmetrical groove doublet configuration

We have provided theoretical study on the spectral and temporal properties of the scattering of pulsed plane wave from a symmetrical groove doublet configuration. Based on the numerical calculation results, we show that the spectrum and the waveform of the scattered field are sensitive to the shape of the rectangular grooves when the grooves are deep enough. In both spectral and temporal domain, a damped oscillatory behavior occurs when the groove spacing increases. Furthermore, the spectral and temporal dependences of the angular distribution are consisted of interference-like fringe patterns. These patterns are sensitive to the size of the groove width and spacing rather than the groove shape when the depth is small enough. Our study takes the analysis of pulse scattering by finite grooves a step further on the theoretical side, and offers opportunities for the control of spectral and temporal properties of pulsed scattered wave in low frequency regime such as THz and microwave domain.

Comprehensive microscopic model of the extraordinary optical transmission

As shown in a recent letter [Nature 452, 728 (2008)] with a microscopic model, the phenomenon of the extraordinary optical transmission (EOT) is intrinsically due to two distinct surface waves: the surface plasmon polariton and the quasi-cylindrical wave (quasi-CW) that efficiently funnel light into the hole aperture at resonance. Here we present a comprehensive microscopic model of the EOT that takes into account the two surface waves. The model preserves the desirable physical insight of the previous approach, but since it additionally takes into account the quasi-CWs, it provides highly accurate predictions over a much broader spectral range, from visible to microwave radiation. The net outcome is a complete understanding of many aspects of the EOT and especially of the role of the metal conductivity that has largely puzzled the initial interpretations. We believe that the main conclusions of the present analysis may be applied to many Wood-type surface resonances on metallic surfaces.

Theoretical reexamination of the cross conversion between surface plasmon polaritons and quasi-cylindrical waves

The cross conversion between surface plasmon polaritons (SPPs) and quasi-cylindrical waves (CWs) is theoretically reexamined. Except for the CW-to-SPP conversion, we find the SPP-to-CW conversion, as well as the reflection and transmission of the CW, plays an indispensable role in the interaction between SPPs and light via periodic grooves. The completeness of the whole scattering coefficients is emphasized by an SPP-CW model proposed to quantitatively predict the SPP excitation efficiency for any number of periodic grooves.

Symmetry in the elementary scattering of surface plasmon polaritons and a generalized symmetry principle

In the elementary scattering of surface plasmon polaritons (SPPs) at individual subwavelength (sub-lambda) objects on metallic surfaces, the in-plane transmission and reflection of SPPs are shown to be two related scattering processes and to satisfy some novel symmetry relations, provided that the objects are mirror symmetric and are narrow enough (<0.1lambda approximately). To interpret these symmetry relations, a new generalized symmetry principle for the scattered field is put forward, which is much less limited than the classical one and is shown to have wide applications for other sub-lambda scattering problems.

Surface plasmon polaritons locally excited on the ridges of metallic gratings

With the perspective to achieve an in-depth understanding of metallic periodic surfaces, we study the surface plasmon polaritons that are locally excited on the ridges (between the indentations) of metallic lamellar gratings composed of slits or grooves. An approximate model and fully vectorial computational results show that the normalized excitation rate is rather small for slit arrays (approximately 10 at maximum) and is surprisingly weakly dependent on the metal permittivity. Additionally, the analysis is supported by an intuitive microscopic model that shines new light on the role of surface plasmons in the transmission and resonance anomalies of periodic metallic surfaces.

A quantitative theory and the generalized Bragg condition for surface plasmon Bragg reflectors

We proposed a quantitative theory based on the surface plasmon polariton (SPP) coupled-mode model for SPP-Bragg reflectors composed of N periodic defects of any geometry and any refractive index profile. A SPP coupled-mode model and its recursive form were developed and shown to be equivalent. The SPP absorption loss, as well as high-order modes in each defect and possible radiation loss, is incorporated without effort. The simple recursive equations derived from the recursive model bridge the reflectance and the transmittance of N periodic defects to those of a single one, resulting in that the computational cost of the geometry optimization or the spectra calculation for N periodic defects is reduced into that for a single one. The model predictions show good agreement with fully vectorial computation data on the reflectance and the transmittance. From the recursive model, the generalized Bragg condition is proposed, which is verified by SPP-Bragg reflectors of various structures. The quantitative theory and the generalized Bragg condition proposed will greatly simplify the design of SPP-Bragg reflectors.

Acoustic surface evanescent wave and its dominant contribution to extraordinary acoustic transmission and collimation of sound

We demonstrate both theoretically and experimentally the physical mechanism that underlies extraordinary acoustic transmission and collimation of sound through a one-dimensional decorated plate. A microscopic theory considers the total field as the sum of the scattered waves by every periodically aligned groove on the plate, which divides the total field into far-field radiative cylindrical waves and acoustic surface evanescent waves (ASEWs). Different from the well-known acoustic surface waves like Rayleigh waves and Lamb waves, ASEW is closely analogous to a surface plasmon polariton in the optical case. By mapping the total field, the experiments well confirm the theoretical calculations with ASEWs excited. The establishment of the concept of ASEW provides a new route for the integration of subwavelength acoustic devices with a structured solid surface.