2018 Vol. 7, No. 5

Original Articles
In situ plasmonic optical fiber detection of the state of charge of supercapacitors for renewable energy storage
Jiajie Lao, Peng Sun, Fu Liu, Xuejun Zhang, Chuanxi Zhao, et al.
Published. 2018, 7(5) : 388-398 doi: 10.1038/s41377-018-0040-y
In situ and continuous monitoring of electrochemical activity is key to understanding and evaluating the operation mechanism and efficiency of energy storage devices. However, this task remains challenging. For example, the present methods are not capable of providing the real-time information about the state of charge (SOC) of the energy storage devices while in operation. To address this, a novel approach based on an electrochemical surface plasmon resonance (SPR) optical fiber sensor is proposed here. This approach offers the capability of in situ comprehensive monitoring of the electrochemical activity (the electrode potential and the SOC) of supercapacitors (used as an example). The sensor adopted is a tilted fiber Bragg grating imprinted in a commercial single-mode fiber and coated with a nanoscale gold film for high-efficiency SPR excitation. Unlike conventional "bulk" detection methods for electrode activity, our approach targets the "localized" (sub-μm-scale) charge state of the ions adjacent to the electrode interface of supercapacitors by monitoring the properties of the SPR wave on the fiber sensor surface located adjacent to the electrode. A stable and reproducible correlation between the real-time charge–discharge cycles of the supercapacitors and the optical transmission of the optical fiber has been found. Moreover, the method proposed is inherently immune to temperature cross-talk because of the presence of environmentally insensitive reference features in the optical transmission spectrum of the devices. Finally, this particular application is ideally suited to the fundamental qualities of optical fiber sensors, such as their compact size, flexible shape, and remote operation capability, thereby opening the way for other opportunities for electrochemical monitoring in various hard-to-reach spaces and remote environments.
Generalization of the optical theorem: experimental proof for radially polarized beams
Alexey V. Krasavin, Paulina Segovia, Rostyslav Dubrovka, Nicolas Olivier, Gregory A. Wurtz, et al.
Published. 2018, 7(5) : 408-417 doi: 10.1038/s41377-018-0025-x
The optical theorem, which is a consequence of the energy conservation in scattering processes, directly relates the forward scattering amplitude to the extinction cross-section of the object. Originally derived for planar scalar waves, it neglects the complex structure of the focused beams and the vectorial nature of the electromagnetic field. On the other hand, radially or azimuthally polarized fields and various vortex beams, essential in modern photonic technologies, possess a prominent vectorial field structure. Here, we experimentally demonstrate a complete violation of the commonly used form of the optical theorem for radially polarized beams at both visible and microwave frequencies. We show that a plasmonic particle illuminated by such a beam exhibits strong extinction, while the scattering in the forward direction is zero. The generalized formulation of the optical theorem provides agreement with the observed results. The reported effect is vital for the understanding and design of the interaction of complex vector beams carrying longitudinal field components with subwavelength objects important in imaging, communications, nanoparticle manipulation, and detection, as well as metrology.
Nanostructured fibers as a versatile photonic platform: radiative cooling and waveguiding through transverse Anderson localization
Norman Nan Shi, Cheng-Chia Tsai, Michael J. Carter, Jyotirmoy Mandal, Adam C. Overvig, et al.
Published. 2018, 7(5) : 399-407 doi: 10.1038/s41377-018-0033-x
Broadband high reflectance in nature is often the result of randomly, three-dimensionally structured materials. This study explores unique optical properties associated with one-dimensional nanostructures discovered in silk cocoon fibers of the comet moth, Argema mittrei. The fibers are populated with a high density of air voids randomly distributed across the fiber cross-section but are invariant along the fiber. These filamentary air voids strongly scatter light in the solar spectrum. A single silk fiber measuring ~50 μm thick can reflect 66% of incoming solar radiation, and this, together with the fibers' high emissivity of 0.88 in the mid-infrared range, allows the cocoon to act as an efficient radiative-cooling device. Drawing inspiration from these natural radiative-cooling fibers, biomimetic nanostructured fibers based on both regenerated silk fibroin and polyvinylidene difluoride are fabricated through wet spinning. Optical characterization shows that these fibers exhibit exceptional optical properties for radiative-cooling applications: nanostructured regenerated silk fibers provide a solar reflectivity of 0.73 and a thermal emissivity of 0.90, and nanostructured polyvinylidene difluoride fibers provide a solar reflectivity of 0.93 and a thermal emissivity of 0.91. The filamentary air voids lead to highly directional scattering, giving the fibers a highly reflective sheen, but more interestingly, they enable guided optical modes to propagate along the fibers through transverse Anderson localization. This discovery opens up the possibility of using wild silkmoth fibers as a biocompatible and bioresorbable material for optical signal and image transport.
Observation of parity-time symmetry in microwave photonics
Yanzhong Liu, Tengfei Hao, Wei Li, Jose Capmany, Ninghua Zhu, et al.
Published. 2018, 7(5) : 379-387 doi: 10.1038/s41377-018-0035-8
Symmetry plays a crucial role in explorations of the laws of nature. Parity-time (PT) symmetry phenomena can lead to entirely real spectra in non-Hermitian systems, which attracts considerable attention in the fields of optics and electronics because these phenomena provide a new tool for the manipulation of oscillation modes and non-reciprocal signal transmission. A potential new field of application is microwave photonics, an interdisciplinary field in which the interaction between microwaves and optical signals is exploited. In this article, we report the experimental use of PT symmetry in an optoelectronic oscillator (OEO), a key microwave photonics system that can generate single-frequency sinusoidal signals with high spectral purity. PT symmetry is theoretically analyzed and experimentally observed in an OEO with two mutually coupled active oscillation cavities via a precise manipulation of the interplay between gain and loss in the two oscillation cavities. Stable single-frequency microwave oscillation is achieved without using any optical/electrical filters for oscillation mode selection, which is an indispensable requirement in traditional OEOs. This observation opens new avenues for signal generation and processing based on the PT symmetry principle in microwave photonics.
Wavelength stability in a hybrid photonic crystal laser through controlled nonlinear absorptive heating in the reflector
Andrei P. Bakoz, Alexandros A. Liles, Alfredo A. Gonzalez-Fernandez, Tatiana Habruseva, Changyu Hu, et al.
Published. 2018, 7(5) : 438-444 doi: 10.1038/s41377-018-0043-8
The need for miniaturized, fully integrated semiconductor lasers has stimulated significant research efforts into realizing unconventional configurations that can meet the performance requirements of a large spectrum of applications, ranging from communication systems to sensing. We demonstrate a hybrid, silicon photonics-compatible photonic crystal (PhC) laser architecture that can be used to implement cost-effective, high-capacity light sources, with high side-mode suppression ratio and milliwatt output output powers. The emitted wavelength is set and controlled by a silicon PhC cavity-based reflective filter with the gain provided by a Ⅲ-Ⅴ-based reflective semiconductor optical amplifier (RSOA). The high power density in the laser cavity results in a significant enhancement of the nonlinear absorption in silicon in the high Q-factor PhC resonator. The heat generated in this manner creates a tuning effect in the wavelength-selective element, which can be used to offset external temperature fluctuations without the use of active cooling. Our approach is fully compatible with existing fabrication and integration technologies, providing a practical route to integrated lasing in wavelength-sensitive schemes.
Experimental demonstration of tunable refractometer based on orbital angular momentum of longitudinally structured light
Ahmed H. Dorrah, Michel Zamboni-Rached, Mo Mojahedi
Published. 2018, 7(5) : 418-429 doi: 10.1038/s41377-018-0034-9
The index of refraction plays a decisive role in the design and classification of optical materials and devices; therefore, its proper and accurate determination is essential. In most refractive index (RI) sensing schemes, however, there is a trade-off between providing high-resolution measurements and covering a wide range of RIs. We propose and experimentally demonstrate a novel mechanism for sensing the index of refraction of a medium by utilizing the orbital angular momentum (OAM) of structured light. Using a superposition of co-propagating monochromatic higher-order Bessel beams with equally spaced longitudinal wavenumbers, in a comb-like setting, we generate non-diffracting rotating light structures in which the orientation of the beam's intensity profile is sensitive to the RI of the medium (here, a fluid). In principle, the sensitivity of this scheme can exceed ~2700°/RI unit (RIU) with a resolution of ~\begin{document}$10^{ - 5}$\end{document} RIU. Furthermore, we show how the unbounded degrees of freedom associated with OAM can be deployed to offer a wide dynamic range by generating structured light that evolves into different patterns based on the change in RI. The rotating light structures are generated by a programmable spatial light modulator. This provides dynamic control over the sensitivity, which can be tuned to perform coarse or fine measurements of the RI in real time. This, in turn, allows high sensitivity and resolution to be achieved simultaneously over a very wide dynamic range, which is a typical trade-off in all RI sensing schemes. We thus envision that this method will open new directions in refractometry and remote sensing.
Quantum-optical spectroscopy of a two-level system using an electrically driven micropillar laser as a resonant excitation source
Sören Kreinberg, Tomislav Grbešić, Max Strauß, Alexander Carmele, Monika Emmerling, et al.
Published. 2018, 7(5) : 445-453 doi: 10.1038/s41377-018-0045-6
Two-level emitters are the main building blocks of photonic quantum technologies and are model systems for the exploration of quantum optics in the solid state. Most interesting is the strict resonant excitation of such emitters to control their occupation coherently and to generate close to ideal quantum light, which is of utmost importance for applications in photonic quantum technology. To date, the approaches and experiments in this field have been performed exclusively using bulky lasers, which hinders the application of resonantly driven two-level emitters in compact photonic quantum systems. Here we address this issue and present a concept for a compact resonantly driven single-photon source by performing quantum-optical spectroscopy of a two-level system using a compact high-β microlaser as the excitation source. The two-level system is based on a semiconductor quantum dot (QD), which is excited resonantly by a fiber-coupled electrically driven micropillar laser. We dress the excitonic state of the QD under continuous wave excitation, and trigger the emission of single photons with strong multi-photon suppression (\begin{document}$\, g^{(2)}(0) = 0.02$\end{document}) and high photon indistinguishability (V = 57±9%) via pulsed resonant excitation at 156 MHz. These results clearly demonstrate the high potential of our resonant excitation scheme, which can pave the way for compact electrically driven quantum light sources with excellent quantum properties to enable the implementation of advanced quantum communication protocols.
Single-shot real-time femtosecond imaging of temporal focusing
Jinyang Liang, Liren Zhu, Lihong V. Wang
Published. 2018, 7(5) : 466-475 doi: 10.1038/s41377-018-0044-7
While the concept of focusing usually applies to the spatial domain, it is equally applicable to the time domain. Real-time imaging of temporal focusing of single ultrashort laser pulses is of great significance in exploring the physics of the space–time duality and finding diverse applications. The drastic changes in the width and intensity of an ultrashort laser pulse during temporal focusing impose a requirement for femtosecond-level exposure to capture the instantaneous light patterns generated in this exquisite phenomenon. Thus far, established ultrafast imaging techniques either struggle to reach the desired exposure time or require repeatable measurements. We have developed single-shot 10-trillion-frame-per-second compressed ultrafast photography (T-CUP), which passively captures dynamic events with 100-fs frame intervals in a single camera exposure. The synergy between compressed sensing and the Radon transformation empowers T-CUP to significantly reduce the number of projections needed for reconstructing a high-quality three-dimensional spatiotemporal datacube. As the only currently available real-time, passive imaging modality with a femtosecond exposure time, T-CUP was used to record the first-ever movie of non-repeatable temporal focusing of a single ultrashort laser pulse in a dynamic scattering medium. T-CUP's unprecedented ability to clearly reveal the complex evolution in the shape, intensity, and width of a temporally focused pulse in a single measurement paves the way for single-shot characterization of ultrashort pulses, experimental investigation of nonlinear light-matter interactions, and real-time wavefront engineering for deep-tissue light focusing.
Boosting third-harmonic generation by a mirror-enhanced anapole resonator
Lei Xu, Mohsen Rahmani, Khosro Zangeneh Kamali, Aristeidis Lamprianidis, Lavinia Ghirardini, et al.
Published. 2018, 7(5) : 430-437 doi: 10.1038/s41377-018-0051-8
We demonstrate that a dielectric anapole resonator on a metallic mirror can enhance the third harmonic emission by two orders of magnitude compared to a typical anapole resonator on an insulator substrate. By employing a gold mirror under a silicon nanodisk, we introduce a novel characteristic of the anapole mode through the spatial overlap of resonantly excited Cartesian electric and toroidal dipole modes. This is a remarkable improvement on the early demonstrations of the anapole mode in which the electric and toroidal modes interfere off-resonantly. Therefore, our system produces a significant near-field enhancement, facilitating the nonlinear process. Moreover, the mirror surface boosts the nonlinear emission via the free-charge oscillations within the interface, equivalent to producing a mirror image of the nonlinear source and the pump beneath the interface. We found that these improvements result in an extremely high experimentally obtained efficiency of 0.01%.
Tailoring spin mixtures by ion-enhanced Maxwell magnetic coupling in color-tunable organic electroluminescent devices
Junwei Xu, Yue Cui, Gregory M. Smith, Peiyun Li, Chaochao Dun, et al.
Published. 2018, 7(5) : 454-465 doi: 10.1038/s41377-018-0046-5
In this work, we show that the spin dynamics of excitons can be dramatically altered by Maxwell magnetic field coupling, together with an ion-enhanced, low-internal-splitting-energy organic semiconducting emitter. By employing a unique, alternating current (AC)-driven organic electroluminescent (OEL) device architecture that optimizes this magnetic field coupling, almost complete control over the singlet-to-triplet ratio (from fluorescent to phosphorescent emission in a single device) is realized. We attribute this spin population control to magnetically sensitive polaron–spin pair intersystem crossings (ISCs) that can be directly manipulated through external driving conditions. As an illustration of the utility of this approach to spin-tailoring, we demonstrate a simple hybrid (double-layer) fluorescence–phosphorescence (F–P) device using a polyfluorene-based emitter with a strong external Zeeman effect and ion-induced long carrier diffusion. Remarkable control over de-excitation pathways is achieved by controlling the device-driving frequency, resulting in complete emission blue–red color tunability. Picosecond photoluminescence (PL) spectroscopy directly confirms that this color control derives from the magnetic manipulation of the singlet-to-triplet ratios. These results may pave the way to far more exotic organic devices with magnetic-field-coupled organic systems that are poised to usher in an era of dynamic spintronics at room temperature.
Repeated photoporation with graphene quantum dots enables homogeneous labeling of live cells with extrinsic markers for fluorescence microscopy
Jing Liu, Ranhua Xiong, Toon Brans, Saskia Lippens, Eef Parthoens, et al.
Published. 2018, 7(5) : 476-485 doi: 10.1038/s41377-018-0048-3
In the replacement of genetic probes, there is increasing interest in labeling living cells with high-quality extrinsic labels, which avoid over-expression artifacts and are available in a wide spectral range. This calls for a broadly applicable technology that can deliver such labels unambiguously to the cytosol of living cells. Here, we demonstrate that nanoparticle-sensitized photoporation can be used to this end as an emerging intracellular delivery technique. We replace the traditionally used gold nanoparticles with graphene nanoparticles as photothermal sensitizers to permeabilize the cell membrane upon laser irradiation. We demonstrate that the enhanced thermal stability of graphene quantum dots allows the formation of multiple vapor nanobubbles upon irradiation with short laser pulses, allowing the delivery of a variety of extrinsic cell labels efficiently and homogeneously into live cells. We demonstrate high-quality time-lapse imaging with confocal, total internal reflection fluorescence (TIRF), and Airyscan super-resolution microscopy. As the entire procedure is readily compatible with fluorescence (super resolution) microscopy, photoporation with graphene quantum dots has the potential to become the long-awaited generic platform for controlled intracellular delivery of fluorescent labels for live-cell imaging.
Second harmonic generation hotspot on a centrosymmetric smooth silver surface
Matan Galanty, Omer Shavit, Adam Weissman, Hannah Aharon, David Gachet, et al.
Published. 2018, 7(5) : 515-522 doi: 10.1038/s41377-018-0053-6
Second harmonic generation (SHG) is forbidden for materials with inversion symmetry, such as bulk metals. Symmetry can be broken by morphological or dielectric discontinuities, yet SHG from a smooth continuous metallic surface is negligible. Using non-linear microscopy, we experimentally demonstrate enhanced SHG within an area of smooth silver film surrounded by nanocavities. Nanocavity-assisted SHG is locally enhanced by more than one order of magnitude compared to a neighboring silver surface area. Linear optical measurements and cathodoluminescence (CL) imaging substantiate these observations. We suggest that plasmonic modes launched from the edges of the nanocavities propagate onto the smooth silver film and annihilate, locally generating SHG. In addition, we show that these hotspots can be dynamically controlled in intensity and location by altering the polarization of the incoming field. Our results show that switchable nonlinear hotspots can be generated on smooth metallic films, with important applications in photocatalysis, single-molecule spectroscopy and non-linear surface imaging.
A hybrid invisibility cloak based on integration of transparent metasurfaces and zero-index materials
Hongchen Chu, Qi Li, Bingbing Liu, Jie Luo, Shulin Sun, et al.
Published. 2018, 7(5) : 507-514 doi: 10.1038/s41377-018-0052-7
The invisibility cloak, a long-standing fantastic dream for humans, has become more tangible with the development of metamaterials. Recently, metasurface-based invisibility cloaks have been proposed and realized with significantly reduced thickness and complexity of the cloaking shell. However, the previous scheme is based on reflection-type metasurfaces and is thus limited to reflection geometry. In this work, by integrating the wavefront tailoring functionality of transparent metasurfaces and the wave tunneling functionality of zero-index materials, we have realized a unique type of hybrid invisibility cloak that functions in transmission geometry. The principle is general and applicable to arbitrary shapes. For experimental demonstration, we constructed a rhombic double-layer cloaking shell composed of a highly transparent metasurface and a double-zero medium consisting of dielectric photonic crystals with Dirac cone dispersions. The cloaking effect is verified by both full-wave simulations and microwave experimental results. The principle also reveals exciting possibilities for realizing skin-thick ultrathin cloaking shells in transmission geometry, which can eliminate the need for spatially varying extreme parameters. Our work paves a path for novel optical and electromagnetic devices based on the integration of metasurfaces and metamaterials.
Hybrid graphene metasurfaces for high-speed mid-infrared light modulation and single-pixel imaging
Beibei Zeng, Zhiqin Huang, Akhilesh Singh, Yu Yao, Abul K. Azad, et al.
Published. 2018, 7(5) : 523-530 doi: 10.1038/s41377-018-0055-4
During the past decades, major advances have been made in both the generation and detection of infrared light; however, its efficient wavefront manipulation and information processing still encounter great challenges. Efficient and fast optoelectronic modulators and spatial light modulators are required for mid-infrared imaging, sensing, security screening, communication and navigation, to name a few. However, their development remains elusive, and prevailing methods reported so far have suffered from drawbacks that significantly limit their practical applications. In this study, by leveraging graphene and metasurfaces, we demonstrate a high-performance free-space mid-infrared modulator operating at gigahertz speeds, low gate voltage and room temperature. We further pixelate the hybrid graphene metasurface to form a prototype spatial light modulator for high frame rate single-pixel imaging, suggesting orders of magnitude improvement over conventional liquid crystal or micromirror-based spatial light modulators. This work opens up the possibility of exploring wavefront engineering for infrared technologies for which fast temporal and spatial modulations are indispensable.
Digital plasmonic holography
Joseph W. Nelson, Greta R. Knefelkamp, Alexandre G. Brolo, Nathan C. Lindquist
Published. 2018, 7(5) : 496-506 doi: 10.1038/s41377-018-0049-2
We demonstrate digital plasmonic holography for direct in-plane imaging with propagating surface-plasmon waves. Imaging with surface plasmons suffers from the lack of simple in-plane lenses and mirrors. Lens-less digital holography techniques, however, rely on digitally decoding an interference pattern between a reference wave and an object wave. With far-field diffractive optics, this decoding scheme provides a full recording, i.e., a hologram, of the amplitude and phase of the object wave, giving three-dimensional information from a two-dimensional recording. For plasmonics, only a one-dimensional recording is needed, and both the phase and amplitude of the propagating plasmons can be extracted for high-resolution in-plane imaging. Here, we demonstrate lens-less, point-source digital plasmonic holography using two methods to record the plasmonic holograms: a dual-probe near-field scanning optical microscope and lithographically defined circular fluorescent screens. The point-source geometry gives in-plane magnification, allowing for high-resolution imaging with relatively lower-resolution microscope objectives. These results pave the way for a new form of in-plane plasmonic imaging, gathering the full complex wave, without the need for plasmonic mirrors or lenses.
Complete polarization control in multimode fibers with polarization and mode coupling
Wen Xiong, Chia Wei Hsu, Yaron Bromberg, Jose Enrique Antonio-Lopez, Rodrigo Amezcua Correa, et al.
Published. 2018, 7(5) : 486-495 doi: 10.1038/s41377-018-0047-4
Multimode optical fibers have seen increasing applications in communication, imaging, high-power lasers, and amplifiers. However, inherent imperfections and environmental perturbations cause random polarization and mode mixing, causing the output polarization states to be different from the input polarization states. This difference poses a serious issue for employing polarization-sensitive techniques to control light–matter interactions or nonlinear optical processes at the distal end of a fiber probe. Here, we demonstrate complete control of polarization states for all output channels by only manipulating the spatial wavefront of a laser beam into the fiber. Arbitrary polarization states for individual output channels are generated by wavefront shaping without constraining the input polarization. The strong coupling between the spatial and polarization degrees of freedom in a multimode fiber enables full polarization control with the spatial degrees of freedom alone; thus, wavefront shaping can transform a multimode fiber into a highly efficient reconfigurable matrix of waveplates for imaging and communication applications.
Probing the limits of plasmonic enhancement using a two-dimensional atomic crystal probe
Wen Chen, Shunping Zhang, Meng Kang, Weikang Liu, Zhenwei Ou, et al.
Published. 2018, 7(5) : 531-541 doi: 10.1038/s41377-018-0056-3
Achieving larger electromagnetic enhancement using a nanogap between neighboring metallic nanostructures has been long pursued for boosting light–matter interactions. However, the quantitative probing of this enhancement is hindered by the lack of a reliable experimental method for measuring the local fields within a subnanometer gap. Here, we use layered MoS2 as a two-dimensional atomic crystal probe in nanoparticle-on-mirror nanoantennas to measure the plasmonic enhancement in the gap by quantitative surface-enhanced Raman scattering. Our designs ensure that the probe filled in the gap has a well-defined lattice orientation and thickness, enabling independent extraction of the anisotropic field enhancements. We find that the field enhancement can be safely described by pure classical electromagnetic theory when the gap distance is no < 1.24 nm. For a 0.62 nm gap, the probable emergence of quantum mechanical effects renders an average electric field enhancement of 114-fold, 38.4% lower than classical predictions.
Single capillary oximetry and tissue ultrastructural sensing by dual-band dual-scan inverse spectroscopic optical coherence tomography
Rongrong Liu, James A. Winkelmann, Graham Spicer, Yunxiao Zhu, Aya Eid, et al.
Published. 2018, 7(5) : 542-554 doi: 10.1038/s41377-018-0057-2
Measuring capillary oxygenation and the surrounding ultrastructure can allow one to monitor a microvascular niche and better understand crucial biological mechanisms. However, capillary oximetry and pericapillary ultrastructure are challenging to measure in vivo. Here we demonstrate a novel optical imaging system, dual-band dual-scan inverse spectroscopic optical coherence tomography (D2-ISOCT), that, for the first time, can simultaneously obtain the following metrics in vivo using endogenous contrast: (1) capillary-level oxygen saturation and arteriolar-level blood flow rates, oxygen delivery rates, and oxygen metabolic rates; (2) spatial characteristics of tissue structures at length scales down to 30 nm; and (3) morphological images up to 2 mm in depth. To illustrate the capabilities of D2-ISOCT, we monitored alterations to capillaries and the surrounding pericapillary tissue (tissue between the capillaries) in the healing response of a mouse ear wound model. The obtained microvascular and ultrastructural metrics corroborated well with each other, showing the promise of D2-ISOCT for becoming a powerful new non-invasive imaging tool.
Modal energy transfer by thermally induced refractive index gratings in Yb-doped fibers
Christoph Stihler, Cesar Jauregui, Andreas Tünnermann, Jens Limpert
Published. 2018, 7(5) : 563-574 doi: 10.1038/s41377-018-0061-6
Thermally induced refractive index gratings in Yb-doped fibers lead to transverse mode instability (TMI) above an average power threshold, which represents a severe problem for many applications. To obtain a deeper understanding of TMI, the evolution of the strength of the thermally induced refractive index grating with the average output power in a fiber amplifier is experimentally investigated for the first time. This investigation is performed by introducing a phase shift between the refractive index grating and modal interference pattern, which is obtained by applying a pump power variation to the fiber amplifier. It is demonstrated that the refractive index grating is sufficiently strong to enable modal energy coupling at powers that are significantly below the TMI threshold if the induced phase shift is sufficiently large. The experiments indicate that at higher powers, the refractive index grating becomes more sensitive to such phase shifts, which will ultimately trigger TMI. Furthermore, the experimental results demonstrate beam cleaning above the TMI threshold via the introduction of a positive phase shift. This finding paves the way for the development of a new class of mitigation strategies for TMI that are based on controlling the phase shift between the thermally induced refractive index grating and modal interference pattern.
Plasmonic nanostructure design and characterization via Deep Learning
Itzik Malkiel, Michael Mrejen, Achiya Nagler, Uri Arieli, Lior Wolf, et al.
Published. 2018, 7(5) : 555-562 doi: 10.1038/s41377-018-0060-7
Nanophotonics, the field that merges photonics and nanotechnology, has in recent years revolutionized the field of optics by enabling the manipulation of light–matter interactions with subwavelength structures. However, despite the many advances in this field, the design, fabrication and characterization has remained widely an iterative process in which the designer guesses a structure and solves the Maxwell's equations for it. In contrast, the inverse problem, i.e., obtaining a geometry for a desired electromagnetic response, remains a challenging and time-consuming task within the boundaries of very specific assumptions. Here, we experimentally demonstrate that a novel Deep Neural Network trained with thousands of synthetic experiments is not only able to retrieve subwavelength dimensions from solely far-field measurements but is also capable of directly addressing the inverse problem. Our approach allows the rapid design and characterization of metasurface-based optical elements as well as optimal nanostructures for targeted chemicals and biomolecules, which are critical for sensing, imaging and integrated spectroscopy applications.
On-chip excitation of single germanium vacancies in nanodiamonds embedded in plasmonic waveguides
Hamidreza Siampour, Shailesh Kumar, Valery A. Davydov, Liudmila F. Kulikova, Viatcheslav N. Agafonov, et al.
Published. 2018, 7(5) : 587-595 doi: 10.1038/s41377-018-0062-5
Monolithic integration of quantum emitters in nanoscale plasmonic circuitry requires low-loss plasmonic configurations capable of confining light well below the diffraction limit. We demonstrated on-chip remote excitation of nanodiamond-embedded single quantum emitters by plasmonic modes of dielectric ridges atop colloidal silver crystals. The nanodiamonds were produced to incorporate single germanium-vacancy (GeV) centres, providing bright, spectrally narrow and stable single-photon sources suitable for highly integrated circuits. Using electron-beam lithography with hydrogen silsesquioxane (HSQ) resist, dielectric-loaded surface plasmon polariton waveguides (DLSPPWs) were fabricated on single crystalline silver plates to contain those of deposited nanodiamonds that are found to feature appropriate single GeV centres. The low-loss plasmonic configuration enabled the 532-nm pump laser light to propagate on-chip in the DLSPPW and reach to an embedded nanodiamond where a single GeV centre was incorporated. The remote GeV emitter was thereby excited and coupled to spatially confined DLSPPW modes with an outstanding figure-of-merit of 180 due to a ~six-fold Purcell enhancement, ~56% coupling efficiency and ~33 μm transmission length, thereby opening new avenues for the implementation of nanoscale functional quantum devices.
Full-space Cloud of Random Points with a Scrambling Metasurface
Zile Li, Qi Dai, Muhammad Q. Mehmood, Guangwei Hu, Boris Luk' yanchuk, et al.
Published. 2018, 7(5) : 596-603 doi: 10.1038/s41377-018-0064-3
With the rapid progress in computer science, including artificial intelligence, big data and cloud computing, full-space spot generation can be pivotal to many practical applications, such as facial recognition, motion detection, augmented reality, etc. These opportunities may be achieved by using diffractive optical elements (DOEs) or light detection and ranging (LIDAR). However, DOEs suffer from intrinsic limitations, such as demanding depth-controlled fabrication techniques, large thicknesses (more than the wavelength), Lambertian operation only in half space, etc. LIDAR nevertheless relies on complex and bulky scanning systems, which hinders the miniaturization of the spot generator. Here, inspired by a Lambertian scatterer, we report a Hermitian-conjugate metasurface scrambling the incident light to a cloud of random points in full space with compressed information density, functioning in both transmission and reflection spaces. Over 4044 random spots are experimentally observed in the entire space, covering angles at nearly 90°. Our scrambling metasurface is made of amorphous silicon with a uniform subwavelength height, a nearly continuous phase coverage, a lightweight, flexible design, and low-heat dissipation. Thus, it may be mass produced by and integrated into existing semiconductor foundry designs. Our work opens important directions for emerging 3D recognition sensors, such as motion sensing, facial recognition, and other applications.
Metasurface-based multi-harmonic free-electron light source
Gilles Rosolen, Liang Jie Wong, Nicholas Rivera, Bjorn Maes, Marin Soljačić, et al.
Published. 2018, 7(5) : 604-615 doi: 10.1038/s41377-018-0065-2
Metasurfaces are subwavelength spatial variations in geometry and material where the structures are of negligible thickness compared to the wavelength of light and are optimized for far-field applications, such as controlling the wavefronts of electromagnetic waves. Here, we investigate the potential of the metasurface near-field profile, generated by an incident few-cycle pulse laser, to facilitate the generation of high-frequency light from free electrons. In particular, the metasurface near-field contains higher-order spatial harmonics that can be leveraged to generate multiple higher-harmonic X-ray frequency peaks. We show that the X-ray spectral profile can be arbitrarily shaped by controlling the metasurface geometry, the electron energy, and the incidence angle of the laser input. Using ab initio simulations, we predict bright and monoenergetic X-rays, achieving energies of 30 keV (with harmonics spaced by 3 keV) from 5-MeV electrons using 3.4-eV plasmon polaritons on a metasurface with a period of 85 nm. As an example, we present the design of a four-color X-ray source, a potential candidate for tabletop multicolor hard X-ray spectroscopy. Our developments could help pave the way for compact multi-harmonic sources of high-energy photons, which have potential applications in industry, medicine, and the fundamental sciences.
Quantifying single plasmonic nanostructure far-fields with interferometric and polarimetric k-space microscopy
Ruslan Röhrich, Chris Hoekmeijer, Clara I. Osorio, A. Femius Koenderink
Published. 2018, 7(5) : 575-586 doi: 10.1038/s41377-018-0059-0
Optically resonant nanoantennae are key building blocks for metasurfaces, nanosensors, and nanophotonic light sources due to their ability to control the amplitude, phase, directivity, and polarization of scattered light. Here, we report an experimental technique for the full recovery of all degrees of freedom encoded in the far-field radiated by a single nanostructure using a high-NA Fourier microscope equipped with digital off-axis holography. This method enables full decomposition of antenna-physics in its multipole contributions and gives full access to the orbital and spin angular momentum properties of light scattered by single nano-objects. Our results demonstrate these capabilities through a quantitative assessment of the purity of the "selection rules" for orbital angular momentum transfer by plasmonic spiral nanostructures.
A deep learning-enabled portable imaging flow cytometer for cost-effective, high-throughput, and label-free analysis of natural water samples
Zoltán Gӧrӧcs, Miu Tamamitsu, Vittorio Bianco, Patrick Wolf, Shounak Roy, et al.
Published. 2018, 7(5) : 616-627 doi: 10.1038/s41377-018-0067-0
We report a deep learning-enabled field-portable and cost-effective imaging flow cytometer that automatically captures phase-contrast color images of the contents of a continuously flowing water sample at a throughput of 100 mL/h. The device is based on partially coherent lens-free holographic microscopy and acquires the diffraction patterns of flowing micro-objects inside a microfluidic channel. These holographic diffraction patterns are reconstructed in real time using a deep learning-based phase-recovery and image-reconstruction method to produce a color image of each micro-object without the use of external labeling. Motion blur is eliminated by simultaneously illuminating the sample with red, green, and blue light-emitting diodes that are pulsed. Operated by a laptop computer, this portable device measures 15.5 cm × 15 cm × 12.5 cm, weighs 1 kg, and compared to standard imaging flow cytometers, it provides extreme reductions of cost, size and weight while also providing a high volumetric throughput over a large object size range. We demonstrated the capabilities of this device by measuring ocean samples at the Los Angeles coastline and obtaining images of its micro- and nanoplankton composition. Furthermore, we measured the concentration of a potentially toxic alga (Pseudo-nitzschia) in six public beaches in Los Angeles and achieved good agreement with measurements conducted by the California Department of Public Health. The cost-effectiveness, compactness, and simplicity of this computational platform might lead to the creation of a network of imaging flow cytometers for large-scale and continuous monitoring of the ocean microbiome, including its plankton composition.
Gain through losses in nonlinear optics
Auro M. Perego, Sergei K. Turitsyn, Kestutis Staliunas
Published. 2018, 7(5) : 336-346 doi: 10.1038/s41377-018-0042-9
Instabilities of uniform states are ubiquitous processes occurring in a variety of spatially extended nonlinear systems. These instabilities are at the heart of symmetry breaking, condensate dynamics, self-organisation, pattern formation, and noise amplification across diverse disciplines, including physics, chemistry, engineering, and biology. In nonlinear optics, modulation instabilities are generally linked to the so-called parametric amplification process, which occurs when certain phase-matching or quasi-phase-matching conditions are satisfied.In the present review article, we summarise the principle results on modulation instabilities and parametric amplification in nonlinear optics, with special emphasis on optical fibres. We then review state-of-the-art research about a peculiar class of modulation instabilities (MIs) and signal amplification processes induced by dissipation in nonlinear optical systems. Losses applied to certain parts of the spectrum counterintuitively lead to the exponential growth of the damped mode themselves, causing gain through losses. We discuss the concept of imaging of losses into gain, showing how to map a given spectral loss profile into a gain spectrum. We demonstrate with concrete examples that dissipation-induced MI, apart from being of fundamental theoretical interest, may pave the way towards the design of a new class of tuneable fibre-based optical amplifiers, optical parametric oscillators, frequency comb sources, and pulsed lasers.
Strategies for reducing speckle noise in digital holography
Vittorio Bianco, Pasquale Memmolo, Marco Leo, Silvio Montresor, Cosimo Distante, et al.
Published. 2018, 7(5) : 347-362 doi: 10.1038/s41377-018-0050-9
Digital holography (DH) has emerged as one of the most effective coherent imaging technologies. The technological developments of digital sensors and optical elements have made DH the primary approach in several research fields, from quantitative phase imaging to optical metrology and 3D display technologies, to name a few. Like many other digital imaging techniques, DH must cope with the issue of speckle artifacts, due to the coherent nature of the required light sources. Despite the complexity of the recently proposed de-speckling methods, many have not yet attained the required level of effectiveness. That is, a universal denoising strategy for completely suppressing holographic noise has not yet been established. Thus the removal of speckle noise from holographic images represents a bottleneck for the entire optics and photonics scientific community. This review article provides a broad discussion about the noise issue in DH, with the aim of covering the best-performing noise reduction approaches that have been proposed so far. Quantitative comparisons among these approaches will be presented.
Looking at sound: optoacoustics with all-optical ultrasound detection
Georg Wissmeyer, Miguel A. Pleitez, Amir Rosenthal, Vasilis Ntziachristos
Published. 2018, 7(5) : 363-378 doi: 10.1038/s41377-018-0036-7
Originally developed for diagnostic ultrasound imaging, piezoelectric transducers are the most widespread technology employed in optoacoustic (photoacoustic) signal detection. However, the detection requirements of optoacoustic sensing and imaging differ from those of conventional ultrasonography and lead to specifications not sufficiently addressed by piezoelectric detectors. Consequently, interest has shifted to utilizing entirely optical methods for measuring optoacoustic waves. All-optical sound detectors yield a higher signal-to-noise ratio per unit area than piezoelectric detectors and feature wide detection bandwidths that may be more appropriate for optoacoustic applications, enabling several biomedical or industrial applications. Additionally, optical sensing of sound is less sensitive to electromagnetic noise, making it appropriate for a greater spectrum of environments. In this review, we categorize different methods of optical ultrasound detection and discuss key technology trends geared towards the development of all-optical optoacoustic systems. We also review application areas that are enabled by all-optical sound detectors, including interventional imaging, non-contact measurements, magnetoacoustics, and non-destructive testing.
Observing charge separation in nanoantennas via ultrafast point-projection electron microscopy
Jan Vogelsang, Germann Hergert, Dong Wang, Petra Groß, Christoph Lienau
Published. 2018, 7(5) : 317-324 doi: 10.1038/s41377-018-0054-5
Observing the motion of electrons on their natural nanometer length and femtosecond time scales is a fundamental goal of and an open challenge for contemporary ultrafast science1-5. At present, optical techniques and electron microscopy mostly provide either ultrahigh temporal or spatial resolution, and microscopy techniques with combined space-time resolution require further development6-11. In this study, we create an ultrafast electron source via plasmon nanofocusing on a sharp gold taper and implement this source in an ultrafast point-projection electron microscope. This source is used in an optical pump—electron probe experiment to study ultrafast photoemissions from a nanometer-sized plasmonic antenna12-15. We probe the real space motion of the photoemitted electrons with a 20-nm spatial resolution and a 25-fs time resolution and reveal the deflection of probe electrons by residual holes in the metal. This is a step toward time-resolved microscopy of electronic motion in nanostructures.
Wireless whispering-gallery-mode sensor for thermal sensing and aerial mapping
Xiangyi Xu, Weijian Chen, Guangming Zhao, Yihang Li, Chenyang Lu, et al.
Published. 2018, 7(5) : 325-330 doi: 10.1038/s41377-018-0063-4
The Internet of Things (IoT)1,2 employs a large number of spatially distributed wireless sensors to monitor physical environments, e.g., temperature, humidity, and air pressure, and has many applications, including environmental monitoring3, health care monitoring4, smart cities5, and precision agriculture. A wireless sensor can collect, analyze, and transmit measurements of its environment1,2. Currently, wireless sensors used in the IoT are predominately based on electronic devices that may suffer from electromagnetic interference in many circumstances. Being immune to the electromagnetic interference, optical sensors provide a significant advantage in harsh environments6. Furthermore, by introducing optical resonance to enhance light–matter interactions, optical sensors based on resonators exhibit small footprints, extreme sensitivity, and versatile functionalities7,8, which can significantly enhance the capability and flexibility of wireless sensors. Here we provide the first demonstration of a wireless photonic sensor node based on a whispering-gallery-mode (WGM) optical resonator, in which light propagates along the circular rim of such a structure like a sphere, a disk, or a toroid by continuous total internal reflection. The sensor node is controlled via a customized iOS app. Its performance was studied in two practical scenarios: (1) real-time measurement of the air temperature over 12 h and (2) aerial mapping of the temperature distribution using a sensor node mounted on an unmanned drone. Our work demonstrates the capability of WGM optical sensors in practical applications and may pave the way for the large-scale deployment of WGM sensors in the IoT.
Optical metasurfaces: new generation building blocks for multi-functional optics
Dragomir Neshev, Igor Aharonovich
Published. 2018, 7(5) : 331-335 doi: 10.1038/s41377-018-0058-1
Optical metasurfaces (OMs) have emerged as promising candidates to solve the bottleneck of bulky optical elements. OMs offer a fundamentally new method of light manipulation based on scattering from resonant nanostructures rather than conventional refraction and propagation, thus offering efficient phase, polarization, and emission control. This perspective highlights state of the art OMs and provides a roadmap for future applications, including active generation, manipulation and detection of light for quantum technologies, holography and sensing.