Intense Pulses Research Articles

Multi-cycle terahertz generation in lithium niobate wafer stacks via mid-infrared pumping

Near-infrared laser-pumped optical rectification (OR) using quasi-phase matching (QPM) in lithium niobate (LN) is widely employed to generate multi-cycle terahertz (THz) pulses, which, however, suffer from low efficiency. Here, we demonstrate that mid-infrared pumping is an effective approach to increase the efficiency of multi-cycle THz generation. By using a 2.3-µm laser to pump a QPM macro-crystal composed of ten x-cut lithium niobate wafers, with their ferroelectric Z axis alternately rotated by π, a laser-to-THz conversion efficiency up to ∼0.4% has been achieved at room temperature, more than twice the efficiencies attained with near-infrared pumping. Electro-optic sampling reveals the generation of five-cycle THz pulses at 0.15 THz for 350-µm-thick wafers and 0.22 THz for 250-µm-thick wafers. Such mid-infrared laser-pumped OR in QPM wafer stacks provides an efficient, controllable, and scalable method for generating intense multi-cycle THz pulses suitable for diverse narrow-bandwidth applications.

Controllable temporal twisting polarization within an ultrafast laser pulse

We propose a method for controlling the time-varying polarization of optical pulses by introducing a quarter-wave plate into a 4-f pulse shaper and using a spatial light modulator to impose a group delay. This setup enables the polarization state of the incident pulse to vary over time. Specifically, for linearly chirped incident pulses, the polarization ellipse twists uniformly over time, while its ellipticity changes monotonically. As the group delay increases, the pulse intensity gradually splits in the time domain, transitioning from a single pulse with uniformly twisting polarization to two circularly polarized pulses with opposite chirality. We provide a detailed and comprehensive explanation of this modulation process using analytical expressions.

Intensity-Dependent Effects of Low-Frequency Subthreshold rTMS on Primary Motor Cortex Excitability and Interhemispheric Inhibition in Elderly Participants: A Randomized Trial.

Low-frequency repetitive transcranial magnetic stimulation (LF-rTMS) protocols targeting primary motor cortex (M1) are used in rehabilitation of neurological diseases for their therapeutic potential, safety, and tolerability. Although lower intensity LF-rTMS can modulate M1 neurophysiology, results are variable, and a systematic assessment of its dose effect is lacking. To determine the dose-response of LF-rTMS on stimulated and non-stimulated M1. In a sham-controlled randomized double-blind crossover study the effect of LF-TMS protocols were determined in 20 right-handed older healthy participants. In 3 sessions, 1 Hz rTMS at 80% (rTMS80), 90% (rTMS90) of motor threshold or sham stimulation were applied to left upper extremity M1. Outcome measures were curve parameters of the stimulus-response curve (maximum motor evoked potential [MEPMAX], slope and the intensity to evoke 50% MEPMAX), short-interval intracortical inhibition (SICI), and interhemispheric inhibition (IHI). Within LF-rTMS sessions, rTMS90, increased MEPMAX in the stimulated M1. Furthermore, rTMS90, increased the slope in the non-stimulated M1. LF-rTMS effects on SICI were dependent on the participants' baseline SICI, hemisphere, and intensity of conditioning pulse. Finally, rTMS90 increased whereas rTMS80 decreased IHI, for both IHI directions. These changes were dependent on baseline IHI and hemisphere and were no longer significant when baseline IHI was accounted for. Intensity of subthreshold LF-rTMS has differential effects on excitation and inhibition of stimulated and non-stimulated M1. The effects were small and were only demonstrated within the LF-rTMS sessions but were not different when compared to sham. rTMS related changes in SICI and IHI were dependent on baseline level. NCT02544503, NCT01726218.

Physical Properties of HfO2 Nano Structures Deposited using PLD

On Quartz substrates, high purity transparent conductive the Di-Oxide of the (HfO2) Nano and micro-structure films were successfully coated. The method of (PLD) Deposition using Pulsed Laser, and the influence of laser energy was investigated. The XRD results revealed two distinct phases, cubic and monoclinic crystal shapes. Peaks in the Fourier-transform infrared (FTIR) spectra showed that HfO2 nanofilm was forming.The findings of the optical characteristics reveal an excellent transparency of nearly 80% (89 percent). As the laser wavelength decreases, the optical energy band gap values for Nanostructure of HfO2 were prepared, with values ranging from 5.24 to 5.53 eV. Also, the EDX results showed the appearance of hafnium peaks and oxygen peaks in varying proportions, which confirms that hafnium oxide is definitely obtained The results of the AFM reveal that when the pulsed laser intensity increases, the average grain diameter values increase from 52.96 to 74.54 nm. With regard to optimum pulsed laser energy, the based on I-V characterization, the generated factor ideality for created diodes was discovered to be decreasing, and the corresponding values of the barrier height grew. The ideality factor of the diode made the optimum pulsed laser energy (1800 mJ) was greater (n=3.1).

XFEL SASE pulses can enhance time-dependent observables

Abstract X-ray free electron lasers (XFELs) have emerged as powerful sources of short and intense X-ray pulses. We propose a simple and robust procedure which takes advantage of the inherent stochasticity of self-amplified stimulated emission (SASE) pulses to enhance the time-resolution and signal strength in the recorded data. Notably, the proposed method is able to enhance the average signal without knowledge of the signal strength of individual shots. Simple metrics for the probe pulses are introduced, such as an effective pulse duration applicable to SASE pulses characterised in the time domain using {\it{e.g.}}\ an X-band transverse cavity (XTCAV). The approach is evaluated using simulated and real pulse data in the context of ultrafast electron dynamics in a molecule. Utilising H$_2$ as a model system, we demonstrate the efficacy of the method theoretically, successfully enhancing the predicted nonresonant ultrafast X-ray scattering signal associated with electron dynamics. The method presented is broadly applicable and offers a general strategy for enhancing time-dependent observables at XFELs.

Observation of molecular resonant double-core excitation driven by intense X-ray pulses

The ultrashort and intense pulses of X-rays produced at X-ray free electron lasers (XFELs) have enabled unique experiments on the atomic level structure and dynamics of matter, with time-resolved studies permitted in the femto- and attosecond regimes. To fully exploit them, it is paramount to obtain a comprehensive understanding of the complex nonlinear interactions that can occur at such extreme X-ray intensities. Herein, we report on the experimental observation of a resonant double-core excitation scheme in N2, where two 1σ core-level electrons are resonantly promoted to unoccupied 1πg*\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$1{\\pi }_{g}^{* }$$\\end{document} molecular orbitals by a single few-femtosecond broad-bandwidth XFEL pulse. The production of these neutral two-site double core hole states is evidenced through their characteristic decay channels, which are observed in good agreement with high-level theoretical calculations. Such multi-core excitation schemes, benefiting from the high interaction cross sections and state- and site-selective nature of resonant X-ray interactions, should be generally accessible in XFEL irradiated molecules, and provide interesting opportunities for chemical analysis and for monitoring ultrafast dynamic processes.

Grating compressor optimization aiming at maximum focal intensity of femtosecond laser pulses

It is shown that the optimal geometry of a Treacy compressor is the full-aperture compressor, in which the beam size at the first diffraction grating is equal to its length. Despite the energy losses and greater size of the focal spot, such a compressor provides considerably higher (by 1.5–2 times) focal intensity than an energy lossless compressor. Decreasing the density of grooves from 1200–1400/mm to about 1000/mm also increases the focal intensity by tens of percent. The constructed theory is generalized to the full-aperture two-grating compressor, which is the best design due to the angle of incidence on the first grating being smaller than the Littrow angle. Two gratings with a length of 138 cm allow obtaining an intensity of 4.09 × 1024W/cm2 and 5.01 × 1024W/cm2 in the focus of F/2 parabola for the projects XCELS and SEL-100PW, reaching the 139 PW and 174 PW power.

A study to determine the efficacy of intense pulse light in vascular mode for topical steroid damaged facies

Objectives: Misuse of topical corticosteroids on the face has gradually evolved over the years and has now reached phenomenal proportions on account of easy availability, widespread misinformation, and the fairness craze sweeping across the land. Topical steroid-damaged/dependent facies (TSDFs) may present with diffuse erythema, telangiectasia, papules, and atrophy. Current modalities used for the management of TSDF, such as topical tacrolimus and oral antibiotics, have shown a variable response. Intense pulsed light (IPL) systems are economical, have a higher safety margin, and are widely available. The goal of treating vascular lesions with IPL is to raise blood vessel temperature high enough to cause coagulation, leading to destruction and healing with fibrosis. In our study, we exploit these properties of IPL for the management of TSDF. Material and Methods: A total of 15 patients of TSDF were recruited and were treated with three sessions of IPL treatments (vascular mode) 3 weeks apart and were assessed based on a clinical score for TSDF, visual analog scale (VAS) for erythema assessed by the physician and patient at the end of 9 weeks. Results: There was a significant response with a reduction in clinical score of 5.4 points (P > 0.001) and a reduction in physician and patient-assessed VAS for erythema of 3.47 and 3.73, respectively. Conclusion: There is a scarcity of data regarding the efficacy of various treatment modalities for TSDF. Our study establishes IPL (vascular mode) as an innovative emerging modality of treatment.

Effects of pulsed magnetic field intensity on the freezing rate and heat loads reduction of harvested mango and tomato

PurposeFresh fruits and vegetables (FV) are crucial global food resources, but the presence of heat loads during harvest adversely impacts their shelf life. While freezing technology provides an effective means of removing heat loads, it is an energy-intensive process and may consequently prove too costly for practical business viability. The growing interest in utilizing magnetic field (MF) technology during the freezing of fresh FV enhances the freezing rate and rapidly removes the heat loads of products.Design/methodology/approachIn the present study, pulsed magnetic field (PMF) pretreatment employing specific field strengths (9 T, 14 T and 20 T) was examined as a preliminary step before freezing mango and tomato and compared to the conventional freezing method (untreated) at − 18 °C.FindingsPMF pretreatment prior to freezing demonstrated a noteworthy enhancement in freezing rate by around 10 and 12% when compared with the conventional (untreated) freezing, which exhibited freezing rates of −0.08 °C/min and −1.10 °C/min for mango and tomato, respectively. The PMF pretreatment (at 20 T) provided a higher freezing rate (at p = 0.05) than the conventional freezing method reduced heat loads amounting to 1.1 × 107 J/kg oC and 2.9 × 106 J/kg oC, significantly (at p = 0.05) from mango and tomato, respectively. These reductions in heat loads were approximately more than 5% of the calculated heat loads removed during conventional freezing.Research limitations/implicationsMango and tomato samples were only tested; the results may lack generalizability. Therefore, researchers are encouraged to test for other products for further studies.Practical implicationsThe paper includes implications for the development of a rapid freezing technique, the development of “pulsed magnetic field” and for eliminating the problem associated with conventional (slow) freezing.Originality/valueThe study holds significance for the production of postharvest freezing technology, providing insightful information on the PMF-assisted freezing of cellular foods.

Measurement of electromagnetic pulse in laser acceleration enhanced by near-critical density targets

High-power laser interactions with solid targets create an abundance of high-energy charged particles, resulting in the generation of intense electromagnetic pulses (EMPs), which are strongly pertinent to the target parameters. In this study, the characteristics of EMPs generated by relativistic femtosecond laser irradiation of double-layer targets composed of near-critical density carbon nanotube foams (CNFs) and an aluminum (Al) foil are investigated. The results demonstrate that the CNF double-layer targets accelerate proton energy by over 1.6 times compared to a single-layer Al plane target, thereby indirectly amplifying the EMP amplitude by over 3.6 times. The findings are beneficial to gaining insight into EMPs induced by femtosecond laser coupling with near-critical density targets and open a new avenue to achieve tunable EMPs by managing the material and structure of the target to optimize the coupling efficiency between the laser and solid targets.

Enhanced pointing and charge properties of electron beams from few-TW laser wakefield acceleration with an asymmetric density profile in a sub-millimeter nitrogen gas jet

This work demonstrates the feasibility of creating a sub-millimeter, subsonic nitrogen gas jet using a 178-μm diameter orifice to conduct laser wakefield acceleration (LWFA) with 1-TW, 40-fs laser pulses. More importantly, our findings reveal that using a blade to impede part of the gas flow and create an asymmetric density profile with a shortened down-ramp leads to a notable reduction in pointing fluctuations and an increase in the total charge of the output electron beams. As evidenced by the corresponding particle-in-cell simulation, the laser intensity is more effectively sustained toward the downstream end of the shaped gas jet, allowing for effective excitation of low-amplitude plasma waves that help preserve the accelerated electrons over the target rear side. In contrast, the pulse intensity drops significantly within the rear side of the unshaped gas jet, resulting in continuously diminishing plasma waves and decreased beam charge. The steeper gradient of the density down-ramp in the shaped gas jet also leads to a more rapid increase in the plasma wavelength over a reduced propagation distance, which helps mitigate the dephasing of accelerated electrons and increase the charge at the high-energy side of the spectrum. Our study paves the way for the future development of few-TW LWFA using a subsonic gas jet with sharp edges to further enhance the properties of output electron beams.

Analysis of Saturated Impulse for Square Plates under Flat Slamming Impact: Experimental and Numerical Investigation

The saturated impulse is a special phenomenon in the dynamic plastic behavior of engineering structures under intensive pulse loading, such as slamming loading. In this study, slamming experiments were performed on steel plates to investigate their slamming pressure and dynamic plastic responses, as well as the saturation phenomenon, and elucidate the effect of the plate thickness and material properties on the dimensionless saturated deflection and saturated impulse in combination with the published test data. The results show that the dimensionless saturated deflection and saturated impulse of the test plates gradually increased as the dimensionless stiffness decreased. After being validated against the experimental results, a numerical method that considered the fluid–structure interaction (FSI) effect was then employed to provide comprehensive insight into the transient plastic responses and saturated impulse of the flat plates under slamming impact. Numerical simulations revealed that the compressed air layer always existed during the effective process of the flat slamming impact. Through the numerical prediction of the dynamic plastic deflection and slamming pulse loading, it was observed that the saturated impulse phenomenon always took place after the time instant of the peak value of the pressure pulse. Furthermore, the analysis of the saturated impulse based on the numerical simulations indicated that the saturation phenomenon was more likely to be achieved as the water impact velocity increased.

Lab study on the physiological thermoregulatory abilities of older people with different frailty levels

The advent of an aging society has increased the need for a better understanding of thermal comfort requirements, especially for older people's health. Differences in thermal comfort needs among older individuals with varying frailty levels are noted, but the impact of health on physiological thermoregulation is underexplored. This study, based on the frailty classification method characterized by Fried, selected 12 non-frail (age:71.00 ± 5.41) and 12 pre-frail (age:72.68 ± 5.7) older individuals for laboratory experiments. This experiment set sudden temperature change experiments to illustrate differences in subjective perceptions and physiological changes between non-frail and pre-frail older people. Results show that non-frail individuals are more likely to complain in warmer environments. Significant differences in perception only emerged when the temperature differential between neutral and warm conditions exceeded 5 °C. Pre-frail individuals exhibited slower skin temperature changes and required more time to stabilize with the environment, particularly in the foot. Pre-frail individuals tend to sweat less in changing environments and require more time to reach stable sweating. There is positive evidence indicating that heart rate of pre-frail individuals is higher than non-frail individuals, with different changing trends observed between the two groups. When the temperature difference exceeds 5 °C, the pulse intensity trend in pre-frail individuals diverges from that of non-frail individuals. In controlling indoor environments for older people, it is essential to consider both subjective comfort and physiological thermoregulation capabilities. For older adults with frailty characteristics, applying localized warming, using small temperature differences, gradually adjusting the environment may better meet their comfort needs.

Peculiarities in Rabi oscillations for fast-relaxing electron spins

Rabi oscillations (transient nutations) are a phenomenon that has proven itself well in EPR for identifying electron spin quantum numbers and electron-spin transitions. They are successfully applied when the Rabi frequency significantly exceeds the spin relaxation rates and therefore does not depend on these rates. However, the short transverse relaxation time, being comparable to or even shorter than the dead time of EPR spectrometers, makes it difficult to observe Rabi oscillations and their frequency depends not only on the intensity of the short microwave pulse, but also on its shape and relaxation rates. Two techniques are considered that are suitable for this case, in which Rabi oscillations are detected by monitoring the FID amplitude as a function of pulse duration or microwave field amplitude. We describe the FID-detected Rabi oscillations analytically or numerically for rectangular or shaped pulses, respectively. The description is confirmed by EPR experiments using DPPH as a model sample.

A CFD‐PBM‐ANN framework to simulate the liquid–liquid two‐phase flow in a pulsed column

AbstractCFD‐PBM numerical simulation is a powerful tool in the research of droplet swarm behavior. In this work, an artificial neural network (ANN) based droplet breakage frequency function is established based on the directly measured data from our previous studies. Then, the weights and biases of ANN are embedded into the CFD‐PBM code in the form of matrices and vectors. For the first time, a CFD‐PBM‐ANN simulation framework is established. Simulation results are in good agreement with the experimental data under different operation conditions. The cumulative droplet size distribution decreases with the increase of interfacial tension and pulse intensity. It is also found by the simulation that the droplet breakage frequency is relatively high at the edge of disc and doughnut plate, which is accordant with the distribution of turbulent energy dissipation and velocity gradient.

Multiphoton ionization in a photonic crystal based on carbon nanotubes under the action of a few cycle optical pulse

We considered a theoretical model of the interaction of a one-dimensional few cycles optical pulse with a nonlinear medium of semiconductor carbon nanotubes, which has a spatial modulation of the refractive index in the direction of pulse propagation (a one-dimensional photonic crystal). The results of the dependence of the rate of one- and two-photon ionization on the intensity of the short-wavelength pulse are shown. The effect of additional external electric and magnetic fields on the photoionization rate is considered.

Generation of optical-terahertz solitons by a few-cycle laser pulse

The generation of broadband terahertz radiation using an extremely short laser pulse of high intensity is considered. Using numerical simulation of the generalized Yajima-Oikawa system, it is shown that in the generation of an optical-terahertz soliton, in contrast to the quasi-monochromatic case, Kerr nonlinearity plays an important role for a low-period pulse, considering its dispersion.

Impact of treatment trajectory on the thermal ablation rate and biological tissue volumetric lesion during irradiation by shock-wave focusing ultrasonic beam

Thermal ablation rates and the shapes of volumetric biological tissue lesion are compared in a numerical experiment, in which biological tissue is exposed to pulsed periodic shock-wave high intensity focused ultrasound. The comparison is performed across three different irradiation sequences of discrete foci placed uniformly within the target area.

Practical twin-field quantum key distribution parameter optimization based on quantum annealing algorithm

Abstract Twin-field quantum key distribution (TF-QKD) is widely studied since it can surpass the key capacity of repeaterless QKD, whereas electromagnetic interference (EMI) is one of the main challenges in its practical applications. This study is based on the Faraday–Michelson TF-QKD. Analyze the effect of EMI on the rotation angle of the Faraday mirror causing an additional quantum bit error rate (QBER). Moreover, the quantum annealing algorithm (QA) based on quantum tunneling mechanism is applied to the optimization of practical TF-QKD. Mapping the secure key rate of TF-QKD into the evaluation function of QA and the transverse magnetic field is introduced to construct the kinetic energy term, which can realize the quantum tunneling effect. Meanwhile, the QA is improved in terms of chaotic optimization to obtain the dynamic initial value of the algorithm, the design of a perturbation method to skip the locally optimal solution, and the use of suitable temperature and magnetic field decay functions. Optimizing TF-QKD with QA, the standard deviation of QBER fluctuation caused by EMI is reduced to 0.206, the mean square error of the signal-state pulse intensity is only 3.38 × 10 − 4 , and the optimization accuracy of the secure key rate can reach 99.8 % . Additionally, this optimization method shortens the runtime and reduces computational resource consumption, making it highly efficient for practical implementations.

Laser-induced modification of an excited-state vibrational wave packet in neutral H2 observed in a pump-control scheme

We observe and modify a molecular vibrational wave packet in an electronically excited state of the neutral hydrogen molecule. In an extreme-ultraviolet (XUV) time-domain absorption spectroscopy experiment, we launch a vibrational wave packet in the DΠu13pπ state of H2 and track its time evolution via the coherent dipole response. The reconstructed time-dependent dipole from experimentally measured XUV absorption spectra provides access to the revival of the vibrational wave packet, which we control via an intense near-infrared (NIR) pulse. Tuning the intensity of the NIR pulse, we observe the revival of the wave packet to be significantly modified, which is supported by the results of a multilevel simulation as well as an analytical model based on state-specific phase shifts. The NIR field is applied only 7 fs after the creation of the wave packet but influences its evolution up to at least its first revival at 270 fs. This experimental approach for nonlocal-in-time laser modification of quantum dynamics in a pump-control scheme enabled by molecular self-probing is generally applicable to a large range of molecules and materials as it only requires the observation of absorption spectra. Published by the American Physical Society 2024