Symmetry‐protected quasi‐bound states in the continuum (BIC) controlled by metasurfaces with broken in‐plane symmetry are widely exploited to achieve highly surface‐sensitive and spectrally sharp ...resonances for nanophotonic biosensors. Through the engineering of silicon‐based asymmetric nanobar pairs, a quasi‐BIC mode is excited showing a dominant toroidal dipole (TD) and electric quadrupole (EQ) resonant feature in the near‐infrared and performs ultrahigh sensitivity in the refractometric monitoring of local environment changes. Contrary to the typical electric and magnetic Mie‐type resonances of dielectric resonators with the enhanced field mostly inside the resonator volume, the TD‐EQ quasi‐BIC mode is found to exhibit strong and tightly confined optical fields at the surface of tilted nanobar pairs, and its refractive‐index (RI) sensitivity can be dramatically increased for nanopillars with larger aspect‐ratio. The measured (simulated) sensitivity and figure of merit for nanobar pairs with a height of 450 nm reach 608 nm/RIU and 46 (612 nm/RIU and 85), respectively. Such ultrahigh sensitive all‐dielectric platform can be fabricated through complementary metal‐oxide‐semiconductor compatible process and is promising for on‐chip integration and sensor miniaturization to a wide range of diagnostic applications.
Quasi‐bound states in the continuum modes in periodic amorphous silicon tilted nanobar pairs are found to exhibit a dominant toroidal dipole and electric quadrupole resonant property. Such higher‐order resonant mode exhibits intriguing near‐field distributions accompanied by a strong electric field tightly confined at the surface of nanobar width and results in ultrahigh refractive index sensitivity for high aspect‐ratio nanopillars.
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Dynamic toroidal dipole (TD) with its peculiar characteristic of broken space‐inversion and time‐reversal symmetries plays an important role in the fundamental physics of light–matter interaction. ...Here, TD metamaterials comprised of amorphous silicon nanopillar arrays embedded in spin‐on‐glass layer are experimentally demonstrated. Upon normal incidence of plane wave, the transverse toroidal moment and the associated anapole‐like state are excited in optical regime. The strong TD response stems from a complete head‐to‐tail configuration of the magnetic dipole moments within each individual nanopillar. Both the experimental and simulation results show that such TD mode sustains a large structural tolerance and can be spectrally tuned by elongating the cylindrical axis perpendicular to the light polarization, corresponding to a cross‐sectional variation from circular to elliptical shapes. The excited TD mode is found to exhibit ultrahigh refractive index sensitivity compared to other multipoles, resulting in a sensitivity of 459 nm (470 nm) per external refractive index change in the experiment (calculation). This approach provides a simple and straightforward path in realizing toroidal metamaterials and establishes a new flat‐optics platform for realizing active metadevices, sensors, and nonlinear nanophotonics.
Optical toroidal dipole (TD) and anapole metamaterials are realized in periodic amorphous silicon nanopillars embedded in a spin‐on glass layer upon normal incidence of plane wave. The TD response can be enhanced and spectrally tuned by elongating cylindrical axis perpendicular to the light polarization and exhibits ultrahigh refractive index sensitivity compared to other multipoles.
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Abstract
The abundant multipolar resonances in all‐dielectric metasurfaces provide a new paradigm to simultaneously induce strong near‐field confinement in the interior of a nanocavity as well as to ...manipulate the far‐field scattering property, which is beneficial for the enhancement of nonlinear effects. Here, third‐harmonic generation (THG) of all‐dielectric silicon metasurfaces that sustain dominant electric dipole (ED), toroidal dipole (TD), and magnetic dipole (MD) moments in near‐infrared is numerically and experimentally studied. The effect of the interplay of these resonant modes on THG is investigated, and a pronounced THG enhancement is observed when these modes become spectrally overlapped, corresponding to the generalized Kerker condition. The constructive interference of the total electric dipole (refers to the summation of the ED and TD scattered fields) and MD modes results in the suppression of the backward scattering along with a strong local‐field enhancement inside the dielectric resonators. The simulation (experimental) results show a 214‐fold (17‐fold) THG enhancement in the vicinity of the generalized Kerker condition compared with the signals of the spectrally separated TD and MD resonances. The silicon‐based metasurfaces with their simple geometry are facile for large‐area fabrication and open new possibilities for the optimization of upconversion processes to achieve efficient nonlinear devices.
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Generating ultrafast pulses with better spectrotemporal control is crucial for optimizing and characterizing nonlinear light–matter responses, yet it is limited by the gain bandwidth of laser media ...or the phase‐matching geometry of nonlinear processes. This work proposes a simple approach to independently manage a femtosecond source's spectral location and bandwidth. Self‐phase‐modulation‐enabled spectral broadening is first analyzed, which is potentially energy‐scalable using hollow‐core capillaries or multipass cells. It is demonstrated that the outmost lobes in the broadened spectrum show different dependencies on the initial pulse energy and duration. A simple yet effective toy model is introduced that successfully predicts broadband spectral tuning, and the impact of other nonlinear effects, dispersion, and input pulse asymmetry on the experimental scenario is also discussed. Thus a fiber‐based versatile source is demonstrated, which is compressible down to its transform‐limit duration, as short as 12.2 fs centered at 920 nm. In addition, bandwidth‐dependent third‐harmonic generation spectroscopy is performed from a dielectric metasurface with an optimized nonlinear response, and the dependency of laser bandwidth and pulse duration is investigated on the signal‐to‐background ratio of two‐photon images. It is believed that this demonstration will advance the investigation of bandwidth‐dependent nonlinear spectroscopy and microscopy.
Independent control of femtosecond sources’ spectral location and bandwidth is realized with a simple approach enabled by self‐phase modulation. This work thus demonstrates a fiber‐based versatile source, delivering pulses compressible down to few‐optical‐cycle regimes. These demonstrations pave the way for deeper insights into the spectral response of nonlinear resonances, further advancing the field of bandwidth‐dependent nonlinear spectroscopy and microscopy.
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Optical tweezers can manipulate micro-particles, which have been widely used in various applications. Here, we experimentally demonstrate that optical tweezers can assemble the micro-particles to ...form stable structures at the glass–solution interface in this paper. Firstly, the particles are driven by the optical forces originated from the diffraction fringes, which of the trapping beam passing through an objective with limited aperture. The particles form stable ring structures when the trapping beam is a linearly polarized beam. The particle distributions in the transverse plane are affected by the particle size and concentration. Secondly, the particles form an incompact structure as two fan-shaped after the azimuthally polarized beam passing through a linear polarizer. Furthermore, the particles form a compact structure when a radially polarized beam is used for trapping. Thirdly, the particle patterns can be printed steady at the glass surface in the salt solution. At last, the disadvantage of diffraction traps is discussed in application of optical tweezers. The aggregation of particles at the interfaces seriously affects the flowing of particles in microfluidic channels, and a total reflector as the bottom surface of sample cell can avoid the optical tweezers induced particle patterns at the interface. The optical trapping study utilizing the diffraction gives an interesting method for binding and assembling microparticles, which is helpful to understand the principle of optical tweezers.
Strong coupling provides a powerful way to modify the nonlinear optical properties of materials. The coupling strength of the state-of-the-art strongly coupled systems is restricted by a weak-field ...confinement of the cavity, which limits the enhancement of the optical nonlinearity. Here, we investigate a strong coupling between Mie resonant modes of high-index dielectric nanocavities and an epsilon-near-zero mode of an ultrathin indium tin oxide film and obtain an anticrossing splitting of 220 meV. Static nonlinear optical measurements reveal a large enhancement in the intensity-independent effective optical nonlinear coefficients, reaching more than 3 orders of magnitude at the coupled resonance. In addition, we observe a transient response of ∼300 fs for the coupled system. The ultrafast and large optical nonlinear coefficients presented here offer a new route towards strong coupling-assisted high-speed photonics.
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We develop a new all-dielectric metasurface for designing high quality-factor (
-factor) quasi-bound states in the continuum (quasi-BICs) using asymmetry kite-shaped nanopillar arrays. The
-factors ...of quasi-BICs follow the quadratic dependence on the geometry asymmetry, and meanwhile their resonant spectral profiles can be readily tuned between Fano and Lorentzian lineshapes through the interplay with the broadband magnetic dipole mode. The third-harmonic signals of quasi-BIC modes exhibit a gain from 43.4- to 634-fold enhancement between samples with an axial-length difference of 15 nm and 75 nm when reducing the numerical aperture of the illuminating objective lenses in nonlinear measurement, which is attributed to the increasing illumination spot size and the less contribution from the large oblique incident light for establishing quasi-BIC modes with high-
spectral profile and strong near-field intensity. The silicon-based metasurfaces with their simple geometry are facile for large-area fabrication and open new possibilities for the optimization of upconversion processes to achieve efficient nonlinear devices.
Toroidal Metamaterials
In article number 2100404, Hui‐Hsin Hsiao, and Ai‐Yin Liu demonstrate optical toroidal dipole and anapole metamaterials through the designs of periodic amorphous silicon ...nano‐pillars embedded in a spin‐on glass layer. Upon normal incidence of plane wave, the transverse toroidal moment, stemming from a complete head‐to‐tail configuration of the magnetic dipole moments within each individual nanopillar, are excited in optical regime. The toroidal response can be enhanced and spectrally tuned by elongating cylindrical axis perpendicular to the light polarization and exhibit ultrahigh refractive index sensitivity compared to other multipoles.
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Ultrafast manipulation of light polarization is crucial in many nonlinear optic and optoelectronic operations. However, most of the configurations are suffering from low modulation speed (gigahertz) ...or small contrast ratio. Here, by taking advantange of the anisotropic nonlinear response of indium tin oxide at its epsilon-near-zero region and plasmonic nanoantennas at their polarization-sensitive resonance, we achieve a large, ultrafast anisotropic modulation of light. A polarization elliptic rotation of 32.5° at 1.23 μm wavelength, and a phase delay between the oscillations of the linear polarization axes of π/7 within 600 fs response time is demonstrated. This approach constitutes a novel, efficient way to implement all-optical high-speed polarization modulators and retarders.
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