Optically Digitalized Holography: A Perspective for All-Optical Machine Learning

Holography, which was invented by Dennis Gabor in 1948, offers an approach to reconstructing both the amplitude and phase information of a three-dimensional (3D) object [1]. Since its invention, the concept of holography has been widely used in various fields, such as microscopy [2], interferometry [3], ultrasonography [4], and holographic display [5]. Optical holography can be divided into two steps: recording and reconstruction. A conventional hologram is recorded onto a photosensitive film as the interference between an object beam carrying the 3D object information and a reference beam. Thereafter, the original object wavefront is reconstructed in the 3D image space by illuminating the reference beam on the recorded hologram.

Tuning the intermediate reaction barriers by a CuPd catalyst to improve the selectivity of CO_(2) electroreduction to C_(2) products

Electrochemical CO2 reduction is a promising strategy for the utilization of CO2 and intermittent excess electricity.Cu is the only single metal catalyst that can electrochemically convert CO2 into multicarbon products.However,Cu exhibits an unfavorable activity and selectivity for the generation of C2 products because of the insufficient amount of CO*provided for the C‐C coupling.Based on the strong CO2 adsorption and ultrafast reaction kinetics of CO*formation on Pd,an intimate CuPd(100)interface was designed to lower the intermediate reaction barriers and improve the efficiency of C2 product formation.Density functional theory(DFT)calculations showed that the CuPd(100)interface enhanced the CO2 adsorption and decreased the CO2*hydrogenation energy barrier,which was beneficial for the C‐C coupling.The potential‐determining step(PDS)barrier of CO2 to C2 products on the CuPd(100)interface was 0.61 eV,which was lower than that on Cu(100)(0.72 eV).Encouraged by the DFT calculation results,the CuPd(100)interface catalyst was prepared by a facile chemical solution method and characterized by transmission electron microscopy.CO2 temperature‐programmed desorption and gas sensor experiments further confirmed the enhancement of the CO2 adsorption and CO2*hydrogenation ability of the CuPd(100)interface catalyst.Specifically,the obtained CuPd(100)interface catalyst exhibited a C2 Faradaic efficiency of 50.3%±1.2%at‒1.4 VRHE in 0.1 M KHCO3,which was 2.1 times higher than that of the Cu catalyst(23.6%±1.5%).This study provides the basis for the rational design of Cu‐based electrocatalysts for the generation of multicarbon products by fine‐tuning the intermediate reaction barriers.

纳米光子学中的光学涡旋

在过去的二十年中,携带轨道角动量的涡旋光引起了研究人员的广泛兴趣。涡旋光不仅在光与物质相互作用中扮演着重要角色,而且可极大拓宽光学信息的承载容量。与此同时,纳米科技的发展使得纳米光子学成为一个新兴学科,开辟了利用纳米结构和器件对光进行调控的新途径。当纳米技术和涡旋光相结合时,衍生出许多新的思路和概念。本文回顾和总结了基于纳米光子学的涡旋光产生、探测及其应用,并对该研究领域的未来进行了展望。

Ultrahigh numerical aperture meta-fibre for flexible optical trapping

Strong focusing on diffraction-limited spots is essential for many photonic applications and is particularly relevant for optical trapping;however,all currently used approaches fail to simultaneously provide flexible transportation of light,straightforward implementation,compatibility with waveguide circuitry,and strong focusing.Here,we demonstrate the design and 3D nanoprinting of an ultrahigh numerical aperture meta-fibre for highly flexible optical trapping.Taking into account the peculiarities of the fibre environment,we implemented an ultrathin meta-lens on the facet of a modified single-mode optical fibre via direct laser writing,leading to a diffraction-limited focal spot with a recordhigh numerical aperture of up to NA≈0.9.The unique capabilities of this flexible,cost-effective,bio-and fibre-circuitrycompatible meta-fibre device were demonstrated by optically trapping microbeads and bacteria for the first time with only one single-mode fibre in combination with diffractive optics.Our study highlights the relevance of the unexplored but exciting field of meta-fibre optics to a multitude of fields,such as bioanalytics,quantum technology and life sciences.

Nanophotonics shines light on hyperbolic metamaterials

Hyperbolic metamaterials with a unique hyperbolic dispersion relation allow propagating waves with infinitely largewavevectors and a high density of states. Researchers from Korea and Singapore provide a comprehensive review ofhyperbolic metamaterials, including artificially structured hyperbolic media and natural hyperbolic materials. Theyexplain key nanophotonic concepts and describe a range of applications for these versatile materials.

Single-step-fabricated disordered metasurfaces for enhanced light extraction from LEDs

While total internal reflection(TIR)lays the foundation for many important applications,foremost fibre optics that revolutionised information technologies,it is undesirable in some other applications such as light-emitting diodes(LEDs),which are a backbone for energy-efficient light sources.In the case of LEDs,TIR prevents photons from escaping the constituent high-index materials.Advances in material science have led to good efficiencies in generating photons from electron–hole pairs,making light extraction the bottleneck of the overall efficiency of LEDs.In recent years,the extraction efficiency has been improved,using nanostructures at the semiconductor/air interface that outcouple trapped photons to the outside continuum.However,the design of geometrical features for light extraction with sizes comparable to or smaller than the optical wavelength always requires sophisticated and timeconsuming fabrication,which causes a gap between lab demonstration and industrial-level applications.Inspired by lightning bugs,we propose and realise a disordered metasurface for light extraction throughout the visible spectrum,achieved with single-step fabrication.By applying such a cost-effective light extraction layer,we improve the external quantum efficiency by a factor of 1.65 for commercialised GaN LEDs,demonstrating a substantial potential for global energy-saving and sustainability.

Unlocking the out-of-plane dimension for photonic bound states in the continuum to achieve maximum optical chirality

The realization of lossless metasurfaces with true chirality crucially requires the fabrication of three-dimensional structures,constraining experimental feasibility and hampering practical implementations.Even though the threedimensional assembly of metallic nanostructures has been demonstrated previously,the resulting plasmonic resonances suffer from high intrinsic and radiative losses.The concept of photonic bound states in the continuum(BICs)is instrumental for tailoring radiative losses in diverse geometries,especially when implemented using lossless dielectrics,but applications have so far been limited to planar structures.Here,we introduce a novel nanofabrication approach to unlock the height of individual resonators within all-dielectric metasurfaces as an accessible parameter for the efficient control of resonance features and nanophotonic functionalities.In particular,we realize out-of-plane symmetry breaking in quasi-BIC metasurfaces and leverage this design degree of freedom to demonstrate an optical all-dielectric quasi-BIC metasurface with maximum intrinsic chirality that responds selectively to light of a particular circular polarization depending on the structural handedness.Our experimental results not only open a new paradigm for all-dielectric BICs and chiral nanophotonics,but also promise advances in the realization of efficient generation of optical angular momentum,holographic metasurfaces,and parity-time symmetry-broken optical systems.

Tunable structural colors on display

Structural coloration takes inspiration from the bright hues found in nature to control the reflection and transmission of light from artificially structured materials.Combining them with active electrical tuning heralds breakthrough applications in optical displays.

Coherent interaction of atoms with a beam of light confined in a light cage

Controlling coherent interaction between optical fields and quantum systems in scalable,integrated platforms is essential for quantum technologies.Miniaturised,warm alkali-vapour cells integrated with on-chip photonic devices represent an attractive system,in particular for delay or storage of a single-photon quantum state.Hollow-core fibres or planar waveguides are widely used to confine light over long distances enhancing light-matter interaction in atomic-vapour cells.However,they suffer from inefficient filling times,enhanced dephasing for atoms near the surfaces,and limited light-matter overlap.We report here on the observation of modified electromagnetically induced transparency for a non-diffractive beam of light in an on-chip,laterally-accessible hollow-core light cage.Atomic layer deposition of an alumina nanofilm onto the light-cage structure was utilised to precisely tune the high-transmission spectral region of the light-cage mode to the operation wavelength of the atomic transition,while additionally protecting the polymer against the corrosive alkali vapour.The experiments show strong,coherent light-matter coupling over lengths substantially exceeding the Rayleigh range.Additionally,the stable non-degrading performance and extreme versatility of the light cage provide an excellent basis for a manifold of quantum-storage and quantumnonlinear applications,highlighting it as a compelling candidate for all-on-chip,integrable,low-cost,vapour-based photon delay.

Radiative suppression of exciton-exciton annihilation in a two-dimensional semiconductor

Two-dimensional(2D)semiconductors possess strongly bound excitons,opening novel opportunities for engineering light-matter interaction at the nanoscale.However,their in-plane confinement leads to large non-radiative exciton–exciton annihilation(EEA)processes,setting a fundamental limit for their photonic applications.In this work,we demonstrate suppression of EEA via enhancement of light-matter interaction in hybrid 2D semiconductor-dielectric nanophotonic platforms,by coupling excitons in WS2 monolayers with optical Mie resonances in dielectric nanoantennas.The hybrid system reaches an intermediate light-matter coupling regime,with photoluminescence enhancement factors up to 102.Probing the exciton ultrafast dynamics reveal suppressed EEA for coupled excitons,even under high exciton densities>10^(12)cm^(−2).We extract EEA coefficients in the order of 10^(−3),compared to 10^(−2)for uncoupled monolayers,as well as a Purcell factor of 4.5.Our results highlight engineering the photonic environment as a route to achieve higher quantum efficiencies,for low-power hybrid devices,and larger exciton densities,towards strongly correlated excitonic phases in 2D semiconductors.

Light tuning CO/H_(2)composition on Ag:Unraveling CO_(2)mass transfer and electron-phonon coupling in plasmon-enhanced electrocatalysis

Plasmon-enhanced electrocatalysis(PEEC)is an emerging approach to mitigate CO_(2)emissions.The mechanisms behind CO_(2)adsorption and reduction at the catalyst-electrolyte interface in PEEC still need to be further explored.Herein,we employ a well-defined Ag nanostructure to elucidate these pivotal issues.By shining light with wavelengths of 625,525,405 nm on Ag,an adjustable CO/H_(2)ratio from 35 to 1 can be obtained.The reaction pathway changing under plasmonic excitation does not originate from the lowered CO_(2)mass transfer in the vicinity of Ag,as the electrochemical quartz crystal microbalance results unravel that a slightly elevated temperature in bulk electrolyte caused by light irradiation cannot weaken the CO_(2)adsorption at the Ag catalyst-electrolyte interface.Theoretical calculations reveal that optical excitation towards shorter wavelengths leads to a progressive lowered energy barrier for H_(2)formation together with an enhanced energy barrier for^(*)COOH formation.Although thermodynamically suppressed,CO_(2)reduction can still be improved kinetically by optimizing the excitation wavelength and intensity,being accompanied with the enhanced photocurrent.Transient absorption spectroscopy results further correlate the higher photocurrent with a prolonged electron-phonon coupling time,verifying that the improvement of CO_(2)reduction kinetics in PEEC can be realized by hot electron harnessing.