Silicon Nanodisks Research Articles

Broadband Active Control of Transverse Scattering from All-Dielectric Nanoparticle

Steering electromagnetic scattering by subwavelength objects is usually accompanied by the excitation of electric and magnetic modes. The Kerker effect, which relies on the precise overlapping between electric and magnetic multipoles, is a potential approach to address this challenge. However, fundamental limitations on the reconfigurability and tunability challenge their future implementation in practical applications. Here, we demonstrate a design approach by applying coherent control to a silicon nanodisk. By utilizing an experimentally feasible two-wave excitation, this coherent light-by-light control enables a highly reconfigurable, broadband, and tunable transverse scattering, extending the feasibility of unidirectional scattering in various practical scenarios, including on-chip integrations and optical communications.

All-dielectric chiral-field-enhanced Raman optical activity

Raman optical activity (ROA) is effective for studying the conformational structure and behavior of chiral molecules in aqueous solutions and is advantageous over X-ray crystallography and nuclear magnetic resonance spectroscopy in sample preparation and cost performance. However, ROA signals are inherently minuscule; 3–5 orders of magnitude weaker than spontaneous Raman scattering due to the weak chiral light–matter interaction. Localized surface plasmon resonance on metallic nanoparticles has been employed to enhance ROA signals, but suffers from detrimental spectral artifacts due to its photothermal heat generation and inability to efficiently transfer and enhance optical chirality from the far field to the near field. Here we demonstrate all-dielectric chiral-field-enhanced ROA by devising a silicon nanodisk array and exploiting its dark mode to overcome these limitations. Specifically, we use it with pairs of chemical and biological enantiomers to show >100x enhanced chiral light–molecule interaction with negligible artifacts for ROA measurements.

Hyper-Doped Silicon Nanoantennas and Metasurfaces for Tunable Infrared Plasmonics

We present the experimental realization of ordered arrays of hyper-doped silicon nanodisks, which exhibit a localized surface plasmon resonance. The plasmon is widely tunable in a spectral window between 2 and 5 μm by adjusting the free carrier concentration between 1020 and 1021 cm–3. We show that strong infrared light absorption can be achieved with all-silicon plasmonic metasurfaces employing nanostructures with dimensions as low as 100 nm in diameter and 23 nm in height. Our numerical simulations show an excellent agreement with the experimental data and provide physical insights on the impact of the nanostructure shape as well as of near-field effects on the optical properties of the metasurface. Our results open highly promising perspectives for integrated all-silicon-based plasmonic devices for instance for chemical or biological sensing or for thermal imaging.

Near-field nonlinear imaging of an anapole mode beyond diffraction limit.

Nonlinear nanophotonics, as an emerging field in nanophotonics, eagerly calls for experimental techniques for probing and analyzing near-field nonlinear optical signals with subwavelength resolution. Here, we report an aperture-type scanning near-field optical microscopic method for probing near-field nonlinear optical processes. As a demonstration, near-field third-harmonic generation from an anapole dark-mode state generated by a silicon nanodisk is probed and imaged. The measured results agree well with the simulations, with a spatial resolution down to $0.14{\lambda _0}$ and a sensitivity of 0.1 nW. This method provides a powerful tool for characterizing nonlinear light-matter interactions at the nanoscale, which can help, for example, to unveil crystal properties involving subwavelength defects or dislocations.

All-Dielectric Metasurface Refractive Index Sensor with Microfluidics

Magnetic resonance and electrical resonance can be excited simultaneously by the incident light source in high refractive index all-dielectric metasurfaces. They can manipulate light waves to a certain extent and can be used in many fields. Silicon is widely used in all-dielectric metasurfaces due to its high refractive index and low loss in near infrared range. In this paper, we use silicon nanodisk structure as refractive index sensor and PDMS microfluidic channels as detection channels to realize liquid refractive index sensing. The magnetic resonance peak of the nanodisk structure is selected for sensing. The position of the magnetic resonance is shifted in the transmission spectrum with the change of the refractive index of the surrounding environment. When the refractive index increases, the resonance peak moves towards the long-wave direction, resulting in red shift, and vice versa. In this paper, the shift of resonance peak reaches 230 nm RIU−1.

Double-layer metasurface for enhanced photon up-conversion

We present a double-layer dielectric metasurface obtained by stacking a silicon nanodisk array and a silicon photonic crystal slab with equal periodicity on top of each other. We focus on the investigation of electric near-field enhancement effects occurring at resonant excitation of the metasurface and study its optical properties numerically and experimentally. We find that the major difference in multi-layer metasurfaces when compared to conventional single-layer structures appears to be in Rayleigh–Wood anomalies: they are split into multiple different modes, which are themselves spectrally broadened. As a proof of concept, we cover a double-layer metasurface with a lanthanide-doped up-conversion particle layer and study its interaction with a 1550 nm photoexcitation. We observe a 2.7-fold enhanced up-conversion photoluminescence by using the stacked metasurface instead of a planar substrate, although only around 1% of the up-conversion material is exposed to enhanced near fields. Two mechanisms are identified explaining this behavior: First, enhanced near fields when exciting the metasurface resonantly, and second, light trapping by total internal reflection in the particle layer when the metasurface redirects light into high angle diffraction orders. These results pave the way for low-threshold and, in particular, broadband photon up-conversion in future solar energy and biosensing applications.

Angular dependence and absorption properties of the anapole mode of Si nano-disks

The polarization- and angle-resolved optical responses of the anapole mode in silicon nano-disks array have been experimentally and theoretically investigated. The good agreement between measured data and simulations yields to a consistent description of the anapole mode behavior that exhibits different features for TE or TM polarization excitation. Scattering matrix calculation allows us to disentangle scattered and diffused light contributions and to provide a quantitative estimation of the absorbance enhancement associated with 2D excitation of the anapole mode. We performed the multipolar decomposition of the far-field scattered radiation for both TE and TM polarizations and unambiguously identified the anapole resonant condition in excellent agreement with the experimental results over a large range of incident angles. Our findings demonstrate the controlled excitation of electromagnetic anapole modes in engineered arrays of silicon nano-disks for the development of optical nanostructures with enhanced light–matter interaction.

Constructive and destructive interference of Kerker-type scattering in an ultrathin silicon Huygens metasurface

High refractive index dielectric nanoparticles have provided a new platform for exotic light manipulation through the interference of multipole modes. The Kerker effect is one example of a Huygens source design. Rather than exploiting interference between the electric dipole and magnetic dipole, as in many conventional Huygens source designs, we explore Kerker-type suppressed backward scattering mediated by the dominant electric dipole, toroidal dipole and magnetic quadrupole. These modes are provided by a designed and fabricated CMOS compatible ultra-thin Silicon nanodisk metasurface with a suppressed magnetic dipole contribution, and verified through multipole decomposition. The non-trivial substrate effect is considered using a semi-analytical transfer matrix model. The model successfully predicts the observed reflection dip. By applying a general criterion for constructive and destructive interference, it is shown that while constructive interference occurs between the electric and toroidal dipole contributions, the experimentally observed suppressed backward Kerker-type scattering arises from the destructive interference between backward scattered contributions due to the total electric dipole and the magnetic quadrupole. Our study paves the way towards new types of Huygens sources or metasurface design, such as for peculiar transverse Kerker scattering.

Controlling light absorption of graphene at critical coupling through magnetic dipole quasi-bound states in the continuum resonance

Enhancing the light-matter interaction in two-dimensional (2D) materials with high-$Q$ resonances in photonic structures has boosted the development of optical and photonic devices. Herein, we intend to build a bridge between the radiation engineering and the bound states in the continuum (BIC), and present a general method to control light absorption at critical coupling through the quasi-BIC resonance. In a single-mode two-port system composed of graphene coupled with silicon nanodisk metasurfaces, the maximum absorption of 0.5 can be achieved when the radiation rate of the magnetic dipole resonance equals to the dissipate loss rate of graphene. Furthermore, the absorption bandwidth can be adjusted more than two orders of magnitude from 0.9 nm to 94 nm by simultaneously changing the asymmetric parameter of metasurfaces, the Fermi level and the layer number of graphene. This work reveals out the essential role of BIC in radiation engineering and provides promising strategies in controlling light absorption of 2D materials for the next-generation optical and photonic devices, e.g., light emitters, detectors, modulators, and sensors.

Fluorescence-Detected Circular Dichroism of a Chiral Molecular Monolayer with Dielectric Metasurfaces.

Strong enhancement of molecular circular dichroism (CD) has the potential to enable efficient asymmetric photolysis, a method of chiral separation that has conventionally been impeded by insufficient yield and low enantiomeric excess. Here, we study experimentally how predicted enhancements in optical chirality density near resonant silicon nanodisks boost CD. We use fluorescence-detected circular dichroism (FDCD) spectroscopy to measure indirectly the differential absorption of circularly polarized light by a monolayer of optically active molecules functionalized to silicon nanodisk arrays. Importantly, the molecules and nanodisk antennas have spectrally coincident resonances, and our fluorescence technique allows us to deconvolute absorption in the nanodisks from the molecules. We find that enhanced FDCD signals depend on nanophotonic resonances, in good agreement with simulated differential absorption and optical chirality density, while no signal is detected from molecules adsorbed on featureless silicon surfaces. These results verify the potential of nanophotonic platforms to be used for asymmetric photolysis with lower energy requirements.

Coupled Toroidal Dipole Modes in Silicon Nanodisk Metasurface: Polarization Independent Narrow Band Absorption and Directional Emission

AbstractBesides the fundamental Mie resonance modes of high‐index dielectric nanostructures, an extended multipole family, i.e., toroidal multipoles, provide great opportunities for engineering a new class of nanophotonic devices. Here, it is demonstrated theoretically and experimentally that a hexagonal array of circular silicon (Si) nanodisks (NDs) supports intra‐ and inter‐ND toroidal dipole (TD) modes. The key concept is the alignment of Si NDs near the anapole state condition for constructive interaction of the TD modes between neighboring NDs. Numerical simulation reveals that strong field confinement by the coupled TD modes enhances the absorption 120‐fold at the resonance wavelength compared to that of a flat Si film with the same thickness. In this study, macroscopic scale hexagonal Si ND arrays are fabricated using a colloidal lithography, and spectrally selective absorption enhancement due to the TD modes is demonstrated. Furthermore, it is demonstrated that a dipolar emitter can also excite TD modes, which modify the spectral shape and the directionality of the emission.

Nonlocal response of Mie-resonant dielectric particles

Mie-resonant high-index dielectric particles are at the core of modern all-dielectric photonics. In many situations, their response to the external fields is well-captured by the dipole model which neglects the excitation of higher-order multipoles. In that case, it is commonly assumed that the dipole moments induced by the external fields are given by the product of particle polarizability tensor and the field in the particle center. Here, we demonstrate that the dipole response of non-spherical subwavelength dielectric particles is significantly more complex since the dipole moments are defined not only by the field in the particle center but also by the second-order spatial derivatives of the field. As we prove, such nonlocal response is especially pronounced in the vicinity of anapole minimum in the scattering cross-section. We examine the excitation of high-index dielectric disk in microwave domain and silicon nanodisk in near infrared applying group-theoretical analysis and retrieving the nonlocal corrections to the dipole moments. Extending the discrete dipole model to include nonlocality of the dipole response, we demonstrate an improved agreement with full-wave numerical simulations. These results provide important insights into meta-optics of Mie-resonant non-spherical particles as well as metamaterials and metadevices based on them.

Efficient Third Harmonic Generation by Doubly Enhanced Electric Dipole Resonance in Metal-Based Silicon Nanodisks

Mie-type electric dipole resonance (EDR) is a commonly feasible Mie-type resonance in all-dielectric metasurfaces. However, conventional EDR suffers from the weak field enhancement and the poor field confinement inside the dielectric, resulting in an inefficient third harmonic generation (THG). Here, by presenting a bottom metal film and manipulating the array period, an effective perfect electric conductor (PEC) mirror effect and a novel coupling effect, as well as their successive combination, are proposed to doubly enhance the EDRs in silicon nanodisks and thus their THG at near-infrared. Numerical frequency and time domain responses demonstrate that the total THG conversion efficiency assisted by doubly enhanced EDR is raised by more than three and eight orders of magnitude compared to those of EDRs with only PEC effect and with a SiO $_2$ substrate, respectively. Furthermore, silicon Kerr effect, describing the field-dependent refractive index with third-order susceptibility, is analyzed under increasing pump intensity. An unprecedented efficiency ∼ $10^{-2}$ under pump intensity 2 GW/cm $^2$ is achieved despite the Kerr effect. This work paves a new way for engineering and boosting the Mie-type EDR, and facilitates its practical applications in quantum sources, spectroscopic and biochemical sensing.

Time-varying optical vortices enabled by time-modulated metasurfaces

Abstract In this paper, generation of optical vortices with time-varying orbital angular momentum (OAM) and topological charge is theoretically demonstrated based on time-modulated metasurfaces with a linearly azimuthal frequency gradient. The topological charge of such dynamic structured light beams is shown to continuously and periodically change with time evolution while possessing a linear dependence on time and azimuthal frequency offset. The temporal variation of OAM yields a self-torqued beam exhibiting a continuous angular acceleration of light. The phenomenon is attributed to the azimuthal phase gradient in space-time generated by virtue of the spatiotemporal coherent path in the interference between different frequencies. In order to numerically authenticate this newly introduced concept, a reflective dielectric metasurface is modelled consisting of silicon nanodisk heterostructures integrated with indium-tin-oxide and gate dielectric layers on top of a mirror-backed silicon slab which renders an electrically tunable guided mode resonance mirror in near-infrared regime. The metasurface is divided into several azimuthal sections wherein nanodisk heterostructures are interconnected via nanobars serving as biasing lines. Addressing azimuthal sections with radio-frequency biasing signals of different frequencies, the direct dynamic photonic transitions of leaky-guided modes are leveraged for realization of an azimuthal frequency gradient in the optical field. Generation of dynamic twisted light beams with time-varying OAM by the metasurface is verified via performing several numerical simulations. Moreover, the role of modulation waveform and frequency gradient on the temporal evolution and diversity of generated optical vortices is investigated which offer a robust electrical control over the number of dynamic beams and their degree of self-torque. Our results point toward a new class of structured light for time-division multiple access in optical and quantum communication systems as well as unprecedented optomechanical manipulation of objects.

Multipole expansion of periodic discrete dipole approximation and its application to subwavelength dielectric arrays

Light-matter interaction in all-dielectric nanoparticles has attracted more attention because of the ability to control and configure light scattering on high-index nanoparticles. In the framework of the discrete dipole approximation, we theoretically applied cartesian multipole expansion to a periodic array of nanoparticles. This allows to analyze the contribution of multipole moments, including toroidal moments, in the scattering spectra of an array of infinite 1D/2D periodic structure of arbitrary shaped nanoparticles. The formulation is based on the Taylor expansion in cartesian coordinate system up to the fifth-order that includes a magnetic 16-pole and an electric 32-pole. Examples of a wide-band reflector and a wide-band transmitter made of silicon nanodisks are studied at IR wavelengths. The results show how constructive and destructive interferences of multipole terms affect the scattering field. The proposed method paves the way to design and analyze novel types of nanoscale subwavelength metasurfaces.

Progressive Self-Boosting Anapole-Enhanced Deep-Ultraviolet Third Harmonic During Few-Cycle Laser Radiation

Nanoantennas made of high-index semiconductors with a strong nonlinearity and supported optical Mie-type resonances offer a promising alternative platform for nonlinear nanophotonics. In this Letter, we employ an array of amorphous silicon nanodisks with varying diameters to produce a broadband deep-ultraviolet third harmonic of a few-cycle Ti:sapphire oscillator. Ultrashort light pulses efficiently deposit their energy at the center of the disks where the electric field is strongly amplified by the anapole states. This leads to a progressive material modification in an extreme multishot (>1010 pulses) and a rather low fluence (<10–3 J/cm2) regime, drastically differing from other known mechanisms, such as nonthermal plasma annealing or thermal melting-induced recrystallization. We suggest that the material modification is due to femtosecond laser-induced excitation of dangling bonds, which leads to a gradual boosting of the third harmonic conversion efficiency and broadening of its spectral bandwidth.

Anapole mediated giant photothermal nonlinearity in nanostructured silicon

Featured with a plethora of electric and magnetic Mie resonances, high index dielectric nanostructures offer a versatile platform to concentrate light-matter interactions at the nanoscale. By integrating unique features of far-field scattering control and near-field concentration from radiationless anapole states, here, we demonstrate a giant photothermal nonlinearity in single subwavelength-sized silicon nanodisks. The nanoscale energy concentration and consequent near-field enhancements mediated by the anapole mode yield a reversible nonlinear scattering with a large modulation depth and a broad dynamic range, unveiling a record-high nonlinear index change up to 0.5 at mild incident light intensities on the order of MW/cm2. The observed photothermal nonlinearity showcases three orders of magnitude enhancement compared with that of unstructured bulk silicon, as well as nearly one order of magnitude higher than that through the radiative electric dipolar mode. Such nonlinear scattering can empower distinctive point spread functions in confocal reflectance imaging, offering the potential for far-field localization of nanostructured Si with an accuracy approaching 40 nm. Our findings shed new light on active silicon photonics based on optical anapoles.

Optical analog computing of spatial differentiation and edge detection with dielectric metasurfaces.

We propose and demonstrate that optical analog computing of spatial differentiation and edge detection can be realized with a single layer of dielectric metasurface. The optical transfer function for second-order derivation is obtained by engineering the spatial dispersion of electric dipole resonance supported by the silicon nanodisks in the metasurface. Benefiting from this unique mechanism of electric dipole resonance, spatial differentiation can be performed for two dimensions and arbitrary polarization with a large spatial bandwidth and high efficiency at the visible wavelength. Explicitly, we have numerically validated the application with one-dimensional spatial functions as well as an image, and the results show excellent performance. Our study can facilitate the research of optical computing with artificial nanostructures.

Topological Space‐Time Photonic Transitions in Angular‐Momentum‐Biased Metasurfaces

AbstractIn this paper, topological space‐time photonic transition of light in angular‐momentum‐biased metasurfaces is established, which yields a superposition of orbital‐angular‐momentum (OAM)‐carrying beams at distinct frequency harmonics upon scattering whose topological charges and frequency shifts are correlated. A reflective dielectric metasurface is considered that consists of silicon nanodisk heterostructures integrated with indium‐tin‐oxide and gate dielectric layers placed on a back mirror forming a dual‐gated field effect modulator. The metasurface is divided into several azimuthal sections wherein the nanodisk heterostructures are interconnected via biasing lines. Addressing each azimuthal section with radio‐frequency biasing signals separately allows for flexible implementation of different spatiotemporal modulation profiles. Active tuning of OAM states with high mode‐purity is demonstrated, which yields minimal cross‐talk between OAM channels in a mode‐multiplexed communication system. Moreover, the role of spatiotemporal modulation profile on the spectral and spatial diversity of OAM states is explored. It is shown that angular‐momentum‐biased metasurfaces allow opportunities for hybridized mode‐division and wavelength‐division multiple access through multiplexing and multicasting across distinct OAM states and wavelengths. The nonreciprocity of topological space‐time photonic transitions across the temporal frequency domain and Hilbert space of OAM states is also investigated giving rise to distinct twisted light channels in up‐ and down‐links.

Excitation of Nonradiating Anapoles in Dielectric Nanospheres.

Although the study of nonradiating anapoles has long been part of fundamental physics, the dynamic anapole at optical frequencies was only recently experimentally demonstrated in a specialized silicon nanodisk structure. We report excitation of the electrodynamic anapole state in isotropic silicon nanospheres using radially polarized beam illumination. The superposition of equal and out-of-phase amplitudes of the Cartesian electric and toroidal dipoles produces a pronounced dip in the scattering spectra with the scattering intensity almost reaching zero-a signature of anapole excitation. The total scattering intensity associated with the anapole excitation is found to be more than 10 times weaker for illumination with radially vs linearly polarized beams. Our approach provides a simple, straightforward alternative path to realizing nonradiating anapole states at the optical frequencies.