Enhanced light-matter interactions in dielectric nanostructures via machine-learning approach

A key concept underlying the specific functionalities of metasurfaces is the use of constituent components to shape the wavefront of the light on demand.Metasurfaces are versatile,novel platforms for manipulating the scattering,color,phase,or intensity of light.Currently,one of the typical approaches for designing a metasurface is to optimize one or two variables among a vast number of fixed parameters,such as various materials’properties and coupling effects,as well as the geometrical parameters.Ideally,this would require multidimensional space optimization through direct numerical simulations.Recently,an alternative,popular approach allows for reducing the computational cost significantly based on a deep-learning-assisted method.We utilize a deep-learning approach for obtaining high-quality factor(high-Q)resonances with desired characteristics,such as linewidth,amplitude,and spectral position.We exploit such high-Q resonances for enhancedlight–matter interaction in nonlinearoptical metasurfaces and optomechanical vibrations,simultaneously.We demonstrate that optimized metasurfaces achieve up to 400-fold enhancement of the third-harmonic generation;at the same time,they also contribute to 100-fold enhancement of the amplitude of optomechanical vibrations.This approach can be further used to realize structures with unconventional scattering responses.

Silicon cuboid nanoantenna with simultaneous large Purcell factor for electric dipole,magnetic dipole and electric quadrupole emission

The Purcell effect is commonly used to increase the spontaneous emission rate by modifying the local environment of a light emitter.Here,we propose a silicon dielectric cuboid nanoantenna for simultaneously enhancing electric dipole(ED),magnetic dipole(MD)and electric quadrupole(EQ)emission.We study the scattering cross section,polarization charge distribution,and electromagnetic field distribution for electromagnetic plane wave illuminating the silicon dielectric cuboid nanoantenna,from which we have identified simultaneous existence of ED,MD and EQ resonance modes in this nanoantenna.We have calculated the Purcell factor of ED,MD and EQ emitters with different moment orientations as a function of radiation wavelength by placing these point radiation source within the nanoantenna,respectively.We find that the resonances wavelengths of the Purcell factor spectrum are matching with the resonance modes in the nanoan-tenna.Moreover,the maximum Purcell factor of these ED,MD and EQ emitters is 18,150 and 118 respectively,occur-ring at the resonance wavelength of 475,750,and 562 nm,respectively,all within the visible range.The polarization charge distribution features allow us to clarify the excitation and radiation of these resonance modes as the physical ori-gin of large Purcell factor simultaneously occurring in this silicon cuboid nanoantenna.Our theoretical results might help to deeply explore and design the dielectric nanoantenna as an ideal candidate to enhance ED,MD and EQ emission simultaneously with very small loss in the visible range,which is superior than the more popular scheme of plasmonic nanoantenna.

Bound states in the continuum in all-dielectric metasurfaces with scaled lattice constants

Bound states in the continuum(BICs)have emerged as an efficient tool for trapping light at the nanoscale,promising several exciting applications in photonics.Breaking the structural symmetry has been proposed as an effective way of exciting quasiBICs(QBICs)and generating high-Q resonances.Herein,we demonstrate that QBICs can be excited in an all-dielectric metasurface by scaling the lattice of the metasurface,causing translational symmetry breaking.The corresponding BICs arise from band folding from the band edge to the Γ point in the first Brillouin zone.Multipole analysis reveals that the toroidal dipole dominates these QBICs.Furthermore,scaling the lattice along different directions provides additional freedom for tailoring QBICs,enabling polarization-dependent or-independent QBICs.In addition,this allows the realization of two QBICs at different wavelengths using plane-wave illumination with different polarizations on the metasurface.We experimentally demonstrated the existence of these BICs by fabricating silicon metasurfaces with scaled lattices and measuring their transmission spectra.The vanished resonant linewidth identifies BICs in the transmission spectrum,and the QBICs are characterized by highQ Fano resonances with the Q-factor reaching 2000.Our results have potential applications in enhancing light-matter interaction,such as laser,nonlinear harmonic generation,and strong coupling.

Spatial and spectral mode mapping of a dielectric nanodot by broadband interferometric homodyne scanning near-field spectroscopy

We investigate the optical properties of nanostructures of antimony sulfide(Sb2S3),a direct-bandgap semiconductor material that has recently sparked considerable interest as a thin film solar cell absorber.Fabrication from a nanoparticle ink solution and two-and three-dimensional nanostructuring with pattern sizes down to 50 nm have recently been demonstrated.Insight into the yet unknown nanoscopic optical properties of these nanostructures is highly desired for their future applications in nanophotonics.We implement a spectrally broadband scattering-type near-field optical spectroscopy technique to study individual Sb2S3 nanodots with a 20-nm spatial resolution,covering the range from 700 to 900 nm.We show that in this below-bandgap range,the Sb2S3 nanostructures act as high-refractive-index,low-loss waveguides with mode profiles close to those of idealized cylindrical waveguides,despite a considerable structural disorder.In combination with their high above-bandgap absorption,this makes them promising candidates for applications as dielectric metamaterials,specifically for ultrafast photoswitching.