Showing papers in "Light-Science & Applications in 2019"

Optical vortices 30 years on: OAM manipulation from topological charge to multiple singularities

Abstract: Thirty years ago, Coullet et al. proposed that a special optical field exists in laser cavities bearing some analogy with the superfluid vortex. Since then, optical vortices have been widely studied, inspired by the hydrodynamics sharing similar mathematics. Akin to a fluid vortex with a central flow singularity, an optical vortex beam has a phase singularity with a certain topological charge, giving rise to a hollow intensity distribution. Such a beam with helical phase fronts and orbital angular momentum reveals a subtle connection between macroscopic physical optics and microscopic quantum optics. These amazing properties provide a new understanding of a wide range of optical and physical phenomena, including twisting photons, spin-orbital interactions, Bose-Einstein condensates, etc., while the associated technologies for manipulating optical vortices have become increasingly tunable and flexible. Hitherto, owing to these salient properties and optical manipulation technologies, tunable vortex beams have engendered tremendous advanced applications such as optical tweezers, high-order quantum entanglement, and nonlinear optics. This article reviews the recent progress in tunable vortex technologies along with their advanced applications.

Probing 10 μK stability and residual drifts in the cross-polarized dual-mode stabilization of single-crystal ultrahigh- Q optical resonators

Abstract: The thermal stability of monolithic optical microresonators is essential for many mesoscopic photonic applications such as ultrastable laser oscillators, photonic microwave clocks, and precision navigation and sensing. Their fundamental performance is largely bounded by thermal instability. Sensitive thermal monitoring can be achieved by utilizing cross-polarized dual-mode beat frequency metrology, determined by the polarization-dependent thermorefractivity of a single-crystal microresonator, wherein the heterodyne radio-frequency beat pins down the optical mode volume temperature for precision stabilization. Here, we investigate the correlation between the dual-mode beat frequency and the resonator temperature with time and the associated spectral noise of the dual-mode beat frequency in a single-crystal ultrahigh-Q MgF2 resonator to illustrate that dual-mode frequency metrology can potentially be utilized for resonator temperature stabilization reaching the fundamental thermal noise limit in a realistic system. We show a resonator long-term temperature stability of 8.53 μK after stabilization and unveil various sources that hinder the stability from reaching sub-μK in the current system, an important step towards compact precision navigation, sensing, and frequency reference architectures. Researchers in California have improved the thermal stability of tiny optical microresonators for use in high-precision timing and global navigation technologies. Ultrahigh-quality whispering gallery optical microresonators work by guiding the light from two differently-polarized lasers around the resonator circumference, which is carefully designed to have particular resonant frequencies. However, microresonators are extremely sensitive to temperature changes, and the impact of laser-induced heating, heat diffusion, and thermal expansion over time is detrimental to performance. Jinkang Lim and Chee Wei Wong at the University of California, US, and co-workers have shown that, by locking the dual-mode beat frequency of the lasers to a radio-frequency clock, the resulting suppression of thermal noise and frequency drift can enhance the long-term thermal stability of optical microresonators. This novel solution could result in microresonators stable enough to be used in space.

Emerging ultra-narrow-band cyan-emitting phosphor for white LEDs with enhanced color rendition.

Abstract: Phosphor-converted white LEDs rely on combining a blue-emitting InGaN chip with yellow and red-emitting luminescent materials. The discovery of cyan-emitting (470–500 nm) phosphors is a challenge to compensate for the spectral gap and produce full-spectrum white light. Na0.5K0.5Li3SiO4:Eu2+ (NKLSO:Eu2+) phosphor was developed with impressive properties, providing cyan emission at 486 nm with a narrow full width at half maximum (FWHM) of only 20.7 nm, and good thermal stability with an integrated emission loss of only 7% at 150 °C. The ultra-narrow-band cyan emission results from the high-symmetry cation sites, leading to almost ideal cubic coordination for UCr4C4-type compounds. NKLSO:Eu2+ phosphor allows the valley between the blue and yellow emission peaks in the white LED device to be filled, and the color-rendering index can be enhanced from 86 to 95.2, suggesting great applications in full-spectrum white LEDs. Luminescent crystals that efficiently emit light in the narrow cyan color wavelength range, from 470 to 500 nanometers, could be used to fill a gap or “valley” in light-emitting diodes (LEDs) intended to mimic the full spectrum of daylight. There is great interest in developing more efficient and cost-effective full daylight spectrum LED lighting sources. These can create more natural indoor lighting conditions that are also believed to be more beneficial for health. Researchers in China and Russia, led by Zhiguo Xia at the University of Science and Technology Beijing, developed a suitable inorganic crystalline compound showing narrow cyan emission, with the formula Na0.5K0.5Li3SiO4:Eu2+. The many possible applications include LEDs for daylight spectrum lamps used to treat the low mood and depression associated with Seasonal Affective Disorder (SAD).

Metasurface interferometry toward quantum sensors.

Abstract: 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. Scientists have developed an optical metasurface capable of entangling and disentangling photon-pairs, providing a path for the development of quantum technologies for applications in computing, imaging, and sensing. Optical metasurfaces are sub-wavelength layers of nanostructures capable of precisely controlling the properties of light. They offer the promise of new miniaturized quantum systems where they remain largely unexplored. Now Thomas Zentgraf and colleagues from the University of Paderborn in Germany, working with researchers from the University of Stuttgart and the Southern University of Science and Technology in China, have developed a nanostructured dielectric metasurface capable of entangling and disentangling the spin states of a photon-pair. Quantum interference of the photons on the metasurface produces a circularly polarized entangled photon-pair, which can be disentangled by passing it through the metasurface a second time.

Smart metasurface with self-adaptively reprogrammable functions

Abstract: Intelligence at either the material or metamaterial level is a goal that researchers have been pursuing. From passive to active, metasurfaces have been developed to be programmable to dynamically and arbitrarily manipulate electromagnetic (EM) wavefields. However, the programmable metasurfaces require manual control to switch among different functionalities. Here, we put forth a smart metasurface that has self-adaptively reprogrammable functionalities without human participation. The smart metasurface is capable of sensing ambient environments by integrating an additional sensor(s) and can adaptively adjust its EM operational functionality through an unmanned sensing feedback system. As an illustrative example, we experimentally develop a motion-sensitive smart metasurface integrated with a three-axis gyroscope, which can adjust self-adaptively the EM radiation beams via different rotations of the metasurface. We develop an online feedback algorithm as the control software to make the smart metasurface achieve single-beam and multibeam steering and other dynamic reactions adaptively. The proposed metasurface is extendable to other physical sensors to detect the humidity, temperature, illuminating light, and so on. Our strategy will open up a new avenue for future unmanned devices that are consistent with the ambient environment.

Single-photon avalanche diode imagers in biophotonics: review and outlook

Abstract: Single-photon avalanche diode (SPAD) arrays are solid-state detectors that offer imaging capabilities at the level of individual photons, with unparalleled photon counting and time-resolved performance. This fascinating technology has progressed at a very fast pace in the past 15 years, since its inception in standard CMOS technology in 2003. A host of architectures have been investigated, ranging from simpler implementations, based solely on off-chip data processing, to progressively "smarter" sensors including on-chip, or even pixel level, time-stamping and processing capabilities. As the technology has matured, a range of biophotonics applications have been explored, including (endoscopic) FLIM, (multibeam multiphoton) FLIM-FRET, SPIM-FCS, super-resolution microscopy, time-resolved Raman spectroscopy, NIROT and PET. We will review some representative sensors and their corresponding applications, including the most relevant challenges faced by chip designers and end-users. Finally, we will provide an outlook on the future of this fascinating technology.

PhaseStain: the digital staining of label-free quantitative phase microscopy images using deep learning.

Abstract: Using a deep neural network, we demonstrate a digital staining technique, which we term PhaseStain, to transform the quantitative phase images (QPI) of label-free tissue sections into images that are equivalent to the brightfield microscopy images of the same samples that are histologically stained. Through pairs of image data (QPI and the corresponding brightfield images, acquired after staining), we train a generative adversarial network and demonstrate the effectiveness of this virtual-staining approach using sections of human skin, kidney, and liver tissue, matching the brightfield microscopy images of the same samples stained with Hematoxylin and Eosin, Jones' stain, and Masson's trichrome stain, respectively. This digital-staining framework may further strengthen various uses of label-free QPI techniques in pathology applications and biomedical research in general, by eliminating the need for histological staining, reducing sample preparation related costs and saving time. Our results provide a powerful example of some of the unique opportunities created by data-driven image transformations enabled by deep learning.

Dielectric metasurfaces for complete and independent control of the optical amplitude and phase

Abstract: Metasurfaces are optically thin metamaterials that promise complete control of the wavefront of light but are primarily used to control only the phase of light. Here, we present an approach, simple in concept and in practice, that uses meta-atoms with a varying degree of form birefringence and rotation angles to create high-efficiency dielectric metasurfaces that control both the optical amplitude and phase at one or two frequencies. This opens up applications in computer-generated holography, allowing faithful reproduction of both the phase and amplitude of a target holographic scene without the iterative algorithms required in phase-only holography. We demonstrate all-dielectric metasurface holograms with independent and complete control of the amplitude and phase at up to two optical frequencies simultaneously to generate two- and three-dimensional holographic objects. We show that phase-amplitude metasurfaces enable a few features not attainable in phase-only holography; these include creating artifact-free two-dimensional holographic images, encoding phase and amplitude profiles separately at the object plane, encoding intensity profiles at the metasurface and object planes separately, and controlling the surface textures of three-dimensional holographic objects.

New strategy for designing orangish-red-emitting phosphor via oxygen-vacancy-induced electronic localization.

Abstract: Phosphor-converted white-light-emitting diodes (pc-WLED) have been extensively employed as solid-state lighting sources, which have a very important role in people’s daily lives. However, due to the scarcity of the red component, it is difficult to realize warm white light efficiently. Hence, red-emitting phosphors are urgently required for improving the illumination quality. In this work, we develop a novel orangish-red La4GeO8:Bi3+ phosphor, the emission peak of which is located at 600 nm under near-ultraviolet (n-UV) light excitation. The full width at half maximum (fwhm) is 103 nm, the internal quantum efficiency (IQE) exceeds 88%, and the external quantum efficiency (EQE) is 69%. According to Rietveld refinement analysis and density functional theory (DFT) calculations, Bi3+ ions randomly occupy all La sites in orthorhombic La4GeO8. Importantly, the oxygen-vacancy-induced electronic localization around the Bi3+ ions is the main reason for the highly efficient orangish-red luminescence. These results provide a new perspective and insight from the local electron structure for designing inorganic phosphor materials that realize the unique luminescence performance of Bi3+ ions. Phosphorescent substances that give white-light-emitting diodes a more natural reddish hue can be made by tuning the electron structure of their activator ions. Jun Lin of China’s Changchun Institute of Applied Chemistry, Chinese Academy of Sciences and colleagues combined germanium, lanthanum and bismuth oxides to manufacture a reddish phosphorescent substance that, when added to blue and green phosphors on an LED chip, produced more natural-looking light compared to conventional phosphor-converted white LEDs. LEDs are constantly being improved as an energy-efficient light source, but the colour of the light they emit can be unnaturally white or blue. Fabricating high quality red-emitting phosphors to change this has been a challenge. The researchers developed a substance with a unique reddish photoluminescence caused by electrons localizing around bismuth ions. Their approach offers a new perspective for exploring luminescence in inorganic materials.

Intelligent metasurface imager and recognizer.

Abstract: There is an increasing need to remotely monitor people in daily life using radio-frequency probe signals. However, conventional systems can hardly be deployed in real-world settings since they typically require objects to either deliberately cooperate or carry a wireless active device or identification tag. To accomplish complicated successive tasks using a single device in real time, we propose the simultaneous use of a smart metasurface imager and recognizer, empowered by a network of artificial neural networks (ANNs) for adaptively controlling data flow. Here, three ANNs are employed in an integrated hierarchy, transforming measured microwave data into images of the whole human body, classifying specifically designated spots (hand and chest) within the whole image, and recognizing human hand signs instantly at a Wi-Fi frequency of 2.4 GHz. Instantaneous in situ full-scene imaging and adaptive recognition of hand signs and vital signs of multiple non-cooperative people were experimentally demonstrated. We also show that the proposed intelligent metasurface system works well even when it is passively excited by stray Wi-Fi signals that ubiquitously exist in our daily lives. The reported strategy could open up a new avenue for future smart cities, smart homes, human-device interaction interfaces, health monitoring, and safety screening free of visual privacy issues. Combining radio-frequency imaging with artificial intelligence could make it easier for computers to interact with individuals using non-verbal cues, such as sign language. Lianlin Li from Peking University in Beijing, China and Tie Jun Cui from Southeast University in Nanjing, China, and co-workers fabricated a meter-scale flat panel containing ‘meta-atoms’, tiny electronic devices that manipulate the phases of light waves, arranged in a grid-like pattern. By emitting microwave signals or manipulating stray Wi-Fi signals and detecting echoes bounced back, the metasurface can collect high-resolution imaging data on multiple non-cooperative subjects, even those behind solid walls. The teams fed the microwave data to a series of artificial intelligence algorithms that first identify human shapes, modify signal distributions to better focus on specific body parts, and recognize people's hand signs and vital signs . Experiments showed this setup could continuously monitor hand signals and breathing, even using stray Wi-Fi signals that ubiquitously exist in the daily lives.

High-efficiency, large-area, topology-optimized metasurfaces

Abstract: Metasurfaces are ultrathin optical elements that are highly promising for constructing lightweight and compact optical systems. For their practical implementation, it is imperative to maximize the metasurface efficiency. Topology optimization provides a pathway for pushing the limits of metasurface efficiency; however, topology optimization methods have been limited to the design of microscale devices due to the extensive computational resources that are required. We introduce a new strategy for optimizing large-area metasurfaces in a computationally efficient manner. By stitching together individually optimized sections of the metasurface, we can reduce the computational complexity of the optimization from high-polynomial to linear. As a proof of concept, we design and experimentally demonstrate large-area, high-numerical-aperture silicon metasurface lenses with focusing efficiencies exceeding 90%. These concepts can be generalized to the design of multifunctional, broadband diffractive optical devices and will enable the implementation of large-area, high-performance metasurfaces in practical optical systems.

High-speed colour-converting photodetector with all-inorganic CsPbBr3 perovskite nanocrystals for ultraviolet light communication

Abstract: Optical wireless communication (OWC) using the ultra-broad spectrum of the visible-to-ultraviolet (UV) wavelength region remains a vital field of research for mitigating the saturated bandwidth of radio-frequency (RF) communication. However, the lack of an efficient UV photodetection methodology hinders the development of UV-based communication. The key technological impediment is related to the low UV-photon absorption in existing silicon photodetectors, which offer low-cost and mature platforms. To address this technology gap, we report a hybrid Si-based photodetection scheme by incorporating CsPbBr3 perovskite nanocrystals (NCs) with a high photoluminescence quantum yield (PLQY) and a fast photoluminescence (PL) decay time as a UV-to-visible colour-converting layer for high-speed solar-blind UV communication. The facile formation of drop-cast CsPbBr3 perovskite NCs leads to a high PLQY of up to ~73% and strong absorption in the UV region. With the addition of the NC layer, a nearly threefold improvement in the responsivity and an increase of ~25% in the external quantum efficiency (EQE) of the solar-blind region compared to a commercial silicon-based photodetector were observed. Moreover, time-resolved photoluminescence measurements demonstrated a decay time of 4.5 ns under a 372-nm UV excitation source, thus elucidating the potential of this layer as a fast colour-converting layer. A high data rate of up to 34 Mbps in solar-blind communication was achieved using the hybrid CsPbBr3–silicon photodetection scheme in conjunction with a 278-nm UVC light-emitting diode (LED). These findings demonstrate the feasibility of an integrated high-speed photoreceiver design of a composition-tuneable perovskite-based phosphor and a low-cost silicon-based photodetector for UV communication. A silicon-based receiver that incorporates perovskite nanocrystals efficiently detects ultraviolet signals, paving the way towards high-speed, high-bandwidth UV wireless communication. The photodetector (PD), developed by Boon S. Ooi of King Abdullah University of Science and Technology (KAUST) and colleagues in Saudi Arabia, is less bulky and cheaper to manufacture than currently available receivers. It builds on technologically advanced silicon-based PDs, which are compact and widely available, but respond best to higher wavelength green light. Incorporating cesium lead bromide (CsPbBr3) perovskite nanocrystals into a silicon-based PD facilitated efficient conversion of UV into green light. The team demonstrated that their receiver could be used in high-speed UV-based communication, paving the way for the use of perovskite-based materials in terrestrial and underwater UV-Internet systems.

Implementation and security analysis of practical quantum secure direct communication

Abstract: Rapid development of supercomputers and the prospect of quantum computers are posing increasingly serious threats to the security of communication. Using the principles of quantum mechanics, quantum communication offers provable security of communication and is a promising solution to counter such threats. Quantum secure direct communication (QSDC) is one important branch of quantum communication. In contrast to other branches of quantum communication, it transmits secret information directly. Recently, remarkable progress has been made in proof-of-principle experimental demonstrations of QSDC. However, it remains a technical feat to bring QSDC into a practical application. Here, we report the implementation of a practical quantum secure communication system. The security is analyzed in the Wyner wiretap channel theory. The system uses a coding scheme of concatenation of low-density parity-check (LDPC) codes and works in a regime with a realistic environment of high noise and high loss. The present system operates with a repetition rate of 1 MHz at a distance of 1.5 kilometers. The secure communication rate is 50 bps, sufficient to effectively send text messages and reasonably sized files of images and sounds. A quantum communication system demonstrated by researchers in China can transfer information securely in a realistic noisy environment. Emerging supercomputers and quantum computers may soon break the classical encryption methods that protect our information, highlighting the need for new cryptographic techniques based on quantum mechanics. Gui-Lu Long at Tsinghua University, Beijing, and co-workers have demonstrated a form of quantum secure direct communication (QSDC) that transfers information directly without the need to distribute keys, which are vulnerable to attacks. The team used a laser to generate single photons, which could carry secure quantum information such as text messages and image files over a distance of 1.5 kilometers. The information was decoded successfully by the receiver, even when the situation was made realistic by causing high photon loss or introducing errors due to noise.

Artificial neural networks enabled by nanophotonics.

Abstract: The growing demands of brain science and artificial intelligence create an urgent need for the development of artificial neural networks (ANNs) that can mimic the structural, functional and biological features of human neural networks. Nanophotonics, which is the study of the behaviour of light and the light-matter interaction at the nanometre scale, has unveiled new phenomena and led to new applications beyond the diffraction limit of light. These emerging nanophotonic devices have enabled scientists to develop paradigm shifts of research into ANNs. In the present review, we summarise the recent progress in nanophotonics for emulating the structural, functional and biological features of ANNs, directly or indirectly.

Deep learning in holography and coherent imaging.

Abstract: Recent advances in deep learning have given rise to a new paradigm of holographic image reconstruction and phase recovery techniques with real-time performance. Through data-driven approaches, these emerging techniques have overcome some of the challenges associated with existing holographic image reconstruction methods while also minimizing the hardware requirements of holography. These recent advances open up a myriad of new opportunities for the use of coherent imaging systems in biomedical and engineering research and related applications.

Soliton Bursts and Deterministic Dissipative Kerr Soliton Generation in Auxiliary-Assisted Microcavities

Abstract: Dissipative Kerr solitons in resonant frequency combs offer a promising route for ultrafast mode-locking, precision spectroscopy and time-frequency standards. The dynamics for the dissipative soliton generation, however, are intrinsically intertwined with thermal nonlinearities, limiting the soliton generation parameter map and statistical success probabilities of the solitary state. Here, via use of an auxiliary laser heating approach to suppress thermal dragging dynamics in dissipative soliton comb formation, we demonstrate stable Kerr soliton singlet formation and soliton bursts. First, we access a new soliton existence range with an inverse-sloped Kerr soliton evolution—diminishing soliton energy with increasing pump detuning. Second, we achieve deterministic transitions from Turing-like comb patterns directly into the dissipative Kerr soliton singlet pulse bypassing the chaotic states. This is achieved by avoiding subcomb overlaps at lower pump power, with near-identical singlet soliton comb generation over twenty instances. Third, with the red-detuned pump entrance route enabled, we uncover unique spontaneous soliton bursts in the direct formation of low-noise optical frequency combs from continuum background noise. The burst dynamics are due to the rapid entry and mutual attraction of the pump laser into the cavity mode, aided by the auxiliary laser and matching well with our numerical simulations. Enabled by the auxiliary-assisted frequency comb dynamics, we demonstrate an application of automatic soliton comb recovery and long-term stabilization against strong external perturbations. Our findings hold potential to expand the parameter space for ultrafast nonlinear dynamics and precision optical frequency comb stabilization. Ultrafast optical states called solitons can be prevented from thermally breaking down by carefully heating them with a laser, researchers in the US and China show. Solitons are optical fields that exist in isolation, like smoke rings in air or bubbles in water, and they could greatly improve precision laser measurements and spectroscopy. However, it is difficult to maintain robust soliton states due to nonlinear thermal effects that cause them to break down. Heng Zhou at UESTC, Chee Wei Wong at UCLA, and co-workers generated solitons by directing a ‘frequency comb’ source (comprising discrete, equally-spaced laser lines) onto a silicon nitride optical microcavity. Crucially, they employed a second laser to provide heating to the system and suppress the thermal nonlinearities. This enabled smooth transitions between useful soliton states, while avoiding chaotic intermediate states.

3D-Integrated metasurfaces for full-colour holography

Abstract: Metasurfaces enable the design of optical elements by engineering the wavefront of light at the subwavelength scale. Due to their ultrathin and compact characteristics, metasurfaces possess great potential to integrate multiple functions in optoelectronic systems for optical device miniaturisation. However, current research based on multiplexing in the 2D plane has not fully utilised the capabilities of metasurfaces for multi-tasking applications. Here, we demonstrate a 3D-integrated metasurface device by stacking a hologram metasurface on a monolithic Fabry-Perot cavity-based colour filter microarray to simultaneously achieve low-crosstalk, polarisation-independent, high-efficiency, full-colour holography, and microprint. The dual functions of the device outline a novel scheme for data recording, security encryption, colour displays, and information processing. Our 3D integration concept can be extended to achieve multi-tasking flat optical systems by including a variety of functional metasurface layers, such as polarizers, metalenses, and others.

Real-time high-resolution mid-infrared optical coherence tomography.

Abstract: The potential for improving the penetration depth of optical coherence tomography systems by using light sources with longer wavelengths has been known since the inception of the technique in the early 1990s. Nevertheless, the development of mid-infrared optical coherence tomography has long been challenged by the maturity and fidelity of optical components in this spectral region, resulting in slow acquisition, low sensitivity, and poor axial resolution. In this work, a mid-infrared spectral-domain optical coherence tomography system operating at a central wavelength of 4 µm and an axial resolution of 8.6 µm is demonstrated. The system produces two-dimensional cross-sectional images in real time enabled by a high-brightness 0.9- to 4.7-µm mid-infrared supercontinuum source with a pulse repetition rate of 1 MHz for illumination and broadband upconversion of more than 1-µm bandwidth from 3.58–4.63 µm to 820–865 nm, where a standard 800-nm spectrometer can be used for fast detection. The images produced by the mid-infrared system are compared with those delivered by a state-of-the-art ultra-high-resolution near-infrared optical coherence tomography system operating at 1.3 μm, and the potential applications and samples suited for this technology are discussed. In doing so, the first practical mid-infrared optical coherence tomography system is demonstrated, with immediate applications in real-time non-destructive testing for the inspection of defects and thickness measurements in samples that exhibit strong scattering at shorter wavelengths. Using longer wavelengths of light in Optical Coherence Tomography (OCT) imaging allows deeper penetration in highly scattering materials, offering possibilities for OCT in non-destructive testing and enhanced non-invasive biomedical imaging. OCT images are based on interference patterns generated by combining light reflected from the examined object with reference light that does not encounter the object. It is currently most widely used to examine the retina of the eye. Researchers at the Technical University of Denmark, together with co-workers in Austria and the UK, overcame several technical challenges to obtain images using mid-infrared light to reveal microscopic structures that are not visible using conventional shorter wavelength near-infrared light.The team combined broadband supercontinuum light and frequency upconversion to achieve high-resolution and real-time image acquisition.Promising applications include advances in defect detection and thickness measurements.

Germanium/perovskite heterostructure for high-performance and broadband photodetector from visible to infrared telecommunication band

Abstract: A high-performance and broadband heterojunction photodetector has been successfully fabricated. The heterostructure device is based on a uniform and pinhole-free perovskite film constructed on top of a single-crystal germanium layer. The perovskite/germanium photodetector shows enhanced performance and a broad spectrum compared with the single-material-based device. The photon response properties are characterized in detail from the visible to near-infrared spectrum. At an optical fibre communication wavelength of 1550 nm, the heterojunction device exhibits the highest responsivity of 1.4 A/W. The performance is promoted because of an antireflection perovskite coating, the thickness of which is optimized to 150 nm at the telecommunication band. At a visible light wavelength of 680 nm, the device shows outstanding responsivity and detectivity of 228 A/W and 1.6 × 1010 Jones, respectively. These excellent properties arise from the photoconductive gain boost in the heterostructure device. The presented heterojunction photodetector provides a competitive approach for wide-spectrum photodetection from visible to optical communication areas. Based on the distinguished capacity of light detection and harvesting from the visible to near-infrared spectrum, the designed germanium/perovskite heterostructure configuration is believed to provide new building blocks for novel optoelectronic devices. A device made from germanium and perovskite layers can detect light in the visible and near-infrared ranges, showing potential for use in a wide range of applications, including in optical communications and next-generation optoelectronics. This heterojunction photodetector fabricated by Chunlai Xue of the Chinese Academy of Sciences and colleagues overcomes problems in single-material photodetectors, which are unable to detect a broad range of light. Recent research into various combinations of semiconducting materials for heterojunction photodetectors has led to devices with poor sensitivity to light or that require a high working voltage. Adding a layer of methylammonium lead triiodide perovskite to a layer of germanium resulted in a highly sensitive photodetector at the optical fibre communication wavelength of 1550 nm (near-infrared range) and the visible light wavelength of 680 nm.

Full-colour nanoprint-hologram synchronous metasurface with arbitrary hue-saturation-brightness control

Abstract: The colour gamut, a two-dimensional (2D) colour space primarily comprising hue and saturation (HS), lays the most important foundation for the colour display and printing industries. Recently, the metasurface has been considered a promising paradigm for nanoprinting and holographic imaging, demonstrating a subwavelength image resolution, a flat profile, high durability, and multi-functionalities. Much effort has been devoted to broaden the 2D HS plane, also known as the CIE map. However, the brightness (B), as the carrier of chiaroscuro information, has long been neglected in metasurface-based nanoprinting or holograms due to the challenge in realising arbitrary and simultaneous control of full-colour HSB tuning in a passive device. Here, we report a dielectric metasurface made of crystal silicon nanoblocks, which achieves not only tailorable coverage of the primary colours red, green and blue (RGB) but also intensity control of the individual colours. The colour gamut is hence extruded from the 2D CIE to a complete 3D HSB space. Moreover, thanks to the independent control of the RGB intensity and phase, we further show that a single-layer silicon metasurface could simultaneously exhibit arbitrary HSB colour nanoprinting and a full-colour hologram image. Our findings open up possibilities for high-resolution and high-fidelity optical security devices as well as advanced cryptographic approaches. Structural colours with hue-saturation-brightness (HSB) control and independent integration with full-colour hologram open up possibilities for optical security devices and advanced cryptographic approaches. Researchers from China and Singapore propose a crystalline-silicon nanoblock metasurface that is able to control the intensity and phase of transmitted red, green, blue (RGB) lights independently. By mixing the three RGB nanoblocks with different intensity proportions together, it is possible to control the HSB value of the transmitted light, pushing the metasurface structural colour from conventional two-dimensional hue-saturation space to a real three-dimensional HSB space. Furthermore, arbitrary HSB colour printing and full-colour hologram images can be integrated in one single metasurface structure. That makes a drastic leap for singlet multifunctional metasurfaces, instead of cascading multiple metasurfaces with one for colourprinting and the other for hologram respectively.

A broadband achromatic metalens array for integral imaging in the visible

Abstract: We realize a polarization-insensitive silicon-nitride metalens-array in visible frequency spectrum, in which there is a set of broadband achromatic metalenses. The achromatic focusing and the achromatic integral imaging are demonstrated for white light.

Optical orbital-angular-momentum-multiplexed data transmission under high scattering

Abstract: Multiplexing multiple orbital angular momentum (OAM) channels enables high-capacity optical communication. However, optical scattering from ambient microparticles in the atmosphere or mode coupling in optical fibers significantly decreases the orthogonality between OAM channels for demultiplexing and eventually increases crosstalk in communication. Here, we propose a novel scattering-matrix-assisted retrieval technique (SMART) to demultiplex OAM channels from highly scattered optical fields and achieve an experimental crosstalk of –13.8 dB in the parallel sorting of 24 OAM channels after passing through a scattering medium. The SMART is implemented in a self-built data transmission system that employs a digital micromirror device to encode OAM channels and realize reference-free calibration simultaneously, thereby enabling a high tolerance to misalignment. We successfully demonstrate high-fidelity transmission of both gray and color images under scattering conditions at an error rate of <0.08%. This technique might open the door to high-performance optical communication in turbulent environments.

Nature-inspired chiral metasurfaces for circular polarization detection and full-Stokes polarimetric measurements.

Abstract: The manipulation and characterization of light polarization states are essential for many applications in quantum communication and computing, spectroscopy, bioinspired navigation, and imaging. Chiral metamaterials and metasurfaces facilitate ultracompact devices for circularly polarized light generation, manipulation, and detection. Herein, we report bioinspired chiral metasurfaces with both strong chiral optical effects and low insertion loss. We experimentally demonstrated submicron-thick circularly polarized light filters with peak extinction ratios up to 35 and maximum transmission efficiencies close to 80% at near-infrared wavelengths (the best operational wavelengths can be engineered in the range of 1.3–1.6 µm). We also monolithically integrated the microscale circular polarization filters with linear polarization filters to perform full-Stokes polarimetric measurements of light with arbitrary polarization states. With the advantages of easy on-chip integration, ultracompact footprints, scalability, and broad wavelength coverage, our designs hold great promise for facilitating chip-integrated polarimeters and polarimetric imaging systems for quantum-based optical computing and information processing, circular dichroism spectroscopy, biomedical diagnosis, and remote sensing applications. Inspired by the polarization-sensitive vision of the compound eyes in a marine crustacean called the Mantis Shrimp, researchers from Arizona State University, US have designed a chiral metasurface for manipulating the polarization of light. The metasurface design consists of a thin nanostructured silicon layer, a dielectric spacer layer and a gold nanowire polarizer, and has a total thickness of less than 1 micrometer. This thin planar surface offers low optical loss with a transmission as high as 80% in the near-infrared wavelength range, and acts as a circular polarization filter with an extinction ratio as high as 35. The circular polarization filters, in combination with linear polarization filters, can enable chip-scale polarimeters for sensing the polarization state of light. This on-chip integrated approach could prove useful in ultra-compact devices for advanced imaging and sensing applications.

"Hot" electrons in metallic nanostructures-non-thermal carriers or heating?

Abstract: Understanding the interplay between illumination and the electron distribution in metallic nanostructures is a crucial step towards developing applications such as plasmonic photocatalysis for green fuels, nanoscale photodetection and more. Elucidating this interplay is challenging, as it requires taking into account all channels of energy flow in the electronic system. Here, we develop such a theory, which is based on a coupled Boltzmann-heat equations and requires only energy conservation and basic thermodynamics, where the electron distribution, and the electron and phonon (lattice) temperatures are determined uniquely. Applying this theory to realistic illuminated nanoparticle systems, we find that the electron and phonon temperatures are similar, thus justifying the (classical) single-temperature models. We show that while the fraction of high-energy "hot" carriers compared to thermalized carriers grows substantially with illumination intensity, it remains extremely small (on the order of 10-8). Importantly, most of the absorbed illumination power goes into heating rather than generating hot carriers, thus rendering plasmonic hot carrier generation extremely inefficient. Our formulation allows for the first time a unique quantitative comparison of theory and measurements of steady-state electron distributions in metallic nanostructures.

Direct observation of ultrafast plasmonic hot electron transfer in the strong coupling regime.

Abstract: Achieving strong coupling between plasmonic oscillators can significantly modulate their intrinsic optical properties. Here, we report the direct observation of ultrafast plasmonic hot electron transfer from an Au grating array to an MoS2 monolayer in the strong coupling regime between localized surface plasmons (LSPs) and surface plasmon polaritons (SPPs). By means of femtosecond pump-probe spectroscopy, the measured hot electron transfer time is approximately 40 fs with a maximum external quantum yield of 1.65%. Our results suggest that strong coupling between LSPs and SPPs has synergetic effects on the generation of plasmonic hot carriers, where SPPs with a unique nonradiative feature can act as an ‘energy recycle bin’ to reuse the radiative energy of LSPs and contribute to hot carrier generation. Coherent energy exchange between plasmonic modes in the strong coupling regime can further enhance the vertical electric field and promote the transfer of hot electrons between the Au grating and the MoS2 monolayer. Our proposed plasmonic strong coupling configuration overcomes the challenge associated with utilizing hot carriers and is instructive in terms of improving the performance of plasmonic opto-electronic devices. A device that taps into two types of surface plasmon waves offers new opportunities to transform light energy into speedy semiconductor charges. When surface plasmons are confined to nanoscale dimensions, they can rapidly decay and produce “hot” electrons with high kinetic energy. Zheyu Fang from Peking University in Beijing, China, and colleagues have constructed a metal–insulator–metal sandwich structure that improves harvesting of hot electrons for applications including photodetectors. To increase extraction under laser excitation, the team coated the insulator with an electron-attracting molybdenum disulfide monolayer. They also identified surface plasmon polaritons—mobile waves at the metal–insulator interface—in the sandwich structure using a gold grating with variable dimensions as the upper metal layer. Laser conditions that coupled mobile and localized plasmons promoted hot electron production instead of thermal dissipation.

Interference-assisted kaleidoscopic meta-plexer for arbitrary spin-wavefront manipulation

Abstract: Achieving simultaneous polarization and wavefront control, especially circular polarization with the auxiliary degree of freedom of light and spin angular momentum, is of fundamental importance in many optical applications. Interferences are typically undesirable in highly integrated photonic circuits and metasurfaces. Here, we propose an interference-assisted metasurface-multiplexer (meta-plexer) that counterintuitively exploits constructive and destructive interferences between hybrid meta-atoms and realizes independent spin-selective wavefront manipulation. Such kaleidoscopic meta-plexers are experimentally demonstrated via two types of single-layer spin-wavefront multiplexers that are composed of spatially rotated anisotropic meta-atoms. One type generates a spin-selective Bessel-beam wavefront for spin-down light and a low scattering cross-section for stealth for spin-up light. The other type demonstrates versatile control of the vortex wavefront, which is also characterized by the orbital angular momentum of light, with frequency-switchable numbers of beams under linearly polarized wave excitation. Our findings offer a distinct interference-assisted concept for realizing advanced multifunctional photonics with arbitrary and independent spin-wavefront features. A variety of applications can be readily anticipated in optical diodes, isolators, and spin-Hall meta-devices without cascading bulky optical elements.

Design of task-specific optical systems using broadband diffractive neural networks

Abstract: Deep learning has been transformative in many fields, motivating the emergence of various optical computing architectures. Diffractive optical network is a recently introduced optical computing framework that merges wave optics with deep-learning methods to design optical neural networks. Diffraction-based all-optical object recognition systems, designed through this framework and fabricated by 3D printing, have been reported to recognize hand-written digits and fashion products, demonstrating all-optical inference and generalization to sub-classes of data. These previous diffractive approaches employed monochromatic coherent light as the illumination source. Here, we report a broadband diffractive optical neural network design that simultaneously processes a continuum of wavelengths generated by a temporally incoherent broadband source to all-optically perform a specific task learned using deep learning. We experimentally validated the success of this broadband diffractive neural network architecture by designing, fabricating and testing seven different multi-layer, diffractive optical systems that transform the optical wavefront generated by a broadband THz pulse to realize (1) a series of tuneable, single-passband and dual-passband spectral filters and (2) spatially controlled wavelength de-multiplexing. Merging the native or engineered dispersion of various material systems with a deep-learning-based design strategy, broadband diffractive neural networks help us engineer the light-matter interaction in 3D, diverging from intuitive and analytical design methods to create task-specific optical components that can all-optically perform deterministic tasks or statistical inference for optical machine learning.

Nonreciprocal metasurface with space-time phase modulation.

Abstract: Creating materials with time-variant properties is critical for breaking reciprocity that imposes fundamental limitations on wave propagation. However, it is challenging to realize efficient and ultrafast temporal modulation in a photonic system. Here, leveraging both spatial and temporal phase manipulation offered by an ultrathin nonlinear metasurface, we experimentally demonstrated nonreciprocal light reflection at wavelengths around 860 nm. The metasurface, with travelling-wave modulation upon nonlinear Kerr building blocks, creates spatial phase gradient and multi-terahertz temporal phase wobbling, which leads to unidirectional photonic transitions in both the momentum and energy spaces. We observed completely asymmetric reflections in forward and backward light propagations over a large bandwidth around 5.77 THz within a sub-wavelength interaction length of 150 nm. Our approach highlights a potential means for creating miniaturized and integratable nonreciprocal optical components. Light propagating in one direction only could pave the way towards miniaturized components for faster, more efficient data transfer in optical circuits. Xingjie Ni and colleagues at the Pennsylvania State University demonstrated the unidirectional flow of near-infrared light in free space based on a 150-nanometre-thick nano-engineered surface, i.e., metasurface. Their approach adds to the body of research examining ways to disrupt the common wisdom that light travelling in one direction can also travel in the other. In the new system, the ultrathin metasurface consisting of a silver reflector plate supporting block-shaped, silicon nanoantennaswas dynamically modulated to generate time-varying parameters. The infrared light interacts with the ultrathin metasurface, ultimately resulting in a reflection that does not propagate in the opposite direction.

Adaptive optics in laser processing.

Abstract: Adaptive optics are becoming a valuable tool for laser processing, providing enhanced functionality and flexibility for a range of systems. Using a single adaptive element, it is possible to correct for aberrations introduced when focusing inside the workpiece, tailor the focal intensity distribution for the particular fabrication task and/or provide parallelisation to reduce processing times. This is particularly promising for applications using ultrafast lasers for three-dimensional fabrication. We review recent developments in adaptive laser processing, including methods and applications, before discussing prospects for the future.

Super-resolution localization photoacoustic microscopy using intrinsic red blood cells as contrast absorbers.

Abstract: Photoacoustic microscopy (PAM) has become a premier microscopy tool that can provide the anatomical, functional, and molecular information of animals and humans in vivo. However, conventional PAM systems suffer from limited temporal and/or spatial resolution. Here, we present a fast PAM system and an agent-free localization method based on a stable and commercial galvanometer scanner with a custom-made scanning mirror (L-PAM-GS). This novel hardware implementation enhances the temporal resolution significantly while maintaining a high signal-to-noise ratio (SNR). These improvements allow us to photoacoustically and noninvasively observe the microvasculatures of small animals and humans in vivo. Furthermore, the functional hemodynamics, namely, the blood flow rate in the microvasculature, is successfully monitored and quantified in vivo. More importantly, thanks to the high SNR and fast B-mode rate (500 Hz), by localizing photoacoustic signals from captured red blood cells without any contrast agent, unresolved microvessels are clearly distinguished, and the spatial resolution is improved by a factor of 2.5 in vivo. L-PAM-GS has great potential in various fields, such as neurology, oncology, and pathology. Photoacoustic microscopy with significantly enhanced temporal and spatial resolution has been demonstrated by scientists in South Korea. Jongbeom Kim and coworkers from Pohang University of Science and Technology (POSTECH) incorporated a fast galvanometer scanner and custom-made scanning mirror into their photoacoustic microscope. The result is an imaging system that is able to visualize very fine blood vessels (microvasculature) in the ear, eye or brain of mice that would usually be very hard to detect or undetectable. The scheme does not require an exogenous contrast agent as the light absorption from red blood cells is sufficiently strong and operates with scan rates of up 500 Hz and with 2.5 times improved spatial resolution by an agent-free localization approach. Potential applications that could benefit from the system include vascularization studies in the areas of dermatology and oncology.