Third-harmonic generation and imaging with resonant Si membrane metasurface

Dielectric metasurfaces play an increasingly important role in enhancing optical nonlinear generations owing to their ability to support strong light-matter interactions based on Mie-type multipolar resonances.Compared to metasurfaces composed of the periodic arrangement of nanoparticles,inverse,so-called,membrane metasurfaces offer unique possibilities for supporting multipolar resonances,while maintaining small unit cell size,large mode volume and high field enhancement for enhancing nonlinear frequency conversion.Here,we theoretically and experimentally investigate the formation of bound states in the continuum(BICs)from silicon dimer-hole membrane metasurfaces.We demonstrate that our BIC-formed resonance features a strong and tailorable electric near-field confinement inside the silicon membrane films.Furthermore,we show that by tuning the gap between the holes,one can open a leaky channel to transform these regular BICs into quasi-BICs,which can be excited directly under normal plane wave incidence.To prove the capabilities of such metasurfaces,we demonstrate the conversion of an infrared image to the visible range,based on the Third-harmonic generation(THG)process with the resonant membrane metasurfaces.Our results suggest a new paradigm for realising efficient nonlinear photonics metadevices and hold promise for extending the applications of nonlinear structuring surfaces to new types of all-optical near-infrared imaging technologies.

Nonlinear frequency conversion in optical nanoantennas and metasurfaces:materials evolution and fabrication

Nonlinear frequency conversion is one of the most fundamental processes in nonlinear optics.It has a wide range of applications in our daily lives,including novel light sources,sensing,and information processing.It is usually assumed that nonlinear frequency conversion requires large crystals that gradually accumulate a strong effect.However,the large size of nonlinear crystals is not compatible with the miniaturisation of modern photonic and optoelectronic systems.Therefore,shrinking the nonlinear structures down to the nanoscale,while keeping favourable conversion efficiencies,is of great importance for future photonics applications.In the last decade,researchers have studied the strategies for enhancing the nonlinear efficiencies at the nanoscale,e.g.by employing different nonlinear materials,resonant couplings and hybridization techniques.In this paper,we provide a compact review of the nanomaterials-based efforts,ranging from metal to dielectric and semiconductor nanostructures,including their relevant nanofabrication techniques.

Dual bound states in the continuum enhanced second harmonic generation with transition metal dichalcogenides monolayer

The emergence of two dimensional(2D)materials has opened new possibilities for exhibiting second harmonic genera-tion(SHG)at the nanoscale,due to their remarkable optical response related to stable excitons at room temperature.However,the ultimate atomic-scale interaction length with light makes the SHG of Transition Metal Dichalcogenides(TM-Ds)monolayers naturally weak.Here,we propose coupling a monolayer of TMDs with a photonic grating slab that works with doubly resonant bound states in the continuum(BIC).The BIC slabs are designed to exhibit a pair of BICs,reson-ant with both the fundamental wave(FW)and the second harmonic wave(SHW).Firstly,the spatial mode matching can be fulfilled by tilting FW's incident angle.We theoretically demonstrate that this strategy leads to more than four orders of magnitude enhancement of SHG efficiency than a sole monolayer of TMDs,under a pump light intensity of 0.1 GW/cm^(2).Moreover,we demonstrate that patterning the TMDs monolayer can further enhance the spatial overlap coefficient,which leads to an extra three orders of magnitude enhancement of SHG efficiency.These results demonstrate remarkable pos-sibilities for enhancing SHG with nonlinear 2D materials,opening many opportunities for chip-based light sources,nano-lasers,imaging,and biochemical sensing.

Dual bound states in the continuum enhanced second harmonic generation with Transition Metal Dichalcogenides monolayer

This file includes:Section 1:Simulation methods Section 2:Field distribution of FW and SHW with homogeneous WS2 monolayer Section 3:Field distribution of FW and SHW with patterned WS2 monolayer Section 4:Asymmetric outcoupling rate of TE-type BIC Section 5:Quality factor of quasi-BIC for a finite size grating Section 6:A flow chart for designing the dual BICs scheme Section 7:TMDs monolayer orientation-dependent polarization-resolved SHG efficiency Section 8:SHG efficiency of homogeneous bulk GaP Section 9:Prospectives for experimental realizations Supplementary information for this paper is available at https://doi.org/10.29026/oea.2022.200097.

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.

Electrically programmable solid-state metasurfaces via flash localised heating

In the last decades,metasurfaces have attracted much attention because of their extraordinary light-scattering properties.However,their inherently static geometry is an obstacle to many applications where dynamic tunability in their optical behaviour is required.Currently,there is a quest to enable dynamic tuning of metasurface properties,particularly with fast tuning rate,large modulation by small electrical signals,solid state and programmable across multiple pixels.Here,we demonstrate electrically tunable metasurfaces driven by thermo-optic effect and flash-heating in silicon.We show a 9-fold change in transmission by<5 V biasing voltage and the modulation rise-time of<625µs.Our device consists of a silicon hole array metasurface encapsulated by transparent conducting oxide as a localised heater.It allows for video frame rate optical switching over multiple pixels that can be electrically programmed.Some of the advantages of the proposed tuning method compared with other methods are the possibility to apply it for modulation in the visible and near-infrared region,large modulation depth,working at transmission regime,exhibiting low optical loss,low input voltage requirement,and operating with higher than video-rate switching speed.The device is furthermore compatible with modern electronic display technologies and could be ideal for personal electronic devices such as flat displays,virtual reality holography and light detection and ranging,where fast,solid-state and transparent optical switches are required.

A noise cross PSD estimator for dual-microphone speech enhancement based on minimum statistics

Some two-microphone noise reduction techniques that work in the frequency domain exploit coherence function between two noisy signals. They have shown good results when noise signals on two sensors are uncorrelated, but their per-formance decreases with correlated noises. Coherence based methods can be improved when the cross power spectral density (CPSD) of correlated noise signals is available. In this paper, we propose a new method for estimation of the CPSD of the noise, which is based on the minimum tracking technique. Despite the fact that the proposed estimator does not need to implement a voice activity detector (VAD), its performance is comparable to a CPSD estimator that uses an ideal VAD.

Infrared upconversion imaging in nonlinear metasurfaces

Infrared imaging is a crucial technique in a multitude of applications,including night vision,autonomous vehicle navigation,optical tomography,and food quality control.Conventional infrared imaging technologies,however,require the use of materials such as narrow bandgap semiconductors,which are sensitive to thermal noise and often require cryogenic cooling.We demonstrate a compact all-optical alternative to perform infrared imaging in a metasurface composed of GaAs semiconductor nanoantennas,using a nonlinear wave-mixing process.We experimentally show the upconversion of short-wave infrared wavelengths via the coherent parametric process of sum-frequency generation.In this process,an infrared image of a target is mixed inside the metasurface with a strong pump beam,translating the image from the infrared to the visible in a nanoscale ultrathin imaging device.Our results open up new opportunities for the development of compact infrared imaging devices with applications in infrared vision and life sciences.

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.

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%.

Surface that perceives depth:3D imaging with metasurfaces

When Galileo Galilei(1564-1642)was building his super bulky lenses for telescopes,he could not imagine that 400 years later,metasurfaces,which are hundred times thinner than a human hair,1–3 could reproduce the function of his lenses.Indeed,for a few centuries,conventional bulky optics,such as lenses,prisms,mirrors,etc.,have been the only tools for shaping the wave-front of light via engineering the optical path of light beams through media of given refractive indices.Recent advances in nanofabrication,characterization,and computational optics techniques have enabled the development of ultrathin metasurfaces,composed of a single layer or few-layer stacks of periodic subwavelength nanostructures,that can reproduce the functions of bulk optics with better performance,4–6 and occasionally offer new functionalities that are not possible with conventional diffractive optics.

Programmable structured surfaces can change the future of wireless communications

An innovative time-varying metasurface has been reported to realise dual-channel data transmissions for light-to-microwave signal conversion.Such a novel technique is a remarkable step forward to realise full-spectrum networks for catering for the growing demand for wireless communications.Moreover,this technique enriches the functionalities of tunable metasurfaces and stimulates new information-oriented applications.