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

Equivalence of photonic sampling to signal holding in channel-interleaved photonic ADCs by controlling photo-detection response

Substantial interests have been attracted in the use of photonic sampling and electronic digitizing for photonic analog-to-digital converter (PADC). However, the nature of that photo-detection with signal holding effects has not been well established. This paper analyzes the equivalence of photonic sampling to signal holding by controlling photo-detection response. In the frequency domain, the high-frequency components generated by the sampling pulse train are folded back into the Nyquist band resulting the signal holding response when the output is digitized. We proposed an approximate response of the photodetector (PD) to verify the theoretical analysis. It is found that the photonic sampling serves as the conventional switch-based sample-and-hold (S&H) circuit in channel-interleaved photonic analog-to-digital converter. In the experiment, the signal holding directly inhibits the timing mismatch without sophisticated calibrations.

Imaging approaches for monitoring three-dimensional cell and tissue culture systems

The past decade has seen an increasing demand for more complex, reproducible and physiologically relevant tissue cultures that can mimic the structural and biological features of living tissues. Monitoring the viability, development and responses of such tissues in real-time are challenging due to the complexities of cell culture physical characteristics and the environments in which these cultures need to be maintained in. Significant developments in optics, such as optical manipulation, improved detection and data analysis, have made optical imaging a preferred choice for many three-dimensional (3D) cell culture monitoring applications. The aim of this review is to discuss the challenges associated with imaging and monitoring 3D tissues and cell culture, and highlight topical label-free imaging tools that enable bioengineers and biophysicists to non-invasively characterise engineered living tissues.

Earth-abundant and environmentally benign Ni-Zn iron oxide intercalated in a polyaniline based nanohybrid as an ultrafast photodetector

Nickel-zinc iron oxide (NZF) was introduced into a polyaniline (PANI) matrix by an in situ chemical oxidation polymerization approach. The surface composition and chemical states were investigated by X-ray photoelectron spectroscopy (XPS), which revealed an Fe 2p spectrum with the two peak positions of Fe 2p_3/2 and Fe 2p_1/2 at 711.00 and 724.48 eV, respectively. Deconvolution of the Fe 2p_3/2 peak revealed two components with binding energies of 713.98 and 718.16 eV, corresponding to the presence of Fe cations in the octahedral and tetrahedral sites. Additionally, the Rietveld refinement of NZF showed a cubic system with the Fd₃m space group. High-resolution transmission electron microscopy (HRTEM) analysis showed that the NZF material strongly interacts with polyaniline, while the selected area electron diffraction (SAED) pattern perfectly matched with the XRD data. Lognormal distribution was used to determine the particle size, which was found to be in the range of 1-100 nm. A flexible photodetector device utilizing the NZF-PANI nanohybrid was fabricated on an environmentally friendly, biodegradable cellulose paper substrate and the device exhibited excellent performance, i.e., a responsivity of 0.069 A W^-1 and detectivity of 7.258 × 10¹⁰ Jones at a very low voltage of 0.1 V. The non-stretched device showed a responsivity of 24.980 A W^-1 at 5 V, whereas at 2 cm^-1 bending curvature, the device showed a responsivity of 20.175 A W^-1, which was much higher than the responsivity of a commercial photodetector (<0.5 A W^-1).

Dual-channel metasurfaces for independent and simultaneous display in near-field and far-field

The operation of near-field and far-field can be employed to display holographic and nanoprinting images, which significantly improves the information density. Previous studies have proposed some approaches to display the images independently or simultaneously, but cannot satisfy these two characteristics in a single structure under the same incident light. Here, a single layer multifunctional metasurface is proposed to display a nanoprinting image and a holographic image independently and simultaneously. By tailoring the dimensions of each nanobricks and adopting different orientation angle, the amplitude and phase can be artificially designed. Moreover, enabled by the simulated annealing algorithm, we take the impact of both amplitude and phase of each nanobrick into consideration, which eliminates the unnecessary influence of amplitude on holographic image. Compared with previous work, our metasurfaces markedly improve the quality of holographic image with simple structures while not affecting the nanoprinting image. To be exact, it breaks the coupling between the near-field and far-field, achieving independent and simultaneous control of both fields. Our proposed metasurfaces carry characteristics of simple manufacture, little crosstalk, and great compactness, which provides novel applications for image displays, optical storage and information technology.

Band-tunable achromatic metalens based on phase change material

Achromatic metalens have the potential to significantly reduce the size and complexity of broadband imaging systems. A large variety of achromatic metalens has been proposed and most of them have the fixed achromatic band that cannot be actively modified. However, band-tunable is an important function in practical applications such as fluorescence microscopic imaging and optical detection. Here, we propose a bilayer metalens that can switch achromatic bands by taking the advantage of the high refractive index contrast of Sb₂S₃ between amorphous and crystalline state. By switching the state of Sb₂S₃, the achromatic band can be reversibly switched between the red region of visible spectrum (650-830 nm) and the near-infrared spectrum (830-1100 nm). This band-tunable design indicates a novel (to our knowledge) method to solve the problem of achromatic focusing in an ultrabroad band. The metalens have an average focusing efficiency of over 35% and 55% in two bands while maintaining diffraction-limited performance. Moreover, through proper design, we can combine different functionalities in two bands such as combining achromatic focusing and diffractive focusing. The proposed metalens have numerous potential applications in tunable displaying, detecting devices and multifunctional devices.

Photonics-enabled spiking timing-dependent convolutional neural network for real-time image classification

A photonics-enabled spiking timing-dependent convolutional neural network (CNN) is proposed by manipulating photonics multidimensional parameters in terms of wavelength, temporal and spatial, which breaks the traditional CNN architecture mapping from a spatially parallel to a time-dependent series structure. The proposed CNN with the application of real-time image recognition comprises a photonics convolution processor to accelerate the computing and an involved electronic full connection to execute the classification task. A timing-dependent series of matrix-matrix operations is conducted in the photonics convolution processor that can be achieved based on multidimensional multiplexing by the accumulation of carriers from an active mode-locked laser, dispersion latency induced by a dispersion compensation fiber, and wavelength spatial separation via a waveshaper. Incorporated with the electronic full connection, a photonics-enabled CNN is proven to perform a real-time recognition task on the MNIST database of handwritten digits with a prediction accuracy of 90.04%. Photonics enables conventional neural networks to accelerate machine learning and neuromorphic computing and has the potential to be widely used in information processing and computing, such as goods classification, vowel recognition, and speech identification.

Inverse design enables large-scale high-performance meta-optics reshaping virtual reality

Meta-optics has achieved major breakthroughs in the past decade; however, conventional forward design faces challenges as functionality complexity and device size scale up. Inverse design aims at optimizing meta-optics design but has been currently limited by expensive brute-force numerical solvers to small devices, which are also difficult to realize experimentally. Here, we present a general inverse-design framework for aperiodic large-scale (20k × 20k λ2) complex meta-optics in three dimensions, which alleviates computational cost for both simulation and optimization via a fast approximate solver and an adjoint method, respectively. Our framework naturally accounts for fabrication constraints via a surrogate model. In experiments, we demonstrate aberration-corrected metalenses working in the visible with high numerical aperture, poly-chromatic focusing, and large diameter up to the centimeter scale. Such large-scale meta-optics opens a new paradigm for applications, and we demonstrate its potential for future virtual-reality platforms by using a meta-eyepiece and a laser back-illuminated micro-Liquid Crystal Display.

Cascadable all-optical NAND gates using diffractive networks

Owing to its potential advantages such as scalability, low latency and power efficiency, optical computing has seen rapid advances over the last decades. Here, we present the design and analysis of cascadable all-optical NAND gates using diffractive neural networks. We encoded the logical values at the input and output planes of a diffractive NAND gate using the relative optical power of two spatially-separated apertures. Based on this architecture, we numerically optimized the design of a diffractive neural network composed of 4 passive layers to all-optically perform NAND operation using diffraction of light, and cascaded these diffractive NAND gates to perform complex logical functions by successively feeding the output of one diffractive NAND gate into another. We numerically demonstrated the cascadability of our diffractive NAND gates by using identical diffractive designs to all-optically perform AND and OR operations, which can be formulated as [Formula: see text] and [Formula: see text], respectively. We also designed an all-optical half-adder that takes two logical values as input and returns their sum and the carry using cascaded diffractive NAND gates. Cascadable all-optical NAND gates composed of spatially-engineered passive diffractive layers can serve optical computing platforms.

12 Gbit/s indoor optical wireless communication system with ultrafast beam-steering using tunable VCSEL

Optical wireless communication (OWC) using line-of-sight connections has great application potential for indoor scenes due to its advantages of high data transmission speed and privacy. In our proposed system, we use infrared tunable vertical-cavity surface-emitting laser (VCSEL) as light source, array waveguide gratings (AWG) combined with 6 × 6 integrated optical fiber arrays as a router to realize ultrafast beam-steering and high capacity point-to-point data transmission, which makes the indoor OWC system compact, low-cost, and easy to install. The high tuning rate of VCSEL enables the channel switching to be completed within 1.7 µs. Based on the modulation format of non-return-to-zero on-off keying (NRZ-OOK), a data rate of 12 Gbit/s per channel can be realized with high sensitivity through 3.1 m free space when the detected optical power is low. The system is proved to be a flexible link with high-speed communication for mobile terminals in a limited space.

Full 2π tunable phase modulation using avoided crossing of resonances

Active metasurfaces have been proposed as one attractive means of achieving high-resolution spatiotemporal control of optical wavefronts, having applications such as LIDAR and dynamic holography. However, achieving full, dynamic phase control has been elusive in metasurfaces. In this paper, we unveil an electrically tunable metasurface design strategy that operates near the avoided crossing of two resonances, one a spectrally narrow, over-coupled resonance and the other with a high resonance frequency tunability. This strategy displays an unprecedented upper limit of 4π range of dynamic phase modulation with no significant variations in optical amplitude, by enhancing the phase tunability through utilizing two coupled resonances. A proof-of-concept metasurface is justified analytically and verified numerically in an experimentally accessible platform using quasi-bound states in the continuum and graphene plasmon resonances, with results showing a 3π phase modulation capacity with a uniform reflection amplitude of ~0.65.

Flat optical phased array receiver incorporating an on-chip metalens concentrator

We propose and design a flat optical phased array (OPA) receiver that consists of a grating antenna, a free-propagation region (FPR) incorporating an on-chip metalens concentrator (OCMC), and an output port of a tapered waveguide. By concatenating the OCMC-integrated FPR with the antenna, the proposed OPA allows light coupled at a slanted ψ angle to be conveyed to the output, thereby resolving the challenges of phase-controlled light detection. To impose a space-dependent phase on the incident light from the antenna, the OCMC is constructed by laterally arranging subwavelength slot meta-atoms with varying slot lengths, which are created in the core layer of a slab and uniformly quantized at 16 phase levels. Hence, without the aid of phase modulators, the light beam emerging from the grating antenna can be focused on the output port through angle-tolerant coupling along the lateral direction. The miniaturized OCMC was confirmed to play a pivotal role in achieving enhanced in-plane coupling efficiency over the field of view.

Perovskite microcells fabricated using swelling-induced crack propagation for colored solar windows

Perovskite microcells have a great potential to be applied to diverse types of optoelectronic devices including light-emitting diodes, photodetectors, and solar cells. Although several perovskite fabrication methods have been researched, perovskite microcells without a significant efficiency drop during the patterning and fabrication process could not be developed yet. We herein report the fabrication of high-efficiency perovskite microcells using swelling-induced crack propagation and the application of the microcells to colored solar windows. The key procedure is a swelling-induced lift-off process that leads to patterned perovskite films with high-quality interfaces. Thus, a power conversion efficiency (PCE) of 20.1 % could be achieved with the perovskite microcell, which is nearly same as the PCE of our unpatterned perovskite photovoltaic device (PV). The semi-transparent PV based on microcells exhibited a light utilization efficiency of 4.67 and a color rendering index of 97.5 %. The metal–insulator–metal structure deposited on the semi-transparent PV enabled to fabricate solar windows with vivid colors and high color purity.

Perovskite microcells can be applied to various types of optoelectronic devices. Here, authors report high efficiency perovskite microcells fabricated using swelling-induced crack propagation and demonstrate solar windows using the microcells.

Automated detection of apoptotic versus nonapoptotic cell death using label-free computational microscopy

Identification of cell death mechanisms, particularly distinguishing between apoptotic versus nonapoptotic pathways, is of paramount importance for a wide range of applications related to cell signaling, interaction with pathogens, therapeutic processes, drug discovery, drug resistance, and even pathogenesis of diseases like cancers and neurogenerative disease among others. Here, we present a novel high-throughput method of identifying apoptotic versus necrotic versus other nonapoptotic cell death processes, based on lensless digital holography. This method relies on identification of the temporal changes in the morphological features of mammalian cells, which are unique to each cell death processes. Different cell death processes were induced by known cytotoxic agents. A deep learning-based approach was used to automatically classify the cell death mechanism (apoptotic vs necrotic vs nonapoptotic) with more than 93% accuracy. This label free approach can provide a low cost (<$250) alternative to some of the currently available high content imaging-based screening tools.

3D k-space reflectance fluorescence tomography via deep learning

We report on the potential to perform image reconstruction in 3D k-space reflectance fluorescence tomography (FT) using deep learning (DL). Herein, we adopt a modified AUTOMAP architecture and develop a training methodology that leverages an open-source Monte-Carlo-based simulator to generate a large dataset. Using an enhanced EMNIST (EEMNIST) dataset as an embedded contrast function allows us to train the network efficiently. The optical strategy utilizes k-space illumination in a reflectance configuration to probe tissue in the mesoscopic regime with high sensitivity and resolution. The proposed DL model training and validation is performed with both in silico data and a phantom experiment. Overall, our results indicate that the approach can correctly reconstruct both single and multiple fluorescent embedding(s) in a 3D volume. Furthermore, the presented technique is shown to outperform the traditional approaches [least-squares (LSQ) and total-variation minimization (TVAL)], especially at higher depths. We, therefore, expect the proposed computational technique to have future implications in preclinical studies.

Performance comparison of high-speed photoacoustic microscopy: opto-ultrasound combiner versus ring-shaped ultrasound transducer

Photoacoustic microscopy (PAM) embedded with a 532 nm pulse laser is widely used to visualize the microvascular structures in both small animals and humans in vivo. An opto-ultrasound combiner (OUC) is often utilized in high-speed PAM to confocally align the optical and acoustic beams to improve the system's sensitivity. However, acoustic impedance mismatch in the OUC results in little improvement in the sensitivity. Alternatively, a ring-shaped ultrasound transducer (RUT) can also accomplish the confocal configuration. Here, we compare the performance of OUC and RUT modules through ultrasound pulse-echo tests and PA imaging experiments. The signal-to-noise ratios (SNRs) of the RUT-based system were 15 dB, 12 dB, and 7 dB higher when compared to the OUC-based system for ultrasound pulse-echo test, PA phantom imaging test, and PA in-vivo imaging test, respectively. In addition, the RUT-based system could image the microvascular structures of small parts of a mouse body in a few seconds with minimal loss in SNR. Thus, with increased sensitivity, improved image details, and fast image acquisition, we believe the RUT-based systems could play a significant role in the design of future fast-PAM systems.

Deep learning enables ultraviolet photoacoustic microscopy based histological imaging with near real-time virtual staining

Histological images can reveal rich cellular information of tissue sections, which are widely used by pathologists in disease diagnosis. However, the gold standard for histopathological examination is based on thin sections on slides, which involves inevitable time-consuming and labor-intensive tissue processing steps, hindering the possibility of intraoperative pathological assessment of the precious patient specimens. Here, by incorporating ultraviolet photoacoustic microscopy (UV-PAM) with deep learning, we show a rapid and label-free histological imaging method that can generate virtually stained histological images (termed Deep-PAM) for both thin sections and thick fresh tissue specimens. With the tissue non-destructive nature of UV-PAM, the imaged intact specimens can be reused for other ancillary tests. We demonstrated Deep-PAM on various tissue preparation protocols, including formalin-fixation and paraffin-embedding sections (7-µm thick) and frozen sections (7-µm thick) in traditional histology, and rapid assessment of intact fresh tissue (~ 2-mm thick, within 15 min for a tissue with a surface area of 5 mm × 5 mm). Deep-PAM potentially serves as a comprehensive histological imaging method that can be simultaneously applied in preoperative, intraoperative, and postoperative disease diagnosis.

Comprehensive deep learning model for 3D color holography

Holography is a vital tool used in various applications from microscopy, solar energy, imaging, display to information encryption. Generation of a holographic image and reconstruction of object/hologram information from a holographic image using the current algorithms are time-consuming processes. Versatile, fast in the meantime, accurate methodologies are required to compute holograms performing color imaging at multiple observation planes and reconstruct object/sample information from a holographic image for widely accommodating optical holograms. Here, we focus on design of optical holograms for generation of holographic images at multiple observation planes and colors via a deep learning model, the CHoloNet. The CHoloNet produces optical holograms which show multitasking performance as multiplexing color holographic image planes by tuning holographic structures. Furthermore, our deep learning model retrieves an object/hologram information from an intensity holographic image without requiring phase and amplitude information from the intensity image. We show that reconstructed objects/holograms show excellent agreement with the ground-truth images. The CHoloNet does not need iteratively reconstruction of object/hologram information while conventional object/hologram recovery methods rely on multiple holographic images at various observation planes along with the iterative algorithms. We openly share the fast and efficient framework that we develop in order to contribute to the design and implementation of optical holograms, and we believe that the CHoloNet based object/hologram reconstruction and generation of holographic images will speed up wide-area implementation of optical holography in microscopy, data encryption, and communication technologies.

Fluorescent concentrator based MISO-NOMA for visible light communications

The experimental realization of multiple-input single-output (MISO) in visible light communication (VLC) has always been difficult due to dynamic channels, complex alignment, and limited capacity. We designed a quantum dot (QD) fluorescent concentrator combined with power domain multiplexing of a non-orthogonal multiple access (NOMA) scheme to provide an estimation-free MISO system. The system supports a large detection area of over 4 cm² and a sum rate up to 120 Mbps over 2.3-m free space, providing a promising connection for the Internet of things (IoT). The concentrator was further implemented in an underwater environment under insufficient incident power. A record data rate of 120 Mbps has been achieved in a 1.5-m underwater MISO system, with a mean bit error rate of 3.24 × 10^-3, which is below the forward error correction criterion.

Orbital angular momentum deep multiplexing holography via an optical diffractive neural network

Orbital angular momentum (OAM) mode multiplexing provides a new strategy for reconstructing multiple holograms, which is compatible with other physical dimensions involving wavelength and polarization to enlarge information capacity. Conventional OAM multiplexing holography usually relies on the independence of physical dimensions, and the deep holography involving spatial depth is always limited for the lack of spatiotemporal evolution modulation technologies. Herein, we introduce a depth-controllable imaging technology in OAM deep multiplexing holography via designing a prototype of five-layer optical diffractive neural network (ODNN). Since the optical propagation with dimensional-independent spatiotemporal evolution offers a unique linear modulation to light, it is possible to combine OAM modes with spatial depths to realize OAM deep multiplexing holography. Exploiting the multi-plane light conversion and in-situ optical propagation principles, we simultaneously modulate both the OAM mode and spatial depth of incident light via unitary transformation and linear modulations, where OAM modes are encoded independently for conversions among holograms. Results show that the ODNN realized light field conversion and evolution of five multiplexed OAM modes in deep multiplexing holography, where the mean square error and structural similarity index measure are 0.03 and 86%, respectively. Our demonstration explores a depth-controllable spatiotemporal evolution technology in OAM deep multiplexing holography, which is expected to promote the development of OAM mode-based optical holography and storage.

Versatile polarization manipulation in vanadium dioxide-integrated terahertz metamaterial

Broadband and switchable versatile polarization metamaterial is crucial in the applications of imaging, sensing and communication, especially in the terahertz frequency. Here, we investigated versatile polarization manipulation in a hybrid terahertz metamaterial with bilayer rectangular rods and a complementary vanadium dioxide (VO₂) layer. The VO₂ phase transition enables a flexible switching from dual-band asymmetric transmission to dual-band reflective half-wave plate. The full width half maximum (FWHM) bandwidths of dual-band asymmetric transmission are 0.77 and 0.21 THz, respectively. The polarization conversion ratio (PCR) of the reflective metamaterial is over 0.9 in the frequency ranges of 1.01-1.17 THz and 1.47-1.95 THz. Angular dependences of multiple polarization properties are studied. The proposed switchable polarization metamaterial is important to the development of multifunctional polarization devices and multichannel polarization detection.

Photonic matrix multiplication lights up photonic accelerator and beyond

Matrix computation, as a fundamental building block of information processing in science and technology, contributes most of the computational overheads in modern signal processing and artificial intelligence algorithms. Photonic accelerators are designed to accelerate specific categories of computing in the optical domain, especially matrix multiplication, to address the growing demand for computing resources and capacity. Photonic matrix multiplication has much potential to expand the domain of telecommunication, and artificial intelligence benefiting from its superior performance. Recent research in photonic matrix multiplication has flourished and may provide opportunities to develop applications that are unachievable at present by conventional electronic processors. In this review, we first introduce the methods of photonic matrix multiplication, mainly including the plane light conversion method, Mach-Zehnder interferometer method and wavelength division multiplexing method. We also summarize the developmental milestones of photonic matrix multiplication and the related applications. Then, we review their detailed advances in applications to optical signal processing and artificial neural networks in recent years. Finally, we comment on the challenges and perspectives of photonic matrix multiplication and photonic acceleration.

Randomized probe imaging through deep k-learning

Randomized probe imaging (RPI) is a single-frame diffractive imaging method that uses highly randomized light to reconstruct the spatial features of a scattering object. The reconstruction process, known as phase retrieval, aims to recover a unique solution for the object without measuring the far-field phase information. Typically, reconstruction is done via time-consuming iterative algorithms. In this work, we propose a fast and efficient deep learning based method to reconstruct phase objects from RPI data. The method, which we call deep k-learning, applies the physical propagation operator to generate an approximation of the object as an input to the neural network. This way, the network no longer needs to parametrize the far-field diffraction physics, dramatically improving the results. Deep k-learning is shown to be computationally efficient and robust to Poisson noise. The advantages provided by our method may enable the analysis of far larger datasets in photon starved conditions, with important applications to the study of dynamic phenomena in physical science and biological engineering.

On-chip asymmetric beam-steering for broadband visible light

Artificial optical nanostructures including three-dimensional (3D) metamaterials and two-dimensional (2D) metasurfaces have shown overwhelming capability to control electromagnetic waves in desirable manners. However, the challenges of manufacturing a complex 3D bulk architecture or achieving nanoscale alignment between multilayers limit their practical applications, and they are unable to be used in on-chip integrated photonic devices. Therefore, the emerging dimensionality-reduction to on-chip metadevices would be of promising research value. Here, we propose a visible-frequency on-chip dual-layer design by cascading one-dimensional (1D) plasmonic metawires with metagratings, which can effectively manipulate surface plasmon polariton (SPP) wavefronts and exhibit on-chip asymmetric beam-steering functionality. Our 1D metawires consist of trapezoidal plasmonic nanoantennas and can enable broadband (460-700 nm) on-chip beam-deflection with a high conversion efficiency. The cascading plasmonic coupling between metawires/metagrating is further demonstrated with broadband asymmetric propagation performance, which is crucial for on-chip plasmonic device development. Finally, we study and theoretically verify a cascade system that integrates a dual-functional (convergent/divergent) lens for the forward/backward propagation, respectively. Compared with conventional free-space multilayer metasurfaces, on-chip 1D metawires enjoy single-time lithography processing and no alignment requirement for implementation in multifunctional devices. We believe that the proof-of-concept on-chip metawires study will pave a new, to the best of our knowledge, way for creating multifunctional photonic integrated devices and hold tremendous potential in realizing on-chip transformation optics, information processing, spectrometers, as well as optical sensors.

Characteristic parameters of photonic nanojets of single dielectric microspheres illuminated by focused broadband radiation

Herein, we report the theoretical investigation on the photonic nanojets (PNJs) of single dielectric microspheres illuminated by focused broadband radiation (polychromatic light) from a Halogen lamp, supercontinuum source, light-emitting diode, and Hg arc lamp. The role of incident beam waist, refractive index of the surrounding medium, and radius of the microsphere on the characteristic parameters such as the electric field intensity enhancement, effective width, and length of the PNJ is studied. Interestingly, the characteristic parameters of the PNJs of solid microspheres obtained for the above-mentioned broadband radiation sources are found close to those observed for the focused monochromatic radiation of wavelengths which are near to the central wavelengths of the sources. Moreover, the characteristic parameters of PNJs of the core-shell microspheres of different thicknesses (t) illuminated by polychromatic radiation from most commonly used sources such as Halogen and Hg arc lamps are studied. For each t value, a suitable wavelength of monochromatic radiation has been found to generate the PNJ with characteristic parameters which are close to those obtained in the case of polychromatic radiation. We believe that the analytical theory and the theoretical simulations reported here would be useful for researchers who work in the fields such as PNJ assisted photoacoustic spectroscopy, white light nanoscopy, low-coherence phase-shifting interference microscopy, and Mirau interferometry.

Bidirectional nanoprinting based on bilayer metasurfaces

Bidirectional nanoprinting, has received significant attention in image display and on-chip integration, due to its superior advantages. By manipulating the amplitude in a narrow- or broad-band wavelength range of forward and backward incident light, different spatially varied intensities or color distributions can be generated on the structure plane. However, the current scheme cannot fully decouple the bidirectional light intensity due to the limitation of design degree of freedom, and it would hinder the development of asymmetric photonic devices. In this paper, we propose and demonstrate bidirectional nanoprinting based on an all-dielectric bilayer metasurface, which can independently control the intensity of forward and backward incident light, resulting in two different continuous grayscale meta-image displaying in the visible region. This asymmetric but still bidirectional optical response is introduced by stacking two layers of nanostructures with different functionality in space, in which the first- and second-layer nanostructures act as a half-wave plate and a polarizer, respectively. Interestingly, these bidirectional nanoprinting metasurfaces have flexible working modes and may bring great convenience for practical applications. Specifically, two different meta-images generated by a bidirectional nanoprinting metasurface can be displayed not only on two sides of the metasurface (working mode in transmission or reflection), but on the same side due to the forward transmitted light and backward reflected light also having asymmetric optical properties. Similar phenomena also exist for forward reflected light and backward transmitted light. Our work extremely expands the design freedom for metasurface devices and may play a significant role in the field of optical display, information multiplexing, etc.

The crystal structure and luminescence properties of the first lithium oxonitridolithosilicate Li3SiNO2:Eu2+

Obtained by a high temperature solid-state synthesis, the compound Li3SiNO2:Eu2+ was characterized via SCXRD, PXRD, and luminescence spectroscopy. This revealed a new structure type and interesting luminescence properties.

Lysosome-targeting luminescent lanthanide complexes: From molecular design to bioimaging

Lysosomes are essential acidic cytoplasmic membrane-bound organelles in human cells that play a critical role in many cellular events. A comprehensive understanding of lysosome-specific imaging can ultimately help us to...

Eu2+-activated MgAl2Si4O6N4: a novel oxonitridoalumosilicate blue phosphor for white LEDs

Recently, blue-emitting phosphors have attracted great interest due to their application in full-spectrum white light illumination.

Nanosecond-resolution photothermal dynamic imaging via MHZ digitization and match filtering

Photothermal microscopy has enabled highly sensitive label-free imaging of absorbers, from metallic nanoparticles to chemical bonds. Photothermal signals are conventionally detected via modulation of excitation beam and demodulation of probe beam using lock-in amplifier. While convenient, the wealth of thermal dynamics is not revealed. Here, we present a lock-in free, mid-infrared photothermal dynamic imaging (PDI) system by MHz digitization and match filtering at harmonics of modulation frequency. Thermal-dynamic information is acquired at nanosecond resolution within single pulse excitation. Our method not only increases the imaging speed by two orders of magnitude but also obtains four-fold enhancement of signal-to-noise ratio over lock-in counterpart, enabling high-throughput metabolism analysis at single-cell level. Moreover, by harnessing the thermal decay difference between water and biomolecules, water background is effectively separated in mid-infrared PDI of living cells. This ability to nondestructively probe chemically specific photothermal dynamics offers a valuable tool to characterize biological and material specimens.

Photothermal microscopy is limited for imaging of thermal dynamics. Here, the authors introduce a lock-in free, mid-infrared photothermal dynamic imaging system, which significantly increases SNR and imaging speed, and demonstrate metabolism analysis at single-cell level and background removal.

Recent advances in high-speed photoacoustic microscopy

Photoacoustic (PA) microscopy (PAM) has achieved remarkable progress in biomedicine in the past decade. It is a fast-rising imaging modality with diverse applications, such as hemodynamics, oncology, metabolism, and neuroimaging. Combining optical excitation and acoustic detection, the hybrid nature of PAM provides advantages of rich contrast and deep penetration. In recent years, high-speed PAM has flourished and enabled high-speed wide-field imaging of functional activity. In this review, we summarize the most recent advances in high-speed PAM technologies, including high-repetition-rate multi-wavelength laser development, fast scanning techniques, and novel PA signal acquisition strategies.

High-speed photoacoustic microscopy: A review dedicated on light sources

In recent years, many methods have been investigated to improve imaging speed in photoacoustic microscopy (PAM). These methods mainly focused upon three critical factors contributing to fast PAM: laser pulse repetition rate, scanning speed, and computing power of the microprocessors. A high laser repetition rate is fundamentally the most crucial factor to increase the PAM speed. In this paper, we review methods adopted for fast PAM systems in detail, specifically with respect to light sources. To the best of our knowledge, ours is the first review article analyzing the fundamental requirements for developing high-speed PAM and their limitations from the perspective of light sources.

Phase retrieval with physics informed zero-shot network

Phase can be reliably estimated from a single diffracted intensity image if faithful prior information about the object is available. Examples include amplitude bounds, object support, sparsity in the spatial or transform domain, deep image prior, and the prior learned from labeled datasets by a deep neural network. Deep learning facilitates state-of-the-art reconstruction quality but requires a large labeled dataset (ground truth measurement pair acquired in the same experimental conditions) for training. To alleviate this data requirement problem, this Letter proposes a zero-shot learning method. The Letter demonstrates that the object prior learned by a deep neural network while being trained for a denoising task can also be utilized for phase retrieval if the diffraction physics is effectively enforced on the network output. The Letter additionally demonstrates that the incorporation of total variation in the proposed zero-shot framework facilitates reconstruction of similar quality in less time (e.g., ∼9 fold, for a test reported in this Letter).

Nanopatterning of oxide 2-dimensional electron systems using low-temperature ion milling

We present a 'top-down' patterning technique based on ion milling performed at low-temperature, for the realization of oxide two-dimensional electron system devices with dimensions down to 160 nm. Using electrical transport and scanning Superconducting QUantum Interference Device measurements we demonstrate that the low-temperature ion milling process does not damage the 2DES properties nor creates oxygen vacancies-related conducting paths in the STO substrate. As opposed to other procedures used to realize oxide 2DES devices, the one we propose gives lateral access to the 2DES along the in-plane directions, finally opening the way to coupling with other materials, including superconductors.

Hybrid WDM-MDM transmitter with an integrated Si modulator array and a micro-resonator comb source

We demonstrate a multi-channel silicon photonic transmitter based on wavelength division multiplexing (WDM) and mode division multiplexing (MDM). The light source is realized by a silicon nitride (Si₃N₄) Kerr frequency comb and optical modulation is realized by silicon electro-optic modulators. Three wavelengths and two modes are employed to increase the optical transmission capacity. The accumulated data rate reaches 150 Gb/s. The dense integration of WDM and MDM components with a compact optical comb source opens new avenues for the future high-capacity multi-dimensional optical transmission.

Deep-learning-based bright-field image generation from a single hologram using an unpaired dataset

We adopted an unpaired neural network training technique, namely CycleGAN, to generate bright-field microscope-like images from hologram reconstructions. The motivation for unpaired training in microscope applications is that the construction of paired/parallel datasets is cumbersome or sometimes not even feasible, for example, lensless or flow-through holographic measuring setups. Our results show that the proposed method is applicable in these cases and provides comparable results to the paired training. Furthermore, it has some favorable properties even though its metric scores are lower. The CycleGAN training results in sharper and-from this point of view-more realistic object reconstructions compared to the baseline paired setting. Finally, we show that a lower metric score of the unpaired training does not necessarily imply a worse image generation but a correct object synthesis, yet with a different focal representation.

Nano-optical trapping using an all-dielectric optical fiber supporting a TEM-like mode

Fiber optical tweezers benefit from compact structures and compatibility with fiber optic technology, however, trapping of nano-objects are rarely demonstrated. Here, we predict stable optical trapping of a 30 nm polystyrene particle using an all-dielectric coaxial optical fiber supporting an axisymmetric TEM-like mode. We demonstrate, via comprehensive finite-difference time-domain simulations, that the trapping behavior arises from a significant shift of the fiber-end-fire radiation directivity originated from the nanoparticle-induced symmetry breaking, rather than the gradient force which assumes an invariant optical field. Fabrication of the fiber involved is entirely feasible with existing techniques, such as thermal-drawn and electrospinning, and therefore can be mass-produced.

High-resolution photoacoustic/ultrasound imaging of the porcine stomach wall: an ex vivo feasibility study

Photoacoustic (PA) imaging has become invaluable in preclinical and clinical research. Endoscopic PA imaging in particular has been explored as a noninvasive imaging modality to view vasculature and diagnose cancers in the digestive system. However, these feasibility studies are still limited to rodents or rabbits. Here, we develop a fully synchronized simultaneous ultrasound and photoacoustic microscopy system using two spectral bands (i.e., the visible and near-infrared) in both optical- and acoustic-resolution modes. We investigate the feasibility of imaging gastric vasculature in an ex vivo porcine model. The entire gastric wall, including the mucosa, submucosa, muscularis propria, and serosa, was excised from fresh porcine stomachs immediately followed by ultrasound and PA imaging being performed within a few hours of sacrifice. PA images of the mucosal vasculature were obtained at depths of 1.90 mm, which is a clinically significant accomplishment considering that the average thickness of the human mucosa is 1.26 mm. The layer structure of the stomach wall could be clearly distinguished in the overlaid PA and US images. Because gastric cancer starts from the mucosal surface and infiltrates into the submucosa, PA imaging can cover a clinically relevant depth in early gastric cancer diagnosis. We were able to detect mucosal vasculature in the entire mucosal layer, suggesting the potential utility of combined PA/US imaging in gastroenterology.

Self-frequency shift of AlN-on-sapphire Kerr solitons

We study the self-frequency shift of continuously pumped Kerr solitons in AlN-on-sapphire microcavities with Raman gain bandwidths narrower than the cavity free-spectral range. Solitons are generated in ∼230GHz microcavities via high-order mode dispersion engineering. The dependence of the self-frequency shift on soliton pulse width is measured and differs from amorphous material microcavities. Our measurement and simulation reveal the impact of frequency detuning between the cavity resonances and Raman gain peaks, as well as the importance of all three Raman gain peaks. The interplay between the Raman effect and dispersive wave recoil and a potential quiet point are also observed.

Optical diffractive deep neural network-based orbital angular momentum mode add-drop multiplexer

Vortex beams have application potential in multiplexing communication because of their orthogonal orbital angular momentum (OAM) modes. OAM add-drop multiplexing remains a challenge owing to the lack of mode selective coupling and separation technologies. We proposed an OAM add-drop multiplexer (OADM) using an optical diffractive deep neural network (ODNN). By exploiting the effective data-fitting capability of deep neural networks and the complex light-field manipulation ability of multilayer diffraction screens, we constructed a five-layer ODNN to manipulate the spatial location of vortex beams, which can selectively couple and separate OAM modes. Both the diffraction efficiency and mode purity exceeded 95% in simulations and four OAM channels carrying 16-quadrature-amplitude-modulation signals were successfully downloaded and uploaded with optical signal-to-noise ratio penalties of ∼1 dB at a bit error rate of 3.8 × 10^-3. This method can break through the constraints of conventional OADM, such as single function and poor flexibility, which may create new opportunities for OAM multiplexing and all-optical interconnection.

Roadmap on digital holography [Invited]

This Roadmap article on digital holography provides an overview of a vast array of research activities in the field of digital holography. The paper consists of a series of 25 sections from the prominent experts in digital holography presenting various aspects of the field on sensing, 3D imaging and displays, virtual and augmented reality, microscopy, cell identification, tomography, label-free live cell imaging, and other applications. Each section represents the vision of its author to describe the significant progress, potential impact, important developments, and challenging issues in the field of digital holography.

Dual-encryption freedom via a monolayer-nanotextured Janus metasurface in the broadband visible

As an emerging category of two-faced 2D architecture, the Janus metasurface aims to explore another universal optical property, that is, the wavevector direction (k-direction), and to enable the asymmetric transmission between the opposite directional incidences. It exhibits significant potential in creating versatile multiplexing metasurfaces and an optical isolator in optical communication applications. However, most previous asymmetric functionality shows merely one-way functionality with the other-way simply muted or demands multilayered nanostructure fabrication and alignment. Hence, it remains a great challenge to make a monolayer-nanotextured Janus metasurface with dual-encryption freedom and conquering the difficulty for multilayer alignment and practical operation bandwidth. In this work, we have proposed and experimentally demonstrated a new strategy of a dual-encryption Janus metasurface design with a simple monolayer-nanotextured metasurface coupled with a commercialized film of the half-wave plate. Utilizing the hybridization from two independent geometrical dimensions of rectangular-antennas, our approach ingeniously transforms the polarization-multiplexing into the dual-directional channels. A series of calculations and experimental results demonstrate that our asymmetric approach simultaneously constructs completely independent imaging encryptions for both forward and backward directions. Additionally, our proposed approach becomes a practical scheme with broadband visible-frequency operation and great simplicity in design and nanofabrication. We believe the universal scheme could facilitate to increase the information encoding capacity and holographic multiplexing channels by expanding the illumination wavevector to the full-space (+/-), and it paves the route toward the potential applications in on-chip integration, telecommunications, encryption, information processing, and communication.

Speckle patterns formed by broadband terahertz radiation and their applications for ghost imaging

Speckle patterns can be very promising for many applications due to their unique properties. This paper presents the possibility of numerically and experimentally formation of speckle patterns using broadband THz radiation. Strong dependence of the statistical parameters of speckles, such as size and sharpness on the parameters of the diffuser are demonstrated: the correlation length and the mean square deviation of the phase surface inhomogeneity. As the surface correlation length is increasing, the speckle size also increases and its sharpness goes down. Alternatively, the magnification of the standard deviation of the surface height leads to the speckle size diminishing and growth of the speckle sharpness. The dimensions of the experimentally formed speckles correspond to the results of numerical simulation. The possibility of utilizing formed speckle patterns for the implementation of the ghost imaging technique has been demonstrated by methods of numerical modeling.

Sensing of ultraviolet light: a transition from conventional to self-powered photodetector

Clouds in the sky pass almost 80% of ultraviolet (UV) radiation to the earth's surface, which has a significant impact on humankind. Conventional UV photodetectors (PDs) require an external battery, which not only increases the device size but also has a limited life span and maintenance costs can be prohibitively expensive. An alternative and more technically-sound solution would be the use of self-powered UV PDs that can operate independently, eliminating the need for an external source. Although many exciting studies have been done and state-of-the-art research is underway to successfully fabricate self-powered UV PDs, periodic reviews on this topic are deemed essential so that the technology's readiness can be properly evaluated and critical challenges can be addressed in a timely manner. In this article, the key issues and most exciting developments made in recent years on built-in electric field assisted self-powered UV PDs based on p-n homojunctions, p-n heterojunctions, and Schottky junctions followed by energy harvester integrated UV PDs are extensively reviewed. Finally, a summary and comparison of different types of self-powered UV PDs as well as future challenges that need to be addressed are discussed. This review sets a foundation providing essential insights into the present status of self-powered UV PDs with which researchers can engage and deal with the major challenges.

An all-sputtered photovoltaic ultraviolet photodetector based on co-doped CuCrO2 and Al-doped ZnO heterojunction

We propose and fabricate a heterojunction between Al-doped ZnO and (Mg, N)-doped CuCrO₂ thin films using the sputtering deposition method. These materials possess wide bandgap that makes them transparent in the visible light but excellent UV-absorbers. On the other hand, the high conductivity of these materials, respectively as n-type and p-type transparent conducting oxides, facilitates the charge transport. We show that the p-n junction fabricated from these materials has the potential to act as a high-performance UV photovoltaic photodetector. The proposed structure, demonstrates fast responses in order of sub seconds, photosensitivity of ~ 41,000, responsivity of 1.645 mA/W, and a detectivity of 3.52 × 10¹² Jones that are significantly improved in comparison with the Al-doped ZnO photoconductor. This excellent improvement is attributed to the capability of the photovoltaic configuration that creates a built-in voltage and facilitates the charge separation and collection rather than recombination in the photoconductor configuration.

Crystal phase transition and polyhedron transformation towards the evolution of photoluminescence and the improvement of thermal stability in efficient blue-emitting Ba0.47-xSr0.50+xAl2Si2O8:Eu2

Investigations into novel single-phase phosphors with outstanding luminescence properties and excellent thermal stability are urgently needed in the lighting field. In this work, a crystal phase transition and polyhedron transformation strategy via cation substitution has been proposed. Via controlling the Sr/Ba ratio, the structural evolution of the phosphor from a monocelsian phase to a hexacelsian or feldspar phase and the variation of the local environments of Eu²⁺ sites are correspondingly studied in Ba_0.47-xSr_0.50+xAl₂Si₂O₈:0.03Eu. Consequently, the optimal Ba_0.17Sr_0.80Al₂Si₂O₈:0.03Eu sample exhibits a higher intensity, up to 15.2-fold that of Ba_0.97Al₂Si₂O₈:0.03Eu. A narrower full-width-at-half-maximum of 73 nm, better color purity of 82.96%, and an internal quantum yield of 82.3% can be realized. With an increase in temperature, the emission intensity losses of samples from x = -10.0-47.0% are no more than 10.0% at 473 K. Moreover, a WLED (CCT = 5210 K; CRI = 90.3) fabricated using Ba_0.17Sr_0.80Al₂Si₂O₈:0.03Eu displays warmer white light than one fabricated using BaMgAl₁₀O₁₇:Eu under the same assembly and test conditions. Analysis shows that the structural evolution with reduced polyhedral symmetry and the condensed crystal structure with fortified rigidity are responsible for the improvement in properties. This discovery demonstrates that the utilization of a crystal phase transition and symmetrical coordination is an efficient way to develop novel efficient phosphors and other related materials.

High-speed multifocus phase imaging in thick tissue

Phase microscopy is widely used to image unstained biological samples. However, most phase imaging techniques require transmission geometries, making them unsuited for thick sample applications. Moreover, when applied to volumetric imaging, phase imaging generally requires large numbers of measurements, often making it too slow to capture live biological processes with fast 3D index-of-refraction variations. By combining oblique back-illumination microscopy and a z-splitter prism, we perform phase imaging that is both epi-mode and multifocus, enabling high-speed 3D phase imaging in thick, scattering tissues with a single camera. We demonstrate here 3D qualitative phase imaging of blood flow in chick embryos over a field of view of 546 × 546 × 137 µm3 at speeds up to 47 Hz.

A photoacoustic finder fully integrated with a solid-state dye laser and transparent ultrasound transducer

The standard-of-care for evaluating lymph node status in breast cancers and melanoma metastasis is sentinel lymph node (SLN) assessment performed with a handheld gamma probe and radioisotopes. However, this method inevitably exposes patients and physicians to radiation, and the special facilities required limit its accessibility. Here, we demonstrate a non-ionizing, cost-effective, handheld photoacoustic finder (PAF) fully integrated with a solid-state dye laser and transparent ultrasound transducer (TUT). The solid-state dye laser handpiece is coaxially aligned with the spherically focused TUT. The integrated finder readily detected photoacoustic signals from a tube filled with methylene blue (MB) beneath a 22 mm thick layer of chicken tissue. In live animals, we also photoacoustically detected both SLNs injected with MB and subcutaneously injected melanomas. We believe that our radiation-free and inexpensive PAF can play a vital role in SLN assessment.

Microsphere-coupled optical tweezers

In this Letter, we study the optical trapping of particles in a focal spot engineered by a combination of a dielectric microsphere and the conventional optical tweezers setup. The dielectric microsphere is placed in the laser path before the focal spot, and its impact on the trapping stiffness is theoretically and experimentally studied in detail. The suggested method for considering the geometric phase shift, due to the presence of the microsphere, into the Debye diffraction integral shows a good agreement with the experiment. We stably trap particles as small as 350 nm in the microsphere-coupled optical trap using a low numerical aperture objective (NA=0.7), while in a conventional optical tweezers setup, it requires at least an NA=1.1. Moreover, the results show that choosing an appropriate microsphere and depth of trapping can enhance the trapping efficiency in the axial direction by a factor of ∼3.8.