2019 Vol. 8, No. 4

Wide-field auroral imager onboard the Fengyun satellite
Xiao-Xin Zhang, Bo Chen, Fei He, Ke-Fei Song, Ling-Ping He, et al.
Published. 2019, 8(4) : 494-505 doi: 10.1038/s41377-019-0157-7
The newly launched Fengyun-3D (FY-3D) satellite carried a wide-field auroral imager (WAI) that was developed by Changchun Institute of Optics, Fine Mechanics and Physics, Chinese Academy of Sciences (CIOMP), which will provide a large field of view (FOV), high spatial resolution, and broadband ultraviolet images of the aurora and the ionosphere by imaging the N2 LBH bands of emissions. The WAI consists of two identical cameras, each with an FOV of 68° in the along-track direction and 10° in the cross-track direction. The two cameras are tilted relative to each other to cover a fan-shaped field of size 130° × 10°. Each camera consists of an unobstructed four-mirror anastigmatic optical system, a BaF2 filter, and a photon-counting imaging detector. The spatial resolution of WAI is ~10 km at the nadir point at a reference height of 110 km above the Earth's surface. The sensitivity is > 0.01 counts s-1 Rayleigh-1 pixel-1 (140-180 nm) for both cameras, which is sufficient for mapping the boundaries and the fine structures of the auroral oval during storms/substorms. Based on the tests and calibrations that were conducted prior to launch, the data processing algorithm includes photon signal decoding, geometric distortion correction, photometric correction, flat-field correction, line-of-sight projection and correction, and normalization between the two cameras. Preliminarily processed images are compared with DMSP SSUSI images. The agreement between the images that were captured by two instruments demonstrates that the WAI and the data processing algorithm operate normally and can provide high-quality scientific data for future studies on auroral dynamics.
Plasmonic metasurfaces with 42.3% transmission efficiency in the visible
Jihua Zhang, Mohamed ElKabbash, Ran Wei, Subhash C. Singh, Billy Lam, et al.
Published. 2019, 8(4) : 506-518 doi: 10.1038/s41377-019-0164-8
Metasurfaces are two-dimensional nanoantenna arrays that can control the propagation of light at will. In particular, plasmonic metasurfaces feature ultrathin thicknesses, ease of fabrication, field confinement beyond the diffraction limit, superior nonlinear properties, and ultrafast performances. However, the technological relevance of plasmonic metasurfaces operating in the transmission mode at optical frequencies is questionable due to their limited efficiency. The state-of-the-art efficiency of geometric plasmonic metasurfaces at visible and near-infrared frequencies, for example, is ≤10%. Here, we report a multipole-interference-based transmission-type geometric plasmonic metasurface with a polarization conversion efficiency that reaches 42.3% at 744 nm, over 400% increase over the state of the art. The efficiency is augmented by breaking the scattering symmetry due to simultaneously approaching the generalized Kerker condition for two orthogonal polarizations. In addition, the design of the metasurface proposed in this study introduces an air gap between the antennas and the surrounding media that confines the field within the gap, which mitigates the crosstalk between meta-atoms and minimizes metallic absorption. The proposed metasurface is broadband, versatile, easy to fabricate, and highly tolerant to fabrication errors. We highlight the technological relevance of our plasmonic metasurface by demonstrating a transmission-type beam deflector and hologram with record efficiencies.
Spatial sampling of terahertz fields with sub-wavelength accuracy via probe-beam encoding
Jiapeng Zhao, Yiwen E, Kaia Williams, Xi-Cheng Zhang, Robert W. Boyd
Published. 2019, 8(4) : 519-526 doi: 10.1038/s41377-019-0166-6
Recently, computational sampling methods have been implemented to spatially characterize terahertz (THz) fields. Previous methods usually rely on either specialized THz devices such as THz spatial light modulators or complicated systems requiring assistance from photon-excited free carriers with high-speed synchronization among multiple optical beams. Here, by spatially encoding an 800-nm near-infrared (NIR) probe beam through the use of an optical SLM, we demonstrate a simple sampling approach that can probe THz fields with a single-pixel camera. This design does not require any dedicated THz devices, semiconductors or nanofilms to modulate THz fields. Using computational algorithms, we successfully measure 128 × 128 field distributions with a 62-μm transverse spatial resolution, which is 15 times smaller than the central wavelength of the THz signal (940 μm). Benefitting from the non-invasive nature of THz radiation and sub-wavelength resolution of our system, this simple approach can be used in applications such as biomedical sensing, inspection of flaws in industrial products, and so on.
MEMS-in-the-lens architecture for a miniature high-NA laser scanning microscope
Tianbo Liu, Milind Rajadhyaksha, David L. Dickensheets
Published. 2019, 8(4) : 527-537 doi: 10.1038/s41377-019-0167-5
Laser scanning microscopes can be miniaturized for in vivo imaging by substituting optical microelectromechanical system (MEMS) devices in place of larger components. The emergence of multifunctional active optical devices can support further miniaturization beyond direct component replacement because those active devices enable diffraction-limited performance using simpler optical system designs. In this paper, we propose a catadioptric microscope objective lens that features an integrated MEMS device for performing biaxial scanning, axial focus adjustment, and control of spherical aberration. The MEMS-in-the-lens architecture incorporates a reflective MEMS scanner between a low-numerical-aperture back lens group and an aplanatic hyperhemisphere front refractive element to support high-numerical-aperture imaging. We implemented this new optical system using a recently developed hybrid polymer/silicon MEMS three-dimensional scan mirror that features an annular aperture that allows it to be coaxially aligned within the objective lens without the need for a beam splitter. The optical performance of the active catadioptric system is simulated and imaging of hard targets and human cheek cells is demonstrated with a confocal microscope that is based on the new objective lens design.
Multichannel direct transmissions of nearfield information
Xiang Wan, Qian Zhang, Tian Yi Chen, Lei Zhang, Wei Xu, et al.
Published. 2019, 8(4) : 538-545 doi: 10.1038/s41377-019-0169-3
A digital-coding programmable metasurface (DCPM) is a type of functional system that is composed of subwavelength-scale digital coding elements with opposite phase responses. By configuring the digital coding elements, a DCPM can construct dynamic near-field image patterns in which the intensity of each pixel of the image can be dynamically and independently modulated. Thus, a DCPM can perform both spatial and temporal modulations. Here, this advantage is used to realize multichannel direct transmissions of near-field information. Three points are selected in the near-field region to form three independent channels. By applying various digital phase codes on the DCPM, independent binary digital symbols defined by amplitude codes (namely, weak and strong amplitudes) are transmitted through the three channels. The measured near-field distributions and temporal transmissions of the system agree with numerical calculations. Compared with the conventional multichannel transmission, the proposed mechanism achieves simultaneous spatial and temporal modulations by treating DCPM as an energy radiator and information modulator, thereby enduing DCPM with high potential in near-field information processing and communications.
Limits of topological protection under local periodic driving
Z. Fedorova (Cherpakova), C. Jörg, C. Dauer, F. Letscher, M. Fleischhauer, et al.
Published. 2019, 8(4) : 546-557 doi: 10.1038/s41377-019-0172-8
The bulk-edge correspondence guarantees that the interface between two topologically distinct insulators supports at least one topological edge state that is robust against static perturbations. Here, we address the question of how dynamic perturbations of the interface affect the robustness of edge states. We illuminate the limits of topological protection for Floquet systems in the special case of a static bulk. We use two independent dynamic quantum simulators based on coupled plasmonic and dielectric photonic waveguides to implement the topological Su-Schriefer-Heeger model with convenient control of the full space- and time-dependence of the Hamiltonian. Local time-periodic driving of the interface does not change the topological character of the system but nonetheless leads to dramatic changes of the edge state, which becomes rapidly depopulated in a certain frequency window. A theoretical Floquet analysis shows that the coupling of Floquet replicas to the bulk bands is responsible for this effect. Additionally, we determine the depopulation rate of the edge state and compare it to numerical simulations.
Giant non-linear susceptibility of hydrogenic donors in silicon and germanium
Nguyen H. Le, Grigory V. Lanskii, Gabriel Aeppli, Benedict N. Murdin
Published. 2019, 8(4) : 558-564 doi: 10.1038/s41377-019-0174-6
Implicit summation is a technique for the conversion of sums over intermediate states in multiphoton absorption and the high-order susceptibility in hydrogen into simple integrals. Here, we derive the equivalent technique for hydrogenic impurities in multi-valley semiconductors. While the absorption has useful applications, it is primarily a loss process; conversely, the non-linear susceptibility is a crucial parameter for active photonic devices. For Si:P, we predict the hyperpolarizability ranges from χ(3)/n3D = 2.9 to 580 × 10-38 m5/V2 depending on the frequency, even while avoiding resonance. Using samples of a reasonable density, n3D, and thickness, L, to produce third-harmonic generation at 9 THz, a frequency that is difficult to produce with existing solid-state sources, we predict that χ(3) should exceed that of bulk InSb and χ(3)L should exceed that of graphene and resonantly enhanced quantum wells.
Ultrasonically sculpted virtual relay lens for in situ microimaging
Matteo Giuseppe Scopelliti, Maysamreza Chamanzar
Published. 2019, 8(4) : 565-579 doi: 10.1038/s41377-019-0173-7
We demonstrate in situ non-invasive relay imaging through a medium without inserting physical optical components. We show that a virtual optical graded-index (GRIN) lens can be sculpted in the medium using in situ reconfigurable ultrasonic interference patterns to relay images through the medium. Ultrasonic wave patterns change the local density of the medium to sculpt a graded refractive index pattern normal to the direction of light propagation, which modulates the phase front of light, causing it to focus within the medium and effectively creating a virtual relay lens. We demonstrate the in situ relay imaging and resolving of small features (22 μm) through a turbid medium (optical thickness = 5.7 times the scattering mean free path), which is normally opaque. The focal distance and the numerical aperture of the sculpted optical GRIN lens can be tuned by changing the ultrasonic wave parameters. As an example, we experimentally demonstrate that the axial focal distance can be continuously scanned over a depth of 5.4 mm in the modulated medium and that the numerical aperture can be tuned up to 21.5%. The interaction of ultrasonic waves and light can be mediated through different physical media, including turbid media, such as biological tissue, in which the ultrasonically sculpted GRIN lens can be used for relaying images of the underlying structures through the turbid medium, thus providing a potential alternative to implanting invasive endoscopes.
Deep-learning-powered photonic analog-to-digital conversion
Shaofu Xu, Xiuting Zou, Bowen Ma, Jianping Chen, Lei Yu, et al.
Published. 2019, 8(4) : 580-590 doi: 10.1038/s41377-019-0176-4
Analog-to-digital converters (ADCs) must be high speed, broadband, and accurate for the development of modern information systems, such as radar, imaging, and communications systems; photonic technologies are regarded as promising technologies for realizing these advanced requirements. Here, we present a deep-learning-powered photonic ADC architecture that simultaneously exploits the advantages of electronics and photonics and overcomes the bottlenecks of the two technologies, thereby overcoming the ADC tradeoff among speed, bandwidth, and accuracy. Via supervised training, the adopted deep neural networks learn the patterns of photonic system defects and recover the distorted data, thereby maintaining the high quality of the electronic quantized data succinctly and adaptively. The numerical and experimental results demonstrate that the proposed architecture outperforms state-of-the-art ADCs with developable high throughput; hence, deep learning performs well in photonic ADC systems. We anticipate that the proposed architecture will inspire future high-performance photonic ADC design and provide opportunities for substantial performance enhancement for the next-generation information systems.
A broadband achromatic metalens array for integral imaging in the visible
Zhi-Bin Fan, Hao-Yang Qiu, Han-Le Zhang, Xiao-Ning Pang, Li-Dan Zhou, et al.
Published. 2019, 8(4) : 600-609 doi: 10.1038/s41377-019-0178-2
Integral imaging is a promising three-dimensional (3D) imaging technique that captures and reconstructs light field information. Microlens arrays are usually used for the reconstruction process to display 3D scenes to the viewer. However, the inherent chromatic aberration of the microlens array reduces the viewing quality, and thus, broadband achromatic imaging remains a challenge for integral imaging. Here, we realize a silicon nitride metalens array in the visible region that can be used to reconstruct 3D optical scenes in the achromatic integral imaging for white light. The metalens array contains 60 × 60 polarization-insensitive metalenses with nearly diffraction-limited focusing. The nanoposts in each high-efficiency (measured as 47% on average) metalens are delicately designed with zero effective material dispersion and an effective achromatic refractive index distribution from 430 to 780 nm. In addition, such an achromatic metalens array is composed of only a single silicon nitride layer with an ultrathin thickness of 400 nm, making the array suitable for on-chip hybrid-CMOS integration and the parallel manipulation of optoelectronic information. We expect these findings to provide possibilities for full-color and aberration-free integral imaging, and we envision that the proposed approach may be potentially applicable in the fields of high-power microlithography, high-precision wavefront sensors, virtual/augmented reality and 3D imaging.
Reconfigurable beam system for non-line-of-sight free-space optical communication
Zizheng Cao, Xuebing Zhang, Gerwin Osnabrugge, Juhao Li, Ivo M. Vellekoop, et al.
Published. 2019, 8(4) : 591-599 doi: 10.1038/s41377-019-0177-3
In this paper, we propose a reconfigurable beam-shaping system to permit energy-efficient non-line-of-sight (NLOS) free-space optical communication. Light is steered around obstacles blocking the direct communication pathway and reaches a receiver after reflecting off of a diffuse surface. A coherent array optical transmitter (CAO-Tx) is used to spatially shape the wavefront of the light incident on a diffuse surface. Wavefront shaping is used to enhance the amount of diffusely reflected light reaching the optical receiver. Synthetic NLOS experiments for a signal reflected over an angular range of 20° are presented. A record-breaking 30-Gbit/s orthogonal frequency-division multiplexing signal is transmitted over a diffused optical wireless link with a > 17-dB gain.
Metasurface interferometry toward quantum sensors
Philip Georgi, Marcello Massaro, Kai-Hong Luo, Basudeb Sain, Nicola Montaut, et al.
Published. 2019, 8(4) : 620-626 doi: 10.1038/s41377-019-0182-6
Optical metasurfaces open new avenues for the precise wavefront control of light for integrated quantum technology. Here, we demonstrate a hybrid integrated quantum photonic system that is capable of entangling and disentangling two-photon spin states at a dielectric metasurface. Via the interference of single-photon pairs at a nanostructured dielectric metasurface, a path-entangled two-photon NOON state with circular polarization that exhibits a quantum HOM interference visibility of 86 ± 4% is generated. Furthermore, we demonstrate nonclassicality andphase sensitivity in a metasurface-based interferometer with a fringe visibility of 86.8 ± 1.1% in the coincidence counts. This high visibility proves the metasurface-induced path entanglement inside the interferometer. Our findings provide a promising way to develop hybrid-integrated quantum technology operating in the high-dimensional mode space in various applications, such as imaging, sensing, and computing.
Non-invasive determination of murine placental and foetal functional parameters with multispectral optoacoustic tomography
Kausik Basak, Xosé Luís Deán-Ben, Sven Gottschalk, Michael Reiss, Daniel Razansky
Published. 2019, 8(4) : 610-619 doi: 10.1038/s41377-019-0181-7
Despite the importance of placental function in embryonic development, it remains poorly understood and challenging to characterize, primarily due to the lack of non-invasive imaging tools capable of monitoring placental and foetal oxygenation and perfusion parameters during pregnancy. We developed an optoacoustic tomography approach for real-time imaging through entire ~4 cm cross-sections of pregnant mice. Functional changes in both maternal and embryo regions were studied at different gestation days when subjected to an oxygen breathing challenge and perfusion with indocyanine green. Structural phenotyping of the cross-sectional scans highlighted different internal organs, whereas multi-wavelength acquisitions enabled non-invasive label-free spectroscopic assessment of blood-oxygenation parameters in foeto-placental regions, rendering a strong correlation with the amount of oxygen administered. Likewise, the placental function in protecting the embryo from extrinsically administered agents was substantiated. The proposed methodology may potentially further serve as a probing mechanism to appraise embryo development during pregnancy in the clinical setting.
Simple and efficient delivery of cell-impermeable organic fluorescent probes into live cells for live-cell superresolution imaging
Meng Zhang, Meihua Li, Wenting Zhang, Yubing Han, Yu-Hui Zhang
Published. 2019, 8(4) : 627-637 doi: 10.1038/s41377-019-0188-0
Numerous commercial organic fluorophores with excellent optical properties are precluded from live-cell superresolution imaging due to poor cell permeability. Here, we develop a simple but effective strategy that renders cells permeable to cell-impermeable, organic fluorescent probes by using a novel peptide vehicle, PV-1. By simple coincubation with PV-1, 22 different cell-impermeable, organic fluorescent probes were efficiently delivered into live cells and specifically labeled a variety of organelles. Moreover, PV-1 can simultaneously transfer up to three different probes into live cells. By using PV-1 and these cell-impermeable fluorescent probes, we obtained multicolor, long-term, live-cell superresolution images of various organelles, which allowed us to study the dynamic interactions between them. PV-1, together with these organic fluorescent probes, will greatly broaden the applications of superresolution imaging technology in diverse live-cell studies and opens up a new avenue in the design and application of peptide vehicles.
Experimental demonstration of linear and spinning Janus dipoles for polarisation- and wavelength-selective near-field coupling
Michela F. Picardi, Martin Neugebauer, Jörg S. Eismann, Gerd Leuchs, Peter Banzer, et al.
Published. 2019, 8(4) : 464-470 doi: 10.1038/s41377-019-0162-x
The electromagnetic field scattered by nano-objects contains a broad range of wavevectors and can be efficiently coupled to waveguided modes. The dominant contribution to scattering from subwavelength dielectric and plasmonic nanoparticles is determined by electric and magnetic dipolar responses. Here, we experimentally demonstrate spectral and phase selective excitation of Janus dipoles, sources with electric and magnetic dipoles oscillating out of phase, in order to control near-field interference and directional coupling to waveguides. We show that by controlling the polarisation state of the dipolar excitations and the excitation wavelength to adjust their relative contributions, directionality and coupling strength can be fully tuned. Furthermore, we introduce a novel spinning Janus dipole featuring cylindrical symmetry in the near and far field, which results in either omnidirectional coupling or noncoupling. Controlling the propagation of guided light waves via fast and robust near-field interference between polarisation components of a source is required in many applications in nanophotonics and quantum optics.
Review Article
Superoscillation: from physics to optical applications
Gang Chen, Zhong-Quan Wen, Cheng-Wei Qiu
Published. 2019, 8(4) : 471-493 doi: 10.1038/s41377-019-0163-9
The resolution of conventional optical elements and systems has long been perceived to satisfy the classic Rayleigh criterion. Paramount efforts have been made to develop different types of superresolution techniques to achieve optical resolution down to several nanometres, such as by using evanescent waves, fluorescence labelling, and postprocessing. Superresolution imaging techniques, which are noncontact, far field and label free, are highly desirable but challenging to implement. The concept of superoscillation offers an alternative route to optical superresolution and enables the engineering of focal spots and point-spread functions of arbitrarily small size without theoretical limitations. This paper reviews recent developments in optical superoscillation technologies, design approaches, methods of characterizing superoscillatory optical fields, and applications in noncontact, far-field and label-free superresolution microscopy. This work may promote the wider adoption and application of optical superresolution across different wave types and application domains.
News & Views
Toward in vivo translation of super-resolution localization photoacoustic computed tomography using liquid-state dyed droplets
Wonseok Choi, Chulhong Kim
Published. 2019, 8(4) : 453-454 doi: 10.1038/s41377-019-0171-9
Color routing at the nanoscale
Qing-Hua Xu
Published. 2019, 8(4) : 455-456 doi: 10.1038/s41377-019-0170-x
Breaking the spatial reciprocity with Janus metamaterials
Lingling Huang
Published. 2019, 8(4) : 457-458 doi: 10.1038/s41377-019-0175-5
Switching of the observation direction as well as the polarization channels of chiral Janus metamaterials may result in different reconstructed images.
Nanosecond modulation of thermal emission
Daniel Wasserman
Published. 2019, 8(4) : 459-461 doi: 10.1038/s41377-019-0179-1
Femtosecond laser pulses are used to modulate the thermal emission from semiconductor materials at the nanosecond timescale. A visible-frequency laser photoexcites energetic free carriers in intrinsic Si and GaAs wafers. As these free carriers return to equilibrium, they not only emit thermal radiation on a picosecond time scale but also modulate the semiconductor thermal emission on a nanosecond to microsecond time scale, offering a novel route towards ultrafast infrared optical pulses.
Terahertz spatial sampling with subwavelength accuracy
Yan Peng, Yiming Zhu, Min Gu, Songlin Zhuang
Published. 2019, 8(4) : 462-463 doi: 10.1038/s41377-019-0185-3
A simple terahertz (THz) spatial sampling method offers kilohertz (kHz) level sampling rates and greatly preserves the energy of a THz pulse, which enables THz imaging detection with a high signal-to-noise ratio, micron-grade accuracy, and subwavelength resolution.