Isabelle Staude

Tailoring Directional Scattering through Magnetic and Electric Resonances in Subwavelength Silicon Nanodisks

Interference of optically induced electric and magnetic modes in high-index all-dielectric nanoparticles offers unique opportunities for tailoring directional scattering and engineering the flow of light. In this article we demonstrate theoretically and experimentally that the interference of electric and magnetic optically induced modes in individual subwavelength silicon nanodisks can lead to the suppression of resonant backscattering and to enhanced resonant forward scattering of light. To this end we spectrally tune the nanodisks fundamental electric and magnetic resonances with respect to each other by a variation of the nanodisk aspect ratio. This ability to tune two modes of different character within the same nanoparticle provides direct control over their interference, and, in consequence, allows for engineering the particles resonant and off-resonant scattering patterns. Most importantly, measured and numerically calculated transmittance spectra reveal that backward scattering can be suppressed and forward scattering can be enhanced at resonance for the particular case of overlapping electric and magnetic resonances. Our experimental results are in good agreement with calculations based on the discrete dipole approach as well as finite-integral frequency-domain simulations. Furthermore, we show useful applications of silicon nanodisks with tailored resonances as optical nanoantennas with strong unidirectional emission from a dipole source.

Photonic metamaterials by direct laser writing and silver chemical vapour deposition

We fabricate planar magnetic photonic metamaterials via direct laser writing and silver chemical vapor deposition, an approach, which is also suitable for three-dimensional structures. Retrieval of the effective metamaterial parameters reveals the importance of bi-anisotropy.

Enhanced Third-Harmonic Generation in Silicon Nanoparticles Driven by Magnetic Response

We observe enhanced third-harmonic generation from silicon nanodisks exhibiting both electric and magnetic dipolar resonances. Experimental characterization of the nonlinear optical response through third-harmonic microscopy and spectroscopy reveals that the third-harmonic generation is significantly enhanced in the vicinity of the magnetic dipole resonances. The field localization at the magnetic resonance results in two orders of magnitude enhancement of the harmonic intensity with respect to unstructured bulk silicon with the conversion efficiency limited only by the two-photon absorption in the substrate.

Polarization-Independent Silicon Metadevices for Efficient Optical Wavefront Control

We experimentally demonstrate a functional silicon metadevice at telecom wavelengths that can efficiently control the wavefront of optical beams by imprinting a spatially varying transmittance phase independent of the polarization of the incident beam. Near-unity transmittance efficiency and close to 0-2π phase coverage are enabled by utilizing the localized electric and magnetic Mie-type resonances of low-loss silicon nanoparticles tailored to behave as electromagnetically dual-symmetric scatterers. We apply this concept to realize a metadevice that converts a Gaussian beam into a vortex beam. The required spatial distribution of transmittance phases is achieved by a variation of the lattice spacing as a single geometric control parameter.

Ultrafast All-Optical Switching with Magnetic Resonances in Nonlinear Dielectric Nanostructures

We demonstrate experimentally ultrafast all-optical switching in subwavelength nonlinear dielectric nanostructures exhibiting localized magnetic Mie resonances. We employ amorphous silicon nanodisks to achieve strong self-modulation of femtosecond pulses with a depth of 60% at picojoule-per-disk pump energies. In the pump-probe measurements, we reveal that switching in the nanodisks can be governed by pulse-limited 65 fs-long two-photon absorption being enhanced by a factor of 80 with respect to the unstructured silicon film. We also show that undesirable free-carrier effects can be suppressed by a proper spectral positioning of the magnetic resonance, making such a structure the fastest all-optical switch operating at the nanoscale.

Observation of Fano Resonances in All‐Dielectric Nanoparticle Oligomers

It is well-known that oligomers made of metallic nanoparticles are able to support sharp Fano resonances originating from the interference of two plasmonic resonant modes with different spectral width. While such plasmonic oligomers suffer from high dissipative losses, a new route for achieving Fano resonances in nanoparticle oligomers has opened up after the recent experimental observations of electric and magnetic resonances in low-loss dielectric nanoparticles. Here, light scattering by all-dielectric oligomers composed of silicon nanoparticles is studied experimentally for the first time. Pronounced Fano resonances are observed for a variety of lithographically-fabricated heptamer nanostructures consisting of a central particle of varying size, encircled by six nanoparticles of constant size. Based on a full collective mode analysis, the origin of the observed Fano resonances is revealed as a result of interference of the optically-induced magnetic dipole mode of the central particle with the collective mode of the nanoparticle structure. This allows for effective tuning of the Fano resonance to a desired spectral position by a controlled size variation of the central particle. Such optically-induced magnetic Fano resonances in all-dielectric oligomers offer new opportunities for sensing and nonlinear applications.

Active Tuning of All-Dielectric Metasurfaces

All-dielectric metasurfaces provide a powerful platform for highly efficient flat optical devices, owing to their strong electric and magnetic dipolar response accompanied by negligible losses at near-infrared frequencies. Here we experimentally demonstrate dynamic tuning of electric and magnetic resonances in all-dielectric silicon nanodisk metasurfaces in the telecom spectral range based on the temperature-dependent refractive-index change of a nematic liquid crystal. We achieve a maximum resonance tuning range of 40 nm and a pronounced change in the transmittance intensity up to a factor of 5. Strongly different tuning rates are observed for the electric and the magnetic response, which allows for dynamically adjusting the spectral mode separation. Furthermore, we experimentally investigate the influence of the anisotropic (temperature-dependent) dielectric environment provided by the liquid crystal on both the electric and magnetic resonances. We demonstrate that the phase transition of the liquid crystal from its nematic to its isotropic phase can be used to break the symmetry of the optical metasurface response. As such, our approach allows for spectral tuning of electric and magnetic resonances of all-dielectric metasurfaces as well as switching of the anisotropy of the optical response of the device.

Resonantly Enhanced Second-Harmonic Generation Using III–V Semiconductor All-Dielectric Metasurfaces

Nonlinear optical phenomena in nanostructured materials have been challenging our perceptions of nonlinear optical processes that have been explored since the invention of lasers. For example, the ability to control optical field confinement, enhancement, and scattering almost independently allows nonlinear frequency conversion efficiencies to be enhanced by many orders of magnitude compared to bulk materials. Also, the subwavelength length scale renders phase matching issues irrelevant. Compared with plasmonic nanostructures, dielectric resonator metamaterials show great promise for enhanced nonlinear optical processes due to their larger mode volumes. Here, we present, for the first time, resonantly enhanced second-harmonic generation (SHG) using gallium arsenide (GaAs) based dielectric metasurfaces. Using arrays of cylindrical resonators we observe SHG enhancement factors as large as 10(4) relative to unpatterned GaAs. At the magnetic dipole resonance, we measure an absolute nonlinear conversion efficiency of ∼2 × 10(-5) with ∼3.4 GW/cm(2) pump intensity. The polarization properties of the SHG reveal that both bulk and surface nonlinearities play important roles in the observed nonlinear process.

Optical Yagi-Uda nanoantennas

Abstract Conventional antennas, which are widely employed to transmit radio and TV signals, can be used at optical frequencies as long as they are shrunk to nanometer-size dimensions. Optical nanoantennas made of metallic or high-permittivity dielectric nanoparticles allow for enhancing and manipulating light on the scale much smaller than wavelength of light. Based on this ability, optical nanoantennas offer unique opportunities regarding key applications such as optical communications, photovoltaics, nonclassical light emission, and sensing. From a multitude of suggested nanoantenna concepts the Yagi-Uda nanoantenna, an optical analogue of the well-established radio-frequency Yagi-Uda antenna, stands out by its efficient unidirectional light emission and enhancement. Following a brief introduction to the emerging field of optical nanoantennas, here we review recent theoretical and experimental activities on optical Yagi-Uda nanoantennas, including their design, fabrication, and applications. We also discuss several extensions of the conventional Yagi-Uda antenna design for broadband and tunable operation, for applications in nanophotonic circuits and photovoltaic devices.

Electro-optical switching by liquid-crystal controlled metasurfaces

We study the optical response of a metamaterial surface created by a lattice of split-ring resonators covered with a nematic liquid crystal and demonstrate millisecond timescale switching between electric and magnetic resonances of the metasurface. This is achieved due to a high sensitivity of liquid-crystal molecular reorientation to the symmetry of the metasurface as well as to the presence of a bias electric field. Our experiments are complemented by numerical simulations of the liquid-crystal reorientation.