Katie E. Chong

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.

Multifold Enhancement of Third-Harmonic Generation in Dielectric Nanoparticles Driven by Magnetic Fano Resonances

Strong Mie-type magnetic dipole resonances in all-dielectric nanostructures provide novel opportunities for enhancing nonlinear effects at the nanoscale due to the intense electric and magnetic fields trapped within the individual nanoparticles. Here we study third-harmonic generation from quadrumers of silicon nanodisks supporting high-quality collective modes associated with the magnetic Fano resonance. We observe nontrivial wavelength and angular dependencies of the generated harmonic signal featuring a multifold enhancement of the nonlinear response in oligomeric systems.

Active Tuning of Spontaneous Emission by Mie-Resonant Dielectric Metasurfaces

Mie-resonant dielectric metasurfaces offer comprehensive opportunities for the manipulation of light fields with high efficiency. Additionally, various strategies for the dynamic tuning of the optical response of such metasurfaces were demonstrated, making them important candidates for reconfigurable optical devices. However, dynamic control of the light-emission properties of active Mie-resonant dielectric metasurfaces by an external control parameter has not been demonstrated so far. Here, we experimentally demonstrate the dynamic tuning of spontaneous emission from a Mie-resonant dielectric metasurface that is situated on a fluorescent substrate and embedded into a liquid crystal cell. By switching the liquid crystal from the nematic state to the isotropic state via control of the cell temperature, we induce a shift of the spectral position of the metasurface resonances. This results in a change of the local photonic density of states, which, in turn, governs the enhancement of spontaneous emission from the substrate. Specifically, we observe spectral tuning of both the electric and magnetic dipole resonances, resulting in a 2-fold increase of the emission intensity at λ ≈ 900 nm. Our results demonstrate a viable strategy to realize flat tunable light sources based on dielectric metasurfaces.

Refractive index sensing with Fano resonances in silicon oligomers

We demonstrate experimentally refractive index sensing with localized Fano resonances in silicon oligomers, consisting of six disks surrounding a central one of slightly different diameter. Owing to the low absorption and narrow Fano-resonant spectral features appearing as a result of the interference of the modes of the outer and the central disks, we demonstrate refractive index sensitivity of more than 150 nm RIU−1 with a figure of merit of 3.8. This article is part of the themed issue ‘New horizons for nanophotonics’.

Emission enhancement from MoS 2 monolayers with silicon nanoantennas

Transition-metal-dichalcogenides (TMDs), which exhibit an indirect electronic band gap as bulk crystals, can become direct semiconductors in the monolayer phase [1]. Such monolayer TMDs show unique optical properties arising from the strong two-dimensional confinement of excitons as well as from the reduction in crystal symmetry. However, the strong mismatch in length scale between the sub-nanometer thickness of an atomically thin crystal sheet and the wavelength of propagating infrared or visible light leads to a rather weak light-matter interaction. By tailoring the near-field environment of monolayer TMDs, resonant optical antennas can strongly modify the excitation response [2]. While research efforts targeted at tailoring and enhancing light-matter interactions in monolayer TMDs have so far been limited to plasmonic nanoantennas, here we concentrate on high-index dielectric nanoantennas, which can show negligible intrinsic losses and thus a high radiation efficiency in the visible and near-infrared spectral range.

Spatial and spectral tailoring of visible light emission with mie resonances in silicon nanoantenna arrays

Similar to their plasmonic counterparts, dielectric nanoantennas have the ability to manipulate the emission properties of nanoscale emitters placed in their vicinity. Most importantly, they can increase the radiative decay rate of emitters by coupling to Mie-type resonances and/or shape the emission into directional patterns [1]. While considerable theoretical work has been dedicated to the study of coupled system consisting of Mie-resonant dielectric nanoantennas and nanoemitters [2, 3], experimental realizations of such systems are sparse due to the difficulty of integrating suitable nanoemitters with designed dielectric nanoantennas in a defined way [4]. Here we experimentally investigate visible light emission from square arrays (lattice constant is 560 nm) of silicon nanoantennas exhibiting dipolar magnetic Mie-type resonances. The nanoantennas are fabricated by electron beam-lithography on thin-films of hydrogenated amorphous silicon (a-Si:H) on a glass substrate. The unstructured wafers show an intrinsic photoluminescence (PL) centered at 740 nm. A scanning-electron micrograph (SEM) of a typical sample is shown in Fig. 1 (a).

Ultra-sensitive biosensing with dielectric nanoantennas

We demonstrate new direction in biosensing based on bio-compatible, non-toxic, robust and low-loss dielectric nanoantennas. Using biotin-coated silicon nanodisks with optically-induced magnetic resonances we detect streptavidin with concentration as low as 10−10 mol/L.