Sub-nanosecond Resolution Research Articles

Comparative analysis of optical gating time characterization methods for ultrafast gated image intensifiers with sub-nanosecond temporal resolution

Ultrafast gated image intensifiers are crucial for capturing high-resolution images with sub-nanosecond temporal resolution. This study compares two methods for characterizing optical gating time in these intensifiers: Method I, involving a “laser pulse walkthrough,” and Method II, utilizing a fiber optic delay array. Both methods were applied to the same intensifier, with results analyzed for consistency and discrepancies. The findings indicate that both methods yield consistent optical gating times within their standard deviations range, though Method II exhibited greater dispersion at the photocathode edges due to spatial non-uniformity. The study further reveals that while the optical gating time distribution across the photocathode is uniformly consistent, a constant delay exists between different regions. To enhance measurement precision it is recommended to subdivide the photocathode for individual calibration of gating time and delay. These insights are pivotal for improving the precision and reliability of optical gating time measurements in applications requiring sub-nanosecond or even picosecond temporal resolution.

Formation of diffuse and spark discharges between two needle electrodes with the scattering of particles

The development of a nanosecond discharge in a pin-to-pin gap filled with air at atmospheric pressure has been studied with high temporal and spatial resolutions from a breakdown start to the spark decay. Positive and negative nanosecond voltage pulses with an amplitude of tens of kilovolts were applied. Time-resolved images of the discharge development were taken with a four-channel Intensified Charge Coupled Device (ICCD) camera. The minimum delay between the camera channels could be as short as ≈ 0.1 ns. This made it possible to study the gap breakdown process with subnanosecond resolution. It was observed that a wide-diameter streamer develops from the high-voltage pointed electrode. The ionization processes near the grounded pin electrode started when the streamer crossed half of the gap. After bridging the gap by the streamer, a diffuse discharge was formed. The development of spark leaders from bright spots on the surface of the pointed electrodes was observed at the next stage. It was found that the rate of development of the spark leader is an order of magnitude lower than that of the wide-diameter streamer. Long thin luminous tracks were observed against the background of a discharge plasma glow. It has been established that the tracks are adjacent to brightly glowing spots on the electrodes and are associated with the flight of small particles.

Radiation hardness and quality validation of the on-detector electronics for the CMS Drift Tubes upgrade

In view of the High Luminosity LHC upgrade, the so-called Phase 2 upgrade, the electronics of the Drift Tubes (DT) subdetector of CMS will undergo a complete innovation. The requirements in terms of trigger rate will exceed the capabilities of the present electronics. Thus, all the on-detector electronics together with the associated back-end need to be replaced. Phase-2 on-detector electronics for DT consist of about 800 FPGAs (Field Programmable Gate Array) based boards called OBDT (On-detector Board for Drift Tubes). These boards are sub-divided in two different categories: 600 OBDT ϕ and 200 OBDT θ, targeting respectively the readout of DT wires parallel and normal to the LHC beams. Each OBDT ϕ is able to time-digitize 240 channels with sub-nanosecond resolution and upstream to the back end using multiple high-speed optical links running at 10.24 Gb/s. The choice of components known to have good resistance to radiation was a requirement in the design of the OBDT. The main component, the FPGA, is a flash-based PolarFire from Microsemi, already qualified in different facilities for radiation hardness tests. As a validation step, a campaign of radiation tests was carried out at the INFN-TIFPA Protontherapy Centre in Trento, Italy, using proton beams. The behavior of an OBDT ϕ board was evaluated during radiation exposure with a total dose much higher than expected to be integrated during 10 years of HL-LHC, which is 0.5 Gy.

Fluorescence lifetime of plant leaves with sub-nanosecond resolution

The study aimed to develop a measurement apparatus for in vivo chlorophyll-a (Chl-a) fluorescence decay measurements of plants by means of time correlated single photon counting. In this approach, sub-nanosecond laser pulses with a repetition rate of 10 MHz are applied to excite the sample, followed by the analysis of arrival times of the emitted fluorescence photons. Photon statistics are generated by iteratively fitting the sum of two exponential functions. The tool was tested on both plastid and in vivo leaf samples of Savoy cabbage (Brassica oleracea var. sabauda) with 3–4 subsequent leaves giving a complete sample coverage starting from the outermost. The Chl-a fluorescence lifetime exhibited a gradual increase in both the isolated plastid suspensions and the in vivo leaf samples towards the innermost leaf layers explained by an increase of natural absence of light (etiolation syndrome). Furthermore, cadmium stress and iron deficiency were investigated on treated sugar beet (Beta vulgaris) samples in vivo using TCSPS measurements. The reduced fluorescence quenching resulted in an increased fluorescence lifetime. Finally, a long-term (10 week) testing of the setup was carried out on Chl-retaining resurrection Haberlea rhodopensis plants protecting themselves by an elevated non-photochemical quenching yielding a decrease of fluorescence lifetime during their desiccation.

Photon counting Raman spectroscopy: a benchmarking study vs surface plasmon enhancement

We demonstrate a single-photon counting Raman spectroscope and benchmark it against conventional and surface-enhanced Raman spectroscopy. For direct comparison without ambiguity, we use the same solutions of Rhodamine 6G and a common optical setup with either a spectrometer or an acousto-optic tunable filter, whereas the surface enhancement is realized with immobilized Ag nanoparticles. Our results find that the single photon counting significantly elevates the detection sensitivity by up to eight orders of magnitude, arriving at a comparable level of surface-enhanced Raman spectroscopy. Another significant advantage is with the time-resolving measurement, where we demonstrate time-gated and time-correlated single-photon counting with sub-nanosecond resolution. It offers insights into the samples’ transient responses and enables the isolation of Raman scattering from fluorescence signals.

Hybrid Approach for Multiscale and Multimodal Time-Resolved Diagnosis of Ultrafast Processes in Materials via Tailored Synchronization of Laser and X-ray Sources at MHz Repetition Rates

The synchronization of laser and X-ray sources is essential for time-resolved measurements in the study of ultrafast processes, including photo-induced piezo-effects, shock wave generation, and phase transitions. On the one hand, optical diagnostics (by synchronization of two laser sources) provides information about changes in vibration frequencies, shock wave dynamics, and linear and nonlinear refractive index behavior. On the other hand, optical pump–X-ray probe diagnostics provide an opportunity to directly reveal lattice dynamics. To integrate two approaches into a unified whole, one needs to create a robust method for the synchronization of two systems with different repetition rates up to the MHz range. In this paper, we propose a universal approach utilizing a field-programmable gate array (FPGA) to achieve precise synchronization between different MHz sources such as various lasers and synchrotron X-ray sources. This synchronization method offers numerous advantages, such as high flexibility, fast response, and low jitter. Experimental results demonstrate the successful synchronization of two different MHz systems with a temporal resolution of 250 ps. This enables ultrafast measurements with a sub-nanosecond resolution, facilitating the uncovering of complex dynamics in ultrafast processes.

High-resolution electric field and temperature distributions in positive streamers

In this work, we aim to take a detailed experimental picture of the positive streamer. We apply optical emission spectroscopy to the first negative system (FNS, B2Σu+→X2Σg+) of N2+ and the second positive system (SPS, C3Πu → B3Πg) of N2. Large, centimeter wide, and highly reproducible streamers are created in pure nitrogen and synthetic air, at pressures ranging from 33 to 266 mbar. Direct time resolved spectral imaging of the space charge layer resulted in spatiotemporal maps of the calculated reduced electric field strength (E/N) and rovibrational temperature in sub-nanosecond and sub-millimetre resolution. The E/N peaks at approximately 540 and 480 Td, directly in front of the space charge layer, for synthetic air and pure nitrogen respectively, as determined by using the intensity ratio method of FNS and SPS. A global model for pure nitrogen in PLASIMO uses the experimentally determined E/N distribution to draw a picture of the gas kinetics around the space charge layer passage. In addition, the results of the global model serve as a reference to interpret the rotational and vibrational temperatures obtained from experimental FNS and SPS emissions.

Kinetic mechanism and sub-ns measurements of the thermal spark in air

This experimental and numerical study is focused on the formation of fully ionized plasmas in ambient air by nanosecond pulsed discharges, namely the thermal spark. The first contribution of this article is the experimental characterization of the electron number density during the pulse. An increase of the electron number density up to 1019 cm−3 was measured with sub-nanosecond resolution via three techniques based on optical emission spectroscopy (OES): Stark broadening of H α , Stark broadening of N+/O+, and the continuum emission of electrons. The discharge diameter is measured with sub-nanosecond resolution using calibrated OES of the N+ and O+ lines. All measurements indicate a transition to a micrometric-size filament of fully ionized plasma in approximately 0.5 ns. The second main contribution of this work is the development of a 0D kinetic mechanism to explain this observation. The mechanism includes 100 reactions, 12 species, and 12 excited electronic states. Particular attention is paid to modeling the N2, N2 +, N, and O electronic state kinetics using the electronic states as additional pseudo-species. Our results show that including the electron-impact ionization of the excited electronic states of N and O, in addition to those of N2, is necessary to explain the experimental results, emphasizing the key role of excited state kinetics in the thermal spark formation.

MA-RTI: Design and Evaluation of a Real-World Multipath-Assisted Device-Free Localization System.

Device-free localization (DFL) systems exploit changes in the radio frequency channel by measuring, for example, the channel impulse response (CIR), to detect and localize obstacles within a target area. However, due to a lack of well-defined interfaces, missing modularization, as well as complex system configuration, it is difficult to deploy DFL systems outside of laboratory setups. This paper focused on the system view and the challenges that come with setting up a DFL system in an indoor environment. We propose MA-RTI, a modular DFL system that is easy to set up, and which utilizes a multipath-assisted (MA) radio-tomographic imaging (RTI) algorithm. To achieve a modular DFL system, we proposed and implemented an architectural model for DFL systems. For minimizing the configuration overhead, we applied a 3D spatial model, that helps in placing the sensors and calculating the required calibration parameters. Therefore, we configured the system solely with idle measurements and a 3D spatial model. We deployed such a DFL system and evaluated it in a real-world office environment with four sensor nodes. The radio technology was ultra-wideband (UWB) and the corresponding signal measurements were CIRs. The DFL system operated with CIRs that provided a sub-nanosecond time-domain resolution. After pre-processing, the update rate was approximately 46 Hz and it provided a localization accuracy of 1.0 m in 50% of all cases and 1.8 m in 80% of all cases. MA fingerprinting approaches lead to higher localization accuracy, but require a labor-intensive training phase.

Terahertz photoconductance dynamics of semiconductors from sub-nanosecond to millisecond timescales

Optical pump terahertz probe spectroscopy (OPTP) is a versatile non-contact technique that measures transient photoconductance decays with femtosecond temporal resolution. However, its maximum temporal range is limited to only a few nanoseconds by the mechanical delay lines used. We extended the temporal range of OPTP to milliseconds and longer while retaining sub-nanosecond resolution. A separate pump laser was electrically synchronized to the probe pulses, allowing the pump–probe delay to be controlled with an electronic delay generator. We demonstrated the capabilities of this technique by examining the photoconductance decays of semiconductors with lifetimes ranging over six orders of magnitude: III-Vs, metal halide perovskites, germanium, and silicon. A direct comparison of results on silicon from OPTP and inductively coupled photoconductance decay highlighted the higher spatial and temporal resolution of OPTP, which allowed in-plane and out-of-plane carrier diffusion to be studied.

Using Artificial Intelligence in the Reconstruction of Signals from the PADME Electromagnetic Calorimeter

The PADME apparatus was built at the Frascati National Laboratory of INFN to search for a dark photon (A′) produced via the process e+e−→A′γ. The central component of the PADME detector is an electromagnetic calorimeter composed of 616 BGO crystals dedicated to the measurement of the energy and position of the final state photons. The high beam particle multiplicity over a short bunch duration requires reliable identification and measurement of overlapping signals. A regression machine-learning-based algorithm has been developed to disentangle with high efficiency close-in-time events and precisely reconstruct the amplitude of the hits and the time with sub-nanosecond resolution. The performance of the algorithm and the sequence of improvements leading to the achieved results are presented and discussed.

<italic>Operando</italic> monitoring microstructures and defects in laser fusion additive manufacturing of metals

<p indent="0mm">Laser fusion additive manufacturing of metals has been growing tremendously over the past decade. Despite its unrivaled capability to directly fabricate complex geometries, the accuracy, repeatability, and reliability of the build remain a major concern. From the micro perspective, it is the unfavorable microstructures and defects/anomalies that deteriorate the service performances such as fatigue life and corrosion resistance. The microstructures here involve not only melt pool and keyhole but also grain morphology and size, crystal orientation, interface structure, composition homogeneity, phase stability, etc. The defects/anomalies mainly include porosity, balling, lack of fusion, spattering, element loss, oxidation, micro residual stress and deformation, microcracking, and rough surface. Accurate detection and characterization of these features are critical for their understanding and control. Traditionally, we use optical microscope, electron microscope, X-ray tomography and diffraction, and so forth, to perform postmortem analysis and track the thermal traces the local material has experienced. However, such information and evidence are typically static and limited. This poses difficulties in recreating and interpreting the microstructural formation. In this review, we summarize recent progress on<italic> in situ</italic>/<italic>operando</italic> monitoring the laser fusion additive manufacturing of metals. It mainly covers high-speed visible light and thermal imaging, laboratory X-ray imaging, ultrasonic measurement, and high-speed synchrotron X-ray imaging and diffraction. Beyond traditional postmortem characterizations, the <italic>in situ</italic>/<italic>operando</italic> techniques have demonstrated the capability to capture the transient and dynamic formation and evolution processes of various microstructures and defects. They are revolutionizing the research mode in the field of metal additive manufacturing, from trial and error to mechanism oriented. In the former, we continue to optimize the objective function (e.g., grain morphology and size) until it arrives at a satisfactory solution, largely based on limited experience and knowledge and costly iterations. In the latter, we quest for the ultimate solution to manufacturing defect-free and perfect structures through direct exploration of the physical origins of the aforementioned features. Among the various techniques, we address high-speed synchrotron X-ray imaging and diffraction. It is an emerging approach in this field and is more suitable to monitor the internal microstructural evolution of metals, e.g., melt pool and keyhole dynamics, pore formation, cracking, rapid solidification, and phase transformation, because of the micrometer spatial resolution, sub-nanosecond temporal resolution, megahertz frame rate, and millimeter penetration capacity. Nevertheless, other techniques like high-speed visible light and thermal imaging have irreplaceable advantages and application scenarios. They can provide complementary information (e.g., thermal distribution on the melt pool surface, vapor plume dynamics, and three-dimensional spatter morphologies) for comprehensive understanding of the fundamental mechanisms in laser melting. Looking ahead to the next decade, the integration of multiple <italic>in situ</italic>/<italic>operando</italic> techniques and the translation among the resultant multi-dimensional signals, as well as big data computing and high precision modeling, will bridge understanding of mechanisms on the microscale and tailoring of properties on the macroscale. This will promote sustainable growth of the additive manufacturing industry.

Development and integration of photonic Doppler velocimetry as a diagnostic for radiation driven experiments on the Z-machine.

Plasma density measurements are key to a wide variety of high-energy-density (HED) and laboratory astrophysics experiments. We present a creative application of photonic Doppler velocimetry (PDV) from which time- and spatially resolved electron density measurements can be made. PDV has been implemented for the first time in close proximity, ∼6 cm, to the high-intensity radiation flux produced by a z-pinch dynamic hohlraum on the Z-machine. Multiple PDV probes were incorporated into the photoionized gas cell platform. Two probes, spaced 4 mm apart, were used to assess plasma density and uniformity in the central region of the gas cell during the formation of the plasma. Electron density time histories with subnanosecond resolution were extracted from PDV measurements taken from the gas cells fielded with neon at 15 Torr. As well, a null shot with no gas fill in the cell was fielded. A major achievement was the low noise high-quality measurements made in the harsh environment produced by the mega-joules of x-ray energy emitted at the collapse of the z-pinch implosion. To evaluate time dependent radiation induced effects in the fiber optic system, two PDV noise probes were included on either side of the gas cell. The success of this alternative use of PDV demonstrates that it is a reliable, precise, and affordable new electron density diagnostic for radiation driven experiments and more generally HED experiments.

Simulation of subnanosecond radio pulse electro-optical repeater

Objectives . The study aimed to construct and analyze a computational model of an electro-optical repeater of radio pulses, capable of reconstructing a pulsed radio image with subnanosecond resolution in a single reception under conditions of additive noise. Methods . Numerical methods of network analysis were used, which are the basis of specialized computer-aided design systems, numerical methods of statistical radio engineering. Results . A radiophotonic radio pulse repeater scheme was constructed, which is implemented based on the principle of fractional multiplexing with delayed feedback. Software simulation of infrared repeaters in the Simulink environment (Trial Version Soft) was developed, which allows for analyzing and investigating the efficiency of the optical reconstruction method of radio pulses using a fractional multiplexing with delayed feedback. It is shown that repeaters schemes, implemented on the principles of fractional multiplexing with delayed feedback, are able to effectively solve the scientific and practical problems of multiple probing of objects with ultrashort pulses (USP) for obtaining a radio image of a target with reliable reproducibility. In the course of numerical simulations, it was found that the two- and four-cascade schemes of delay lines with feedback do not provide reliable reproducibility in the case of reconstruction of an ultrashort pulse with a complex time profile. At the same time, the scheme with a cascade of 8 delay lines showed good results, providing a correlation reproducibility of more than 0.9. In this case, the scheme of an electro-optical repeater with a cascade of 16 delay lines did not make a significant contribution to increasing the accuracy of the USP reconstruction if compared to the scheme with a cascade of 8 lines; therefore, the latter can be determined as an optimal solution. An electro-optical method was proposed for solving the radio engineering problem of stroboscopic registration and reconstruction of subnanosecond radio pulses, which represent the signature of the radio image of dynamic objects for active radio imaging systems. Conclusions . It was found that an electro-optical repeater with 8 delay lines is able to recover a complex pulse reflected from a target in 30 iterations with a correlation coefficient greater than 0.9 between the reference and reconstructed pulses at a signal-to-noise ratio of at least 9 dB.

High Resolution Fluorescence Lifetime Maps from Minimal Photon Counts.

Fluorescence lifetime imaging microscopy (FLIM) may reveal subcellular spatial lifetime maps of key molecular species. Yet, such a quantitative picture of life necessarily demands high photon budgets at every pixel under the current analysis paradigm, thereby increasing acquisition time and photodamage to the sample. Motivated by recent developments in computational statistics, we provide a direct means to update our knowledge of the lifetime maps of species of different lifetimes from direct photon arrivals, while accounting for experimental features such as arbitrary forms of the instrument response function (IRF) and exploiting information from empty laser pulses not resulting in photon detection. Our ability to construct lifetime maps holds for arbitrary lifetimes, from short lifetimes (comparable to the IRF) to lifetimes exceeding interpulse times. As our method is highly data efficient, for the same amount of data normally used to determine lifetimes and photon ratios, working within the Bayesian paradigm, we report direct blind unmixing of lifetimes with subnanosecond resolution and subpixel spatial resolution using standard raster scan FLIM images. We demonstrate our method using a wide range of simulated and experimental data.

Spectrally filtered ps–ns emission dynamics of atmospheric-pressure nanosecond pulsed plasmas

The temporal dynamics of atmospheric-pressure nanosecond pulsed plasma discharges in a pin-to-pin electrode configuration are studied using streak-camera line imaging of the interelectrode gap with a time resolution as short as ∼25 ps. Discharge emission initiates homogeneously throughout the interelectrode gap with no detectable streamer propagation and then temporally decays in two distinct phases. Plasma emission bands attributed to various electronic transitions are tracked for single discharges in air and N2. Spectral filtering of the excited molecular states reveals that the N2(C–B) and N2(B–A) emission bands evolve in distinct early and late phases, respectively, with a time separation of ∼15–20 ns. Furthermore, significant differences in the temporal dynamics of plasma discharges in air and N2 are observed. High levels of excited-state atomic oxygen and NO appear after the initial decay of the N2(C) state and coincide primarily with the latter phases of plasma evolution in air environments. From temporal traces of discharge emission, the formation and relaxation timescales of the electronically excited states of N2 are quantified in pure N2 and air environments with sub-nanosecond resolution. The streak-OES (optical emission spectroscopy) technique enables quantitative time-resolved studies of key chemical species for model validation in ultra-short-pulsed plasmas.

High-Dimensional Acousto-optoelectric Correlation Spectroscopy Reveals Coupled Carrier Dynamics in Polytypic Nanowires

The authors combine acousto-optoelectric and multi-channel photon correlation spectroscopy to probe spatio-temporal carrier dynamics induced by a piezoelectric surface acoustic wave (SAW). The technique is implemented by combining phase-locked optical micro-photoluminescence spectroscopy and simultaneous three-channel time resolved detection. From the recorded time correlated single photon counting data the time transients of individual channels and the second and third order correlation functions are obtained with sub-nanosecond resolution. The method is validated by probing the correlations SAW-driven carrier dynamics between three decay channels of a single polytypic semiconductor nanowire on a conventional LiNbO$_\mathrm{3}$ SAW delay line chip. The method can be readily applied to other types of nanosystems and probe SAW-regulated charge state preparation in quantum dots, charge transfer processes in van der Waals heterostructures or other types of hybrid nanoarchitectures.

Recording Finite Radio Images in the Subnanosecond Resolution Signal Radio Vision

Recording Finite Radio Images in the Subnanosecond Resolution Signal Radio Vision

Time-resolved cathodoluminescence in an ultrafast transmission electron microscope

Ultrafast transmission electron microscopy (UTEM) combines sub-picosecond time-resolution with the versatility of TEM spectroscopies. It allows us to study the ultrafast materials' response using complementary techniques. However, until now, time-resolved cathodoluminescence was unavailable in UTEM. In this paper, we report time-resolved cathodoluminescence measurements in an ultrafast transmission electron microscope. We mapped the spatial variations of the emission dynamics from nano-diamonds with a high density of NV centers with a 12 nm spatial resolution and sub-nanosecond temporal resolution. This development will allow us to study the emission dynamics from quantum emitters with a unique spatiotemporal resolution and benefit from the wealth of complementary signals provided by transmission electron microscopes. It will further expand the possibilities of ultrafast transmission electron microscopes, paving the way to the investigation of the quantum aspects of an electron/sample interaction.

Gigahertz Frame Rate Imaging of Charge-Injection Dynamics in a Molecular Light Source.

Light sources on the scale of single molecules can be addressed and characterized at their proper sub-nanometer scale by scanning tunneling microscopy-induced luminescence (STML). Such a source can be driven by defined short charge pulses while the luminescence is detected with sub-nanosecond resolution. We introduce an approach to concurrently image the molecular emitter, which is based on an individual defect, with its local environment along with its luminescence dynamics at a resolution of a billion frames per second. The observed dynamics can be assigned to the single electron capture occurring in the low-nanosecond regime. While the emitter’s location on the surface remains fixed, the scanning of the tip modifies the energy landscape for charge injection into the defect. The principle of measurement is extendable to fundamental processes beyond charge transfer, like exciton diffusion.