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Showing papers in "Light-Science & Applications in 2017"


Journal ArticleDOI
TL;DR: This review highlights the latest optical trapping configurations and their applications in bioscience, as well as recent advances down to the nanoscale, and discusses the future prospects of nanomanipulation.
Abstract: Since the invention of optical tweezers, optical manipulation has advanced significantly in scientific areas such as atomic physics, optics and biological science. Especially in the past decade, numerous optical beams and nanoscale devices have been proposed to mechanically act on nanoparticles in increasingly precise, stable and flexible ways. Both the linear and angular momenta of light can be exploited to produce optical tractor beams, tweezers and optical torque from the microscale to the nanoscale. Research on optical forces helps to reveal the nature of light-matter interactions and to resolve the fundamental aspects, which require an appropriate description of momenta and the forces on objects in matter. In this review, starting from basic theories and computational approaches, we highlight the latest optical trapping configurations and their applications in bioscience, as well as recent advances down to the nanoscale. Finally, we discuss the future prospects of nanomanipulation, which has considerable potential applications in a variety of scientific fields and everyday life.

424 citations


Journal ArticleDOI
Yicheng Zhao1, Wenke Zhou1, Xu Zhou1, Kaihui Liu1, Dapeng Yu1, Qing Zhao1 
TL;DR: A novel light-assisted method of catalyzing ionic interdiffusion between CH3NH3I and PbI2 stacking layers in sequential deposition perovskite synthesis is designed, which enables fine control of the reaction depth in perovSKite synthesis and, in turn, supports light-enhanced ionic transport.
Abstract: Ionic transport in organometal halide perovskites is of vital importance because it dominates anomalous phenomena in perovskite solar cells, from hysteresis to switchable photovoltaic effects. However, excited state ionic transport under illumination has remained elusive, although it is essential for understanding the unusual light-induced effects (light-induced self-poling, photo-induced halide segregation and slow photoconductivity response) in organometal halide perovskites for optoelectronic applications. Here, we quantitatively demonstrate light-enhanced ionic transport in CH3NH3PbI3 over a wide temperature range of 17–295 K, which reveals a reduction in ionic transport activation energy by approximately a factor of five (from 0.82 to 0.15 eV) under illumination. The pure ionic conductance is obtained by separating it from the electronic contribution in cryogenic galvanostatic and voltage-current measurements. On the basis of these findings, we design a novel light-assisted method of catalyzing ionic interdiffusion between CH3NH3I and PbI2 stacking layers in sequential deposition perovskite synthesis. X-ray diffraction patterns indicate a significant reduction of PbI2 residue in the optimized CH3NH3PbI3 thin film produced via light-assisted sequential deposition, and the resulting solar cell efficiency is increased by over 100% (7.5%–15.7%) with little PbI2 residue. This new method enables fine control of the reaction depth in perovskite synthesis and, in turn, supports light-enhanced ionic transport. Using light to excite ions in perovskite thin films can improve the conductivity and synthetic deposition of low-cost solar cells. Organometal halide perovskites have a suitable bandgap for photovoltaics and are compatible with solution processing, but tend to degrade after long exposure to sunlight. A team led by Qing Zhao from Peking University now reports that excited state ionic transport is the key to understanding perovskite’s poor photostability. Through video snapshots and quantitative conductivity extractions, their analysis revealed that illumination drops the energy barrier needed to activate ionic transport by almost five fold—an enhancement that may induce disorder of electronic structure in the solar cell over time. Intriguingly, the light-enhanced ionic transport can also catalyze removal of metal halide precipitates during thin film annealing in sequential deposition reaction, boosting the device efficiency from 7.5 to 15.7% after just 10 minutes of light exposure.

327 citations


Journal ArticleDOI
TL;DR: This work presents a novel design concept for highly integrated active optical components that employs a combination of resonant plasmonic metasurfaces and the phase-change material Ge3Sb2Te6, and demonstrates beam switching and bifocal lensing.
Abstract: Compact nanophotonic elements exhibiting adaptable properties are essential components for the miniaturization of powerful optical technologies such as adaptive optics and spatial light modulators. While the larger counterparts typically rely on mechanical actuation, this can be undesirable in some cases on a microscopic scale due to inherent space restrictions. Here, we present a novel design concept for highly integrated active optical components that employs a combination of resonant plasmonic metasurfaces and the phase-change material Ge3Sb2Te6. In particular, we demonstrate beam switching and bifocal lensing, thus, paving the way for a plethora of active optical elements employing plasmonic metasurfaces, which follow the same design principles.

313 citations


Journal ArticleDOI
TL;DR: It is demonstrated that by exploiting the random rolling of cells while they are flowing along a microfluidic channel, it is possible to obtain in-line phase-contrast tomography, if smart strategies for wavefront analysis are adopted.
Abstract: High-throughput single-cell analysis is a challenging task. Label-free tomographic phase microscopy is an excellent candidate to perform this task. However, in-line tomography is very difficult to implement in practice because it requires a complex set-up for rotating the sample and examining the cell along several directions. We demonstrate that by exploiting the random rolling of cells while they are flowing along a microfluidic channel, it is possible to obtain in-line phase-contrast tomography, if smart strategies for wavefront analysis are adopted. In fact, surprisingly, a priori knowledge of the three-dimensional position and orientation of rotating cells is no longer needed because this information can be completely retrieved through digital holography wavefront numerical analysis. This approach makes continuous-flow cytotomography suitable for practical operation in real-world, single-cell analysis and with a substantial simplification of the optical system; that is, no mechanical scanning or multi-direction probing is required. A demonstration is given for two completely different classes of biosamples: red blood cells and diatom algae. An accurate characterization of both types of cells is reported, despite their very different nature and material content, thus showing that the proposed method can be extended by adopting two alternate strategies of wavefront analysis to many classes of cells.

306 citations


Journal ArticleDOI
Chao Xie1, Peng You1, Zhike Liu1, Li Li1, Feng Yan1 
TL;DR: The first report of low-voltage high-gain phototransistors based on perovskite/organic-semiconductor vertical heterojunctions, which show ultrahigh responsivities and specific detectivities in a broadband region from the ultraviolet to the near infrared are presented.
Abstract: Organolead halide perovskites have emerged as the most promising materials for various optoelectronic devices, especially solar cells, because of their excellent optoelectronic properties. Here, we present the first report of low-voltage high-gain phototransistors based on perovskite/organic-semiconductor vertical heterojunctions, which show ultrahigh responsivities of ~109A W–1 and specific detectivities of ~1014 Jones in a broadband region from the ultraviolet to the near infrared. The high sensitivity of the devices is attributed to a pronounced photogating effect that is mainly due to the long carrier lifetimes and strong light absorption in the perovskite material. In addition, flexible perovskite photodetectors have been successfully prepared via a solution process and show high sensitivity as well as excellent flexibility and bending durability. The high performance and facile solution-based fabrication of the perovskite/organic-semiconductor phototransistors indicate their promise for potential application for ultrasensitive broadband photodetection. High-gain phototransistors that operate from the ultraviolet to the near infrared could enable sensitive, broadband light detection. The excellent optoelectronic properties of organolead halide perovskites make them promising for various optoelectronic devices. Now, FY and co-workers from The Hong Kong Polytechnic University have fabricated low-voltage, high-gain phototransistors based on a vertical heterojunction consisting of a perovskite (CH3NH3PbI3-xClx) and an organic-semiconductor (PEDOT:PSS). Their high sensitivity is attributed mainly to the strong light absorption and long charge carrier lifetimes of the perovskite. The phototransistors can be fabricated on plastic substrates by solution processing, making them both flexible and compatible with cost-effective mass-production. Tests with light-emitting diodes that emit light at wavelengths of 370, 598 and 895 nanometers and a monochromator confirmed the broadband operation of the phototransistors.

247 citations


Journal ArticleDOI
TL;DR: Recent advances in the realisation of integrated sources of photonic quantum states are reviewed, focusing on approaches based on nonlinear optics that are compatible with contemporary optical fibre telecommunications and quantum memory platforms as well as with chip-scale semiconductor technology.
Abstract: The ability to generate complex optical photon states involving entanglement between multiple optical modes is not only critical to advancing our understanding of quantum mechanics but will play a key role in generating many applications in quantum technologies. These include quantum communications, computation, imaging, microscopy and many other novel technologies that are constantly being proposed. However, approaches to generating parallel multiple, customisable bi- and multi-entangled quantum bits (qubits) on a chip are still in the early stages of development. Here, we review recent advances in the realisation of integrated sources of photonic quantum states, focusing on approaches based on nonlinear optics that are compatible with contemporary optical fibre telecommunications and quantum memory platforms as well as with chip-scale semiconductor technology. These new and exciting platforms hold the promise of compact, low-cost, scalable and practical implementations of sources for the generation and manipulation of complex quantum optical states on a chip, which will play a major role in bringing quantum technologies out of the laboratory and into the real world. Several new platforms are promising for generating and manipulating complex quantum optical states on a chip. Chip-based sources of quantum states of light are needed to bring quantum technologies out of the lab and into the real world, but such sources are still immature. David Moss at Swinburne University of Technology, Australia, and an international team have reviewed progress in developing and characterizing such sources. Waveguide, cavity and ring resonator devices made from nonlinear materials such as silicon, silicon nitride, silicon oxynitride, Hydex and periodically poled lithium niobate offer scientists a rich variety of sources. Furthermore, many of these technologies can be integrated with silicon CMOS photonics, providing a path for building sophisticated, scalable optical integrated circuits for generating and manipulating quantum optical states for applications in quantum information processing and communications.

228 citations


Journal ArticleDOI
TL;DR: An on-chip parametric down-conversion source of photon pairs based on second-order nonlinearity in an aluminum-nitride microring resonator is demonstrated, providing great potential for realizing sufficient on- chip filtering and monolithic integration of quantum light sources, waveguide circuits and single-photon detectors.
Abstract: Quantum-photonic chips, which integrate quantum light sources alongside active and passive optical elements, as well as single-photon detectors, show great potential for photonic quantum information processing and quantum technology. Mature semiconductor nanofabrication processes allow for scaling such photonic integrated circuits to on-chip networks of increasing complexity. Second-order nonlinear materials are the method of choice for generating photonic quantum states in the overwhelming majority of linear optic experiments using bulk components, but integration with waveguide circuitry on a nanophotonic chip proved to be challenging. Here, we demonstrate such an on-chip parametric down-conversion source of photon pairs based on second-order nonlinearity in an aluminum-nitride microring resonator. We show the potential of our source for quantum information processing by measuring the high visibility anti-bunching of heralded single photons with nearly ideal state purity. Our down-conversion source yields measured coincidence rates of 80 Hz, which implies MHz generation rates of correlated photon pairs. Low noise performance is demonstrated by measuring high coincidence-to-accidental ratios. The generated photon pairs are spectrally far separated from the pump field, providing great potential for realizing sufficient on-chip filtering and monolithic integration of quantum light sources, waveguide circuits and single-photon detectors. A chip-based source of photon pairs for applications in quantum information processing has been built by a team of scientists in the US. Xiang Guo and co-workers from Yale University built the quantum light source from an aluminum nitride microring resonator. The strong second-order nonlinearity of the aluminum nitride yields efficient parametric down-conversion. This allows 775-nm-wavelength pump photons to be converted into a pair of entangled photons in the telecommunications window of 1550 nm. Tests indicated that the heralded photons generated by the microring source are anti-bunched and have high visibility and a high purity of state, indicating that they are highly suitable for use in quantum optics experiments. In principle, the on-chip source is compatible with megahertz generation rates and large-scale manufacture of integrated optical circuits.

215 citations


Journal ArticleDOI
TL;DR: Meta-axicons with high NA up to 0.9 capable of generating Bessel beams with full width at half maximum about as small as ~λ/3 (λ=405 nm) have transverse intensity profiles independent of wavelength across the visible spectrum.
Abstract: Bessel beams are of great interest due to their unique non-diffractive properties Using a conical prism or an objective paired with an annular aperture are two typical approaches for generating zeroth-order Bessel beams However, the former approach has a limited numerical aperture (NA), and the latter suffers from low efficiency, as most of the incident light is blocked by the aperture Furthermore, an additional phase-modulating element is needed to generate higher-order Bessel beams, which in turn adds complexity and bulkiness to the system We overcome these problems using dielectric metasurfaces to realize meta-axicons with additional functionalities not achievable with conventional means We demonstrate meta-axicons with high NA up to 09 capable of generating Bessel beams with full width at half maximum about as small as ~λ/3 (λ=405 nm) Importantly, these Bessel beams have transverse intensity profiles independent of wavelength across the visible spectrum These meta-axicons can enable advanced research and applications related to Bessel beams, such as laser fabrication, imaging and optical manipulation

186 citations


Journal ArticleDOI
TL;DR: The performance limitations of interleaved nanoantenna arrays are studied by means of a Wigner phase-space distribution to establish the ultimate information capacity of a metasurface-based photonic system, and the possibility of achieving complete real-time control and measurement of the fundamental, intrinsic properties of light.
Abstract: Shared-aperture technology for multifunctional planar systems, performing several simultaneous tasks, was first introduced in the field of radar antennas. In photonics, effective control of the electromagnetic response can be achieved by a geometric-phase mechanism implemented within a metasurface, enabling spin-controlled phase modulation. The synthesis of the shared-aperture and geometric-phase concepts facilitates the generation of multifunctional metasurfaces. Here shared-aperture geometric-phase metasurfaces were realized via the interleaving of sparse antenna sub-arrays, forming Si-based devices consisting of multiplexed geometric-phase profiles. We study the performance limitations of interleaved nanoantenna arrays by means of a Wigner phase-space distribution to establish the ultimate information capacity of a metasurface-based photonic system. Within these limitations, we present multifunctional spin-dependent dielectric metasurfaces, and demonstrate multiple-beam technology for optical rotation sensing. We also demonstrate the possibility of achieving complete real-time control and measurement of the fundamental, intrinsic properties of light, including frequency, polarization and orbital angular momentum.

177 citations


Journal ArticleDOI
TL;DR: It will be shown how combining the concepts of spatial and spectral-band broadening led to the improvement in compactness that is uniquely enabled by freeform optics.
Abstract: We present optical designs with freeform optics in the context of hyperspectral imaging. Results show designs that are 5 × more compact in volume than similar designs using conventional spherical or aspherical surfaces. We will show how combining the concepts of spatial and spectral-band broadening, which will be introduced in this paper, led to the improvement in compactness that is uniquely enabled by freeform optics.

175 citations


Journal ArticleDOI
TL;DR: This work develops a powerful strategy to realize chiral microstructures in isotropic material by coaxial interference of a vortex beam and a plane wave, which produces three-dimensional (3D) spiral optical fields.
Abstract: Optical vortices, a type of structured beam with helical phase wavefronts and ‘doughnut’-shaped intensity distributions, have been used to fabricate chiral structures in metals and spiral patterns in anisotropic polarization-dependent azobenzene polymers. However, in isotropic polymers, the fabricated microstructures are typically confined to non-chiral cylindrical geometry due to the two-dimensional ‘doughnut’-shaped intensity profile of the optical vortices. Here we develop a powerful strategy to realize chiral microstructures in isotropic material by coaxial interference of a vortex beam and a plane wave, which produces three-dimensional (3D) spiral optical fields. These coaxial interference beams are generated by designing contrivable holograms consisting of an azimuthal phase and an equiphase loaded on a liquid-crystal spatial light modulator. In isotropic polymers, 3D chiral microstructures are achieved under illumination using coaxial interference femtosecond laser beams with their chirality controlled by the topological charge. Our further investigation reveals that the spiral lobes and chirality are caused by interfering patterns and helical phase wavefronts, respectively. This technique is simple, stable and easy to perform, and it offers broad applications in optical tweezers, optical communications and fast metamaterial fabrication. Helical microstructures can be directly polymerized into standard photoresists using beams derived from interfering holograms. Recent studies have shown that optical vortices can pattern polymer surfaces with the same left- or right-handed chirality of the spinning light beam, but only if the material’s structure has a built-in asymmetry. Dong Wu and co-workers from the University of Science and Technology of China report that optical vortices generated by liquid-crystal devices called spatial light modulators (SLMs) are stable enough to engrave chiral microstructures into more-common isotropic polymers. Directing femtosecond laser pulses onto an SLM produced holograms and plane waves that interfered and co-propagated into helices without the phase sensitivity of typical split-beam setups. This approach enabled controllable fabrication of spiral patterns with different lobes and orientations over large areas with a 100-nanometer-scale precision.

Journal ArticleDOI
TL;DR: The lossy nature of plasmonic wave due to absorption is shown to become an advantage for scaling-up a large area surface nanotexturing of transparent dielectrics and semiconductors by a self-organized sub-wavelength energy deposition leading to an ablation pattern—ripples—using this plAsmonic nano-printing.
Abstract: The lossy nature of plasmonic wave due to absorption is shown to become an advantage for scaling-up a large area surface nanotexturing of transparent dielectrics and semiconductors by a self-organized sub-wavelength energy deposition leading to an ablation pattern—ripples—using this plasmonic nano-printing. Irreversible nanoscale modifications are delivered by surface plasmon polariton (SPP) using: (i) fast scan and (ii) cylindrical focusing of femtosecond laser pulses for a high patterning throughput. The mechanism of ripple formation on ZnS dielectric is experimentally proven to occur via surface wave at the substrate–plasma interface. The line focusing increase the ordering quality of ripples and facilitates fabrication over wafer-sized areas within a practical time span. Nanoprinting using SPP is expected to open new applications in photo-catalysis, tribology, and solar light harvesting via localized energy deposition rather scattering used in photonic and sensing applications based on re-scattering of SPP modes into far-field modes. Femtosecond laser pulses focused onto a dielectric induce ripples on its surface that are useful for photovoltaic and sensing applications. Light-driven excitation of surface charge waves—plasmons—achieves light localization on surfaces down to the deep-subwavelength nanoscale. Now, Hong-Bo Sun of Jilin University in China and co-workers from China and Australia have used surface plasmon polaritons to create permanent surface modifications whose period is smaller than the laser wavelength. They achieved this by using a cylindrical lens to create a line focus for near-infrared ultrashort laser pulses on silicon and zinc sulfide surfaces. This nanoprinting technique is well suited for texturing large areas on a wafer scale, since the patterning speed is determined simply by the repetition rate of the laser used. Potential applications include enhanced light harvesting for solar cells and improved photocatalysis.

Journal ArticleDOI
TL;DR: This work shows that it can automatically unscramble optical beams that have been arbitrarily mixed in a multimode waveguide, undoing the scattering and mixing between the spatial modes through a mesh of silicon photonics tuneable beam splitters.
Abstract: Propagation of light beams through scattering or multimode systems may lead to the randomization of the spatial coherence of the light. Although information is not lost, its recovery requires a coherent interferometric reconstruction of the original signals, which have been scrambled into the modes of the scattering system. Here we show that we can automatically unscramble optical beams that have been arbitrarily mixed in a multimode waveguide, undoing the scattering and mixing between the spatial modes through a mesh of silicon photonics tuneable beam splitters. Transparent light detectors integrated in a photonic chip are used to directly monitor the evolution of each mode along the mesh, allowing sequential tuning and adaptive individual feedback control of each beam splitter. The entire mesh self-configures automatically through a progressive tuning algorithm and resets itself after significantly perturbing the mixing, without turning off the beams. We demonstrate information recovery by the simultaneous unscrambling, sorting and tracking of four mixed modes, with residual cross-talk of −20 dB between the beams. Circuit partitioning assisted by transparent detectors enables scalability to meshes with a higher port count and to a higher number of modes without a proportionate increase in the control complexity. The principle of self-configuring and self-resetting in optical systems should be applicable in a wide range of optical applications. A silicon photonics chip featuring a mesh of tunable beam splitters can unscramble mode mixing that occurs in multimode waveguides. Scattering or multimode systems can randomize the spatial coherence of light beams. Francesco Morichetti and co-workers from Politecnico di Milano, Italy, and Stanford University, USA, have fabricated a chip-based descrambler that can automatically unscramble optical beams. A progressive tuning algorithm that monitors the output of the chip enables the mesh to self-configure so that it can unscramble and sort different spatial modes. In a demonstration of the device, four optical beams containing mixed modes were unmixed and separated into outputs with a residual crosstalk of less than −20 dB between the modes. The approach is scalable to a higher number of modes and is promising for optical communication systems employing mode division multiplexing.

Journal ArticleDOI
TL;DR: In vivo imaging of superficial microvasculature and melanoma tumors was demonstrated with ~2.7±0.5 μm lateral resolution and Phantom studies confirmed signal dependence on optical absorption, index contrast and excitation fluence.
Abstract: Elasto-optical refractive index modulation due to photoacoustic initial pressure transients produced significant reflection of a probe beam when the absorbing interface had an appreciable refractive index difference This effect was harnessed in a new form of non-contact optical resolution photoacoustic microscopy called photoacoustic remote sensing microscopy A non-interferometric system architecture with a low-coherence probe beam precludes detection of surface oscillations and other phase-modulation phenomenon The probe beam was confocal with a scanned excitation beam to ensure detection of initial pressure-induced intensity reflections at the subsurface origin where pressures are largest Phantom studies confirmed signal dependence on optical absorption, index contrast and excitation fluence In vivo imaging of superficial microvasculature and melanoma tumors was demonstrated with ~27±05 μm lateral resolution A new design for a photoacoustic microscope capable of high-quality, real-time in vivo imaging has been developed by scientists in Canada Parsin Hajireza and co-workers from the University of Alberta and the company Illumisonics report that, unlike other designs, their approach does not rely on interferometric detection of photoacoustic stress, which can be problematic Instead, it involves making time-varying intensity measurements of the reflection of a probe beam from the sample A high signal-to-noise ratio and a working distance of 25 centimetres between the sample and the system's objective lens are achievable The researchers demonstrate the potential of their scheme for biomedical applications by using to perform in vivo imaging of microvasculature and melanoma tumours in chicken embryos with a spatial resolution of 27 micrometres

Journal ArticleDOI
TL;DR: The hybrid plasmonic UC nanostructures with an optimal shell thickness exhibit an improved bioimaging performance compared with bare UCNCs, and a polarized nature of the light at both UC emission bands is observed, which stems from the relationship between the excitation polarization and GNR orientation.
Abstract: Lanthanide-doped upconversion nanocrystals (UCNCs) have recently become an attractive nonlinear fluorescence material for use in bioimaging because of their tunable spectral characteristics and exceptional photostability Plasmonic materials are often introduced into the vicinity of UCNCs to increase their emission intensity by means of enlarging the absorption cross-section and accelerating the radiative decay rate Moreover, plasmonic nanostructures (eg, gold nanorods, GNRs) can also influence the polarization state of the UC fluorescence-an effect that is of fundamental importance for fluorescence polarization-based imaging methods yet has not been discussed previously To study this effect, we synthesized GNR@SiO2@CaF2:Yb3+,Er3+ hybrid core-shell-satellite nanostructures with precise control over the thickness of the SiO2 shell We evaluated the shell thickness-dependent plasmonic enhancement of the emission intensity in ensemble and studied the plasmonic modulation of the emission polarization at the single-particle level The hybrid plasmonic UC nanostructures with an optimal shell thickness exhibit an improved bioimaging performance compared with bare UCNCs, and we observed a polarized nature of the light at both UC emission bands, which stems from the relationship between the excitation polarization and GNR orientation We used electrodynamic simulations combined with Forster resonance energy transfer theory to fully explain the observed effect Our results provide extensive insights into how the coherent interaction between the emission dipoles of UCNCs and the plasmonic dipoles of the GNR determines the emission polarization state in various situations and thus open the way to the accurate control of the UC emission anisotropy for a wide range of bioimaging and biosensing applications

Journal ArticleDOI
Kaikai Du1, Qiang Li1, Yanbiao Lyu1, Jichao Ding1, Yue Lu1, Zhiyuan Cheng1, Min Qiu1 
TL;DR: In this article, the authors demonstrated control over the emissivity of a thermal emitter consisting of a film of phase-changing material Ge2Sb2Te5 (GST) on top of a metal film.
Abstract: Controlling the emissivity of a thermal emitter has attracted growing interest, with a view toward a new generation of thermal emission devices. To date, all demonstrations have involved using sustained external electric or thermal consumption to maintain a desired emissivity. In the present study, we demonstrated control over the emissivity of a thermal emitter consisting of a film of phase-changing material Ge2Sb2Te5 (GST) on top of a metal film. This thermal emitter achieves broad wavelength-selective spectral emissivity in the mid-infrared. The peak emissivity approaches the ideal blackbody maximum, and a maximum extinction ratio of >10 dB is attainable by switching the GST between the crystalline and amorphous phases. By controlling the intermediate phases, the emissivity can be continuously tuned. This switchable, tunable, wavelength-selective and thermally stable thermal emitter will pave the way toward the ultimate control of thermal emissivity in the field of fundamental science as well as for energy harvesting and thermal control applications, including thermophotovoltaics, light sources, infrared imaging and radiative coolers. The use of phase-change materials can provide tunable control over the emissivity of a thermal emitter. This discovery, made by Kaikai Du and co-workers at Zhejiang University in China, could be useful for applications in thermophotovoltaics, infrared imaging and radiative cooling. The team coated gold substrates with thin layers of the phase-change material Ge2Sb2Te5 (GST). They found that the emissivity of these samples changed with the thickness of the GST layer and the sample temperature. Specifically, the wavelength of the emissivity peak shifted from around 9 to 13 micrometres as the thickness of the GST layer increased from 360 to 540 nanometres. Furthermore, gradually changing the temperature of the GST layer to switch it between its amorphous and crystalline states provided continuous control over the emissivity.

Journal ArticleDOI
TL;DR: A new backlight system incorporating a functional reflective polarizer and a patterned half-wave plate to decouple the polarization states of the blue light and the green/red lights is proposed, which goes beyond the color gamut limit achievable by a conventional LCD.
Abstract: In this study, we analyze how a backlight’s peak wavelength, full-width at half-maximum (FWHM), and color filters affect the color gamut of a liquid crystal display (LCD) device and establish a theoretical limit, even if the FWHM approaches 1 nm. To overcome this limit, we propose a new backlight system incorporating a functional reflective polarizer and a patterned half-wave plate to decouple the polarization states of the blue light and the green/red lights. As a result, the crosstalk between three primary colors is greatly suppressed, and the color gamut is significantly widened. In the experiment, we prepare a white-light source using a blue light-emitting diode (LED) to pump green perovskite polymer film and red quantum dots and demonstrate an exceedingly large color gamut (95.8% Rec. 2020 in Commission internationale de l'eclairage (CIE) 1931 color space and 97.3% Rec. 2020 in CIE 1976 color space) with commercial high-efficiency color filters. These results are beyond the color gamut limit achievable by a conventional LCD. Our design works equally well for other light sources, such as a 2-phosphor-converted white LED. A liquid-crystal display that produces a wider range of colours than current commercial devices has been created. Shin-Tson Wu from the University of Central Florida and co-workers achieved this by determining the optimal optical characteristics of a liquid-crystal display backlight. Colour liquid-crystal displays work by using a layer of electrically controlled molecules to block emission from a white-light source. The optical properties of this white backlight determine the full range of colours created by the display, referred to as its gamut. Wu and colleagues created a backlight based on a blue light-emitting diode combined with quantum dots and a perovskite polymer film. They then optimized the emission by adding a reflective polarizer and a half-wave plate, and hence increased the gamut of the display.

Journal ArticleDOI
TL;DR: A new approach based on difference-frequency generation of frequency-agile near-infrared frequency combs that are produced using electro-optic modulators is demonstrated, which holds promise for fast and sensitive time-resolved studies of, for example, trace gases.
Abstract: Absorption spectroscopy of fundamental ro-vibrational transitions in the mid-infrared region provides a powerful tool for studying the structure and dynamics of molecules in the gas phase and for sensitive and quantitative gas sensing. Laser frequency combs permit novel approaches to perform broadband molecular spectroscopy. Multiplex dual-comb spectroscopy without moving parts can achieve particularly high speed, sensitivity and resolution. However, achieving Doppler-limited resolution in the mid-infrared still requires overcoming instrumental challenges. Here we demonstrate a new approach based on difference-frequency generation of frequency-agile near-infrared frequency combs that are produced using electro-optic modulators. The combs have a remarkably flat intensity distribution, and their positions and line spacings can be freely selected by simply dialing a knob. Using the proposed technique, we record, in the 3-μm region, Doppler-limited absorption spectra with resolved comb lines within milliseconds, and precise molecular line parameters are retrieved. Our technique holds promise for fast and sensitive time-resolved studies of, for example, trace gases.

Journal ArticleDOI
TL;DR: It is demonstrated that the infrared plasmonic response of a graphene multilayer stack is analogous to that of a highly doped single layer of graphene, preserving mobility and supporting plasMonic resonances with higher oscillator strength than previously explored single-layer devices.
Abstract: Graphene is emerging as a promising material for photonic applications owing to its unique optoelectronic properties Graphene supports tunable, long-lived and extremely confined plasmons that have great potential for applications such as biosensing and optical communications However, in order to excite plasmonic resonances in graphene, this material requires a high doping level, which is challenging to achieve without degrading carrier mobility and stability Here, we demonstrate that the infrared plasmonic response of a graphene multilayer stack is analogous to that of a highly doped single layer of graphene, preserving mobility and supporting plasmonic resonances with higher oscillator strength than previously explored single-layer devices Particularly, we find that the optically equivalent carrier density in multilayer graphene is larger than the sum of those in the individual layers Furthermore, electrostatic biasing in multilayer graphene is enhanced with respect to single layer due to the redistribution of carriers over different layers, thus extending the spectral tuning range of the plasmonic structure The superior effective doping and improved tunability of multilayer graphene stacks should enable a plethora of future infrared plasmonic devices with high optical performance and wide tunability

Journal ArticleDOI
TL;DR: This work presents a new pulse-stretching technique, termed free-space angular-chirp-enhanced delay (FACED), with three distinguishing features absent in the prevailing dispersive-fiber-based implementations, and demonstrates not only ultrafast laser-scanning time-stretch imaging with superior bright-field image quality compared with previous work but also, for the first time, MHz fluorescence and colorized time-Stretch microscopy.
Abstract: Optical time-stretch imaging enables the continuous capture of non-repetitive events in real time at a line-scan rate of tens of MHz—a distinct advantage for the ultrafast dynamics monitoring and high-throughput screening that are widely needed in biological microscopy. However, its potential is limited by the technical challenge of achieving significant pulse stretching (that is, high temporal dispersion) and low optical loss, which are the critical factors influencing imaging quality, in the visible spectrum demanded in many of these applications. We present a new pulse-stretching technique, termed free-space angular-chirp-enhanced delay (FACED), with three distinguishing features absent in the prevailing dispersive-fiber-based implementations: (1) it generates substantial, reconfigurable temporal dispersion in free space (>1 ns nm−1) with low intrinsic loss (<6 dB) at visible wavelengths; (2) its wavelength-invariant pulse-stretching operation introduces a new paradigm in time-stretch imaging, which can now be implemented both with and without spectral encoding; and (3) pulse stretching in FACED inherently provides an ultrafast all-optical laser-beam scanning mechanism at a line-scan rate of tens of MHz. Using FACED, we demonstrate not only ultrafast laser-scanning time-stretch imaging with superior bright-field image quality compared with previous work but also, for the first time, MHz fluorescence and colorized time-stretch microscopy. Our results show that this technique could enable a wider scope of applications in high-speed and high-throughput biological microscopy that were once out of reach. A new pulse-stretching technique has enabled ultrafast laser-scanning time-stretch imaging to be achieved in the important visible region. Optical time-stretching is used to realize real-time continuous imaging at ultrahigh frame rates, but current technologies based on dispersive fibers are generally restricted to near-infrared wavelengths. Now, a team at the University of Hong Kong led by Kevin Tsia has overcome this limitation by developing a pulse-stretching technique that they dub free-space angular-chirp-enhanced delay. It has the advantages of generating a large dispersion in free space with low loss and of enabling wavelength-invariant stretching. The researchers demonstrated its potential by realizing ultrafast laser-scanning time-stretch imaging with excellent bright-field image quality. They also used it to achieve megahertz fluorescence and color time-stretch microscopy at the optical wavelength of 700 nm.

Journal ArticleDOI
Sofie Abé1, Jonas Joos1, Lisa I. D. J. Martin1, Zeger Hens1, Philippe Smet1 
TL;DR: General guidelines are derived to optimize both the cost and efficiency of CdSe/CdS and other (potentially cadmium-free) quantum dot systems and when reabsorption of the green and/or red emission is significant compared to the absorption strength for the blue emission of the pumping light emitting diode, the hybrid remote phosphor approach becomes beneficial.
Abstract: Quantum dots are ideally suited for color conversion in light emitting diodes owing to their spectral tunability, high conversion efficiency and narrow emission bands These properties are particularly important for display backlights; the highly saturated colors generated by quantum dots justify their higher production cost Here, we demonstrate the benefits of a hybrid remote phosphor approach that combines a green-emitting europium-doped phosphor with red-emitting CdSe/CdS core/shell quantum dots Different stacking geometries, including mixed and separate layers of both materials, are studied at the macroscopic and microscopic levels to identify the configuration that achieves maximum device efficiency while minimizing material usage The influence of reabsorption, optical outcoupling and refractive index-matching between the layers is evaluated in detail with respect to device efficiency and cost From the findings of this study, general guidelines are derived to optimize both the cost and efficiency of CdSe/CdS and other (potentially cadmium-free) quantum dot systems When reabsorption of the green and/or red emission is significant compared to the absorption strength for the blue emission of the pumping light emitting diode, the hybrid remote phosphor approach becomes beneficial

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TL;DR: A demonstration of a III-V-on-Si comb laser that can function as a compact, low-cost frequency comb generator after frequency stabilization and the use of low-loss passive silicon waveguides enables the integration of a long laser cavity, which enables the laser to be locked in the passive mode at a record-low 1 GHz repetition rate.
Abstract: Optical frequency combs emerge as a promising technology that enables highly sensitive, near-real-time spectroscopy with a high resolution The currently available comb generators are mostly based on bulky and high-cost femtosecond lasers for dense comb generation (line spacing in the range of 100 MHz to 1 GHz) However, their integrated and low-cost counterparts, which are integrated semiconductor mode-locked lasers, are limited by their large comb spacing, small number of lines and broad optical linewidth In this study, we report a demonstration of a III-V-on-Si comb laser that can function as a compact, low-cost frequency comb generator after frequency stabilization The use of low-loss passive silicon waveguides enables the integration of a long laser cavity, which enables the laser to be locked in the passive mode at a record-low 1 GHz repetition rate The 12-nm 10-dB output optical spectrum and the notably small optical mode spacing results in a dense optical comb that consists of over 1400 equally spaced optical lines The sub-kHz 10-dB radio frequency linewidth and the narrow longitudinal mode linewidth (<400 kHz) indicate notably stable mode-locking Such integrated dense comb lasers are very promising, for example, for high-resolution and real-time spectroscopy applications

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TL;DR: This letter describes the realization of a sub-shot-noise wide field microscope based on spatially multi-mode non-classical photon number correlations in twin beams, achieving the best sensitivity per incident photon reported in absorption microscopy.
Abstract: Recently, several proof of principle experiments have demonstrated the advantages of quantum technologies over classical schemes. The present challenge is to surpass the limits of proof of principle demonstrations to approach real applications. This letter presents such an achievement in the field of quantum enhanced imaging. In particular, we describe the realization of a sub-shot-noise wide field microscope based on spatially multi-mode non-classical photon number correlations in twin beams. The microscope produces realtime images of 8000 pixels at full resolution, for a 500 μm2 field of view, with noise reduced to 80% of the shot noise level (for each pixel), which is suitable for absorption imaging of complex structures. By fast post-elaboration, specifically applying a quantum enhanced median filter, the noise can be further reduced (to <30% of the shot noise level) by setting a trade-off with the resolution, thus achieving the best sensitivity per incident photon reported in absorption microscopy.

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TL;DR: In this article, the authors proposed an OAM complex spectrum analyzer that enables simultaneous measurements of the power and phase distributions of OAM modes by employing the rotational Doppler effect.
Abstract: The ability to measure the orbital angular momentum (OAM) distribution of vortex light is essential for OAM applications. Although there have been many studies on the measurement of OAM modes, it is difficult to quantitatively and instantaneously measure the power distribution among different OAM modes, let alone measure the phase distribution among them. In this work, we propose an OAM complex spectrum analyzer that enables simultaneous measurements of the power and phase distributions of OAM modes by employing the rotational Doppler effect. The original OAM mode distribution is mapped to an electrical spectrum of beat signals using a photodetector. The power and phase distributions of superimposed OAM beams are successfully retrieved by analyzing the electrical spectrum. We also extend the measurement technique to other spatial modes, such as linear polarization modes. These results represent a new landmark in spatial mode analysis and show great potential for applications in OAM-based systems and optical communication systems with mode-division multiplexing. An instrument that can map the power and phase distributions of vortex light beams that have orbital angular momentum (OAM) has been built. The ability to measure the OAM distribution of vortex light is essential for OAM applications, but it is challenging to instantaneously measure the power and phase distribution of different OAM modes. The OAM spectrum analyzer developed by Jianji Dong of Huazhong University of Science and Technology and co-workers characterizes a beam with unknown OAM by sending it and a reference beam to a spinning object, which scatters the light, imparting a rotational Doppler frequency shift in the process. The scattered light is then passed through a mode filter and collected by a photodetector for analysis. The frequency shift of the scattered light reveals information about the OAM content of the unknown beam.

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TL;DR: The demonstrated TMDC–silicon photonic hybrid integration opens the door to second-order nonlinear effects within the silicon photonic platform, including efficient frequency conversion, parametric amplification and the generation of entangled photon pairs.
Abstract: Two-dimensional transition-metal dichalcogenides (TMDCs) with intrinsically broken crystal inversion symmetry and large second-order nonlinear responses have shown great promise for future nonlinear light sources. However, the sub-nanometer monolayer thickness of such materials limits the length of their nonlinear interaction with light. Here, we experimentally demonstrate the enhancement of the second-harmonic generation from monolayer MoSe2 by its integration onto a 220-nm-thick silicon waveguide. Such on-chip integration allows for a marked increase in the interaction length between the MoSe2 and the waveguide mode, further enabling phase matching of the nonlinear process. The demonstrated TMDC-silicon photonic hybrid integration opens the door to second-order nonlinear effects within the silicon photonic platform, including efficient frequency conversion, parametric amplification and the generation of entangled photon pairs.

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TL;DR: It is demonstrated for the first time that a single achiral nanoaperture can be utilized as a meta-macromolecule to achieve giant angular spin Hall effect of light and can enable full control of the phase gradient at a deep-subwavelength level.
Abstract: With properties not previously available, optical metamaterials and metasurfaces have shown their great potential in the precise control of light waves at the nanoscale. However, the use of current metamaterials and metasurfaces is limited by the collective response of the meta-atoms/molecules, which means that a single element cannot provide the functionalities required by most applications. Here, we demonstrate for the first time that a single achiral nanoaperture can be utilized as a meta-macromolecule to achieve giant angular spin Hall effect of light. By controlling the spin-related momenta, we show that these nanoapertures can enable full control of the phase gradient at a deep-subwavelength level, thus forming unique building blocks for optical metasurfaces. As a proof-of-concept demonstration, a miniaturized Bessel-like beam generator and flat lens are designed and experimentally characterized. The results presented here may open a door for the development of meta-macromolecule-based metasurfaces for integrated optical systems and nanophotonics.

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TL;DR: This spatially incoherent lensless imaging technique is simple and capable of variable focusing with adjustable depths of focus that enables depth sensing of 3D objects that are concealed by the diffusing medium.
Abstract: Scattering media, such as diffused glass and biological tissue, are usually treated as obstacles in imaging. To cope with the random phase introduced by a turbid medium, most existing imaging techniques recourse to either phase compensation by optical means or phase recovery using iterative algorithms, and their applications are often limited to two-dimensional imaging. In contrast, we utilize the scattering medium as an unconventional imaging lens and exploit its lens-like properties for lensless three-dimensional (3D) imaging with diffraction-limited resolution. Our spatially incoherent lensless imaging technique is simple and capable of variable focusing with adjustable depths of focus that enables depth sensing of 3D objects that are concealed by the diffusing medium. Wide-field imaging with diffraction-limited resolution is verified experimentally by a single-shot recording of the 1951 USAF resolution test chart, and 3D imaging and depth sensing are demonstrated by shifting focus over axially separated objects.

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TL;DR: By tuning the spacing between axial positions of the interference pump patterns, the mode intensity profiles of single-bottle WGMs can be spatially overlapped with the interference stripes, intrinsically enabling single-mode lasing and selection.
Abstract: Single-mode lasing in whispering-gallery mode (WGM) microresonators is challenging to achieve. In bottle microresonators, the highly non-degenerated WGMs are spatially well-separated along the long-axis direction and provide mode-selection capability. In this work, by engineering the pump intensity to modify the spatial gain profiles of bottle microresonators, we demonstrate a simple and general approach to realizing single-mode WGM lasing in polymer bottle microresonators. The pump intensity is engineered into an interference distribution on the bottle microresonator surface. By tuning the spacing between axial positions of the interference pump patterns, the mode intensity profiles of single-bottle WGMs can be spatially overlapped with the interference stripes, intrinsically enabling single-mode lasing and selection. Attractive advantages of the system, including high side-mode suppression factors >20 dB, large spectral tunability >8 nm, low-lasing threshold and reversible control, are presented. Our demonstrated approach may have a variety of promising applications, ranging from tunable single-mode lasing and sensing to nonlinear optics. By engineering the spatial profile of a pump beam, scientists in China have made a bottle-shaped microresonator lase in a single mode. Whispering-gallery-mode microresonators, which rely on light circulating in a closed path around a disk, sphere, toroid or bottle, are popular due to their small size and high quality factor. However, they tend to be multimodal when lasing due to the lack of a mode-selection element. Fuxing Gu at the University of Shanghai for Science and Technology and co-workers overcame this limitation by using a pump beam with a striped interference pattern to excite a single transverse mode of a small polymer bottle microresonator. The result was a miniature single-mode laser with a low lasing threshold and high suppression of unwanted side modes.

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TL;DR: The first demonstration of a proof-of-principle optical fiber ‘meta-tip’, which integrates a phase-gradient plasmonic metasurface on the fiber tip is reported, which constitutes a first step toward the integration of unprecedented (metasurfaces-enabled) light-manipulation capabilities in optical-fiber technology.
Abstract: We report on the first demonstration of a proof-of-principle optical fiber 'meta-tip', which integrates a phase-gradient plasmonic metasurface on the fiber tip. For illustration and validation purposes, we present numerical and experimental results pertaining to various prototypes implementing generalized forms of the Snell's transmission/reflection laws at near-infrared wavelengths. In particular, we demonstrate several examples of beam steering and coupling with surface waves, in fairly good agreement with theory. Our results constitute a first step toward the integration of unprecedented (metasurface-enabled) light-manipulation capabilities in optical-fiber technology. By further enriching the emergent 'lab-on-fiber' framework, this may pave the way for the widespread diffusion of optical metasurfaces in real-world applications to communications, signal processing, imaging and sensing.

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TL;DR: A field-portable cost-effective platform for high-throughput quantification of particulate matter using computational lens-free microscopy and machine-learning, which can be adaptively tailored to detect specific particles in air, for example, various types of pollen and mold.
Abstract: Rapid, accurate and high-throughput sizing and quantification of particulate matter (PM) in air is crucial for monitoring and improving air quality. In fact, particles in air with a diameter of ≤2.5 μm have been classified as carcinogenic by the World Health Organization. Here we present a field-portable cost-effective platform for high-throughput quantification of particulate matter using computational lens-free microscopy and machine-learning. This platform, termed c-Air, is also integrated with a smartphone application for device control and display of results. This mobile device rapidly screens 6.5 L of air in 30 s and generates microscopic images of the aerosols in air. It provides statistics of the particle size and density distribution with a sizing accuracy of ~93%. We tested this mobile platform by measuring the air quality at different indoor and outdoor environments and measurement times, and compared our results to those of an Environmental Protection Agency–approved device based on beta-attenuation monitoring, which showed strong correlation to c-Air measurements. Furthermore, we used c-Air to map the air quality around Los Angeles International Airport (LAX) over 24 h to confirm that the impact of LAX on increased PM concentration was present even at >7 km away from the airport, especially along the direction of landing flights. With its machine-learning-based computational microscopy interface, c-Air can be adaptively tailored to detect specific particles in air, for example, various types of pollen and mold and provide a cost-effective mobile solution for highly accurate and distributed sensing of air quality. Accurate on-site air-quality monitoring can be performed using lens-free microscopy on a chip coupled with machine learning. To monitor and enhance air quality, it is vital to realize rapid, accurate and high-throughput sizing of airborne particles. A portable system built by Aydogan Ozcan and co-workers from the University of California, Los Angeles, generates statistics of particle size and density from microscopic images of particulate matter in air. A sticky coverslip captures airborne particles and then light from three LEDs (red, green and blue) creates holograms of the particle distribution, captured on a CMOS image sensor and processed. The system can screen 6.5 litres of air in about 30 s and has a particle sizing accuracy of about 93%. Results obtained using this technology achieved a strong correlation with those acquired using conventional particle-sizing devices.