Yakir Hadad

Space-time gradient metasurfaces

Metasurfaces characterized by a transverse gradient of local impedance have recently opened exciting directions for light manipulation at the nanoscale. Here we add a temporal gradient to the picture, showing that spatio-temporal variations over a surface may largely extend the degree of light manipulation in metasurfaces, and break several of their constraints associated to symmetries. As an example, we synthesize a non-reciprocal classical analogue to electromagnetic induced transparency, opening a narrow window of one-way transmission in an otherwise opaque surface. These properties pave the way to magnetic free, planarized non-reciprocal ultrathin surfaces for free-space isolation.

Magnetized Spiral Chains of Plasmonic Ellipsoids for One-Way Optical Waveguides

When a linear chain of plasmonic nanoparticles is subject to longitudinal magnetic field, it exhibits optical Faraday rotation. If the magnetized nanoparticles are plasmonic ellipsoids arranged as a spiral chain, the interplay between the Faraday rotation and the geometrical spiral rotation (structural chirality) can strongly enhance nonreciprocity. This interplay forms a waveguide that permits one-way propagation only, within four disjoint frequency bands, two bands for each direction.

Breaking temporal symmetries for emission and absorption

Significance Antennas, from radiofrequencies to optics, are forced to transmit and receive with the same efficiency to/from the same direction. The same constraint applies to thermophotovoltaic systems, which are forced to emit as well as they can absorb, limiting their efficiency. In this paper, we show that it is possible to efficiently overcome these bounds using temporally modulated traveling-wave circuits. Beyond the basic physics interest of our theoretical and experimental findings, we also prove that the proposed temporally modulated antenna can be efficiently used to transmit without being forced to listen to echoes and reflections, with important implications for radio-wave communications. Similar concepts may be extended to infrared frequencies, with relevant implications for energy harvesting. Time-reversal symmetries impose stringent constraints on emission and absorption. Antennas, from radiofrequencies to optics, are bound to transmit and receive signals equally well from the same direction, making a directive antenna prone to receive echoes and reflections. Similarly, in thermodynamics Kirchhoff’s law dictates that the absorptivity and emissivity are bound to be equal in reciprocal systems at equilibrium, e(ω,θ)=a(ω,θ), with important consequences for thermal management and energy applications. This bound requires that a good absorber emits a portion of the absorbed energy back to the source, limiting its overall efficiency. Recent works have shown that weak time modulation or mechanical motion in suitably designed structures may largely break reciprocity and time-reversal symmetry. Here we show theoretically and experimentally that a spatiotemporally modulated device can be designed to have drastically different emission and absorption properties. The proposed concept may provide significant advances for compact and efficient radiofrequency communication systems, as well as for energy harvesting and thermal management when translated to infrared frequencies.

Self-induced topological transitions and edge states supported by nonlinear staggered potentials

The canonical Su-Schrieffer-Heeger (SSH) array is one of the basic geometries that have spurred significant interest in topological band-gap modes. Here, we show that the judicious inclusion of third-order Kerr nonlinearities in SSH arrays opens rich physics in topological insulators, including the possibility of supporting self-induced topological transitions, as a function of the applied intensity. We highlight the emergence of a class of topological solutions in nonlinear SSH arrays localized at the array edges and with unusual properties. As opposed to their linear counterparts, these nonlinear states decay to a plateau of nonzero amplitude inside the array, highlighting the local nature of topologically nontrivial band gaps in nonlinear systems. We study the conditions under which these states can be excited and their temporal dynamics as a function of the applied excitation, paving the way to interesting directions in the physics of topological edge states with robust propagation properties based on nonlinear interactions in suitably designed periodic arrays.

Parameterization of the Tilted Gaussian Beam Waveobjects

Novel time-harmonic beam fields have been recently obtained by utilizing a non-orthogonal coordinate system which is a priori matched to the field’s planar linearly-phased Gaussian aperture distribution. These waveobjects were termed tilted Gaussian beams. The present investigation is concerned with parameterization of these time-harmonic tilted Gaussian beams and of the wave phenomena associated with them. Specific types of tilted Gaussian beams that are characterized by their aperture complex curvature matrices, are parameterized in term of beam-widths, waist-locations, collimationlengths, radii of curvature, and other features. Emphasis is placed on the difference in the parameterization between the conventional (orthogonal coordinates) beams and the tilted ones.

Non-Orthogonal Domain Parabolic Equation and Its Tilted Gaussian Beam Solutions

A non-orthogonal coordinate system which is a priori matched to localized initial field distributions for time-harmonic wave propagation is presented. Applying, in addition, a rigorous paraxial-asymptotic approximation, results in a novel parabolic wave equation for beam-type field propagation in 3D homogeneous media. Localized solutions to this equation that exactly match linearly-phased Gaussian aperture distributions are termed tilted Gaussian beams. These beams serve as the building blocks for various beam-type expansion schemes. Application of the scalar waveobjects to electromagnetic field beam-type expansion, as well as reflection and transmission of these waveobjects by planar velocity (dielectric) discontinuity are presented. A numerical example which demonstrates the enhanced accuracy of the tilted Gaussian beams over the conventional ones concludes the paper.

A four quadrants HF AC chopper with no deadtime

A four quadrant HF AC chopper is described, analyzed and verified experimentally. The main advantage of the proposed topology is the ability to handle reactive loads without the need for a deadtime between switches. This is accomplished by dynamically configuring the chopper as positive or negative buck or backward boost converters corresponding to the momentary polarities of the input voltage and output filter inductor current. A 300W breadboard unit was built to test the proposed concept. The experimental results conform to the expected behavior of proposed chopper

One way optical waveguides for matched non-reciprocal nanoantennas with dynamic beam scanning functionality.

Matching circuits for waveguide-nanoantenna connections are difficult to implement. However, if the waveguide permits only one-way propagation, the matching issue disappears since back-reflections cannot take place; the feed signal is converted to radiation at high efficiency. Hence, a terminated one-way waveguide may serve as an assembly consisting of a waveguide, a matching mechanism, and an antenna. Since one-way structures are inherently non-reciprocal, this antenna possesses different transmit and receive patterns. We test and demonstrate this concept on a recently suggested new class of one-way plasmonic waveguides and present an additional significant dynamic beam scanning functionality.

Tilted Gaussian beam propagation in inhomogeneous media

The conventional form of Gaussian beam propagation in inhomogeneous media is based on paraxial approximation in an orthogonal ray-centered coordinate system. However, phase-space spectral distributions of wave fields require beam solutions in which the initial Gaussian distribution is given on a plane which is generally inclined to the beam propagation direction. Thus, the conventional paraxial approximation is not valid for large-angle spectral components. The current research is dealing with paraxial beam solutions in inhomogeneous media, which are asymptotically valid for all angles of propagation. By applying a novel non-orthogonal ray-centered coordinate system to the inhomogeneous wave equation and using asymptotic (paraxial) considerations, the wave equation is reduced into a new form of a Parabolic wave equation. Solutions to this equation, form a new kind of beam waveobjects which serve as building blocks for phase-space representations. The characteristics of these wave fields are investigated, as well as the novel wave phenomena associated with them.

Time-Dependent Tilted Pulsed-Beams and Their Properties

Novel time-dependent wavepacket equation and its pulsed field solutions are obtained by utilizing a non-orthogonal coordinate system which is a priori matched to the fields planar linearly-delayed pulsed localized aperture distributions. These waveobjects that serve as the building blocks for various time-dependent beam-expansion schemes, are termed tilted pulsed-beams. Iso-axial pulsed-beams are parameterized in term of beam-widths, waist-locations, collimation-lengths, wave-front radii of curvature, and other features. Emphasis is placed on a direct time-domain derivation. A numerical example is presented in which the enhanced accuracy of the tilted pulsed-beams over the conventional (orthogonal coordinates) ones in the well-collimated zone is demonstrated.