Time-resolved Imaging System Research Articles

In vitro Assessment of lesion activity using simultaneous time-resolved reflectance imaging at 1300 and 1950nm.

The aim of this study was to explore the feasibility of a time-resolved reflectance imaging system employing a single photodetector to assess the activity of caries lesions that exploits the differential absorption of water at 1300 and 1950nm. The time-resolved reflectivity of 10 active and 10 arrested lesions on the proximal surfaces and 5 active and 5 arrested lesions on the occlusal surfaces of extracted teeth were monitored simultaneously at 1300 and 1950nm during forced air drying for 60s. The presence of a highly mineralized surface zone measured with microcomputed tomography (microCT) was used to indicate lesion activity. Multiple kinetic parameters were extracted from the acquired short wavelength infrared (SWIR) intensity versus time dehydration curves and used to assess lesion activity. Differences in the reflectivity between curves acquired at 1300 and 1950nm due to differential absorption of water provided improved discrimination between active and arrested lesions over the use of 1950nm alone. This study demonstrates that it is feasible to use a device with a single photodetector operating at 1950nm to collect dehydration curves for the assessment of lesion activity and that a system employing two SWIR wavelengths with differential water absorption can improve the performance of lesion activity assessment.

Transient cavity-cavity strong coupling at terahertz frequency on LiNbO3 chips.

Terahertz (THz) microcavities have garnered considerable attention for their ability to localize and confine THz waves, allowing for strong coupling to remarkably enhance the light-matter interaction. These properties hold great promise for advancing THz science and technology, particularly for high-speed integrated THz chips where transient interaction between THz waves and matter is critical. However, experimental study of these transient time-domain processes requires high temporal and spatial resolution since these processes, such as THz strong coupling, occur in several picoseconds and microns. Thus, most literature studies rarely cover temporal and spatial processes at the same time. In this work, we thoroughly investigate the transient cavity-cavity strong-coupling phenomena at THz frequency and find a Rabi-like oscillation in the microcavities, manifested by direct observation of a periodic energy exchange process via a phase-contrast time-resolved imaging system. Our explanation, based on the Jaynes-Cummings model, provides theoretical insight into this transient strong-coupling process. This work provides an opportunity to deeply understand the transient strong-coupling process between THz microcavities, which sheds light on the potential of THz microcavities for high-speed THz sensor and THz chip design.

Morphological and Optical Modification of Melanosomes in Fish Integuments upon Oxidation

Reactive oxygen species (ROS) such as superoxide radicals O2−, hydroxyl radicals OH−, and hydrogen peroxide H2O2 may have detrimental effects on marine organisms, including their integuments and visual appearances. Although some studies have described the impact of ROS on marine ecosystems and species ecology, the influence on the optical response of the integuments of marine species and on their visual appearances remains unknown. In this article, we used histology and optical characterisation to show, for the first time, that skin melanophores (melanin-containing chromophores) of the coral reef fish, Stegastes apicalis, change their shapes and fluorescent proprieties upon oxidation with H2O2 radicals. Our observations also suggest that pheomelanosomes may occur in fish integuments, where, previously, it was thought that fish melanosomes only contain eumelanin. This investigation relied on light and electron microscopy and steady-state fluorimetry, as well as time-resolved streak imaging systems. We suggest that the changes in the morphological and spectral characteristics of melanophores can be used as a marker of physiological stress induced by environmental factors such as ROS. Moreover, S. apicalis may be used as a potential model for studying the interaction between the surrounding environment and natural organisms in biologically diverse ecosystems, such as the Great Barrier Reef in Australia.

Investigation and analysis of pin-point damage and damage growth characteristics in KDP and DKDP crystals.

In this paper, we report the damage and damage growth in potassium dihydrogen phosphate and its deuterated analog crystals. A time-resolved shadow imaging system was used to investigate the damage behavior in the bulk and on the rear surface. The damage images show differences in the damage sizes of the crystals with different deuterization rates. Theoretical simulations demonstrated that this may be due to differences in the crystallographic defects. The experimental results showed that the development of crystal damage was not only manifested as the expansion of damage on the rear surface of the crystal but also as an increase in pin-point density and size within the crystal. Crystals with higher deuterization rates had higher probability of the increasing of initial damage size, rather than the increasing of pin-point density.

Effects and mechanism of focusing condition on machining quality and bubble behaviors in femtosecond laser micro-grooving of silicon wafer in liquid

In this paper, the effect and mechanism of how varying focusing conditions would affect the machining quality of microgrooves and bubble behaviors (cavitation bubbles (CB) and persistent bubbles (PB)) in liquid-assisted femtosecond laser micro-grooving of silicon wafer were investigated. Firstly, the width and depth of the microgroove machined under negative defocusing condition (NDC) were considerably uniform, which showed an increase by 100 % and 125 % compared with that machined under positive defocusing condition (PDC). Secondly, the temporal evolution images of CB were captured by a time-resolved shadowgraph imaging system. Compared with PDC, the radius and lifetime of CB induced under NDC increased by 71 % and 100 %, and the time evolution curve of the radius of CB exhibited a high consistence with the Rayleigh distribution, while that induced under PDC fluctuated irregularly. Thirdly, the PB induced under NDC showed a backward flush and contributed to the formation of tendentious liquid flow, while that induced under PDC occupied the laser transmission path and caused disturbances on subsequent laser pulses. Finally, the differences in laser transmission mechanism under both the negative and positive defocusing conditions were discussed and the NDC was confirmed as the stable and efficient processing method. Through the comprehensive study of the bubble behaviors, the correlation between focusing conditions and machining quality of liquid-assisted laser cutting was established.

Nanosecond laser-induced multi-focusing damage in the bulk of fused silica

A time-resolved imaging system was utilized in this work to study the focusing behavior during laser-induced filamentary damage in the bulk of fused silica. The results show that focusing behavior has a significant dependence on laser energy. With the increased laser energy, single focusing is changed into multi-focusing. The probability of multi-focusing appearances also increases. During this process, the plasma distribution interval increases, which expands the area affected by the damage. The difference in the stress wave velocity indicates that the multi-focusing formation has temporal and spatial order. Finally, the multi-focusing damage’s physical mechanism and formation process are discussed. The results provide a reference for the mechanism of nanosecond laser-induced multi-focusing damage to transparent media.

Elucidating ejection regimes of metal microdroplets in voxel-based laser-induced forward transfer

Voxel-based laser-induced forward transfer (VB-LIFT) is a promising micro additive manufacturing process to efficiently print microstructures by ejecting a metal microdroplet array. Elucidating the ejection regimes of microdroplets is crucial to manipulating the printing process. This study investigates the ejecting behaviors of microdroplets induced by irradiating a femtoliter metal voxel with an ultraviolet nanosecond laser. A time-resolved dark-field imaging system was developed to capture the ejecting microdroplets. The ejecting velocity increased linearly with the laser fluence but was independent of the voxel side length. The analysis of the laser energy conversion revealed that one-thousandth of the laser energy was converted to the kinetic energy of the ejecting microdroplets, and the majority of the remaining were converted into internal energy. Meanwhile, the relaxation of the thermal compressive stress induced by laser heating drove the metal voxel detaching from the donor glass to generate a microdroplet. Furtherly, the ejection map in the VB-LIFT is proposed, including the non-detaching, detaching without melting, detaching and melting, and broken regimes. This study provides insights to eject debris-free metal microdroplets with the desired temperature and velocity in laser-induced forward transfer. • We develop a dark-field imaging system to capture the ejecting metal microdroplets. • Ejecting velocity is proportional to laser fluence but independent of voxel length. • Metal voxel detaching is driven by thermal compressive stress due to laser heating. • An ejection map containing four regimes is proposed for the VB-LIFT process.

EFFECT OF PARTICLE LOADING ON THE BURNING CHARACTERISTICS OF BORON-LADEN GEL FUEL DROPLET

Droplet combustion of pure and boron-loaded gel fuels is studied to understand the combustion behavior of gel fuel under atmospheric conditions. Here, Jet A-1 is taken as a base fuel, Thixatrol ST as gellant, and boron as energetic particles. Four kinds of gel fuels are taken for this study with varying boron loadings of 0%, 10%, 20%, and 30% (by weight). Before combustion studies, the rheological properties of all gel fuels were determined. The results show a linear variation of viscosity with an increasing shear rate and with increasing boron content. Combustion characteristics of all these fuels were studied and analyzed. The droplet combustion process was captured using a time-resolved high-speed imaging system. The evolution of droplet profile shows that droplet regresses smoothly for pure gel (PG) sample (0% boron), whereas for the GB10 and GB20 cases with 10% and 20% boron, respectively, the droplet regresses with puffing, disruption, and is followed by micro-explosions. For the GB30 case with 30% boron, the phenomena of micro-explosion occur immediately after the ignition of the droplet. Flame standoff distance decreases with the higher boron-loaded gel fuel. Analysis of secondary droplets explains that: (1) puffing induces a smaller diameter, while micro-explosion ejects daughter droplets of larger diameter. (2) Average secondary flame length increases with the increase of boron loading. As a result, the boron component in gel fuel appears to have a good impact on the whole combustion process. Scanning electron microscopy (SEM) image shows numerous micropores (blow holes) on particle surface in case of GB30 sample while no such pores were present on other counterparts. The micrograph also reveals eroded, flaky, and ashy-like structures in the residue of the GB30 case.

Explosion plume on the exit surface of fused silica during UV laser- induced damage

Explosion plume on the exit surface of fused silica during UV laser-induced damage was investigated using a two-color interferometric time-resolved side-viewed imaging system. Phase shifts and refractive index variation on specific positions, induced by the plume at different delays, were obtained by processing the interferogram. Assisted with the observation of the collected ejection, the material species, distributions and thermodynamic behaviors of ejections were presented. Based on findings, the temporal evolvement of plume was described. The key transition nodes of the response behaviors during laser induced damage were captured. Furthermore, the dissipation of plasma and the cessation of material evaporation were presented. These results provided valuable insight into the non-equilibrium physics involved during the interaction process of laser and surface material.

A time-resolved imaging system for the diagnosis of x-ray self-emission in high energy density physics experiments.

A diagnostic capable of recording spatially and temporally resolved x-ray self-emission data was developed to characterize experiments on the MAGPIE pulsed-power generator. The diagnostic used two separate imaging systems: a pinhole imaging system with two-dimensional spatial resolution and a slit imaging system with one-dimensional spatial resolution. The two-dimensional imaging system imaged light onto the image plate. The one-dimensional imaging system imaged light onto the same piece of image plate and a linear array of silicon photodiodes. This designallowed the cross-comparison of different images, allowing a picture of the spatial and temporal distribution of x-ray self-emission to be established. The designwas tested in a series of pulsed-power-driven magnetic-reconnection experiments.

Parametric Study of Jet/Droplet Formation Process during LIFT Printing of Living Cell-Laden Bioink.

Bioprinting offers great potential for the fabrication of three-dimensional living tissues by the precise layer-by-layer printing of biological materials, including living cells and cell-laden hydrogels. The laser-induced forward transfer (LIFT) of cell-laden bioinks is one of the most promising laser-printing technologies enabling biofabrication. However, for it to be a viable bioprinting technology, bioink printability must be carefully examined. In this study, we used a time-resolved imaging system to study the cell-laden bioink droplet formation process in terms of the droplet size, velocity, and traveling distance. For this purpose, the bioinks were prepared using breast cancer cells with different cell concentrations to evaluate the effect of the cell concentration on the droplet formation process and the survival of the cells after printing. These bioinks were compared with cell-free bioinks under the same printing conditions to understand the effect of the particle physical properties on the droplet formation procedure. The morphology of the printed droplets indicated that it is possible to print uniform droplets for a wide range of cell concentrations. Overall, it is concluded that the laser fluence and the distance of the donor–receiver substrates play an important role in the printing impingement type; consequently, a careful adjustment of these parameters can lead to high-quality printing.

Study of the effects of stress wave behavior in laser-induced exit-surface damage of fused silica using time-resolved multi-polariscopic imaging

During laser-induced exit-surface damage of fused silica components, strain and stress are generated and accompanied by cracks in the substrate, further deteriorating the component. The authors studied the behaviors of surface and shear waves quantitatively using a time-resolved multipolariscopic imaging system, showing their potential to affect the development of circumferential and longitudinal cracks. When the material received a cumulative shot, a greater stress intensity was observed, and the stress wave had a greater impact on the laser-induced damage. The results indicate that the two stress waves from the cumulated shots dominated the developmental characteristics of the damage morphology.

Adaptive optics for a time-resolved Förster resonance energy transfer (FRET) and fluorescence lifetime imaging microscopy (FLIM) in vivo.

Förster resonance energy transfer (FRET) and fluorescence lifetime imaging (FLIM) have been coupled with multiphoton microscopy to image in vivo dynamics. However, the increase in optical aberrations as a function of depth significantly reduces the fluorescent signal, spatial resolution, and fluorescence lifetime accuracy. We present the development of a time-resolved FRET-FLIM imaging system with adaptive optics. We demonstrate the improvement of our adaptive optics (AO)-FRET-FLIM instrument over standard multiphoton FRET-FLIM imaging. We validate our approach using fixed cellular samples with FRET standards and in vivo with live imaging in a mouse kidney.

Investigation of stress wave and damage morphology growth generated by laser-induced damage on rear surface of fused silica.

In this study, stress wave and damage morphology on the rear surface of silica during laser induced damage with cumulative UV pump shots is investigated. Time resolved imaging system, along with stress induced birefringence detecting, is used to demonstrate the development of stress wave. The properties of three types of stress waves, namely, compressive wave (P-wave), shear wave (S-wave), and Rayleigh wave (R-wave) are recorded and studied during the damage process. The experimental and simulated results indicated that R-wave exhibits the highest intensity among all the three stress waves. Because the R-wave is mainly localized at the region near the surface, it is responsible for the mechanical damage along the surface; in addition, it rapidly increases the damage diameter, which was observed from the front view of the damage.

Non-line-of-sight Imaging with Partial Occluders and Surface Normals

Imaging objects obscured by occluders is a significant challenge for many applications. A camera that could “see around corners” could help improve navigation and mapping capabilities of autonomous vehicles or make search and rescue missions more effective. Time-resolved single-photon imaging systems have recently been demonstrated to record optical information of a scene that can lead to an estimation of the shape and reflectance of objects hidden from the line of sight of a camera. However, existing non-line-of-sight (NLOS) reconstruction algorithms have been constrained in the types of light transport effects they model for the hidden scene parts. We introduce a factored NLOS light transport representation that accounts for partial occlusions and surface normals. Based on this model, we develop a factorization approach for inverse time-resolved light transport and demonstrate high-fidelity NLOS reconstructions for challenging scenes both in simulation and with an experimental NLOS imaging system.

Conversion from terahertz-guided waves to surface waves with metasurface.

Surface waves (SWs) have attracted a widespread attention due to the characteristic of subwavelength confinement and convenient manipulation in photonic integrated circuits. Though metasurface provides a powerful tool in realizing the conversion between freely propagating waves and surface modes in recent years, a gulf between guided waves (GWs) and SWs in terahertz (THz) range still exists as a bottleneck for on-chip photonic integrated devices. Here, we implemented the conversion from THz GWs to SWs through the coupling of a lithium niobate (LN) subwavelength waveguide and metasurface antennas on an all-feature on-chip THz integrated platform. The conversion process and transmission mode of the THz waves were directly visualized via a time-resolved imaging system. Based on the dynamic process, the formation of SWs could be clarified through analyzing the dispersion relation of propagating modes, which is in good agreement with numerical models. In further, relying on the numerical simulation, SWs were induced from the collective oscillations of the metasurface antenna array and the maximum coupling efficiency was around 62.6 percent. Our work provides an efficient approach to control of GWs, and promotes the practicability of THz surface integrated devices, including THz surface spectroscopy sensing.

Propagation of THz pulses in rectangular subwavelength dielectric waveguides

Rectangular subwavelength waveguides are necessary for the development of micro/nanophotonic devices and on-chip platforms. Using a time-resolved imaging system, we studied the transient properties and the propagation modes of THz pulses in rectangular subwavelength dielectric waveguides. The dynamic process of THz pulses was systematically recorded to a movie. In addition, an anomalous group velocity dispersion was demonstrated in rectangular subwavelength waveguides. By using the effective index method, we theoretically calculated the modes in rectangular subwavelength waveguides, which agree well with the experiments and simulations. This work provides the opportunity to improve the analysis and design of the integrated platforms and photonic devices.

Dual-Emissive Phosphorescent Polymer Probe for Accurate Temperature Sensing in Living Cells and Zebrafish Using Ratiometric and Phosphorescence Lifetime Imaging Microscopy.

Temperature plays an important part in many biochemical processes. Accurate diagnosis and proper treatment usually depend on precise measurement of temperature. In this work, a dual-emissive phosphorescent polymer temperature probe, composed of iridium(III) complexes as temperature sensitive unit with phosphorescence lifetime of ∼500 ns and europium(III) complexes as reference unit with lifetime of ∼400 μs, has been rationally designed and synthesized. Upon the increase of the temperature, the luminescence intensity from the iridium(III) complexes is enhanced, while that from the europium(III) complexes remains unchanged, which makes it possible for the ratiometric detection of temperature. Furthermore, the polymer also displays a significant change in emission lifetime accompanied by the temperature variation. By utilizing the laser scanning confocal microscope and time-resolved luminescence imaging systems, ratiometric and time-resolved luminescence imaging in Hela cells and zebrafish have been carried out. Notably, the intensity ratio and long-lifetime-based imaging can offer higher sensitivity, decrease the detection limit, and minimize the background interference from biosamples.

Time-Resolved Diffuse Optical Spectroscopy and Imaging Using Solid-State Detectors: Characteristics, Present Status, and Research Challenges.

Diffuse optical spectroscopy (DOS) and diffuse optical imaging (DOI) are emerging non-invasive imaging modalities that have wide spread potential applications in many fields, particularly for structural and functional imaging in medicine. In this article, we review time-resolved diffuse optical imaging (TR-DOI) systems using solid-state detectors with a special focus on Single-Photon Avalanche Diodes (SPADs) and Silicon Photomultipliers (SiPMs). These TR-DOI systems can be categorized into two types based on the operation mode of the detector (free-running or time-gated). For the TR-DOI prototypes, the physical concepts, main components, figures-of-merit of detectors, and evaluation parameters are described. The performance of TR-DOI prototypes is evaluated according to the parameters used in common protocols to test DOI systems particularly basic instrumental performance (BIP). In addition, the potential features of SPADs and SiPMs to improve TR-DOI systems and expand their applications in the foreseeable future are discussed. Lastly, research challenges and future developments for TR-DOI are discussed for each component in the prototype separately and also for the entire system.

Depth position recognition-related laser-induced damage test method based on initial transient damage features.

Even absorptive defects or inner cracks hiding several micrometers to a few dozen micrometers beneath the top surface can induce damage to transmission elements in the ultraviolet band. The extremely small size and disordered state of such defects or cracks hinder their detection using conventional methods. Therefore, the diagnosis of factors that limit damage resistance performance is a key technique for improving the fabrication technology of optical elements. With a focus on laser damage to third-harmonic transmission elements, this study establishes a micron space-resolved and nanosecond time-resolved imaging system on the basis of the pump-probe detection technique. The changes in the properties of defect-induced laser damage in the time domain are clarified. A diagnostic method for original damage depth in micron precision is proposed according to damage behaviors. This method can retrieve initial information on damage inducement and depth position. The recognition and diagnostic capabilities of such a technique are calibrated with artificial samples and then used to analyze real samples.