Tal Ellenbogen

Multicolored Vertical Silicon Nanowires

We demonstrate that vertical silicon nanowires take on a surprising variety of colors covering the entire visible spectrum, in marked contrast to the gray color of bulk silicon. This effect is readily observable by bright-field microscopy, or even to the naked eye. The reflection spectra of the nanowires each show a dip whose position depends on the nanowire radii. We compare the experimental data to the results of finite difference time domain simulations to elucidate the physical mechanisms behind the phenomena we observe. The nanowires are fabricated as arrays, but the vivid colors arise not from scattering or diffractive effects of the array, but from the guided mode properties of the individual nanowires. Each nanowire can thus define its own color, allowing for complex spatial patterning. We anticipate that the color filter effect we demonstrate could be employed in nanoscale image sensor devices.

Chromatic Plasmonic Polarizers for Active Visible Color Filtering and Polarimetry

Color filters are widely used in color displays, optical measurement devices, and imaging devices. Conventional color filters have usually only one fixed output color. However developing active color filters with controllable color output can lead to more compact and sophisticated color filter-based devices and applications. Recent progress in nanotechnology and new knowledge of the interaction of light with metal nanostructures allow us to capture and control light better than ever. Here we use it to fabricate active color filters, based on arrays of metallic optical nanoantennas that are tailored to interact with light at visible frequencies via excitation of localized surface plasmons. This interaction maps the polarization state of incident white light to visible color. Similarly, it converts unpolarized white light to chromatically polarized light. We experimentally demonstrate a wide range of applications including active color pixels, chromatically switchable and invisible tags, and polarization imaging based on these engineered colored metasurfaces.

Switching the acceleration direction of Airy beams by a nonlinear optical process.

We present experimental control of the acceleration direction of Airy beams generated by nonlinear three-wave mixing processes in an asymmetrically poled nonlinear photonic crystal. Changing the crystal temperature enabled us to switch the phase matching condition between second-harmonic generation and difference-frequency generation in the same nonlinear crystal and thereby to change the acceleration direction and the wavelength of the output Airy beam. All-optical control of the acceleration direction can be also realized at a fixed crystal temperature by using a tunable pump source and selecting the proper crystal poling period.

Aluminum Nanoantenna Complexes for Strong Coupling between Excitons and Localized Surface Plasmons

We study the optical dynamics in complexes of aluminum nanoantennas coated with molecular J-aggregates and find that they provide an excellent platform for the formation of hybrid exciton-localized surface plasmons. Giant Rabi splitting of 0.4 eV, which corresponds to ∼10 fs energy transfer cycle, is observed in spectral transmittance. We show that the nanoantennas can be used to manipulate the polarization of hybrid states and to confine their mode volumes. In addition, we observe enhancement of the photoluminescence due to enhanced absorption and increase in the local density of states at the exciton-localized surface plasmon energies. With recent emerging technological applications based on strongly coupled light-matter states, this study opens new possibilities to explore and utilize the unique properties of hybrid states over all of the visible region down to ultraviolet frequencies in nanoscale, technologically compatible, integrated platforms based on aluminum.

Control of free space propagation of Airy beams generated by quadratic nonlinear photonic crystals

We present experimentally the control of free space propagation of an Airy beam. This beam is generated by a nonlinear wave mixing process in an asymmetrically poled nonlinear photonic crystal. Changing the quasi-phase matching conditions, e.g., the crystal temperature or pump wavelength, alters the location of the Airy beam peak intensity along the same curved trajectory. We explain that the variation in the beam shape is caused by noncollinear interactions. Owing to the highly asymmetric shape of nonlinear crystal in the Fourier space, these noncollinear interactions are still relatively efficient for positive (nonzero) phase mismatch.

Metasurfaces based dual wavelength diffractive lenses

We demonstrate experimentally and by simulations a method for using thin nanostructured plasmonic metasurfaces to design diffractive Fresnel zone plate lenses that focus pairs of wavelengths to a single focal point. The metasurfaces are made of tightly packed cross and rod shaped optical nanoantennas with strong polarization and wavelength selectivity. This selectivity allows multiplexing two different lenses with low spectral crosstalk on the same substrate and to address any superposition of the two colors at the focus of the lenses by controlling the polarization of light. This concept can open the door to use ultrathin diffractive lenses in fluorescence microscopy and in stimulated emission depletion microscopy.

Noncollinear double quasi phase matching in one-dimensional poled crystals

We demonstrate simultaneous phase matching of two different nonlinear processes, using a noncollinear interaction in periodically poled crystal with single grating. The noncollinear scheme provides phase-matching solutions over continuous regions of the optical spectrum and can be used for multiple-harmonic generation as well as all-optical effects. We have demonstrated experimentally third-harmonic generation of a 3 microm pump wavelength in a noncollinear configuration using a periodically poled LiNbO3 crystal. We observed, in good agreement with theoretical calculation, very broad spectral and thermal acceptance bandwidths, as well as a relatively narrow angular bandwidth.

Composite functional metasurfaces for multispectral achromatic optics

Nanostructured metasurfaces offer unique capabilities for subwavelength control of optical waves. Based on this potential, a large number of metasurfaces have been proposed recently as alternatives to standard optical elements. In most cases, however, these elements suffer from large chromatic aberrations, thus limiting their usefulness for multiwavelength or broadband applications. Here, in order to alleviate the chromatic aberrations of individual diffractive elements, we introduce dense vertical stacking of independent metasurfaces, where each layer is made from a different material, and is optimally designed for a different spectral band. Using this approach, we demonstrate a triply red, green and blue achromatic metalens in the visible range. We further demonstrate functional beam shaping by a self-aligned integrated element for stimulated emission depletion microscopy and a lens that provides anomalous dispersive focusing. These demonstrations lead the way to the realization of ultra-thin superachromatic optical elements showing multiple functionalities—all in a single nanostructured ultra-thin element.

Exciton-polariton emission from organic semiconductor optical waveguides

We photoexcite slab polymer waveguides doped with J-aggregating dye molecules and measure the leaky emission from strongly coupled waveguide exciton polariton modes at room temperature. We show that the momentum of the waveguide exciton polaritons can be controlled by modifying the thickness of the excitonic waveguide. Nonresonantly pumped excitons in the slab excitonic waveguide decay into transverse electric and transverse magnetic strongly coupled exciton waveguide modes with radial symmetry. These leak to cones of light with radial and azimuthal polarizations. The emission of light by recombination of bound electronhole pairs (excitons) in excitonic materials, e.g., semiconductors, quantum wells, and dye molecules, is used for a wide range of applications, including lasers, light-emitting diodes, 1 and fluorescent tags in biology. 2 The majority of the applications based on photoemission from excitons operate in the weak-coupling regime, where the exciton is annihilated and, as a consequence, a photon is emitted either by spontaneous emission or by stimulated emission. The exciton and emitted photon can therefore be treated as two separate physical entities. On the other hand, in the strong-coupling regime, the exciton and photon mode can exchange energy at a rate faster than the decay rate of the exciton and the escape rate of the photon, and the two particles create a hybrid state known as exciton-polariton (EP). 3,4 The EP can be considered to be a quasi-particle with mixed properties of light and matter. The masses of cavity EPs are 10 −4 ‐10 −5

Radially symmetric nonlinear photonic crystals

Different types of quadratic, radially symmetric, nonlinear photonic crystals are presented. The modulation of the nonlinear coefficient may be a periodic or an aperiodic function of the radial coordinate, whereas the azimuthal symmetry of the crystal may be either continuous or discrete. Nonlinear interactions within these structures are studied in two orientations, transverse and longitudinal, for which the interacting beams propagate either perpendicularly or in the plane of modulation. We show that radially symmetric structures can phase match multiple arbitrary processes in any direction. We study multiple wavelength three-wave mixing interactions and multiple direction interactions and analyze spatially dependent polarization states of the generated harmonics.