All-passive nonreciprocal metastructure.

One-way propagation of light, analogous to the directional flow of electrons in the presence of electric potential difference, has been an important goal in the wave-matter interaction. Breaking time-reversal symmetry in photonic flows is faced with challenges different from those for electron flows. In recent years several approaches and methods have been offered towards achieving this goal. Here we investigate another systematic approach to design all-passive relatively high-throughput metastructures that exhibit nonreciprocal properties and achieve wave-flow isolation. Moreover, we build on those findings and propose a paradigm for a quasi-two-dimensional metastructure that mimics the nonreciprocal property of Faraday rotation without using any magnetic or electric biasing. We envision that the proposed approaches may serve as a building block for all-passive time-reversal symmetry breaking with potential applications for future nonreciprocal systems and devices.

NEMS With Broken T Symmetry: Graphene Based Unidirectional Acoustic Transmission Lines.

In this work we discuss the idea of one-way acoustic signal isolation in low dimensional nanoelectromechanical oscillators. We report a theoretical study showing that one-way conversion between in-phase and anti-phase vibrational modes of a double layer graphene nanoribbon is achieved by introducing spatio-temporal modulation of system properties. The required modulation length in order to reach full conversion between the two modes is subsequently calculated. Generalization of the method beyond graphene nanoribbons and realization of a NEMS signal isolator are also discussed.

Hotspots from nonreciprocal surface waves.

We present theoretical and numerical results for a new method of obtaining hotspots that relies on nonreciprocal "one-way" propagation as opposed to the well-known case of resonance-based hotspots. The nonreciprocal propagation is achieved by the breaking of time-reversal symmetry through the use of magnetically biased medium. The location and existence of the hotspots depends on the magnetic bias that results in the breaking of the time-reversal symmetry. This results in the intriguing possibility of switching and spatial control of the hotspots through the magnetic bias.

Nonreciprocal rotating power flow within plasmonic nanostructures.

We theoretically explore the notion of nonreciprocal near-zone manipulation of electromagnetic fields within subwavelength plasmonic nanostructures embedded in magneto-optical materials. We derive an analytical model predicting a strong, magneto-optically induced time-reversal symmetry breaking of localized plasmonic resonances in topologically symmetric structures. Our numerical simulations of plasmon excitations reveal a considerable near-zone power flow rotation within such hybrid nanostructures, demonstrating nanoscale nonreciprocity. This can be considered as another mechanism for tuning plasmonic phenomena at the nanoscale.

Self-similar parabolic plasmonic beams.

We demonstrate that an interplay between diffraction and defocusing nonlinearity can support stable self-similar plasmonic waves with a parabolic profile. Simplicity of a parabolic shape combined with the corresponding parabolic spatial phase distribution creates opportunities for controllable manipulation of plasmons through a combined action of diffraction and nonlinearity.

Optical isolation with epsilon-near-zero metamaterials.

We suggest a principle for isolation of circularly polarized waves in magnetically active extreme-parameter metamaterials. Using theoretical analysis and numerical simulations, we show that metamaterials with extreme parameters, such as epsilon-near-zero materials (ENZ), when merged with magneto-optical materials, become transparent for forward circularly polarized waves of a given handedness and opaque for backward propagating waves of the same handedness. We theoretically study two possible implementations of such hybrid materials: (1) the case of metal-dielectric stacks; and (2) rectangular waveguide near its cut-off frequency. We prove that these structures can be utilized as compact isolators for circularly polarized waves.

Theory of wave propagation in magnetized near-zero-epsilon metamaterials: evidence for one-way photonic states and magnetically switched transparency and opacity.

We study propagation of transverse-magnetic electromagnetic waves in the bulk and at the surface of a magnetized epsilon-near-zero (ENZ) medium in a Voigt configuration. We reveal that in a certain range of material parameters novel regimes of wave propagation emerge; we show that the transparency of the medium can be altered with the magnetization leading either to magnetically induced Hall opacity or Hall transparency of the ENZ. In our theoretical study, we demonstrate that surface waves at the interface between either a transparent or an opaque Hall medium and a homogeneous medium may, under certain conditions, be predominantly one way. Moreover, we predict that one-way photonic surface states may exist at the interface of an opaque Hall ENZ and a regular metal, giving rise to the possibility for backscattering immune wave propagation and isolation.

Tailoring terahertz near-field enhancement via two-dimensional plasmons.

We suggest a novel possibility for electrically tunable terahertz near-field enhancement in flatland electronic materials supporting two-dimensional plasmons, including recently discovered graphene. We employ electric-field effect modulation of electron density in such materials and induce a periodic plasmonic lattice with a defect cavity. We demonstrate that the plasmons resonantly excited in such a periodic plasmonic lattice by an incident terahertz radiation can strongly pump the cavity plasmon modes leading to a deep subwavelength concentration of terahertz energy, beyond λ/1000, with giant electric-field enhancement factors up to 10(4), which is 2 orders of magnitude higher than achieved previously in metal-based terahertz field concentrators.

Symmetry breaking in plasmonic waveguides with metal nonlinearities.

We studied nonlinear effects in plasmonic metal film waveguides and couplers stimulated by third-order optical response due to ponderomotive metal nonlinearities. We analyzed the structure and dispersion of nonlinear plasmonic guided modes and predicted the bifurcations and symmetry breaking of nonlinear modes for the critical powers, depending on the structure dimensions.

Nonlinear nanofocusing in tapered plasmonic waveguides.

We suggest using tapered waveguides for compensating losses of surface plasmon-polaritons in order to enhance nonlinear effects at the nanoscale. We study nonlinear plasmon self-focusing in tapered metal-dielectric-metal slot waveguides and demonstrate that, by an appropriate choice of the taper angle, we can effectively suppress the mode attenuation achieving stable propagation of a spatial plasmon soliton. For larger tapering angles we observe plasmon-beam nanofocusing in both spatial dimensions.

Quadratic phase matching in nonlinear plasmonic nanoscale waveguides.

We analyze phase matching in metal-dielectric nonlinear structures which support highly localized plasmon polariton modes. We reveal that quadratic phase matching between the plasmon modes of different symmetries becomes possible in planar waveguide geometries. We discuss the example of a nonlinear LiNbO(3) waveguide sandwiched between two silver plates, and demonstrate that second-harmonic generation can be achieved for interacting plasmonic modes.

Self-focusing and spatial plasmon-polariton solitons.

We study nonlinear propagation of surface plasmon polaritons along an interface between metal and nonlinear Kerr dielectric. We demonstrate numerically self-focusing of a plasmon beam at large powers and the formation of slowly decaying spatial soliton in the presence of losses. We develop an analytical model for describing the evolution of spatial plasmon-solitons and observe a good agreement with numerical results.

Nonlinear plasmonic slot waveguides.

We study nonlinear modes in a subwavelength slot waveguide created by a nonlinear dielectric slab sandwiched between two metals. We present the dispersion diagrams of the families of nonlinear guided modes and reveal that the symmetric mode undergoes the symmetry-breaking bifurcation and becomes primarily localized near one of the interfaces. We also find that the antisymmetric mode may split into two brunches giving birth to two families of nonlinear antisymmetric modes.

Bloch oscillations in chirped layered structures with metamaterials.

We analyze the Bloch oscillations of electromagnetic waves in chirped layered structures with alternating layers of negative-index metamaterial and conventional dielectric under the condition of the zero average refractive index. We consider the case when the chirp is introduced by varying the thickness of the layers linearly across the structure. We demonstrate that such structures can support three different types of the Bloch oscillations for electromagnetic waves associated with either propagating or evanescent guided modes. In particular, we predict a novel type of the Bloch oscillations associated with coupling between surface waves excited at the interfaces separating the layers of negative-index metamaterial and the layers of the conventional dielectric.