Volume 9, Issue 15

Electron beams in electron microscopes are efficient probes of optical near-fields, thanks to spectroscopy tools like electron energy-loss spectroscopy and cathodoluminescence spectroscopy. Nowadays, we can acquire multitudes of information about nanophotonic systems by applying space-resolved diffraction and time-resolved spectroscopy techniques. In addition, moving electrons interacting with metallic materials and optical gratings appear as coherent sources of radiation. A swift electron traversing metallic nanostructures induces polarization density waves in the form of electronic collective excitations, i.e., the so-called plasmon polariton. Propagating plasmon polariton waves normally do not contribute to the radiation; nevertheless, they diffract from natural and engineered defects and cause radiation. Additionally, electrons can emit coherent light waves due to transition radiation, diffraction radiation, and Smith-Purcell radiation. Some of the mechanisms of radiation from electron beams have so far been employed for designing tunable radiation sources, particularly in those energy ranges not easily accessible by the state-of-the-art laser technology, such as the THz regime. Here, we review various approaches for the design of coherent electron-driven photon sources. In particular, we introduce the theory and nanofabrication techniques and discuss the possibilities for designing and realizing electron-driven photon sources for on-demand radiation beam shaping in an ultrabroadband spectral range to be able to realize ultrafast few-photon sources. We also discuss our recent attempts for generating structured light from precisely fabricated nanostructures. Our outlook for the realization of a correlative electron-photon microscope/spectroscope, which utilizes the above-mentioned radiation sources, is also described.

With their ultrathin characteristics as well as the powerful and flexible capabilities of wavefront modulation, optical metasurfaces have brought a new understanding of the interaction between light and matter and provided a powerful way to constrain and manage light. However, the unmodifiable structures and the immutable materials used in the construction lead to the unsatisfactory applications in most functional devices. The emergence of tunable optical metasurfaces breaks the aforementioned limitations and enables us to achieve dynamic control of the optical response. The work in recent years has focused on achieving tunability of optical metasurfaces through material property transition and structural reconfiguration. In this review, some tunable optical metasurfaces in recent years are introduced and summarized, as well as the advantages and limitations of various materials and mechanisms used for this purpose. The corresponding applications in functional devices based on tunability are also discussed. The review is terminated with a short section on the possible future developments and perspectives for future applications.

Recent developments in the application of plasmonic nanoparticles have showcased the importance of understanding in detail their plasmonic resonances at the single-particle level. These resonances can be excited and probed through various methods, which can be grouped in four categories, depending on whether excitation and detection involve electrons (electron energy loss spectroscopy), photons (e.g., dark-field microscopy), or both (cathodoluminescence and photon-induced near-field electron microscopy). While both photon-based and electron-based methods have made great strides toward deepening our understanding of known plasmonic properties and discovering new ones, they have in general progressed in parallel, without much cross-pollination. This evolution can be primarily attributed to the different theoretical approaches driving these techniques, mainly dictated by the inherent different nature of electrons and photons. The discrepancies that still exist among them have hampered the development of a holistic approach to the characterization of plasmonic materials. In this review therefore, we aim to briefly present those electron-based and photon-based methods fundamental to the study of plasmonic properties at the single-particle level, with an eye to new behaviors involving multipolar, propagating, and bulk modes coexisting in colloidal nanostructures. By exploring the key fundamental discoveries in nanoparticle plasmonics achieved with these techniques, herein we assess how integrating this information could encourage the creation of a unified understanding of the various phenomena occurring in individual nanoparticles, which would benefit the plasmonics and electron microscopy communities alike.

Photonic biosensors are a major topic of research that continues to make exciting advances. Technology has now improved sufficiently for photonics to enter the realm of microbiology and to allow for the detection of individual bacteria. Here, we discuss the different nanophotonic modalities used in this context and highlight the opportunities they offer for studying bacteria. We critically review examples from the recent literature, starting with an overview of photonic devices for the detection of bacteria, followed by a specific analysis of photonic antimicrobial susceptibility tests. We show that the intrinsic advantage of matching the optical probed volume to that of a single, or a few, bacterial cell, affords improved sensitivity while providing additional insight into single-cell properties. We illustrate our argument by comparing traditional culture-based methods, which we term macroscopic, to microscopic free-space optics and nanoscopic guided-wave optics techniques. Particular attention is devoted to this last class by discussing structures such as photonic crystal cavities, plasmonic nanostructures and interferometric configurations. These structures and associated measurement modalities are assessed in terms of limit of detection, response time and ease of implementation. Existing challenges and issues yet to be addressed will be examined and critically discussed.

Topological photonics has emerged as a novel paradigm for the design of electromagnetic systems from microwaves to nanophotonics. Studies to date have largely focused on the demonstration of fundamental concepts, such as nonreciprocity and waveguiding protected against fabrication disorder. Moving forward, there is a pressing need to identify applications where topological designs can lead to useful improvements in device performance. Here, we review applications of topological photonics to ring resonator–based systems, including one- and two-dimensional resonator arrays, and dynamically modulated resonators. We evaluate potential applications such as quantum light generation, disorder-robust delay lines, and optical isolation, as well as future research directions and open problems that need to be addressed.

We show that the second-harmonic generation (SHG) is enhanced in the chiral one-dimensional electron currents in a broad frequency range. The origin of the enhancement is twofold: first, the linear dispersion of the quasiparticles and the associated plasmonic mode as well as the quasi-linear dispersion of plasmon-polariton result in the lift of the phase-matching condition. Moreover, the strong field localization leads to the further increase of the SHG in the structure. The results suggest that the chiral currents localized at the domain walls of magnetic topological insulators can be an efficient source of the second-harmonic signal in the terahertz frequency range.

Vertically aligned nanowire arrays, with high surface-to-volume ratio and efficient light-trapping absorption, have attracted much attention for photoelectric devices. In this paper, vertical β-Ga 2 O 3 nanowire arrays with an average diameter/height of 110/450 nm have been fabricated by the inductively coupled plasma etching technique. Then a metal-semiconductor-metal structured solar-blind photodetector (PD) has been fabricated by depositing interdigital Ti/Au electrodes on the nanowire arrays. The fabricated β-Ga 2 O 3 nanowire PD exhibits ∼10 times higher photocurrent and responsivity than the corresponding film PD. Moreover, it also possesses a high photocurrent to dark current ratio ( I light / I dark ) of ∼10 4 and a ultraviolet/visible rejection ratio ( R 260 nm / R 400 nm ) of 3.5 × 10 3 along with millisecond-level photoresponse times.

Laser threshold magnetometry using the negatively charged nitrogen-vacancy (NV − ) centre in diamond as a gain medium has been proposed as a technique to dramatically enhance the sensitivity of room-temperature magnetometry. We experimentally explore a diamond-loaded open tunable fibre-cavity system as a potential contender for the realisation of lasing with NV − centres. We observe amplification of the transmission of a cavity-resonant seed laser at 721 nm when the cavity is pumped at 532 nm and attribute this to stimulated emission. Changes in the intensity of spontaneously emitted photons accompany the amplification, and a qualitative model including stimulated emission and ionisation dynamics of the NV − centre captures the dynamics in the experiment very well. These results highlight important considerations in the realisation of an NV − laser in diamond.

The single-layer graphene (SLG)-coupled nanowire (NW) hybrid plasmonic gap mode (PGM)-driven molecular catalytic reaction was investigated experimentally and theoretically. First, an SLG-coupled NW was constructed, then the surface-enhanced Raman scattering (SERS) effect of graphene in the hybrid plasmonic gap was studied via the normal and oblique incidence of excitation light. The SERS peaks of the D and G of graphene are more intensely enhanced by oblique incidence than by normal incidence. Furthermore, the catalytic reaction of the dimerization of the 4-nitrobenzenethiol molecule to p , p ′-dimercaptoazobenzene molecule driven by PGM was carried out by SERS. It was demonstrated that the efficiency of the PGM-driven catalytic reaction is much higher for oblique incidence than that for normal incidence. The mechanism of the PGM-driven catalytic reaction was studied by a finite-difference time-domain numerical simulation. When the PGM is excited by oblique incidence with θ  = 30°, the coupling between the NW and SLG/SiO 2 substrate increases to the maximum value. This is clearly evidenced by the excitation of a vertical bonding dipolar plasmon mode under the dipole approximation. The theoretical and experimental results were consistent with each other. This research may open up a pathway toward controlling PGM-driven catalytic reactions through polarization changes in excitation laser incidence on single anisotropic nanostructures.

Metasurfaces have been widely used to control beam propagation e.g. transmission, reflection, and absorption on an interface through a thin layer of nanoantennas with the thickness smaller than the wavelength. However, previous study of metasurfaces typically focused on controlling only one form of these propagations. In this work, we propose and demonstrate a multipolar plasmonic metasurface that can simultaneously realize antireflection (AR) and antitransmission (AT) in the visible and near-infrared regions. The AR and AT arise from destructive multipolar interferences in the backward and forward directions, respectively, i.e., through the generalized Kerker effect. By engineering the multipolar interference, we show that the AR and AT can happen at different or similar wavelength ranges, which can be used for low-absorption spectral filters due to off-resonance operation or inversely strong optical absorbers through near-resonance operation, respectively. We also present a simple two-dimensional design of the multipolar metasurface that supports AT for one polarization and AR for another polarization over a broadband, which is applicable to broadband transmissive polarizers with efficiency over 90% and the extinction ratio over 18 dB. By tuning the dimension and thus the multipolar interference, the transmitted polarization and operation wavelength are both controllable.

An efficient electro-optic transition control is reported in all-fiber graphene devices over a broad spectral range from visible to near-infrared. The ion liquid–based gating device fabricated onto a side-polished fiber with high numerical aperture significantly enhances the light-matter interaction with graphene, resulting in strong and nonresonant electro-optic absorption of up to 25.5 dB in the wavelength ranging from 532 to 1950 nm. A comprehensive analysis of the optical and electrical properties of the device fabricated with monolayer and bilayer graphene revealed that the number of graphene layers significantly impacts on the performance of the device, including modulation depth and driving voltage. Wavelength-dependent optical response is also measured, which clearly characterizes the electronic bandgap dispersion of graphene. The device exhibited more efficient electro-optic modulation in the longer wavelength region, where the maximum light modulation efficiency of 286.3%/V is achieved at a wavelength of 1950 nm.

Three-dimensional (3D) tracking of nano-objects represents a novel pathway for understanding dynamic nanoscale processes within bioanalytics and life science. Here we demonstrate 3D tracking of diffusing 100 nm gold nanosphere within a water-filled optofluidic fiber via elastic light scattering–based position retrieval. Specifically, the correlation between intensity and position inside a region of a fiber-integrated microchannel has been used to decode the axial position from the scattered intensity, while image processing–based tracking was used in the image plane. The 3D trajectory of a diffusing gold nanosphere has been experimentally determined, while the determined diameter analysis matches expectations. Beside key advantages such as homogenous light-line illumination, low-background scattering, long observation time, large number of frames, high temporal and spatial resolution and compatibility with standard microscope, the particular properties of operating with water defines a new bioanalytical platform that is highly relevant for medical and life science applications.

Two-dimensional iron phosphorus trichalcogenide (FePS 3 ) has attracted significant attention for its use in electricity, magnetism and optical fields due to its outstanding physical and chemical properties. Herein, FePS 3 was prepared using the chemical vapor transportation (CVT) method and then exfoliated by using fast electrochemical exfoliation. After gradient centrifugation, FePS 3 nanosheets with thicknesses ranging from 1.5 to 20 nm and lateral dimensions of 0.5–3 μm were obtained. By utilizing the spatial self-phase modulation (SSPM) effect, the relationships between the nonlinear refractive index and the size of the FePS 3 nanosheets were investigated in detail which revealed that the nonlinear refractive index can be effectively controlled by the size of the FePS 3 nanosheets. It is worth noting that the optimal FePS 3 nanosheets (3–5 layers thick and ∼1 μm in lateral dimensions) displayed the highest nonlinear refractive index of ∼10 −5  cm 2  W −1 . This work demonstrates the potential that FePS 3 nanosheets have for use in nonlinear optics or nonlinear optical devices.

Bound states in the continuum (BICs) or trapped modes can provide an important new avenue for strong light confinement via destructive interference. Dielectric photonic structures have been extensively studied for optical BICs. However, BICs in plasmonic nanostructures have not been explored much yet. Herein, we present a thorough experimental study of plasmonic BICs via Fourier-plane spectroscopy and imaging. Optical mode dispersion in a metal grating covered by a dielectric layer is directly measured in an angle-resolved white light reflection spectrum. Two dielectric layer thicknesses are considered. Both plasmonic and photonics modes are supported in the visible range using a thicker dielectric film; hence, either hybrid or purely plasmonic BICs can be formed. With a thinner dielectric layer, only plasmonic modes are strongly excited and purely plasmonic BICs appear. Our measurements exhibit all features expected for BICs, including a substantial increase in the Q factor. We also demonstrate that the BIC position can be switched from one optical mode branch to the other by tuning a metal grating parameter. Moreover, by mixing luminescent dyes in a dielectric layer, light emission coupling into BICs is investigated. We find that the photoluminescence peak disappears at the BIC condition, which is attributed to the trapping of molecular emission at plasmonic BICs. Therefore, both white light reflection and dye emission measurements in the Fourier plane clearly indicate the formation of trapped modes in plasmonic nanostructures. Our observation implies that plasmonic BICs can enable a highly effective light trapping device despite the simple structure of the device geometry. Plasmonic supercavity design based on the BIC concept may provide many interesting future opportunities for nanolasers, optical sensing, and nonlinear enhancement.

Integrated photonics offers attractive solutions for realizing combinational logic for high-performance computing. The integrated photonic chips can be further optimized using multiplexing techniques such as wavelength-division multiplexing (WDM). In this paper, we propose a WDM-based electronic–photonic switching network (EPSN) to realize the functions of the binary decoder and the multiplexer, which are fundamental elements in microprocessors for data transportation and processing. We experimentally demonstrate its practicality by implementing a 3–8 (three inputs, eight outputs) switching network operating at 20 Gb/s. Detailed performance analysis and performance enhancement techniques are also given in this paper.

Deflection, a basic functionality of wavefront manipulation is usually associated with the phase-gradient metasurfaces and the classical blazed gratings. We numerically and experimentally demonstrate an unusually wideband and simultaneously wide-angle deflection achieved at near-infrared in reflection mode for a periodic (nongradient), ultrathin meta-array comprising only one silicon nanorod (Mie resonator) per period. It occurs in the range where only the first negative diffraction order and zero order may propagate. Deflection serves as the enabler for multifunctional operation. Being designed with the main goal to obtain ultra-wideband and wide-angle deflection, the proposed meta-array is also capable in spatial filtering and wide-angle splitting. Spatial filtering of various types can be obtained in one structure by exploiting either deflection in nonzero diffraction orders, or the specular-reflection (zero-order) regime. Thus, the role of different diffraction orders is clarified. Moreover, on – off switching of deflection and related functionalities is possible by changing polarization state of the incident wave. The suggested device is simple to fabricate and only requires cost-effective materials, so it is particularly appropriate for the large-area fabrication using nanoprint lithography. Ultra-wideband wide-angle and other deflection scenarios, along with the other functionalities, are promising for applications in optical communications, laser optics, sensing, detection, and imaging.

Interfacial solar vapor generation has revived the solar-thermal-based desalination due to its high conversion efficiency of solar energy. However, most solar evaporators reported so far suffer from severe salt-clogging problems during solar desalination, leading to performance degradation and structural instability. Here, we demonstrate a free-standing salt-rejecting reduced graphene oxide (rGO) membrane serving as an efficient, stable, and antisalt-fouling solar evaporator. The evaporation rate of the membrane reaches up to 1.27 kg m −2  h −1 (solar–thermal conversion efficiency ∼79%) under one sun, out of 3.5 wt% brine. More strikingly, due to the tailored narrow interlayer spacing, the rGO membrane can effectively reject ions, preventing salt accumulation even for high salinity brine (∼8 wt% concentration). With enabled salt-antifouling capability, flexibility, as well as stability, our rGO membrane serves as a promising solar evaporator for high salinity brine treatment.

Broadband dispersion free, large and ultrafast nonlinear material platforms comprise the essential foundation for the study of nonlinear optics, integrated optics, intense field optical physics, and quantum optics. Despite substantial research efforts, such material platforms have not been established up to now because of intrinsic contradictions between large nonlinear optical coefficient, broad operating bandwidth, and ultrafast response time. In this work, a broadband dispersion free, large and ultrafast nonlinear material platform based on broadband epsilon-near-zero (ENZ) material is experimentally demonstrated, which is designed through a novel physical mechanism of combining structural dispersion and material dispersion. The broadband ENZ material is constructed of periodically nanostructured indium tin oxide (ITO) films, and the structure is designed with the help of theoretical predictions combined with algorithm optimization. Within the whole broad ENZ wavelength range (from 1300 to 1500 nm), a wavelength-independent and large average nonlinear refractive index of −4.85 × 10 −11  cm 2 /W, which is enlarged by around 20 times than that of an unstructured ITO film at its single ENZ wavelength, and an ultrafast response speed at the scale of Tbit/s are experimentally reached simultaneously. This work not only provides a new approach for constructing nonlinear optical materials but also lays the material foundation for the application of nanophotonics.

Photonic skyrmions have applications in many areas, including the vectorial and chiral optics, optical manipulation, deep-subwavelength imaging and nanometrology. Much effort has been focused on the experimental characterization of photonic skyrmions. Here, we give an insight into the spin and orbital features of photonic skyrmions constructed by the p -polarized and s -polarized surface waves at an interface with various electric and magnetic properties by analyzing the continuity of chirality, energy flow and momentum densities through the electric and magnetic interface. The continuity of chirality density indicates that the photonic skyrmion has a property of the optical transverse spin. Most importantly, the continuity of energy flow and momentum densities results in four spin–orbit interaction quantities, which indicate the gradient of electric polarizability or permeability governs the spin–orbit interaction of photonic skyrmions and leads to the discontinuity and even the reversal of spin orientation through the optical interface. Our investigations on the spin–orbit properties of photonic skyrmions, which can give rise to the spin-dependent force and topological unidirectional transportation, is thorough and can be extended to other classical wave, such as acoustic and fluid waves. The findings help in understanding the spin–orbit feature of photonic topological texture and in constructing further optical manipulation, sensing, quantum and topological techniques.