Shaping the third-harmonic radiation from silicon nanodimers

Deterministic Excitation of Polarization-Sensitive Extrinsic Anapole State in Si Nanodisk Clusters

Nanoparticle clusters provide new degrees of freedom for light control due to their mutual interaction compared with an individual one. Here, the authors demonstrate theoretically and experimentally a type of optical anapole (a nonradiating state) termed as extrinsic anapole, with mode field spreading across Si nanodisk dimers unlike the intrinsic one that is confined within individual nanodisks. The extrinsic anapole is sensitive to the polarized excitation. When the electric vector E of excitation is perpendicular to the dimer axis, the coupled toroidal dipole (TD) mode is largely enhanced and broadened to be spectrally overlapped with the electric dipole (ED) mode. The destructive interference of these two modes results in the generation of the extrinsic anapole. However, it vanishes when E is parallel to the dimer axis. Such polarization dependence can be relieved with the participation of the third nanodisk. Scattering spectra of Si nanodisk trimers stay almost unchanged under different polarized excitations, although the near-field distributions are quite different. Finally, enhanced white-light emission is observed in Si nanodisk clusters, which can be attributed to the near-infrared absorption enhancement induced by extrinsic anapole states. The findings manifest that high-index all-dielectric nanodisk clusters are promising for light manipulation based on mode interference.

Third- and Second-Harmonic Generation in All-Dielectric Nanostructures: A Mini Review

Frequency conversion such as harmonic generation is a fundamental physical process in nonlinear optics. The conventional nonlinear optical systems suffer from bulky size and cumbersome phase-matching conditions due to the inherently weak nonlinear response of natural materials. Aiming at the manipulation of nonlinear frequency conversion at the nanoscale with favorable conversion efficiencies, recent research has shifted toward the integration of nonlinear functionality into nanophotonics. Compared with plasmonic nanostructures showing high dissipative losses and thermal heating, all-dielectric nanostructures have demonstrated many excellent properties, including low loss, high damage threshold, and controllable resonant electric and magnetic optical nonlinearity. In this review, we cover the recent advances in nonlinear nanophotonics, with special emphasis on third- and second-harmonic generation from all-dielectric nanoantennas and metasurfaces. We discuss the main theoretical concepts, the design principles, and the functionalities of third- and second-harmonic generation processes from dielectric nanostructures and provide an outlook on the future directions and developments of this research field.

Dielectric optical nanoantennas

Nanophotonics allows the manipulation of light on the subwavelength scale. Optical nanoantennas are nanoscale elements that enable increased resolution in bioimaging, novel photon sources, solar cells with higher absorption, and the detection of fluorescence from a single molecule. While plasmonic nanoantennas have been extensively explored in the literature, dielectric nanoantennas have several advantages over their plasmonic counterparts, including low dissipative losses and near-field enhancement of both electric and magnetic fields. Nanoantennas increase the optical density of states, which increase the rate of spontaneous emission due to the Purcell effect. The increase is quantified by the Purcell factor, which depends on the mode volume and the quality factor. It is one of the main performance parameters for nanoantennas. One particularly interesting feature of dielectric nanoantennas is the possibility of integrating them into optical resonators with a high quality-factor, further improving the performance of the nanoantennas and giving very high Purcell factors. This review introduces the properties and parameters of dielectric optical nanoantennas, and gives a classification of the nanoantennas based on the number and shape of the nanoantenna elements. An overview of recent progress in the field is provided, and a simulation is included as an example. The simulated nanoantenna, a dimer consisting of two silicon nanospheres separated by a gap, is shown to have a very small mode volume, but a low quality-factor. Some recent works on photonic crystal resonators are reviewed, including one that includes a nanoantenna in the bowtie unit-cell. This results in an enormous increase in the calculated Purcell factor, from 200 for the example dimer, to 8 × 10⁶ for the photonic crystal resonator. Some applications of dielectric nanoantennas are described. With current progress in the field, it is expected that the number of applications will grow and that nanoantennas will be incorporated into new commercial products. A list of relevant materials with high refractive indexes and low losses is presented and discussed. Finally, prospects and major challenges for dielectric nanoantennas are addressed.

Dielectric travelling wave antennas for directional light emission

We present a combined experimental and numerical study of the far-field emission properties of optical travelling wave antennas made from low-loss dielectric materials. The antennas considered here are composed of two simple building blocks, a director and a reflector, deposited on a glass substrate. Colloidal quantum dots placed in the feed gap between the two elements serve as internal light source. The emission profile of the antenna is mainly formed by the director while the reflector suppresses backward emission. Systematic studies of the director dimensions as well as variation of antenna material show that the effective refractive index of the director primarily governs the far-field emission pattern. Below cut off, i.e., if the director's effective refractive index is smaller than the refractive index of the substrate, the main lobe results from leaky wave emission along the director. In contrast, if the director supports a guided mode, the emission predominately originates from the end facet of the director.

Tailorable Dynamics in Nonlinear Optical Metasurfaces

Controlling light with light is essential for all-optical switching, data processing in optical communications and computing. Until now, all-optical control of light has relied almost exclusively on nonlinear optical interactions in materials. Achieving giant nonlinearities under low light intensity is essential for weak-light nonlinear optics. In the past decades, such weak-light nonlinear phenomena have been demonstrated in photorefractive and photochromic materials. However, their bulky size and slow speed have hindered practical applications. Metasurfaces, which enhance light-matter interactions at the nanoscale, provide a new framework with tailorable nonlinearities for weak-light nonlinear dynamics. Current advances in nonlinear metasurfaces are introduced, with a special emphasis on all-optical light controls. The tuning of the nonlinearity values using metasurfaces, including enhancement and sign reversal is presented. The tailoring of the transient behaviors of nonlinearities in metasurfaces to achieve femtosecond switching speed is also discussed. Furthermore, the impact of quantum effects from the metasurface on the nonlinearities is introduced. Finally, an outlook on the future development of this energetic field is offered.

From Single-Dimensional to Multidimensional Manipulation of Optical Waves with Metasurfaces

Metasurfaces, 2D artificial arrays of subwavelength elements, have attracted great interest from the optical scientific community in recent years because they provide versatile possibilities for the manipulation of optical waves and promise an effective way for miniaturization and integration of optical devices. In the past decade, the main efforts were focused on the realization of single-dimensional (amplitude, frequency, polarization, or phase) manipulation of optical waves. Compared to the metasurfaces with single-dimensional manipulation, metasurfaces with multidimensional manipulation of optical waves show significant advantages in many practical application areas, such as optical holograms, sub-diffraction imaging, and the design of integrated multifunctional optical devices. Nowadays, with the rapid development of nanofabrication techniques, the research of metasurfaces has been inevitably developed from single-dimensional manipulation toward multidimensional manipulation of optical waves, which greatly boosts the application of metasurfaces and further paves the way for arbitrary design of optical devices. Herein, the recent advances in metasurfaces are briefly reviewed and classified from the viewpoint of different dimensional manipulations of optical waves. Single-dimensional manipulation and 2D manipulation of optical waves with metasurfaces are discussed systematically. In conclusion, an outlook and perspectives on the challenges and future prospects in these rapidly growing research areas are provided.

Nonlinear Diffraction in Asymmetric Dielectric Metasurfaces

Metasurfaces provide new and promising mechanisms with which to control and manipulate light at the nanoscale. While most metasurfaces are designed to operate in the linear regime, it was recently shown that such metasurfaces may also generate nonlinear signals by manipulation of the higher-order susceptibility terms. As such, metasurfaces can generate additional harmonics without the need for light propagation, as typically occurs in nonlinear crystals. While such demonstrations typically rely on the nonlinear properties of metals, we hereby report the design, fabrication, and experimental characterization of a resonant dielectric metasurface made of amorphous silicon to create and manipulate second harmonic light and control its diffraction patterns. As shown in the paper, the second harmonic generation of light follows selection rules that rely on the asymmetry of the meta-atom. Given the fact that silicon crystals are centrosymmetric, the generation of the second harmonic signal in amorphous silicon is intriguing. In fact, the second harmonic signal is generated mostly from the surface of the meta-atom. It is the use of nanostructures that increases the surface-to-volume ratio and enables second harmonic generation. Additionally, the meta-atom is designed to exploit its spectral resonances in the principal and the second harmonic frequencies for providing electromagnetic field enhancement, which assists in boosting the generation of second harmonic signals.

Directional second-harmonic generation controlled by sub-wavelength facets of an organic mesowire

Directional harmonic generation is an important property characterizing the ability of nonlinear optical antennas to diffuse the signal in a well-defined region of space. Herein, we show how sub-wavelength facets of an organic molecular mesowire crystal can be utilized to systematically vary the directionality of second-harmonic generation (SHG) in the forward-scattering geometry. We demonstrate this capability on crystalline diamonoanthraquinone (DAAQ) mesowires with sub-wavelength facets. We observed that the radial angles of the SHG emission can be tuned over a range of 130 deg. This angular variation arises due to spatially distributed nonlinear dipoles in the focal volume of the excitation as well as the geometrical cross section and facet orientation of the mesowire. Numerical simulations of the near-field excitation profile corroborate the role of the mesowire geometry in localizing the electric field. In addition to directional SHG from the mesowire, we experimentally observe optical waveguiding of the nonlinear two-photon excited fluorescence (TPEF). Interestingly, we observed that for a given pump excitation, the TPEF signal is isotropic and delocalized, whereas the SHG emission is directional and localized at the location of excitation. All the observed effects have direct implications not only in active nonlinear optical antennas but also in nonlinear signal processing.

Nonlinear Wavefront Control with All-Dielectric Metasurfaces

Metasurfaces, two-dimensional lattices of nanoscale resonators, offer unique opportunities for functional flat optics and allow the control of the transmission, reflection, and polarization of a wavefront of light. Recently, all-dielectric metasurfaces reached remarkable efficiencies, often matching or out-performing conventional optical elements. The exploitation of the nonlinear optical response of metasurfaces offers a paradigm shift in nonlinear optics, and dielectric nonlinear metasurfaces are expected to enrich subwavelength photonics by enhancing substantially nonlinear response of natural materials combined with the efficient control of the phase of nonlinear waves. Here, we suggest a novel and rather general approach for engineering the wavefront of parametric waves of arbitrary complexity generated by a nonlinear metasurface. We design all-dielectric nonlinear metasurfaces, achieve a highly efficient wavefront control of a third-harmonic field, and demonstrate the generation of nonlinear beams at a designed angle and the generation of nonlinear focusing vortex beams. Our nonlinear metasurfaces produce phase gradients over a full 0-2π phase range with a 92% diffraction efficiency.

All-silicon-based nano-antennas for wavelength and polarization demultiplexing

We propose an all-silicon-based nano-antenna that functions as not only a wavelength demultiplexer but also a polarization one. The nano-antenna is composed of two silicon cuboids with the same length and height but with different widths. The asymmetric structure of the nano-antenna with respect to the electric field of the incident light induced an electric dipole component in the propagation direction of the incident light. The interference between this electric dipole and the magnetic dipole induced by the magnetic field parallel to the long side of the cuboids is exploited to manipulate the radiation direction of the nano-antenna. The radiation direction of the nano-antenna at a certain wavelength depends strongly on the phase difference between the electric and magnetic dipoles interacting coherently, offering us the opportunity to realize wavelength demultiplexing. By varying the polarization of the incident light, the interference of the magnetic dipole induced by the asymmetry of the nano-antenna and the electric dipole induced by the electric field parallel to the long side of the cuboids can also be used to realize polarization demultiplexing in a certain wavelength range. More interestingly, the interference between the dipole and quadrupole modes of the nano-antenna can be utilized to shape the radiation directivity of the nano-antenna. We demonstrate numerically that radiation with adjustable direction and high directivity can be realized in such a nano-antenna which is compatible with the current fabrication technology of silicon chips.

High-contrast and reversible scattering switching via hybrid metal-dielectric metasurfaces

Novel types of optical hybrid metasurfaces consist of metallic and dielectric elements are designed and proposed for controlling the interference between magnetic and electric modes of the system, in a reversible manner. By employing the thermo-optical effect of silicon and gold nanoantennas we demonstrate an active control on the excitation and interference between electric and magnetic modes, and subsequently, the Kerker condition, as a directive radiation pattern with zero backscattering, via temperature control as a versatile tool. This control allows precise tuning optical properties of the system and stimulating switchable sharp spectral Fano-like resonance. Furthermore, it is shown that by adjusting the intermediate distance between metallic and dielectric elements, opposite scattering directionalities are achievable in an arbitrary wavelength. Interestingly, this effect is shown to have a direct influence on nonlinear properties, too, where 10-fold enhancement in the intensity of third harmonic light can be obtained for this system, via heating. This hybrid metasurface can find a wide range of applications in slow light, nonlinear optics and bio-chemical sensing.

Boosting third-harmonic generation by a mirror-enhanced anapole resonator

We demonstrate that a dielectric anapole resonator on a metallic mirror can enhance the third harmonic emission by two orders of magnitude compared to a typical anapole resonator on an insulator substrate. By employing a gold mirror under a silicon nanodisk, we introduce a novel characteristic of the anapole mode through the spatial overlap of resonantly excited Cartesian electric and toroidal dipole modes. This is a remarkable improvement on the early demonstrations of the anapole mode in which the electric and toroidal modes interfere off-resonantly. Therefore, our system produces a significant near-field enhancement, facilitating the nonlinear process. Moreover, the mirror surface boosts the nonlinear emission via the free-charge oscillations within the interface, equivalent to producing a mirror image of the nonlinear source and the pump beneath the interface. We found that these improvements result in an extremely high experimentally obtained efficiency of 0.01%.

Efficient Third Harmonic Generation from Metal-Dielectric Hybrid Nanoantennas

High refractive index dielectric nanoantennas are expected to become key elements for nonlinear nano-optics applications due to their large nonlinearities, low energy losses, and ability to produce high electric field enhancements in relatively large nanoscale volumes. In this work, we show that the nonlinear response from a high-index dielectric nanoantenna can be significantly improved by adding a metallic component to build a metal-dielectric hybrid nanostructure. We demonstrate that the plasmonic resonance of a Au nanoring can boost the anapole mode supported by a Si nanodisk, strongly enhancing the electric field inside the large third-order susceptibility dielectric. As a result, a high third harmonic conversion efficiency, which reaches 0.007% at a third harmonic wavelength of 440 nm, is obtained. In addition, by suitably modifying geometrical parameters of the hybrid nanoantenna, we tune the enhanced third harmonic emission throughout the optical regime. Coupling metallic and dielectric nanoantennas to expand the potential of subwavelength structures opens new paths for efficient nonlinear optical effects in the visible range on the nanoscale.