Tunable photon-induced spatial modulation of free electrons.

Spatial modulation of electron beams is an essential tool for various applications such as nanolithography and imaging, yet its conventional implementations are severely limited and inherently non-tunable. Conversely, proposals of light-driven electron spatial modulation promise tunable electron wavefront shaping, for example, using the mechanism of photon-induced near-field electron microscopy. Here we present tunable photon-induced spatial modulation of electrons through their interaction with externally controlled surface plasmon polaritons (SPPs). Using recently developed methods of shaping SPP patterns, we demonstrate a dynamic control of the electron beam with a variety of electron distributions and verify their coherence through electron diffraction. Finally, the nonlinearity stemming from energy post-selection provides us with another avenue for controlling the electron shape, generating electron features far below the SPP wavelength. Our work paves the way to on-demand electron wavefront shaping at ultrafast timescales, with prospects for aberration correction, nanofabrication and material characterization.

Topological Transitions and Surface Umklapp Scattering in Weakly Modulated Periodic Metasurfaces.

Controlling and manipulating surface waves is highly beneficial for imaging applications, nanophotonic device design, and light-matter interactions. While deep-subwavelength structuring of the metal-dielectric interface can influence surface waves by forming strong effective anisotropy, it disregards important structural degrees of freedom such as the interplay between corrugation periodicity and depth and its effect on the beam transport. Here, we unlock these degrees of freedom, introducing weakly modulated metasurfaces, structured metal-dielectric surfaces beyond effective medium. We utilize groove-structuring with varying depths and periodicities to demonstrate control over the transport of surface waves, dominated by the depth-period interplay. We show unique backward focusing of surface waves driven by an umklapp process-momentum relaxation empowered by the periodic nature of the structure and discover a yet unexplored, dual-stage topological transition. Our findings can be applied to any type of guided wave, introducing a simple and versatile approach for controlling wave propagation in artificial media.

Nonlinear Forced Response of Plasmonic Nanostructures.

Incorporating optical surface waves in nonlinear processes unlocks unique and sensitive nonlinear interactions wherein highly confined surface states can be accessed and explored. Here, we unravel the rich physics of modal-nonmodal state pairs of short-range surface plasmons in thin metal films by leveraging "dark nonlinearity"-a nonradiating nonlinear source. We control and observe the nonlinear forced response of these modal-nonmodal pairs and present nonlinearly mediated direct access to nonmodal plasmons in a lossless regime. Our study can be generalized to other forms of surface waves or optical nonlinearities, toward on-chip nonlinearly controlled nanophotonic devices.

Miniaturizing nanoantennas with hybrid photonic-plasmonic modes for improved metasurfaces.

The increasing interest in manipulating light on scales much smaller than its wavelength has driven intensive research on designing high efficiency optical antennas for near and far field applications. In particular, such nanoantennas serve as the main building block of metasurfaces, which were identified as an emerging technology for their capability in constructing versatile optical and electromagnetic devices. Hence, reducing the antennas' dimensions without compromising on their scattering efficiency is of utmost importance. In this Letter, we show that nanoantennas carved from hybrid plasmonic-dielectric waveguides preserve the unique properties of the hybrid modes, showing stronger confinement and better tunability at a relatively low loss, emanating from the coupling between the dielectric and plasmonic modes. This enables a design of high performance ultrasmall antennas that outperform dielectric and plasmonic nanoantennas at similar dimensions. We demonstrate this capability by simulating the performance of metasurfaces made of ultrasmall hybrid nanoantennas, proven to be superior over their dielectric and plasmonic counterparts. Using such hybrid nanoantennas as unit-cells in metasurfaces holds a great promise for designing new tunable, multifunctional, and low-loss nano-optical materials and applications.

Spin-Orbit Interaction of Light in Plasmonic Lattices.

In the past decade, the spin-orbit interaction (SOI) of light has been a driving force in the design of metamaterials, metasurfaces, and schemes for light-matter interaction. A hallmark of the spin-orbit interaction of light is the spin-based plasmonic effect, converting spin angular momentum of propagating light to near-field orbital angular momentum. Although this effect has been thoroughly investigated in circular symmetry, it has yet to be characterized in a noncircular geometry, where whirling, periodic plasmonic fields are expected. Using phase-resolved near-field microscopy, we experimentally demonstrate the SOI of circularly polarized light in nanostructures possessing dihedral symmetry. We show how interaction with hexagonal slits results in four topologically different plasmonic lattices, controlled by engineered boundary conditions, and reveal a cyclic nature of the spin-based plasmonic effect which does not exist for circular symmetry. Finally, we calculate the optical forces generated by the plasmonic lattices, predicting that light with mere spin angular momentum can exert torque on a multitude of particles in an ordered fashion to form an optical nanomotor array. Our findings may be of use in both biology and chemistry, as a means for simultaneous trapping, manipulation, and excitation of multiple objects, controlled by the polarization of light.

Linearly dichroic plasmonic lens and hetero-chiral structures.

We present an experimental study of Hetero-Chiral (HC) plasmonic lenses, comprised of constituents with opposite chirality, demonstrating linearly dichroic focusing. The lenses focus only light with a specific linear polarization and result in a dark focal spot for the orthogonal polarization state. We introduce the design concepts and quantitatively compare several members of the HC family, deriving necessary conditions for linear dichroism and several comparative engineering parameters. The HC lenses were experimentally investigated using aperture-less near field scanning microscope collecting the amplitude of the plasmonic near-field. Our results exhibit an excellent match to the simulation predictions. The demonstrated ability for linearly dichroic functional focusing could lead to novel sensing applications.

Double moiré structured illumination microscopy with high-index materials.

Structured illumination microscopy utilizes illumination of periodic light patterns to allow reconstruction of high spatial frequencies, conventionally doubling the microscope's resolving power. This Letter presents a structured illumination microscopy scheme with the ability to achieve 60 nm resolution by using total internal reflection of a double moiré pattern in high-index materials. We propose a realization that provides dynamic control over relative amplitudes and phases of four coherently interfering beams in gallium phosphide and numerically demonstrate its capability.

Metafocusing by a Metaspiral Plasmonic Lens.

We designed and realized a metasurface (manipulating the local geometry) spiral (manipulating the global geometry) plasmonic lens, which fundamentally overcomes the multiple efficiency and functionality challenges of conventional in-plane plasmonic lenses. The combination of spirality and metasurface achieves much more efficient and uniform linear-polarization-independent plasmonic focusing. As for functionality, under matched circularly polarized illumination the lens directs all of the power coupled to surface plasmon polaritons (SPPs) into the focal spot, while the orthogonal polarization excites only diverging SPPs that do not penetrate the interior of the lens, achieving 2 orders of magnitude intensity contrast throughout the entire area of the lens. This optimal functional focusing is clearly demonstrated by near-field optical microscopy measurements that are in excellent agreement with simulations and are supported by a detailed theoretical interpretation of the underlying mechanisms. Our results advance the field of plasmonics toward functional detection and the employment of SPPs in smart pixels, near-field microscopy, lithography, and particle manipulation.

Spin-patterned plasmonics: towards optical access to topological-insulator surface states.

Topological insulators (TI) are new phases of matter with topologically protected surface states (SS) possessing novel physical properties such as spin-momentum locking. Coupling optical angular momentum to the SS is of interest for both fundamental understanding and applications in future spintronic devices. However, due to the nanoscale thickness of the surface states, the light matter interaction is dominated by the bulk. Here we propose and experimentally demonstrate a plasmonic cavity enabling both nanoscale light confinement and control of surface plasmon-polariton (SPP) spin angular momentum (AM)--towards coupling to topological-insulator SS. The resulting SPP field components within the cavity are arranged in a chess-board-like pattern. Each chess-board square exhibits approximately a uniform circular polarization (spin AM) of the local in-plane field interleaved by out-of-plane field vortices (orbital AM). As the first step, we demonstrate the predicted pattern experimentally by near-field measurements on a gold-air interface, with excellent agreement to our theory. Our results pave the way towards efficient optical access to topological-insulator surface states using plasmonics.

Plasmon lasers at deep subwavelength scale.

Laser science has been successful in producing increasingly high-powered, faster and smaller coherent light sources. Examples of recent advances are microscopic lasers that can reach the diffraction limit, based on photonic crystals, metal-clad cavities and nanowires. However, such lasers are restricted, both in optical mode size and physical device dimension, to being larger than half the wavelength of the optical field, and it remains a key fundamental challenge to realize ultracompact lasers that can directly generate coherent optical fields at the nanometre scale, far beyond the diffraction limit. A way of addressing this issue is to make use of surface plasmons, which are capable of tightly localizing light, but so far ohmic losses at optical frequencies have inhibited the realization of truly nanometre-scale lasers based on such approaches. A recent theoretical work predicted that such losses could be significantly reduced while maintaining ultrasmall modes in a hybrid plasmonic waveguide. Here we report the experimental demonstration of nanometre-scale plasmonic lasers, generating optical modes a hundred times smaller than the diffraction limit. We realize such lasers using a hybrid plasmonic waveguide consisting of a high-gain cadmium sulphide semiconductor nanowire, separated from a silver surface by a 5-nm-thick insulating gap. Direct measurements of the emission lifetime reveal a broad-band enhancement of the nanowire's exciton spontaneous emission rate by up to six times owing to the strong mode confinement and the signature of apparently threshold-less lasing. Because plasmonic modes have no cutoff, we are able to demonstrate downscaling of the lateral dimensions of both the device and the optical mode. Plasmonic lasers thus offer the possibility of exploring extreme interactions between light and matter, opening up new avenues in the fields of active photonic circuits, bio-sensing and quantum information technology.

Field enhancement and resonance phenomena in complex three-dimensional nanoparticles: efficient computation using the source-model technique.

We present a semi-analytical method for computing the electromagnetic field in and around 3D nanoparticles (NP) of complex shape and demonstrate its power via concrete examples of plasmonic NPs that have nonsymmetrical shapes and surface areas with very small radii of curvature. In particular, we show the three axial resonances of a 3D cashew-nut and the broadband response of peanut-shell NPs. The method employs the source-model technique along with a newly developed intricate source distributing algorithm based on the surface curvature. The method is simple and can outperform finite-difference time domain and finite-element-based software tools in both its efficiency and accuracy.

Exact modeling of cylindrical metal-dielectric multilayers beyond the effective medium approximation.

We present a semi-analytical method for accurate modeling of wave propagation in cylindrically symmetric subwavelength metal-dielectric multilayers. Utilizing a cylindrical transfer matrix method, we compute the amplitude transfer function of cylindrical hyperlens, simulate the exact field distribution and propagation for a given source and compare it to that in effective hyperbolic medium. We investigate the conditions under which the effective medium theory (EMT) is valid and show that in cylindrical configuration, a new degree of freedom is present in the applicability of the EMT-the ratio between the inner radius of the structure and the unit cell size.

Plasmon-enhanced four-wave mixing for superresolution applications.

We introduce a nonlinear optical approach to transform spatial information stored in evanescent waves into propagating ones: we study analytically the use of partially degenerate four-wave mixing in thin metallic film to map a band of evanescent waves at a given frequency into a propagating-wave band at a different one. The relatively low efficiency of this process is compensated by setting the pump beam, mediating this transformation, to be a surface plasmon polariton, whose field enhancement increases the nonlinear interaction strength. This setting can be utilized for nonresonant plasmon-assisted superresolution applications that support transverse-electric polarization, in contrast to linear plasmonic imaging (such as superlens) that can only transfer transverse-magnetic waves.

Subwavelength multilayer dielectrics: ultrasensitive transmission and breakdown of effective-medium theory.

We show that a purely dielectric structure made of alternating layers of deep subwavelength thicknesses exhibits novel transmission effects which completely contradict conventional effective medium theories exactly in the regime in which those theories are commonly used. We study waves incident at the vicinity of the effective medium's critical angle for total internal reflection and show that the transmission through the multilayer structure depends strongly on nanoscale variations even at layer thicknesses smaller than λ/50. In such deep subwavelength structures, we demonstrate dramatic changes in the transmission for variations in properties such as periodicity, order of the layers, and their parity. In addition to its conceptual importance, such sensitivity has important potential applications in sensing and switching.

Three-dimensional optical metamaterial with a negative refractive index.

Metamaterials are artificially engineered structures that have properties, such as a negative refractive index, not attainable with naturally occurring materials. Negative-index metamaterials (NIMs) were first demonstrated for microwave frequencies, but it has been challenging to design NIMs for optical frequencies and they have so far been limited to optically thin samples because of significant fabrication challenges and strong energy dissipation in metals. Such thin structures are analogous to a monolayer of atoms, making it difficult to assign bulk properties such as the index of refraction. Negative refraction of surface plasmons was recently demonstrated but was confined to a two-dimensional waveguide. Three-dimensional (3D) optical metamaterials have come into focus recently, including the realization of negative refraction by using layered semiconductor metamaterials and a 3D magnetic metamaterial in the infrared frequencies; however, neither of these had a negative index of refraction. Here we report a 3D optical metamaterial having negative refractive index with a very high figure of merit of 3.5 (that is, low loss). This metamaterial is made of cascaded 'fishnet' structures, with a negative index existing over a broad spectral range. Moreover, it can readily be probed from free space, making it functional for optical devices. We construct a prism made of this optical NIM to demonstrate negative refractive index at optical frequencies, resulting unambiguously from the negative phase evolution of the wave propagating inside the metamaterial. Bulk optical metamaterials open up prospects for studies of 3D optical effects and applications associated with NIMs and zero-index materials such as reversed Doppler effect, superlenses, optical tunnelling devices, compact resonators and highly directional sources.

Three-dimensional nanometer-scale optical cavities of indefinite medium.

Miniaturization of optical cavities has numerous advantages for enhancing light-matter interaction in quantum optical devices, low-threshold lasers with minimal power consumption, and efficient integration of optoelectronic devices at large scale. However, the realization of a truly nanometer-scale optical cavity is hindered by the diffraction limit of the nature materials. In addition, the scaling of the photon life time with the cavity size significantly reduces the quality factor of small cavities. Here we theoretically present an approach to achieve ultrasmall optical cavities using indefinite medium with hyperbolic dispersion, which allows propagation of electromagnetic waves with wave vectors much larger than those in vacuum enabling extremely small 3D cavity down to (λ/20)(3). These cavities exhibit size-independent resonance frequencies and anomalous scaling of quality factors in contrast to the conventional cavities, resulting in nanocavities with both high Q/V(m) ratio and broad bandwidth.

Finite element simulation of a perturbed axial-symmetric whispering-gallery mode and its use for intensity enhancement with a nanoparticle coupled to a microtoroid.

We present an optical mode solver for a whispering gallery resonator coupled to an adjacent arbitrary shaped nano-particle that breaks the axial symmetry of the resonator. Such a hybrid resonator-nanoparticle is similar to what was recently used for bio-detection and for field enhancement. We demonstrate our solver by parametrically studying a toroid-nanoplasmonic device and get the optimal nano-plasmonic size for maximal enhancement. We investigate cases near a plasmonic resonance as well as far from a plasmonic resonance. Unlike common plasmons that typically benefit from working near their resonance, here working far from plasmonic resonance provides comparable performance. This is because the plasmonic resonance enhancement is accompanied by cavity quality degradation through plasmonic absorption.

Nonlinear hyperlens.

The performance of an optical hyperlens made of metal-dielectric layers can be improved by incorporating self-focusing nonlinearity in the dielectric layers. Using a modified beam propagation method in cylindrical coordinates, we show increased bandwidth and better propagation length, which can improve the spatial and temporal resolution of the device.

Transport and Anderson localization in disordered two-dimensional photonic lattices.

One of the most interesting phenomena in solid-state physics is Anderson localization, which predicts that an electron may become immobile when placed in a disordered lattice. The origin of localization is interference between multiple scatterings of the electron by random defects in the potential, altering the eigenmodes from being extended (Bloch waves) to exponentially localized. As a result, the material is transformed from a conductor to an insulator. Anderson's work dates back to 1958, yet strong localization has never been observed in atomic crystals, because localization occurs only if the potential (the periodic lattice and the fluctuations superimposed on it) is time-independent. However, in atomic crystals important deviations from the Anderson model always occur, because of thermally excited phonons and electron-electron interactions. Realizing that Anderson localization is a wave phenomenon relying on interference, these concepts were extended to optics. Indeed, both weak and strong localization effects were experimentally demonstrated, traditionally by studying the transmission properties of randomly distributed optical scatterers (typically suspensions or powders of dielectric materials). However, in these studies the potential was fully random, rather than being 'frozen' fluctuations on a periodic potential, as the Anderson model assumes. Here we report the experimental observation of Anderson localization in a perturbed periodic potential: the transverse localization of light caused by random fluctuations on a two-dimensional photonic lattice. We demonstrate how ballistic transport becomes diffusive in the presence of disorder, and that crossover to Anderson localization occurs at a higher level of disorder. Finally, we study how nonlinearities affect Anderson localization. As Anderson localization is a universal phenomenon, the ideas presented here could also be implemented in other systems (for example, matter waves), thereby making it feasible to explore experimentally long-sought fundamental concepts, and bringing up a variety of intriguing questions related to the interplay between disorder and nonlinearity.