Single-pulse ultrafast real-time simultaneous planar imaging of femtosecond laser-nanoparticle dynamics in flames

The creation of carbonaceous nanoparticles and their dynamics in hydrocarbon flames are still debated in environmental,combustion,and material sciences.In this study,we introduce single-pulse femtosecond laser sheetcompressed ultrafast photography(fsLS-CUP),an ultrafast imaging technique specifically designed to shed light on and capture ultrafast dynamics stemming from interactions between femtosecond lasers and nanoparticles in flames in a single-shot.fsLS-CUP enables the first-time real-time billion frames-per-second(Gfps)simultaneous twodimensional(2D)imaging of laser-induced fluorescence(LIF)and laser-induced heating(LIH)that are originated from polycyclic aromatic hydrocarbons(PAHs)and soot particles,respectively.Furthermore,fsLS-CUP provides the real-time spatiotemporal map of femtosecond laser-soot interaction as elastic light scattering(ELS)at an astonishing 250 Gfps.In contrast to existing single-shot ultrafast imaging approaches,which are limited to millions of frames per second only and require multiple laser pulses,our method employs only a single pulse and captures the entire dynamics of laserinduced signals at hundreds of Gfps.Using a single pulse does not change the optical properties of nanoparticles for a following pulse,thus allowing reliable spatiotemporal mapping.Moreover,we found that particle inception and growth are derived from precursors.In essence,as an imaging modality,fsLS-CUP offers ultrafast 2D diagnostics,contributing to the fundamental understanding of nanoparticle’s inception and broader applications across different fields,such as material science and biomedical engineering.

Single-pulse real-time billion-frames-per-second planar imaging of ultrafast nanoparticle-laser dynamics and temperature in flames

Unburnt hydrocarbon flames produce soot,which is the second biggest contributor to global warming and harmful to human health.The state-of-the-art high-speed imaging techniques,developed to study non-repeatable turbulent flames,are limited to million-frames-per-second imaging rates,falling short in capturing the dynamics of critical species.Unfortunately,these techniques do not provide a complete picture of flame-laser interactions,important for understanding soot formation.Furthermore,thermal effects induced by multiple consecutive pulses modify the optical properties of soot nanoparticles,thus making single-pulse imaging essential.Here,we report single-shot laser-sheet compressed ultrafast photography(LS-CUP)for billion-frames-per-second planar imaging of flame-laser dynamics.We observed laser-induced incandescence,elastic light scattering,and fluorescence of soot precursors-polycyclic aromatic hydrocarbons(PAHs)in real-time using a single nanosecond laser pulse.The spatiotemporal maps of the PAHs emission,soot temperature,primary nanoparticle size,soot aggregate size,and the number of monomers,present strong experimental evidence in support of the theory and modeling of soot inception and growth mechanism in flames.LS-CUP represents a generic and indispensable tool that combines a portfolio of ultrafast combustion diagnostic techniques,covering the entire lifecycle of soot nanoparticles,for probing extremely short-lived(picoseconds to nanoseconds)species in the spatiotemporal domain in non-repeatable turbulent environments.Finally,LS-CUP’s unparalleled capability of ultrafast wide-field temperature imaging in real-time is envisioned to unravel mysteries in modern physics such as hot plasma, sonoluminescence, and nuclear fusion.

Recent Advances in Photoacoustic Tomography

Photoacoustic tomography(PAT)that integrates the molecular contrast of optical imaging with the high spatial resolution of ultrasound imaging in deep tissue has widespread applications in basic biological science,preclinical research,and clinical trials.Recently,tremendous progress has been made in PAT regarding technical innovations,preclinical applications,and clinical translations.Here,we selectively review the recent progresses and advances in PAT,including the development of advanced PAT systems for small-animal and human imaging,newly engineered optical probes for molecular imaging,broad-spectrum PAT for label-free imaging of biological tissues,high-throughput snapshot photoacoustic topography,and integration of machine learning for image reconstruction and processing.We envision that PAT will have further technical developments and more impactful applications in biomedicine.

Single-shot real-time femtosecond imaging of temporal focusing

While the concept of focusing usually applies to the spatial domain,it is equally applicable to the time domain.Realtime imaging of temporal focusing of single ultrashort laser pulses is of great significance in exploring the physics of the space–time duality and finding diverse applications.The drastic changes in the width and intensity of an ultrashort laser pulse during temporal focusing impose a requirement for femtosecond-level exposure to capture the instantaneous light patterns generated in this exquisite phenomenon.Thus far,established ultrafast imaging techniques either struggle to reach the desired exposure time or require repeatable measurements.We have developed single-shot 10-trillion-frame-per-second compressed ultrafast photography(T-CUP),which passively captures dynamic events with 100-fs frame intervals in a single camera exposure.The synergy between compressed sensing and the Radon transformation empowers T-CUP to significantly reduce the number of projections needed for reconstructing a high-quality three-dimensional spatiotemporal datacube.As the only currently available real-time,passive imaging modality with a femtosecond exposure time,T-CUP was used to record the first-ever movie of nonrepeatable temporal focusing of a single ultrashort laser pulse in a dynamic scattering medium.T-CUP’s unprecedented ability to clearly reveal the complex evolution in the shape,intensity,and width of a temporally focused pulse in a single measurement paves the way for single-shot characterization of ultrashort pulses,experimental investigation of nonlinear light-matter interactions,and real-time wavefront engineering for deep-tissue light focusing.

Physical picture of the optical memory effect

The optical memory effect is an interesting phenomenon that has attracted considerable attention in recent decades. Here, we present a new physical picture of the optical memory effect, in which the memory effect and the conventional spatial shift invariance are united. Based on this picture we depict the role of thickness, scattering times, and anisotropy factor and derive equations to calculate the ranges of the angular memory effect(AME) of different scattering components(ballistic light, singly scattered, doubly scattered, etc.), and hence a more accurate equation for the real AME ranges of volumetric turbid media. A conventional random phase mask model is modified according to the new picture. The self-consistency of the simulation model and its agreement with the experiment demonstrate the rationality of the model and the physical picture, which provide powerful tools for more sophisticated studies of the memory-effect-related phenomena and wavefront-sensitive techniques, such as wavefront shaping, optical phase conjugation, and optical trapping in/through scattering media.

Deep learning acceleration of multiscale superresolution localization photoacoustic imaging

A superresolution imaging approach that localizes very small targets,such as red blood cells or droplets of injected photoacoustic dye,has significantly improved spatial resolution in various biological and medical imaging modalities.However,this superior spatial resolution is achieved by sacrificing temporal resolution because many raw image frames,each containing the localization target,must be superimposed to form a sufficiently sampled high-density superresolution image.Here,we demonstrate a computational strategy based on deep neural networks(DNNs)to reconstruct high-density superresolution images from far fewer raw image frames.The localization strategy can be applied for both 3D label-free localization optical-resolution photoacoustic microscopy(OR-PAM)and 2D labeled localization photoacoustic computed tomography(PACT).For the former,the required number of raw volumetric frames is reduced from tens to fewer than ten.For the latter,the required number of raw 2D frames is reduced by 12 fold.Therefore,our proposed method has simultaneously improved temporal(via the DNN)and spatial(via the localization method)resolutions in both label-free microscopy and labeled tomography.Deep-learning powered localization PA imaging can potentially provide a practical tool in preclinical and clinical studies requiring fast temporal and fine spatial resolutions.

Learning-based super-resolution interpolation for sub-Nyquist sampled laser speckles

Information retrieval from visually random optical speckle patterns is desired in many scenarios yet considered challenging.It requires accurate understanding or mapping of the multiple scattering process,or reliable capability to reverse or compensate for the scattering-induced phase distortions.In whatever situation,effective resolving and digitization of speckle patterns are necessary.Nevertheless,on some occasions,to increase the acquisition speed and/or signal-to-noise ratio(SNR),speckles captured by cameras are inevitably sampled in the sub-Nyquist domain via pixel binning(one camera pixel contains multiple speckle grains)due to finite size or limited bandwidth of photosensors.Such a down-sampling process is irreversible;it undermines the fine structures of speckle grains and hence the encoded information,preventing successful information extraction.To retrace the lost information,super-resolution interpolation for such sub-Nyquist sampled speckles is needed.In this work,a deep neural network,namely SpkSRNet,is proposed to effectively up sample speckles that are sampled below 1/10 of the Nyquist criterion to well-resolved ones that not only resemble the comprehensive morphology of original speckles(decompose multiple speckle grains from one camera pixel)but also recover the lost complex information(human face in this study)with high fidelity under normal-and low-light conditions,which is impossible with classic interpolation methods.These successful speckle super-resolution interpolation demonstrations are essentially enabled by the strong implicit correlation among speckle grains,which is non-quantifiable but could be discovered by the well-trained network.With further engineering,the proposed learning platform may benefit many scenarios that are physically inaccessible,enabling fast acquisition of speckles with sufficient SNR and opening up new avenues for seeing big and seeing clearly simultaneously in complex scenarios.

Non-invasive photoacoustic computed tomography of rat heart anatomy and function

Complementary to mainstream cardiac imaging modalities for preclinical research,photoacoustic computed tomography(PACT)can provide functional optical contrast with high imaging speed and resolution.However,PACT has not been demonstrated to reveal the dynamics of whole cardiac anatomy or vascular system without surgical procedure(thoracotomy)for tissue penetration.Here,we achieved non-invasive imaging of rat hearts using the recently developed three-dimensional PACT(3D-PACT)platform,demonstrating the regulated illumination and detection schemes to reduce the effects of optical attenuation and acoustic distortion through the chest wall;thereby,enabling unimpeded visualization of the cardiac anatomy and intracardiac hemodynamics following rapidly scanning the heart within 10 s.We further applied 3D-PACT to reveal distinct cardiac structural and functional changes among the healthy,hypertensive,and obese rats,with optical contrast to uncover differences in cardiac chamber size,wall thickness,and hemodynamics.Accordingly,3D-PACT provides high imaging speed and nonionizing penetration to capture the whole heart for diagnosing the animal models,holding promises for clinical translation to cardiac imaging of human neonates.

Deep learning-enhanced microscopy with extended depth-of-field

A computational imaging platform utilizing a physics-incorporated,deep-learned design of binary phase filter and a jointly optimized deconvolution neural network has been reported,achieving high-resolution,high-contrast imaging over extended depth ranges without the need for serial refocusing.

Polarization patterns of light enable geolocalization in oceans

The deep ocean,characterized by its immense depths and absence of global positioning system(GPS)functionality,presents considerable challenges for search and rescue missions.Inspired by the geolocalization capabilities of migratory marine animals,Bai et al.present a novel method for underwater geolocalization using the polarization patterns of light in the underwater environment.Emulating a sextant using these underwater polarization patterns,the study determines geolocation in underwater settings.Despite prior skepticism,even in low-visibility waters,these patterns,learned through a deep neural network,provide geolocation accuracies of 55 km at 8m and 255 km at 50 m.This pioneering approach offers implications for search and rescue and hints at navigation mechanisms in marine animals.

Focusing light into scattering media with ultrasound-induced field perturbation

Focusing light into scattering media,although challenging,is highly desirable in many realms.With the invention of time-reversed ultrasonically encoded(TRUE)optical focusing,acousto-optic modulation was demonstrated as a promising guidestar mechanism for achieving noninvasive and addressable optical focusing into scattering media.Here,we report a new ultrasound-assisted technique,ultrasound-induced field perturbation optical focusing,abbreviated as UFP.Unlike in conventional TRUE optical focusing,where only the weak frequency-shifted first-order diffracted photons due to acousto-optic modulation are useful,here UFP leverages the brighter zeroth-order photons diffracted by an ultrasonic guidestar as information carriers to guide optical focusing.We find that the zeroth-order diffracted photons,although not frequency-shifted,do have a field perturbation caused by the existence of the ultrasonic guidestar.By detecting and time-reversing the differential field of the frequency-unshifted photons when the ultrasound is alternately ON and OFF,we can focus light to the position where the field perturbation occurs inside the scattering medium.We demonstrate here that UFP optical focusing has superior performance to conventional TRUE optical focusing,which benefits from the more intense zeroth-order photons.We further show that UFP optical focusing can be easily and flexibly developed into double-shot realization or even single-shot realization,which is desirable for high-speed wavefront shaping.This new method upsets conventional thinking on the utility of an ultrasonic guidestar and broadens the horizon of light control in scattering media.We hope that it provides a more efficient and flexible mechanism for implementing ultrasound-guided wavefront shaping.

In vivo superresolution photoacoustic computed tomography by localization of single dyed droplets

Photoacoustic(PA)computed tomography(PACT)is a noninvasive hybrid imaging technique that combines optical excitation and acoustic detection to realize high contrast,high resolution,and deep penetration in biological tissues.However,the spatial resolution of PACT is limited by acoustic diffraction.Here,we report in vivo superresolution PACT,which breaks the acoustic diffraction limit by localizing the centers of single dyed droplets that are flowing in blood vessels.The droplets were prepared by dissolving hydrophobic absorbing dye in oil,followed by mixing with water.The dyed droplets generate much higher-amplitude PA signals than blood and can flow smoothly in vessels;thus,they are excellent tracers for localization-based superresolution imaging.The in vivo resolution enhancement was demonstrated by continuously imaging the cortical layer of a mouse brain during droplet injection.The droplets that were flowing in the vessels were localized,and their center positions were used to construct a superresolution image that exhibits sharper features and more finely resolved vascular details.An improvement in spatial resolution by a factor of 6 has been realized in vivo by the droplet localization technique.

Harnessing a multi-dimensional fibre laser using genetic wavefront shaping

The multi-dimensional laser is a fascinating platform not only for the discovery and understanding of new higherdimensional coherent lightwaves but also for the frontier study of the complex three-dimensional(3D)nonlinear dynamics and solitary waves widely involved in physics,chemistry,biology and materials science.Systemically controlling coherent lightwave oscillation in multi-dimensional lasers,however,is challenging and has largely been unexplored;yet,it is crucial for both designing 3D coherent light fields and unveiling any underlying nonlinear complexities.Here,for the first time,we genetically harness a multi-dimensional fibre laser using intracavity wavefront shaping technology such that versatile lasing characteristics can be manipulated.We demonstrate that the output power,mode profile,optical spectrum and mode-locking operation can be genetically optimized by appropriately designing the objective function of the genetic algorithm.It is anticipated that this genetic and systematic intracavity control technology for multi-dimensional lasers will be an important step for obtaining high-performance 3D lasing and presents many possibilities for exploring multi-dimensional nonlinear dynamics and solitary waves that may enable new applications.

Multifocal photoacoustic microscopy using a single-element ultrasonic transducer through an ergodic relay

Optical-resolution photoacoustic microscopy(OR-PAM)has demonstrated high-spatial-resolution imaging of optical absorption in biological tissue.To date,most OR-PAM systems rely on mechanical scanning with confocally aligned optical excitation and ultrasonic detection,limiting the wide-field imaging speed of these systems.Although several multifocal OR-PA(MFOR-PA)systems have attempted to address this limitation,they are hindered by the complex design in a constrained physical space.Here,we present a two-dimensional(2D)MFOR-PAM system that utilizes a 2D microlens array and an acoustic ergodic relay.Using a single-element ultrasonic transducer,this system can detect PA signals generated from 400 optical foci in parallel and then raster scan the optical foci patterns to form an MFOR-PAM image.This system improves the imaging resolution of an acoustic ergodic relay system from 220 to 13μm and enables 400-folds shorter scanning time than that of a conventional OR-PAM system at the same resolution and laser repetition rate.We demonstrated the imaging ability of the system with both in vitro and in vivo experiments.

Rapid fabrication of microrings with complex cross section using annular vortex beams

A ring-shaped focus, such as a focused vortex beam, has played an important role in microfabrication and optical tweezers.The shape and diameter of the ring-shaped focus can be easily adjusted by the topological charge of the vortex. However,the flow energy is also related to the topological charge, making the individual control of diameter and flow energy of the vortex beam impossible. Meanwhile, the shape of the focus of the vortex beam remains in the hollow ring. Expanding the shape of focus of structural light broadens the applications of the vortex beam in the field of microfabrication. Here, we proposed a ring-shaped focus with controllable gaps by multiplexing the vortex beam and annular beam. The multiplexed beam has several advantages, such as the diameter and flow energy of the focal point can be individually controlled and are not affected by the zero-order beam, and the gap size and position are controllable.