Effect Of Pulse Length Research Articles

Pulse length effects in the nonresonant long-wave infrared nonlinear optical response of n-Ge, GaAs, and ZnSe

Nonlinear optical refraction and nonlinear absorption are characterized in important long-wave infrared optical materials with picosecond and femtosecond laser pulses. The effective nonlinear refractive indices are found to be constant across a range of pulse parameters. Nonlinear absorption far from resonance is observed at relatively low (∼1GW/cm2) intensities in these materials, and the onset intensity and fluence scale strongly with pulse length. A free carrier dominated nonlinear absorption mechanism is identified for picosecond pulses, whereas nonperturbative photoionization causes femtosecond absorption.

Numerical Investigation of Ultrasound-Induced Bubble Motion and Coalescence

Abstract Ultrasound-induced bubble motion and coalescence can be a unique approach to neutralize cavitation clouds in many biomedical applications. However, the detailed mechanisms of bubbles’ collective behaviours have not been fully understood. This work aims to develop an Eulerian-Lagrangian computational framework to simulate the bubble motion and coalescence driven by an ultrasound field. Specifically, the bubbles are modeled as oscillating spheres governed by the Keller-Miksis equation and are tracked as Lagrangian particles moving through the background Eulerian mesh. Several forces that drive bubble dynamics are modeled, including primary and secondary Bjerknes forces, drag force. As two bubbles contacting with each other, coalescence may happen and the film drainage model is applied to simulate the process. Using the new framework, the effects of pulse amplitude, frequency and length on the motion and coalescence of bubbles with different sizes and void fraction are investigated. The new knowledge can be applied to identify the optimal pulse parameters to better control the bubble motion and coalescence, improving the effectiveness of relevant ultrasound techniques.

Analysis of the Effect of Pulse Length and Magnetic Field Strength on Nonlinear Thomson Scattering

Analysis of the Effect of Pulse Length and Magnetic Field Strength on Nonlinear Thomson Scattering

Effect of plasma discharge pulse length for GaN film crystallinity on sapphire substrate by high density convergent plasma sputtering device

Effect of plasma discharge pulse length for GaN film crystallinity on sapphire substrate by high density convergent plasma sputtering device

Variability of pulse width in transcranial magnetic stimulation

Objective. There is a high variability in the physiological effects of transcranial magnetic brain stimulation, resulting in limited generalizability of measurements. The cause of the variability is assumed to be primarily based on differences in brain function and structure of the stimulated individuals, while the variability of the physical properties of the magnetic stimulus has so far been largely neglected. Thus, this study is dedicated to the systematic investigation of variability in the pulse width of different TMS pulse sources at different stimulation intensities. Approach. The pulse widths of seven MagVenture® pulse sources were measured at the output of 10%–100% stimulation intensity in 10% increments via Near Field Probe and oscilloscope. The same C-B60 coil was used to deliver biphasic pulses. Pulse widths were compared between pulse sources and stimulation intensities. Main results. The mean sample pulse width was 288.11 ± 0.37 µs, which deviates from the value of 280 µs specified by the manufacturer. The pulse sources and stimulation intensities differ in their average pulse width (p’s < .001). However, the coefficient of variation within the groups (pulse source; stimulation intensity) were moderately low (CV = 0.13%–0.67%). Significance. The technical parameter of pulse width shows deviations from the proposed manufacturer value. According to our data, within a pulse source of the same manufacturer, the pulse width variability is minimal, but varies between pulse sources of the same and other pulse source models. Whether the observed variability in pulse width has potential physiological relevance was tested in a pilot experiment on a single healthy subject, showing no significant difference in motor evoked potential amplitude and significant difference in latencies. Future research should systematically investigate the physiological effects of different pulse lengths. Furthermore, potential hardware ageing effects and pulse amplitude should be investigated.

High-power impulse magnetron sputter deposition of TiBx thin films: Effects of pulse length and peak current density

We report on a systematic study of the effect of pulse length (ton=25−200μs), and peak target current density (JT,peak=0.25−2.0A/cm2) during HiPIMS deposition of AlB2-phase TiBx thin films from a TiB2 target at a pressure of pAr=1.33Pa(10mTorr) and substrate temperature Ts=500°C. All films are under-stoichiometric with B/Ti = 1.36–1.89, with the higher values corresponding to longer pulses and higher JT,peak values. While the deposition flux, including both ions and neutrals, in general increases with increasing ton and JT,peak, the Ti+ ion flux saturates, resulting in the higher B/Ti values under these conditions. Thus, the relative amount of Ti ionization, and the degree to which these ions are guided toward the substrate by magnetic fields, are main modulators determining the composition of TiBx thin films.

Effect of Frequency Chirp and Pulse Length on Laser Wakefield Excitation in Under-Dense Plasma

Effect of Frequency Chirp and Pulse Length on Laser Wakefield Excitation in Under-Dense Plasma

Optically modified second harmonic generation in silicon oxynitride thin films via local layer heating

Strong second harmonic generation (SHG) in silicon nitride has been extensively studied—among others, in terms of laser-induced SHG enhancement in Si3N4 waveguides. This enhancement has been ascribed to the all-optical poling induced by the coherent photogalvanic effect. Yet, an analogous process for Si3N4 thin films has not been reported. Our article reports on the observation of laser-induced threefold SHG enhancement in Si3N4 thin films. The observed enhancement has many features similar to all-optical poling, such as highly nonlinear power dependence, cumulative effect, or connection to the Si3N4–Si interface. However, identical experiments for low-oxygen silicon oxynitride thin films lead to complex behavior, including laser-induced SHG reduction. Following a thorough experimental study, including the effects of repetition rate or pulse length, the observed results were ascribed to heat-induced SHG variation. In addition to revealing a new mechanism of laser-induced SHG variation, our results also provide a means to identify this mechanism.

Pulse length effects in long wavelength driven non-sequential double ionization

Pulse length effects in long wavelength driven non-sequential double ionization

Pulse length effects in long wavelength driven non-sequential double ionization

We present a joint experimental and theoretical study of non-sequential double ionization (NSDI) in argon driven by a 3100 nm laser source. The correlated photoelectron momentum distribution (PMD) shows a strong dependence on the pulse duration, and the evolution of the PMD can be explained by an envelope-induced intensity effect. Determined by the time difference between tunneling and rescattering, the laser vector potential at the ionization time of the bound electron will be influenced by the pulse duration, leading to different drift momenta. Such a mechanism is extracted through a classical trajectory Monte Carlo-based model and it can be further confirmed by quantum mechanical simulations. This work sheds light on the importance of the pulse duration in NSDI and improves our understanding of the strong field tunnel-recollision dynamics under mid-IR laser fields.

Effect of Ultrasound Pulse Length on Sonoreperfusion Therapy.

In recent years, long- and short-pulse ultrasound (US)-targeted microbubble cavitation (UTMC) has been found to increase perfusion in healthy and ischemic skeletal muscle, in pre-clinical animal models of microvascular obstruction and in the myocardium of patients presenting with acute myocardial infarction. There is evidence that the observed microvascular vasodilation is driven by the nitric oxide pathway and purinergic signaling, but the time course of the response and the dependency on US pulse length are not well elucidated. Because our prior data supported that sonoreperfusion efficacy is enhanced by long-pulse US versus short-pulse US, in this study, we sought to compare long-pulse (5000 cycles) and short-pulse (500×10 cycles) US at a pressure of 1.5 MPa with an equivalent total number of acoustical cycles, hence constant acoustic energy, and at the same frequency (1 MHz), in a rodent hind limb model with and without microvascular obstruction (MVO). In quantifying perfusion using burst replenishment contrast-enhanced US imaging, we made three findings: (i) Long and short pulses result in different vasodilation kinetics in an intact hind limb model. The long pulse causes an initial spasmic reduction in flow that spontaneously resolved at 4 min, followed by sustained higher flow rates (approximately twofold) compared with baseline, starting 10 min after therapy (p < 0.05). The short pulse caused a short-lived approximately twofold increase in flow rate that peaked at 4 min (p < 0.05), but without the initial spasm. (ii) The sustained increased response with the long pulse is not simply reactive hyperemia. (iii) Both pulses are effective in reperfusion of MVO in our hindlimb model by restoring blood volume, but only the long pulse caused an increase in flow rate after treatment ii, compared with MVO (p < 0.05). Histological analysis of hind limb muscle post-UTMC with either pulse configuration indicates no evidence of tissue damage or hemorrhage. Our findings indicate that the microbubble oscillation induces vasodilation, and therapeutic efficacy for the treatment of MVO can be tuned by varying pulse length; relative to short-pulse US, longer pulses drive greater microbubble cavitation and more rapid microvascular flow rate restoration after MVO, warranting further optimization of the pulse length for sonoreperfusion therapy.

Understanding Cantilever Transduction Efficiency and Spatial Resolution in Nanoscale Infrared Microscopy

Photothermal induced resonance (PTIR), also known as AFM-IR, enables nanoscale infrared (IR) imaging and spectroscopy by using the tip of an atomic force microscope to transduce the local photothermal expansion and contraction of a sample. The signal transduction efficiency and spatial resolution of PTIR depend on a multitude of sample, cantilever, and illumination source parameters in ways that are not yet well understood. Here, we elucidate and separate the effects of laser pulse length, pulse shape, sample thermalization time (τ), interfacial thermal conductance, and cantilever detection frequency by devising analytical and numerical models that link a sample's photothermal excitations to the cantilever dynamics over a broad bandwidth (10 MHz). The models indicate that shorter laser pulses excite probe oscillations over broader bandwidths and should be preferred for measuring samples with shorter thermalization times. Furthermore, we show that the spatial resolution critically depends on the interfacial thermal conductance between dissimilar materials and improves monotonically, but not linearly, with increasing cantilever detection frequencies. The resolution can be enhanced for samples that do not fully thermalize between pulses (i.e., laser repetition rates ≳ 1/3τ) as the probed depth becomes smaller than the film thickness. We believe that the insights presented here will accelerate the adoption and impact of PTIR analyses across a wide range of applications by informing experimental designs and measurement strategies as well as by guiding future technical advances.

Molecular Emissions from Stretched Excitation Pulse in Nanosecond Phase-Selective Laser-Induced Breakdown Spectroscopy of TiO2 Nanoaerosols.

In phase-selective laser-induced breakdown spectroscopy (PS-LIBS), gas-borne nanoparticles are irradiated with laser pulses (∼2.4GW/cm2) resulting in breakdown of the nanoparticle phase but not the surrounding gas phase. In this work, the effect of excitation laser-pulse duration and energy on the intensity and duration of TiO2-nanoparticle PS-LIBS emission signal is investigated. Laser pulses from a frequency-doubled neodymium-doped yttrium aluminum garnet (Nd:YAG) laser (532nm) are stretched from 8ns (full width at half maximum, FWHM) up to ∼30ns at fixed pulse energy using combinations of two optical cavities. The intensity of the titanium atomic emissions at around 500nm wavelength increases by ∼60%, with the stretched pulse and emissions at around 482nm, attributed to TiO, enhanced over 10 times. While the atomic emissions rise with the stretched laser pulse and decay around 20ns after the end of the laser pulse, the TiO emissions reach their peak intensity at about 20ns later and last longer. At low laser energy (i.e., 1mJ/pulse, or 80MW/cm2), the TiO emissions dominate, but their increase with laser energy is lower compared to the atomic emissions. The origin of the 482nm emission is explored by examining several different aerosol setups, including Ti-O, Ti-N, and Ti-O-N from a spark particle generator and Ti-O-N-C-H aerosol from flame synthesis. The 482nm emissions are attributed to electronically excited TiO, likely resulting from the reaction of excited titanium atoms with surrounding oxidizing (carbonaceous and/or radical) species. The effects of pulse length are attributed to the shift of absorption from the initial interaction with the particle to the prolonged interaction with the plasma through inverse bremsstrahlung.

Tellurite glass based optical fiber for the investigation of supercontinuum generation and nonlinear properties

This manuscript introduces a unique tellurite core-based photonic crystal fiber (PCF) with silica clad and circular air holes, which manifests highly birefringent and non-linear characteristics. Several optical features, such as birefringence (Br), nonlinear coefficients (NLC), dispersion (D), confinement loss (CL), material loss, etc are thoroughly analyzed and explored by applying the finite element method (FEM). The simulated outcomes validate that by optimizing the formation of the cladding region, a large NLC of as well as an ultra-high Br of and zero-dispersion can be accomplished in the offered PCF design at 1.56 μm wavelength. Moreover, the evaluated findings indicate that the stated fiber structure is capable of generating a wide supercontinuum spectrum spanning from to when augmented with a input power and a pulse duration of Calculations and analyses have been carried out on the effects of higher-order dispersion co-efficients, pulse length and input power on spectrum broadening. The advanced PCF design will be a suitable candidate for practical applications in numerous fields, including bio photonics, biomedical imaging, biosensing, spectroscopy and ultra-broadband signal amplification, etc.

Growth behavior of Ir metal formed by atomic layer deposition in the nanopores of anodic aluminum oxide.

The conformal coating or surface modification in high aspect ratio nanostructures is a tough challenge using traditional physical/chemical vapor deposition, especially for metal deposition. In this work, the growth behavior of iridium (Ir) metal formed by atomic layer deposition (ALD) in anodic aluminum oxide (AAO) templates was explored deeply. It is found that the surface hydrophilicity is crucial for the nucleation of ALD Ir. An in situ ALD Al2O3 layer with an ultra-hydrophilic surface can greatly promote the nucleation of ALD Ir in AAO nanopores. The effect of the Ir precursor pulse time, diameter, and length of AAO nanopores on the infiltration depth of ALD Ir was investigated systematically. The results show that the infiltration depth of ALD Ir in AAO nanopores is in proportion to the pore diameter and the square root of the Ir precursor pulse time, which follows a diffusion-limited model. Furthermore, the Ir precursor pulse time to obtain conformal Ir coating throughout all the AAO channels is in proportion to the square of the aspect ratio of AAO templates. In addition, the conformal Ir deposition in AAO nanopores is also related to the Ir precursor purge time and the O2 partial pressure. Insufficient Ir purge time could cause a CVD-like reaction, leading to the reduction of the infiltration depth in AAO. Higher O2 partial pressure can facilitate Ir nucleation with more Ir precursor consumption at the entrance of nanopores, decreasing the infiltration depth in AAO nanopores, so appropriate O2 partial pressure should be chosen for ALD Ir in high aspect ratio materials. Above all, our research is valuable for surface modification or coating of metal by ALD in high aspect ratio nanostructures for 3D microelectronics, nano-fabrication, catalysis and energy fields.

The effects of laser pulse length and collisional ionization on the acceleration of titanium ions

The interaction of a relativistic laser pulse (1018 W cm−2 µm2) with foil targets can accelerate ions to energies of tens of MeV u−1 with optimized laser and target parameters. We report the results on simulations of the interaction of a 6.0 × 1020 W cm−2 laser pulse incident on ultrathin (10–500 nm) titanium foils to investigate the roles of laser pulse duration and ionization mechanisms in the acceleration of titanium ions. While holding peak intensity constant, two laser pulse durations were investigated, 140 and 650 fs. The optimum thickness is dependent on pulse duration, as it requires the concurrence of target transparency with the incidence of the peak laser intensity. The collisional processes do not play a significant role in Ti ion beam generation from the 140 fs laser pulse duration at the optimum thickness (30 nm). However, for the 650 fs laser optimum, collisions improve the conversion efficiency of highly energetic (10 MeV u−1), high charge titanium (Ti21 − 22 +) by a factor of 20, and the titanium ion cutoff energy by ∼15%. This improvement is due to the fact that collisional ionization increases the electron density of the plasma, which delays the time of relativistic transparency, causing collisions to decrease the optimum foil thickness from 150 to 100 nm. Additionally, collisional ionization increases the charge-to-mass ratio of the titanium, and injects more electrons into the accelerating sheath field. At the optimum thickness, target normal sheath acceleration is the dominant mechanism of acceleration, with additional contributions from radiation pressure and shock wave acceleration.

Pulse length effects on autoionizing states under the influence of intense SASE XUV fields

The Fano absorption line shape of an autoionizing state encodes information on its internal atomic structure and dynamics. When driven near-resonantly with intense extreme ultraviolet (XUV) electric fields, the absorption profile can be deliberately modified, including observable changes of both the line-shape asymmetry and strength of the resonance, revealing information on the underlying dynamics of the system in response to such external driving. We report on the influence of the XUV pulse parameters at high intensity that can be achieved with a free-electron laser (FEL) with statistically broadened spectra based on self-amplified spontaneous emission (SASE). More specifically, the impact of the FEL pulse duration is studied for the example of the doubly excited 2s2p resonance in helium, where line-shape modifications have been measured with XUV transient absorption spectroscopy in Fraunhofer-type transmission geometry. A computational few-level-model provides insight into the impact of different average pulse durations of the stochastic FEL pulses. These findings are supported by measurements performed at the Free-Electron Laser in Hamburg (FLASH) and provide further insight into XUV strong-coupling dynamics of resonant transitions driven by intense high-frequency FEL sources.

Ion energy distributions at substrate in bipolar HiPIMS: effect of positive pulse delay, length and amplitude

A bipolar HiPIMS discharge with a rectangular positive voltage pulse (with controllable amplitude, delay after the main negative pulse and positive pulse length) was systematically investigated by mass spectroscopy. The time-averaged spectra of ions measured at the substrate position exhibit a prominent high-energy peak. It is shown that the position of the peak can be varied by the positive pulse amplitude, its magnitude scales with the pulse length and its width can be slightly influenced by the length of the delay interval. Measurements of the plasma potential at the mass spectrometer position and time-resolved mass spectroscopy clearly show that the high-energy peak is formed by ions accelerated by the elevated plasma potential during the positive pulse. In addition, fine details of the ion energy distribution functions related to the plasma potential transients at the start of the positive pulse are identified. The presented results are beneficial for the optimisation of the parameters of the positive pulse in experiments implementing the bipolar HiPIMS technology.

Multiphoton microscopy: a personal historical review, with some future predictions.

.The historical development of multiphoton microscopy is described, starting with a review of two-photon absorption, and including two- and three-photon fluorescence microscopies, and second- and third-harmonic generation microscopies. The effects of pulse length on signal strength and breakdown are considered. Different contrast mechanisms, including use of nanoparticles, are discussed. Two new promising techniques that can be applied to multiphoton microscopy are described.

The effect of ultrasound pulse length on microbubble cavitation induced antibody accumulation and distribution in a mouse model of breast cancer

In solid tumors, the limited diffusion of therapeutic molecules in the perivascular space is a known limitation impacting treatment efficacy. Ultrasound Targeted Microbubble Cavitation (UTMC) has been shown to increase vascular permeability and improve the delivery of therapeutic compounds including small molecules, antibodies (mAb), nanoparticles and even cells, notably across the blood-brain-barrier (BBB). In this study, we hypothesized that UTMC could improve the accumulation and biodistribution of mAb targeting the adenosinergic pathway (i.e. CD73) in mice bearing bilateral subcutaneous 4T1 mammary carcinoma. METHODS: A bolus of fluorescently labeled mAb was given intravenously, followed by a slow infusion of microbubbles. UTMC therapy (1 MHz, 850 kPa) was given under ultrasound image guidance for 5 minutes to the right side tumor only, using three different pulse lengths with identical ultrasound energy (5000cyc “long”, 125x40cyc “mid” and 500x10cyc “short”), and leaving the left tumor as a paired control. Longitudinal accumulation at 0 h, 4 h and 24 h was measured using whole-body biofluorescence and confocal microscopy. RESULTS: Our data support an increase in antibody accumulation and extravasation (# extravasated vessels and extravasated signal intensity) at 0 h for all pulses and at 4 h for the mid and short pulses when compared to the control non treated side. However, this difference was not found at 24 h post UTMC, indicative of the transient nature of UTMC. Interestingly, confocal data supported that the highest extravasation range was obtained at 0 h with the long pulse and that the short pulse caused no increase in the extravasation range. Overall, the mid pulse was the only pulse to increase all our metrics (biofluorescence, fraction of extravasated vessels, amount of extravasated Ab, and extravasation range) at 0 h and 4 h time points. CONCLUSIONS: Our results support that UTMC can enhance antibody accumulation in solid tumors at the macroscopic and microscopic levels. This preferential accumulation was evident at early time points (0 h and 4 h) but had started to fade by 24 h, a time dependence that is consistent with the ultrasound blood brain barrier opening literature. Further development and optimization of this theranostic platform, such as repeated UTMC, could help improve antibody based therapies against solid cancer.