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Chiral nanogap antennas, which exhibit specific chiroptical responses, hold exceptional potential in various chiral applications, including polarization converters, asymmetric catalysis, label‐free chiral recognition, and chiral sensing. However, characterizing the chiral optical fields within nanogaps presents considerable challenges owing to their complex light field responses and spatial dimensions. In this study, the strong modulation of helicity‐resolved surface‐enhanced Raman spectroscopy (SERS) signals in graphene by chiral plasmonic antennas, offering a promising approach to characterize chiral near fields within nanogaps is demonstrated. The SERS of achiral monolayer graphene is performed within plasmonic nanogap antennas, including single gold nanorods (GNRs) and GNR dimers on gold microflakes. The Raman scattering intensity of graphene within these nanogap antennas is enhanced approximately sixfold. Furthermore, the degree of cross‐circularly polarized Raman intensity reaches up to ≈45%, indicating a robust chiral optical field. Moreover, under opposite circularly polarized light excitation, the SERS of graphene reveals varying emission intensities attributed to the modulation arising from the chiral antennas in both the excitation and emission processes of Raman scattering. Furthermore, the degree of chirality depends on the antenna configuration and the excitation laser wavelength. [ABSTRACT FROM AUTHOR]

Higher‐order topological insulators hosting topological modes at hinges and corners provide a new avenue for disorder‐immune light transport, which prospects great potential in applications of integrated photonics devices. However, the active control of topological modes in 3D higher‐order topological insulators is not realized yet, and its construction is too complicated and huge to analyze. Here, a method is proposed to construct effective Hamiltonians for higher‐order topological insulators, which provides a correct physical picture of the states and the corresponding energies at the domain walls. A non‐Hermitian 3D honeycomb lattice is constructed, which can generate arbitrary‐located, arbitrary‐shaped, and robust topological hinge states propagating at gain‐loss domain walls. The 3D honeycomb lattice can also appear as non‐Hermitian third‐order TIs exhibiting corner states, which can be dynamically controlled and show new topologically protected confinement rules. This work expands the understanding of the topological properties in non‐Hermitian systems and enables the dynamic control of topological states of different dimensions. [ABSTRACT FROM AUTHOR]

Machine‐learning has proven useful in distinguishing topological phases. However, there is still a lack of relevant research in the non‐Hermitian community, especially from the perspective of the momentum‐space. Here, an unsupervised machine‐learning method, diffusion maps, is used to study non‐Hermitian topologies in the momentum‐space. Choosing proper topological descriptors as input datasets, topological phases are successfully distinguished in several prototypical cases, including a line‐gapped tight‐binding model, a line‐gapped Floquet model, and a point‐gapped tight‐binding model. The datasets can be further reduced when certain symmetries exist. A mixed diffusion kernel method is proposed and developed, which could study several topologies at the same time and give hierarchical clustering results. As an application, a novel phase transition process is discovered in a non‐Hermitian honeycomb lattice without tedious numerical calculations. This study characterizes band properties without any prior knowledge, which provides a convenient and powerful way to study topology in non‐Hermitian systems. [ABSTRACT FROM AUTHOR]

Supersymmetry (SUSY) laser array with superpartner structure can suppress excess modes to achieve high‐intensity and high‐coherent radiation. Compared with complex superpartner, using parity time (PT) symmetry broken to manipulate SUSY laser arrays is a more flexible approach. Herein, based on ultrathin perovskite single crystal, a SUSY laser array is constructed without superpartner, but with auxiliary gain–loss structure. PT symmetry broken with high‐order exceptional point (EP) is realized in the visible spectral range. It intrinsically avoids superpartner field mismatch and improves side mode suppression ratio 8.8–12 dB for single‐mode lasing. Furthermore, variations in EP‐order enable the transition from single‐mode to dual‐mode or tri‐mode radiation and retain amplified radiation characteristics. In addition, these SUSY laser arrays with the high‐order EP have the potential to be small‐volume optical sensor devices. The new design combining SUSY and PT symmetry broken presents its potential in micro‐nanophotonic devices with the benefit of small size, flexibility in spectral control, expandability, simplicity, and multifunctionality feature. [ABSTRACT FROM AUTHOR]

The external quantum efficiency (EQE) of state‐of‐the‐art near‐infrared perovskite light‐emitting diodes (PeLEDs) is now approaching the limit set by the out‐coupling efficiency. However, there is plenty of room for enhancing these devices' radiance and the EQE roll‐off. This work reports how conductive and passivating additives construct efficient high‐radiance near‐infrared PeLEDs with improved EQE roll‐off. Specifically, the synergistic effect of 1,4‐phenylenediacetic acid (PDA) and 5‐aminovaleric acid (5‐AVA) is used to regulate the quality of the perovskite emissive layer, thereby enhancing the roll‐off threshold of injection currents (a value at which the device EQE or radiance starts degrading). According to in situ structural evolution analyses from the perovskite precursor to the perovskite crystal, the presence of PDA within the perovskite precursor solution can produce an intermediate phase with 5‐AVA and formanmidinium cation, which can retard the perovskite nucleation and crystal growth, leading to improved perovskite film quality. The resulting champion PeLED delivers among the highest radiance of 505 W sr−1 m−2 and a peak EQE of 17.56%. More importantly, the improved PeLED shows a well‐retained radiance of 80% at a current injection density of up to ≈1800 mA cm−1, opening a new avenue for high‐radiance PeLEDs with improved roll‐off degradation. [ABSTRACT FROM AUTHOR]

Vectorial metasurface holograms ar realized through segmenting or interleaving nanostructures to modulate spatial polarization of the reconstructed holography on the imaging plane, but it is still a challenge to achieve full‐color vectorial meta‐holograms with subwavelength meta‐molecules and independent spatial polarization control of reconstructed images. Here, this problem is solved by multiplexing the hologram phase for one circularly polarized light and the conjugated hologram phase for another orthogonal circularly polarized light using only a single nanostructure and then interleaving three kinds of metasurfaces for red, green, and blue light in subwavelength meta‐molecules. By independent control of the phase difference of two overlapped circularly polarized holographic images for each color, the vectorial field on the imaging plane can be freely manipulated at the pixel scale and dynamically changed with the incident polarization. Therefore, the local color and even contrast of the reconstructed full‐color image can be tuned by selecting the incident and output polarization states, demonstrating great potentials in secure optical encryption and dynamic color display. [ABSTRACT FROM AUTHOR]

Understanding in situ crystallization of perovskite materials is fundamentally important for perovskite‐based materials, components, and devices. Here, in situ crystallization of perovskites in three dimensions is investigated using two‐photon excitation fluorescence microscopy with diffraction‐limited spatial resolution. By taking a perovskite precursor droplet as an example, the spatiotemporal kinetics of nanocrystal formation during crystallization is demonstrated, taking advantage of the perovskites' unique advantage of in situ synthesis from solutions. Based on the time‐lapsed 3D two‐photon excitation fluorescence imaging, the in situ formation of the "crystal crust" is observed and the results suggest a centripetal direction of the crystallization starting from the free surface of the droplet. Analysis based on fluorescence intensity further confirms the evaporation kinetics in a sessile precursor droplet. The reported results demonstrate a promising framework for utilizing two‐photon excitation fluorescence microscopy as a powerful tool to study the in situ crystallization of perovskite materials as well as more general fluorescence‐related chemical reactions in three dimensions. [ABSTRACT FROM AUTHOR]

Metasurfaces achieving arbitrary phase profiles within ultrathin thickness, emerge as miniaturized, ultracompact, and kaleidoscopic nanophotonic platforms. However, it is often required to segment or interleave independent subarray metasurfaces to multiplex holograms in a single nanodevice, which in turn affects the device's compactness and channel capacity. Here, a flexible strategy is proposed for multiplexing vectorial holographic images by controlling the phase distributions of holographic images in far field. Benefitting from precisely controlling the phase difference of reconstructed images through the modified Gerchberg–Saxton algorithm, two different holographic images are independently designed for the circular light by two interleaved metasurfaces and an extra vectorial hologram is flexibly encrypted in far field without additional set of structures on the metasurface plane. An unlimited number of polarization can be achieved in the holographic image and additional information can be decrypted when different polarization‐dependent holographic images overlap. By continually varying phase difference between the incident right and left circular polarized light, the image within the overlap area can be modulated. The silicon dielectric metahologram with record absolute multiplexed efficiency (>25%) is achieved in the experiment. This technique, as far as it is known, promises an enormous data capacity as well as a high level of information security. [ABSTRACT FROM AUTHOR]

Quantum topological photonics is a new research field with great potential that is based on developments in both quantum optics and topological photonics. Topological photonics offers unique properties, including topological robustness and an anti‐backscattering property, and these advantages are strongly required in quantum optics. Quantum technology, which includes quantum optics, represents an important direction for future technological development. However, existing quantum light sources are unstable and quantum information may easily be lost during transmission. These disadvantages have troubled researchers for a long time and no perfect solution is available thus far. Fortunately, application of topological photonics to quantum optics can help to generate robust quantum light sources and protect photons from decoherence during photon propagation. This allows the correlation and entanglement to be maintained even when photons travel over long distances. To date, quantum topological photonics has provided major breakthroughs in certain quantum devices. This Review presents the basic concepts of quantum topological photonics and summarizes how the topological protection property works in quantum light sources, quantum information transmission, and other quantum devices. Finally, an outlook is provided on the remaining challenges and potential future directions of quantum topological photonics, which can aid in exploration of additional new phenomena. [ABSTRACT FROM AUTHOR]

High‐fidelity quantum state transfer, lying in the heart of the scalable quantum network, requires the frequency linkage among different quantum elements. Here, a protocol of quantum state transfer with seamless frequency‐connection between the solid spin and phonon as well as between arbitrary frequency and telecom photons through the diamond optomechanical cavity is proposed. When the diamond optomechanical cavity acts as a quantum node of the network, within each node, the spin state can map to the phonon state with arbitrary frequency via the sideband transition between spins and phonons, and simultaneously such phonon state can convert to photon state without any frequency selection through optomechanical interaction. Furthermore, the communication among quantum nodes is implemented by the evanescent field of nanofiber. The scheme not only enables universal scalable quantum networks for using robust quantum spins, but also offers a paradigm of the building block for optical quantum communications. [ABSTRACT FROM AUTHOR]

Electron behaviors are expected to differ in circumstances with polarization. Changes in electron dynamics further result in changes in the physical properties, especially at reduced dimensions. In this paper, threshold photoemission is used to image a 2D electron gas induced on a SrTiO3 surface with regions polarized by defect‐based localized dipolar moments. It is found that the induction of surface polarization results in decrease of the surface itinerant electron density and the magnetic moments at room temperature. Such effects are attributed to the polarization‐based trapping of the surface itinerant electrons. It is also demonstrated that those changes induced by surface polarization can be manipulated by varying temperature. These findings address the importance of polarization in low‐dimensional electron dynamics, which offers an alternative way to engineer the properties of the SrTiO3‐based 2D electron gas. [ABSTRACT FROM AUTHOR]

Cesium‐based all‐inorganic halide perovskites solar cells (PSCs) have recently attracted increasing attention. Currently, due to the existence of high defects density and unoptimized interfacial morphology, "state‐of‐the‐art" performances of all‐inorganic PSCs are still far away from their theoretical limits. Although commonly used two‐step passivation methods can effectively passivate the perovskite surface, they will inevitably detriment the original perovskite morphology due to the use of weak‐polarity solvents. This will potentially result in the unintentional doping, uncontrollable interfacial band alignment, and the additional defects formation. Hence, a spontaneous interfacial manipulation (SIM) method is developed to self‐organize a 2D/3D multidimensional perovskite top interface. It is demonstrated that the spontaneously formed ultrathin 2D perovskite can not only eliminate the interfacial defects, but also effectively prevent moisture penetration. As a result, a significant power conversion efficiency enhancement from 13.64% to over 18% is obtained along with greatly extended device lifetime, for CsPbIxBr3‐x‐based all‐inorganic PSC. [ABSTRACT FROM AUTHOR]

The deterministic integration of single‐photon emitters with large Purcell enhancements and waveguides is vital for integrated quantum circuits and quantum chips. Purcell enhancements are not only related to waveguide structures, but are also greatly affected by the position of single‐photon emitters in waveguides. Here, the insertion of a thin dielectric film with a controllable thickness between a dielectric strip and a metal surface to precisely position a single‐photon emitter in a dielectric‐load surface‐plasmon‐polariton (DLSPP) waveguide is proposed, and the Purcell enhancement is greatly enlarged. Simulations show that the vertical position of the single‐photon emitter above the metal surface is more important than its in‐plane position in the DLSPP waveguide for achieving enlarged Purcell enhancements. In the experiment, the vertical position of the single‐photon emitter in the modified DLSPP waveguide is controlled by evaporating a thin MgF2 film, and the in‐plane position is controlled by combining aligned electron beam lithography and photoluminescence imaging. The Purcell factor in the emitter‐waveguide system is measured to be Fp = 22.4, which is greater than those obtained using conventional DLSPP waveguides (Fp ≤ 6.0) in previous studies. [ABSTRACT FROM AUTHOR]

Abstract: Electronic–photonic hybrid integrated circuits represent the essential basis of next‐generation ultrahigh‐speed information processing chips, which will require ultrafast and ultralow‐energy‐consumption electronic and photonic information interconversion and modulation, i.e., on‐chip dual electro‐optic and optoelectric modulation. However, this type of modulation has not been realized to date because of an absence of materials that demonstrate sufficient intrinsic electro‐optic and optoelectric responses simultaneously. On‐chip dual electro‐optic and optoelectric modulation is experimentally realized here based on refractive index variation of a ZnO nanowire driven by the gate voltage and electrical conductivity changes in this nanowire caused by the strong localized light field of a silicon nitride photonic crystal nanocavity mode. A low gate voltage of 1.5 V induces a large shift of 7 nm in the Fano‐like resonance wavelength and modulates the propagation state of an 800 nm light signal with a large modulation depth of 75%. Additionally, weak signal light with power as low as 0.4 µW induces a modulation depth of 60% in the electric signal. On‐chip conversion between electronic and photonic signals thus is achieved. This work paves the way toward realization of novel nanoscale multifunctional optoelectronic integrated devices and provides an on‐chip platform for the study of new optoelectronic functional materials. [ABSTRACT FROM AUTHOR]

Abstract: Formamidinium (FA)‐based lead iodide perovskites have emerged as the most promising light‐absorber materials in the prevailing perovskite solar cells (PSCs). However, they suffer from the phase‐instability issue in the ambient atmosphere, which is holding back the realization of the full potential of FA‐based PSCs in the context of high efficiency and stability. Herein, the tetraethylorthosilicate hydrolysis process is integrated with the solution crystallization of FA‐based perovskites, forming a new film structure with individual perovskite grains encapsulated by amorphous silica layers that are in situ formed at the nanoscale. The silica not only protects perovskite grains from the degradation but also enhances the charge‐carrier dynamics of perovskite films. The underlying mechanism is discussed using a joint experiment‐theory approach. Through this in situ grain encapsulation method, PSCs show an efficiency close to 20% with an impressive 97% retention after 1000‐h storage under ambient conditions. [ABSTRACT FROM AUTHOR]

Abstract: Epsilon‐near‐zero (ENZ) photonics is the study of light–matter interactions in the presence of structures with near‐zero permittivity and has been emerging as an important field of research in recent years. The introduction of zero permittivity structures also introduces a number of unique features to traditional photonic systems, including decoupling of their spatial and temporal field variations, tunneling through arbitrary channels, constant phase transmission, strong field confinement, and ultrafast phase transitions. Along with the continued developments in the theoretical research on ENZ photonics, many novel functional photonic devices are proposed and demonstrated experimentally, thus indicating the broad prospects of ENZ photonics for fabrication of high‐performance integrated photonic chips. Zero‐epsilon materials, which represent a singular point in optical materials, are expected to lead to remarkable developments in the fields of integrated photonic devices and optical interconnections. This review summarizes the underlying principles, the related novel physical effects, the fundamental principles for realization of ENZ photonic systems, and the integrated device applications of ENZ photonics. The review concludes with a brief overview of the challenges to be confronted and the potential development directions that may be pursued to realize extensive applications of ENZ photonics in the field of integrated photonic signal processing. [ABSTRACT FROM AUTHOR]

Abstract: Organic–inorganic hybrid perovskite photodetectors have been reported to possess superior optoelectronic properties, such as high sensitivity, ultrafast response, and capability of strongly absorbing the light in the visible range. While in the near‐infrared range, the performances of these photodetectors deteriorate seriously, originating from the weak coupling of infrared light to the perovskites. In this study, an organic–inorganic hybrid perovskite photodetector on arrays of Au nanostructures is fabricated, which exhibits a remarkable photocurrent enhancement in the visible and near‐infrared range, ascribed to the huge localized electric field induced by the surface plasmon resonance of Au nanostructures. The maximum external quantum efficiency of this photodetector is as high as about 65%, 2.5 times that on a usual Si/SiO2 substrate without Au nanostructures. Obviously, by exploiting Au nanostructures, the light harvesting ability of the photodetector breaks through the perovskite absorption edge, and extends to the near‐infrared range. [ABSTRACT FROM AUTHOR]

Abstract: Electron or energy transfer process is critical to understand the interaction between graphene and metal nanostructures. Combining atomic force microscope nanomanipulation with single‐particle spectroscopy enables in situ investigation of the interaction between single gold nanorod and monolayer graphene. This paper reveals that scattering and photoluminescence intensity of the nanorod decreased after coupling with monolayer graphene. The coupling strength is distance dependent and different for diverse nanoparticles, resulting in intensity quenching ratio varying from 20% to 60%, and broadening of resonance linewidth varying from 18 to 38 meV. These results clarify previous controversy between ensemble studies and single‐particle measurements, and that contributes to the deeper understanding of plasmonic graphene hybrids. [ABSTRACT FROM AUTHOR]

Abstract: An on‐chip integrated one‐dimension topological insulator in the optical communication range is realized directly in an integrated photonic circuit. The system takes on a configuration of a 220 nm thick 1D photonic crystal heterostructure sandwiched between two gold films. A photonic topological edge state centered at 1550 nm is obtained for the chip‐integrated one‐dimension topological insulator made of a silicon/SiO2 photonic crystal heterostructure with a feature size of only 2.25 µm integrated with a silicon waveguide. On/off switching of the photonic topological edge state was also achieved in a 1D topological insulator made of a VO2/SiO2 photonic crystal heterostructure based on a thermally induced insulator‐to‐metal transition of VO2. This system not only paves the way for practical applications of photonic topological insulators in integrated photonic devices and chips but also provides a platform for fundamental studies of topological photonics. [ABSTRACT FROM AUTHOR]

On-chip-triggered all-optical switching is a key component of ultrahigh-speed and ultrawide-band information processing chips. This switching technique, the operating states of which are triggered by a remote control light, paves the way for the realization of cascaded and complicated logic processing circuits and quantum solid chips. Here, a strategy is reported to realize on-chip remotely-triggered, ultralow-power, ultrafast, and nanoscale all-optical switching with high switching efficiency in integrated photonic circuits. It is based on control-light induced dynamic modulation of the coupling properties of two remotely-coupled silicon photonic crystal nanocavities, and extremely large optical nonlinearity enhancement associated with epsilon-near-zero multi-component nanocomposite achieved through dispersion engineering. Compared with previous reports of on-chip direct-triggered all-optical switching, the threshold control intensity, 560 kW/cm2, is reduced by four orders of magnitude, while maintaining ultrafast switching time of 15 ps. This not only provides a strategy to construct photonic materials with ultrafast and large third-order nonlinearity, but also offers an on-chip platform for the fundamental study of nonlinear optics. [ABSTRACT FROM AUTHOR]

We investigate the high performance of an apertureless near-field probe consisting of a tapered metal tip with periodic shallow surface grooves. The spontaneous emission of a single emitter near the apex is systematically analyzed for side-illumination tip enhanced spectroscopy scheme. In contrast to a conventional bare tapered tip, the corrugated probe not only strongly enhances the local excitation field, but also modulates the emission directivity, leading to high collection efficiency and signal-to-noise ratio. Specifically, we propose that an asymmetric tip enhanced spectroscopy probe containing two different length nanorods at the apex realizes unidirectional emissions. The radiation pattern is sensitive to the emission wavelengths and the emitter positions, which can increase the signal-to-noise ratio through suppression of the undesired signal. The proposed asymmetrical corrugated probe possesses a wide range of potential applications, including increasing the detection efficiency of tip enhanced spectroscopy at the single molecule level. Copyright © 2016 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

During the past 6 years, perovskite solar cells have experienced a rapid development and shown great potential as the next-generation photovoltaics. For the perovskite solar cells with regular structure (n-i-p structure), device efficiency has reached over 20% after the intense efforts of researchers from all over the world. Recently, perovskite solar cells with the inverted structure (p-i-n structure) have been becoming more and more attractive, owing to their easy-fabrication, cost-effectiveness, and suppressed hysteresis characteristics. Some recent progress in their device performance and stability has indicated their promising future. Here, recent developments and future perspectives about inverted perovskite solar cells are reviewed. Interface engineering, film morphology control, device stability, hysteresis phenomena and other research hotspots are discussed to present the roadmap for the development of inverted perovskite solar cells. [ABSTRACT FROM AUTHOR]

Transparent conducting electrodes (TCEs) with multi-length scaled structure are promising candidates as a potential replacement for indium tin oxide (ITO). In this work, multi-length scaled silver nanowire (AgNW) grids are demonstrated as TCEs for organic solar cells. The multi-length scale silver nanowire grids are prepared by top-down patterning using a neutral vapor etching process. Patterning AgNW film into multi-length scale grid structures could improve the optical transmittance and enhance the use of incident photons. Based on these multi-length scale AgNW grids, inverted bulk heterojunction polymer solar cells with power conversion efficiency up to 9.02% are fabricated, which are higher than that based on the original AgNW films and comparable to that based on ITO. [ABSTRACT FROM AUTHOR]

Ultracompact directional optical nanoantennas are rapidly gaining popularity yet still challenging tasks to construct at the nanoscale. Here, we experimentally demonstrate unidirectional emission from a fluorescent nanodiamond coupled with a single gold nanorod. Different configurations of the assembled hybrid nanostructures were realized via step-by-step atomic force microscope nanomanipulation. The emission patterns can be controlled by adjusting the configurations, i.e., the gold nanorod orientation and separation with respect to the nanodiamond. Numerical simulation results reveal that the unidirectional emission can be ascribed to the interference between the electromagnetic fields produced by the dipole-like source and the out-of-phase dipole induced in the gold nanorod. The proposed hybrid nanostructures remarkably exhibit highly unidirectional emission even when the emitter is positioned up to 50 nm away from the nanorod antenna and present a broad working spectral bandwidth of ∼200 nm. The distinct features of this hybrid structure suggest potential applications for novel nanoscale light sources, sensing, and quantum information. [ABSTRACT FROM AUTHOR]

Organic-inorganic lead halide perovskites are emerging materials for the next-generation photovoltaics. Lead halides are the most commonly used lead precursors for perovskite active layers. Recently, lead acetate (Pb(Ac)2) has shown its superiority as the potential replacement for traditional lead halides. Here, we demonstrate a strategy to improve the efficiency for the perovskite solar cell based on lead acetate precursor. We utilized methylammonium bromide as an additive in the Pb(Ac)2 and methylammonium iodide precursor solution, resulting in uniform, compact and pinhole-free perovskite films. We observed enhanced charge carrier extraction between the perovskite layer and charge collection layers and delivered a champion power conversion efficiency of 18.3% with a stabilized output efficiency of 17.6% at the maximum power point. The optimized devices also exhibited negligible current density-voltage ( J- V) hysteresis under the scanning conditions. [ABSTRACT FROM AUTHOR]

A mesoporous PbI2 scaffold is demonstrated to achieve high‐performance planar heterojunction perovskite solar cells. A remarkable power conversion efficiency (PCE) of 15.7% is achieved under AM1.5G 100 mW cm−2 solar irradiation with almost no hysteresis. The morphologies of the PbI2 scaffolds can be programmed by a controlled nucleation and growth mechanism. Correlations between the PbI2 film morphology and device performance are established. [ABSTRACT FROM AUTHOR]

Optical whispering-gallery mode (WGM) microcavities featuring ultrahigh Q factors and small mode volumes enhance significantly the interaction between light and matter, becoming an excellent platform for achieving ultralow-threshold microlasers. However, the emission of traditional WGMs is isotropic due to the rotational symmetry of cavity geometries, which hinders the potential photonics applications. In this review, the progress in WGM microcavities towards unidirectional laser emission is summarized. When a subwavelength scatterer is placed on the boundary of the microcavity, the unidirectional emission occurs due to the collimation effect of the microcavity-enhanced scattering field. Furthermore, microcavities deformed from the circular shapes can not only produce the chaos-assisted unidirectional emission, but also maintain high Q factors by special design and fabrication processes. Finally, gratings along the circumference of the WGM microdisk or microring can scatter the WGMs in the vertical direction. The review also lists several important applications of these types of microcavities, such as wide-band laser illumination source, free-space coupling, evanescent-field enhancement, optical energy storage, and sensing. [ABSTRACT FROM AUTHOR]

Stimulated emission depletion (STED) microscopy has become a powerful imaging and localized excitation method, breaking the diffraction barrier for improved spatial resolution in cellular imaging, lithography, etc. Because of specimen-induced aberrations and scattering distortion, it is a great challenge for STED to maintain consistent lateral resolution deep inside specimens. Here we report on deep imaging STED microscopy using a Gaussian beam for excitation and a hollow Bessel beam for depletion (GB-STED). The proposed scheme shows an improved imaging depth of up to about 155 μm in a solid agarose sample, 115 μm in polydimethylsiloxane, and 100 μm in a phantom of gray matter in brain tissue with consistent super resolution, while standard STED microscopy shows a significantly reduced lateral resolution at the same imaging depth. The results indicate the excellent imaging penetration capability of GB-STED, paving the way for deep tissue super-resolution imaging and three-dimensional precise laser fabrication. [ABSTRACT FROM AUTHOR]

Directional side scattering of light by individual gold nanoparticles (AuNPs) trimers assembled by the atomic force microscope (AFM) nanomanipulation method is investigated in experiment and theory. The AFM nanomanipulation approach brings an active way to construct ultracompact and effective optical nanoantennas. Different configurations of the trimers are constructed in situ via AFM nanomanipulation. Unidirectional side scattering of light by a single trimer is demonstrated with a broad response bandwidth over 400 nm and directivity up to ∼7.8 dB in experiments. The near-field plasmon coupling of the AuNPs is simulated with the 3D finite-difference time-domain method and the far-field radiation patterns are calculated by employing near-field-to-far-field transformation methods. The calculated results are in agreement with the experiments qualitatively. The physical origin is revealed intuitively by employing a simple phenomenological 'two-dipole' model. The unidirectional light scattering is due to the interference between multiple plasmonic resonance modes of the trimers. The study contributes to the understanding of the optical response of complex nanostructures and optimizing nanoantenna performances for practical applications, e.g. increasing the detection efficiency of surface-enhanced spectroscopy. [ABSTRACT FROM AUTHOR]

Surface plasmon polaritons (SPPs) have sparked enormous interest on nanophotonics beyond the diffraction limit for their remarkable capabilities of subwavelength confinements and strong enhancements. Due to the inherent polarization sensitivity of the SPPs [transverse-magnetic (TM) polarization], it is a great challenge to couple the s-polarized free-space light to the SPPs. Here, an ultrasmall defect aperture (< λ2/2) is designed to directionally couple both the p- and s-polarized incident beams to the single SPP mode in a broad bandwidth, which is guided by a subwavelength plasmonic waveguide. Simulations show that hot spots emerge at the sharp corners of the defect aperture when the incident beams illuminate it from the back side. The strong radiative fields from the hot spots are directionally coupled to the SPP mode because of the symmetry breaking of the defect aperture. By adjusting the structural parameters, both the unidirectional and bidirectional SPP coupling from the two orthogonal linear-polarization incident beams are experimentally demonstrated. The polarization-free coupling of the SPPs is of importance in circuits for fully optical processing of information with a deep-subwavelength footprint. [ABSTRACT FROM AUTHOR]