2021 Vol. 10, No. 5

News & Views
Whispering-gallery microlasers for cell tagging and barcoding: the prospects for in vivo biosensing
Nikita Toropov, Frank Vollmer
Published. 2021, 10(5) : 710-712 doi: 10.1038/s41377-021-00517-6
Researchers in the field of whispering-gallery-mode (WGM) microresonators have proposed biointegrated low-threshold WGM lasers, to enable large-scale parallel single-cell tracking and barcoding. Although the reported devices have so far been primarily investigated in model applications, most recent results represent important steps towards the development of in vivo tags and sensors that utilize the unique and narrow spectral features of miniature WGM lasers.
Entanglement goes classically high-dimensional
Qiwen Zhan
Published. 2021, 10(5) : 713-714 doi: 10.1038/s41377-021-00521-w
Laser beams from a customarily designed resonator can produce vectorial structured light fields as classical analogs to high-dimensional multipartite quantum entangled states.
Quasi-two-dimensional perovskite light emitting diodes for bright future
Jin-Wook Lee, Nam-Gyu Park
Published. 2021, 10(5) : 715-716 doi: 10.1038/s41377-021-00528-3
The fundamentals, promise and challenges of metal halide quasi-two-dimensional (quasi-2D) perovskites for a next generation emitter in light emitting diode devices are systematically reviewed.
Quantum optics with swift electrons
Nahid Talebi
Published. 2021, 10(5) : 717-719 doi: 10.1038/s41377-021-00530-9
Stimulated and spontaneous interactions of electron wavepackets with optical near fields were explored with complementary techniques. In striking agreement with theory, scientists have demonstrated the dependence of spontaneous and stimulated quantum mechanical processes on the spatial distribution of optical modes.
Interlayer exciton formation, relaxation, and transport in TMD van der Waals heterostructures
Ying Jiang, Shula Chen, Weihao Zheng, Biyuan Zheng, Anlian Pan
Published. 2021, 10(5) : 720-748 doi: 10.1038/s41377-021-00500-1
Van der Waals (vdW) heterostructures based on transition metal dichalcogenides (TMDs) generally possess a type-Ⅱ band alignment that facilitates the formation of interlayer excitons between constituent monolayers. Manipulation of the interlayer excitons in TMD vdW heterostructures holds great promise for the development of excitonic integrated circuits that serve as the counterpart of electronic integrated circuits, which allows the photons and excitons to transform into each other and thus bridges optical communication and signal processing at the integrated circuit. As a consequence, numerous studies have been carried out to obtain deep insight into the physical properties of interlayer excitons, including revealing their ultrafast formation, long population recombination lifetimes, and intriguing spin-valley dynamics. These outstanding properties ensure interlayer excitons with good transport characteristics, and may pave the way for their potential applications in efficient excitonic devices based on TMD vdW heterostructures. At present, a systematic and comprehensive overview of interlayer exciton formation, relaxation, transport, and potential applications is still lacking. In this review, we give a comprehensive description and discussion of these frontier topics for interlayer excitons in TMD vdW heterostructures to provide valuable guidance for researchers in this field.
Circularly polarized luminescence from organic micro-/nano-structures
Yongjing Deng, Mengzhu Wang, Yanling Zhuang, Shujuan Liu, Wei Huang, et al.
Published. 2021, 10(5) : 749-766 doi: 10.1038/s41377-021-00516-7
Circularly polarized light exhibits promising applications in future displays and photonic technologies. Circularly polarized luminescence (CPL) from chiral luminophores is an ideal approach to directly generating circularly polarized light, in which the energy loss induced by the circularly polarized filters can be reduced. Among various chiral luminophores, organic micro-/nano-structures have attracted increasing attention owing to the high quantum efficiency and luminescence dissymmetry factor. Herein, the recent progress of CPL from organic micro-/nano-structures is summarized. Firstly, the design principles of CPL-active organic micro-/nano-structures are expounded from the construction of micro-/nano-structure and the introduction of chirality. Based on these design principles, several typical organic micro-/nano-structures with CPL activity are introduced in detail, including self-assembly of small molecules, self-assembly of π-conjugated polymers, and self-assembly on micro-/nanoscale architectures. Subsequently, we discuss the external stimuli that can regulate CPL performance, including solvents, pH value, metal ions, mechanical force, and temperature. We also summarize the applications of CPL-active materials in organic light-emitting diodes, optical information processing, and chemical and biological sensing. Finally, the current challenges and prospects in this emerging field are presented. It is expected that this review will provide a guide for the design of excellent CPL-active materials.
Silica optical fiber integrated with two-dimensional materials: towards opto-electro-mechanical technology
Jin-hui Chen, Yi-feng Xiong, Fei Xu, Yan-qing Lu
Published. 2021, 10(5) : 767-784 doi: 10.1038/s41377-021-00520-x
In recent years, the integration of graphene and related two-dimensional (2D) materials in optical fibers have stimulated significant advances in all-fiber photonics and optoelectronics. The conventional passive silica fiber devices with 2D materials are empowered for enhancing light-matter interactions and are applied for manipulating light beams in respect of their polarization, phase, intensity and frequency, and even realizing the active photo-electric conversion and electro-optic modulation, which paves a new route to the integrated multifunctional all-fiber optoelectronic system. This article reviews the fast-progress field of hybrid 2D-materials-optical-fiber for the opto-electro-mechanical devices. The challenges and opportunities in this field for future development are discussed.
Quantum-dot microlasers based on whispering gallery mode resonators
A. E. Zhukov, N. V. Kryzhanovskaya, E. I. Moiseev, M. V. Maximov
Published. 2021, 10(5) : 785-795 doi: 10.1038/s41377-021-00525-6
The subject of this paper is microlasers with the emission spectra determined by the whispering gallery modes. Owing to the total internal reflection of light on the sidewalls, a high Q-factor is achieved until the diameter is comparable to the wavelength. The light emission predominantly occurs in the plane of the structure, which facilitates the microlaser integration with other elements. We focus on microdisk lasers with various types of the In(Ga)As quantum dots (QDs). Deep localization of charge carriers in spatially separated regions suppresses the lateral diffusion and makes it possible to overcome the undesirable effect of non-radiative recombination in deep mesas. Thus, using conventional epitaxial structures and relatively simple post-growth processing methods, it is possible to realize small microlasers capable of operating without temperature stabilization at elevated temperatures. The low sensitivity of QDs to epitaxial and manufacturing defects allows fabricating microlasers using Ⅲ-Ⅴ heterostructures grown on silicon.
Creation and control of high-dimensional multi-partite classically entangled light
Yijie Shen, Isaac Nape, Xilin Yang, Xing Fu, Mali Gong, et al.
Published. 2021, 10(5) : 796-805 doi: 10.1038/s41377-021-00493-x
Vector beams, non-separable in spatial mode and polarisation, have emerged as enabling tools in many diverse applications, from communication to imaging. This applicability has been achieved by sophisticated laser designs controlling the spin and orbital angular momentum, but so far is restricted to only two-dimensional states. Here we demonstrate the first vectorially structured light created and fully controlled in eight dimensions, a new state-of-the-art. We externally modulate our beam to control, for the first time, the complete set of classical Greenberger–Horne–Zeilinger (GHZ) states in paraxial structured light beams, in analogy with high-dimensional multi-partite quantum entangled states, and introduce a new tomography method to verify their fidelity. Our complete theoretical framework reveals a rich parameter space for further extending the dimensionality and degrees of freedom, opening new pathways for vectorially structured light in the classical and quantum regimes.
Highly efficient THz four-wave mixing in doped silicon
Nils Dessmann, Nguyen H. Le, Viktoria Eless, Steven Chick, Kamyar Saeedi, et al.
Published. 2021, 10(5) : 806-811 doi: 10.1038/s41377-021-00509-6
Third-order non-linearities are important because they allow control over light pulses in ubiquitous high-quality centro-symmetric materials like silicon and silica. Degenerate four-wave mixing provides a direct measure of the third-order non-linear sheet susceptibility χ(3)L (where L represents the material thickness) as well as technological possibilities such as optically gated detection and emission of photons. Using picosecond pulses from a free electron laser, we show that silicon doped with P or Bi has a value of χ(3)L in the THz domain that is higher than that reported for any other material in any wavelength band. The immediate implication of our results is the efficient generation of intense coherent THz light via upconversion (also a χ(3) process), and they open the door to exploitation of non-degenerate mixing and optical nonlinearities beyond the perturbative regime.
High-fidelity structured illumination microscopy by point-spread-function engineering
Gang Wen, Simin Li, Linbo Wang, Xiaohu Chen, Zhenglong Sun, et al.
Published. 2021, 10(5) : 812-823 doi: 10.1038/s41377-021-00513-w
Structured illumination microscopy (SIM) has become a widely used tool for insight into biomedical challenges due to its rapid, long-term, and super-resolution (SR) imaging. However, artifacts that often appear in SIM images have long brought into question its fidelity, and might cause misinterpretation of biological structures. We present HiFi-SIM, a high-fidelity SIM reconstruction algorithm, by engineering the effective point spread function (PSF) into an ideal form. HiFi-SIM can effectively reduce commonly seen artifacts without loss of fine structures and improve the axial sectioning for samples with strong background. In particular, HiFi-SIM is not sensitive to the commonly used PSF and reconstruction parameters; hence, it lowers the requirements for dedicated PSF calibration and complicated parameter adjustment, thus promoting SIM as a daily imaging tool.
Dynamical machine learning volumetric reconstruction of objects' interiors from limited angular views
Iksung Kang, Alexandre Goy, George Barbastathis
Published. 2021, 10(5) : 824-844 doi: 10.1038/s41377-021-00512-x
Limited-angle tomography of an interior volume is a challenging, highly ill-posed problem with practical implications in medical and biological imaging, manufacturing, automation, and environmental and food security. Regularizing priors are necessary to reduce artifacts by improving the condition of such problems. Recently, it was shown that one effective way to learn the priors for strongly scattering yet highly structured 3D objects, e.g. layered and Manhattan, is by a static neural network [Goy et al. Proc. Natl. Acad. Sci. 116, 19848-19856 (2019)]. Here, we present a radically different approach where the collection of raw images from multiple angles is viewed analogously to a dynamical system driven by the object-dependent forward scattering operator. The sequence index in the angle of illumination plays the role of discrete time in the dynamical system analogy. Thus, the imaging problem turns into a problem of nonlinear system identification, which also suggests dynamical learning as a better fit to regularize the reconstructions. We devised a Recurrent Neural Network (RNN) architecture with a novel Separable-Convolution Gated Recurrent Unit (SC-GRU) as the fundamental building block. Through a comprehensive comparison of several quantitative metrics, we show that the dynamic method is suitable for a generic interior-volumetric reconstruction under a limited-angle scheme. We show that this approach accurately reconstructs volume interiors under two conditions: weak scattering, when the Radon transform approximation is applicable and the forward operator well defined; and strong scattering, which is nonlinear with respect to the 3D refractive index distribution and includes uncertainty in the forward operator.
Polarization-insensitive 3D conformal-skin metasurface cloak
He-Xiu Xu, Guangwei Hu, Yanzhao Wang, Chaohui Wang, Mingzhao Wang, et al.
Published. 2021, 10(5) : 845-857 doi: 10.1038/s41377-021-00507-8
Electromagnetic metasurface cloaks provide an alternative paradigm toward rendering arbitrarily shaped scatterers invisible. Most transformation-optics (TO) cloaks intrinsically need wavelength-scale volume/thickness, such that the incoming waves could have enough long paths to interact with structured meta-atoms in the cloak region and consequently restore the wavefront. Other challenges of TO cloaks include the polarization-dependent operation to avoid singular parameters of composite cloaking materials and limitations of canonical geometries, e.g., circular, elliptical, trapezoidal, and triangular shapes. Here, we report for the first time a conformal-skin metasurface carpet cloak, enabling to work under arbitrary states of polarization (SOP) at Poincaré sphere for the incident light and arbitrary conformal platform of the object to be cloaked. By exploiting the foundry three-dimensional (3D) printing techniques to fabricate judiciously designed meta-atoms on the external surface of a conformal object, the spatial distributions of intensity and polarization of its scattered lights can be reconstructed exactly the same as if the scattering wavefront were deflected from a flat ground at any SOP, concealing targets under polarization-scanning detections. Two conformal-skin carpet cloaks working for partial- and full-azimuth plane operation are respectively fabricated on trapezoid and pyramid platforms via 3D printing. Experimental results are in good agreement with numerical simulations and both demonstrate the polarization-insensitive cloaking within a desirable bandwidth. Our approach paves a deterministic and robust step forward to the realization of interfacial, free-form, and full-polarization cloaking for a realistic arbitrary-shape target in real-world applications.
Engineering single-molecule fluorescence with asymmetric nano-antennas
Wenqi Zhao, Xiaochaoran Tian, Zhening Fang, Shiyi Xiao, Meng Qiu, et al.
Published. 2021, 10(5) : 858-866 doi: 10.1038/s41377-021-00522-9
As a powerful tool for studying molecular dynamics in bioscience, single-molecule fluorescence detection provides dynamical information buried in ensemble experiments. Fluorescence in the near-infrared (NIR) is particularly useful because it offers higher signal-to-noise ratio and increased penetration depth in tissue compared with visible fluorescence. The low quantum yield of most NIR fluorophores, however, makes the detection of single-molecule fluorescence difficult. Here, we use asymmetric plasmonic nano-antenna to enhance the fluorescence intensity of AIEE1000, a typical NIR dye, by a factor up to 405. The asymmetric nano-antenna achieve such an enhancement mainly by increasing the quantum yield (to ~80%) rather than the local field, which degrades the molecules' photostability. Our coupled-mode-theory analysis reveals that the enhancements stem from resonance-matching between antenna and molecule and, more importantly, from optimizing the coupling between the near- and far-field modes with designer asymmetric structures. Our work provides a universal scheme for engineering single-molecule fluorescence in the near-infrared regime.
Spontaneous and stimulated electron-photon interactions in nanoscale plasmonic near fields
Matthias Liebtrau, Murat Sivis, Armin Feist, Hugo Lourenço-Martins, Nicolas Pazos-Pérez, et al.
Published. 2021, 10(5) : 867-880 doi: 10.1038/s41377-021-00511-y
The interplay between free electrons, light, and matter offers unique prospects for space, time, and energy resolved optical material characterization, structured light generation, and quantum information processing. Here, we study the nanoscale features of spontaneous and stimulated electron-photon interactions mediated by localized surface plasmon resonances at the tips of a gold nanostar using electron energy-loss spectroscopy (EELS), cathodoluminescence spectroscopy (CL), and photon-induced near-field electron microscopy (PINEM). Supported by numerical electromagnetic boundary-element method (BEM) calculations, we show that the different coupling mechanisms probed by EELS, CL, and PINEM feature the same spatial dependence on the electric field distribution of the tip modes. However, the electron-photon interaction strength is found to vary with the incident electron velocity, as determined by the spatial Fourier transform of the electric near-field component parallel to the electron trajectory. For the tightly confined plasmonic tip resonances, our calculations suggest an optimum coupling velocity at electron energies as low as a few keV. Our results are discussed in the context of more complex geometries supporting multiple modes with spatial and spectral overlap. We provide fundamental insights into spontaneous and stimulated electron-light-matter interactions with key implications for research on (quantum) coherent optical phenomena at the nanoscale.
Intrinsic in-plane nodal chain and generalized quaternion charge protected nodal link in photonics
Dongyang Wang, Biao Yang, Qinghua Guo, Ruo-Yang Zhang, Lingbo Xia, et al.
Published. 2021, 10(5) : 881-889 doi: 10.1038/s41377-021-00523-8
Nodal lines are degeneracies formed by crossing bands in three-dimensional momentum space. Interestingly, these degenerate lines can chain together via touching points and manifest as nodal chains. These nodal chains are usually embedded in two orthogonal planes and protected by the corresponding mirror symmetries. Here, we propose and demonstrate an in-plane nodal chain in photonics, where all chained nodal lines coexist in a single mirror plane instead of two orthogonal ones. The chain point is stabilized by the intrinsic symmetry that is specific to electromagnetic waves at the Г point of zero frequency. By adding another mirror plane, we find a nodal ring that is constructed by two higher bands and links with the in-plane nodal chain. The nodal link in momentum space exhibits non-Abelian characteristics on a C2T - invariant plane, where admissible transitions of the nodal link structure are determined by generalized quaternion charges. Through near-field scanning measurements of bi-anisotropic metamaterials, we experimentally mapped out the in-plane nodal chain and nodal link in such systems.
Quantification of electron accumulation at grain boundaries in perovskite polycrystalline films by correlative infrared-spectroscopic nanoimaging and Kelvin probe force microscopy
Ting-Xiao Qin, En-Ming You, Mao-Xin Zhang, Peng Zheng, Xiao-Feng Huang, et al.
Published. 2021, 10(5) : 890-897 doi: 10.1038/s41377-021-00524-7
Organic-inorganic halide perovskites are emerging materials for photovoltaic applications with certified power conversion efficiencies (PCEs) over 25%. Generally, the microstructures of the perovskite materials are critical to the performances of PCEs. However, the role of the nanometer-sized grain boundaries (GBs) that universally existing in polycrystalline perovskite films could be benign or detrimental to solar cell performance, still remains controversial. Thus, nanometer-resolved quantification of charge carrier distribution to elucidate the role of GBs is highly desirable. Here, we employ correlative infrared-spectroscopic nanoimaging by the scattering-type scanning near-field optical microscopy with 20 nm spatial resolution and Kelvin probe force microscopy to quantify the density of electrons accumulated at the GBs in perovskite polycrystalline thin films. It is found that the electron accumulations are enhanced at the GBs and the electron density is increased from 6 × 1019 cm−3 in the dark to 8 × 1019 cm−3 under 10 min illumination with 532 nm light. Our results reveal that the electron accumulations are enhanced at the GBs especially under light illumination, featuring downward band bending toward the GBs, which would assist in electron-hole separation and thus be benign to the solar cell performance.
Revealing unconventional host-guest complexation at nanostructured interface by surface-enhanced Raman spectroscopy
Gan-Yu Chen, Yi-Bin Sun, Pei-Chen Shi, Tao Liu, Zhi-Hao Li, et al.
Published. 2021, 10(5) : 898-905 doi: 10.1038/s41377-021-00526-5
Interfacial host-guest complexation offers a versatile way to functionalize nanomaterials. However, the complicated interfacial environment and trace amounts of components present at the interface make the study of interfacial complexation very difficult. Herein, taking the advantages of near-single-molecule level sensitivity and molecular fingerprint of surface-enhanced Raman spectroscopy (SERS), we reveal that a cooperative effect between cucurbit[7]uril (CB[7]) and methyl viologen (MV2+2I) in aggregating Au NPs originates from the cooperative adsorption of halide counter anions I, MV2+, and CB[7] on Au NPs surface. Moreover, similar SERS peak shifts in the control experiments using CB[n]s but with smaller cavity sizes suggested the occurrence of the same guest complexations among CB[5], CB[6], and CB[7] with MV2+. Hence, an unconventional exclusive complexation model is proposed between CB[7] and MV2+ on the surface of Au NPs, distinct from the well-known 1:1 inclusion complexation model in aqueous solutions. In summary, new insights into the fundamental understanding of host-guest interactions at nanostructured interfaces were obtained by SERS, which might be useful for applications related to host-guest chemistry in engineered nanomaterials.
Super-resolution vibrational microscopy by stimulated Raman excited fluorescence
Hanqing Xiong, Naixin Qian, Yupeng Miao, Zhilun Zhao, Chen Chen, et al.
Published. 2021, 10(5) : 906-915 doi: 10.1038/s41377-021-00518-5
Inspired by the revolutionary impact of super-resolution fluorescence microscopy, super-resolution Raman imaging has been long pursued because of its much higher chemical specificity than the fluorescence counterpart. However, vibrational contrasts are intrinsically less sensitive compared with fluorescence, resulting in only mild resolution enhancement beyond the diffraction limit even with strong laser excitation power. As such, it is still a great challenge to achieve biocompatible super-resolution vibrational imaging in the optical far-field. In 2019 Stimulated Raman Excited Fluorescence (SREF) was discovered as an ultrasensitive vibrational spectroscopy that combines the high chemical specificity of Raman scattering and the superb sensitivity of fluorescence detection. Herein we developed a novel super-resolution vibrational imaging method by harnessing SREF as the contrast mechanism. We first identified the undesired role of anti-Stokes fluorescence background in preventing direct adoption of super-resolution fluorescence technique. We then devised a frequency-modulation (FM) strategy to remove the broadband backgrounds and achieved high-contrast SREF imaging. Assisted by newly synthesized SREF dyes, we realized multicolor FM-SREF imaging with nanometer spectral resolution. Finally, by integrating stimulated emission depletion (STED) with background-free FM-SREF, we accomplished high-contrast super-resolution vibrational imaging with STED-FM-SREF whose spatial resolution is only determined by the signal-to-noise ratio. In our proof-of-principle demonstration, more than two times of resolution improvement is achieved in biological systems with moderate laser excitation power, which shall be further refined with optimized instrumentation and imaging probes. With its super resolution, high sensitivity, vibrational contrast, and mild laser excitation power, STED-FM-SREF microscopy is envisioned to aid a wide variety of applications.
Compressively sampling the optical transmission matrix of a multimode fibre
Shuhui Li, Charles Saunders, Daniel J. Lum, John Murray-Bruce, Vivek K Goyal, et al.
Published. 2021, 10(5) : 916-930 doi: 10.1038/s41377-021-00514-9
The measurement of the optical transmission matrix (TM) of an opaque material is an advanced form of space-variant aberration correction. Beyond imaging, TM-based methods are emerging in a range of fields, including optical communications, micro-manipulation, and computing. In many cases, the TM is very sensitive to perturbations in the configuration of the scattering medium it represents. Therefore, applications often require an up-to-the-minute characterisation of the fragile TM, typically entailing hundreds to thousands of probe measurements. Here, we explore how these measurement requirements can be relaxed using the framework of compressive sensing, in which the incorporation of prior information enables accurate estimation from fewer measurements than the dimensionality of the TM we aim to reconstruct. Examples of such priors include knowledge of a memory effect linking the input and output fields, an approximate model of the optical system, or a recent but degraded TM measurement. We demonstrate this concept by reconstructing the full-size TM of a multimode fibre supporting 754 modes at compression ratios down to ~5% with good fidelity. We show that in this case, imaging is still possible using TMs reconstructed at compression ratios down to ~1% (eight probe measurements). This compressive TM sampling strategy is quite general and may be applied to a variety of other scattering samples, including diffusers, thin layers of tissue, fibre optics of any refractive profile, and reflections from opaque walls. These approaches offer a route towards the measurement of high-dimensional TMs either quickly or with access to limited numbers of measurements.
Ultra-broadband reflectionless Brewster absorber protected by reciprocity
Jie Luo, Hongchen Chu, Ruwen Peng, Mu Wang, Jensen Li, et al.
Published. 2021, 10(5) : 931-940 doi: 10.1038/s41377-021-00529-2
The Brewster's law predicts zero reflection of p-polarization on a dielectric surface at a particular angle. However, when loss is introduced into the permittivity of the dielectric, the Brewster condition breaks down and reflection unavoidably appears. In this work, we found an exception to this long-standing dilemma by creating a class of nonmagnetic anisotropic metamaterials, where anomalous Brewster effects with independently tunable absorption and refraction emerge. This loss-independent Brewster effect is bestowed by the extra degrees of freedoms introduced by anisotropy and strictly protected by the reciprocity principle. The bandwidth can cover an extremely wide spectrum from dc to optical frequencies. Two examples of reflectionless Brewster absorbers with different Brewster angles are both demonstrated to achieve large absorbance in a wide spectrum via microwave experiments. Our work extends the scope of Brewster effect to the horizon of nonmagnetic absorptive materials, which promises an unprecedented wide bandwidth for reflectionless absorption with high efficiency.