Longer Pulse Duration Research Articles

Generation and detection of mode-locked spin coherence in (In,Ga)As/GaAs quantum dots by laser pulses of long duration

Using optical pulses of variable duration up to 80 ps, we report on spin coherence initialization and its subsequent detection in n-type singly-charged quantum dots, subject to a transverse magnetic field, by pump-probe techniques. We demonstrate experimentally and theoretically that the spin coherence generation and readout efficiencies are determined by the ratio of laser pulse duration to spin precession period: An increasing magnetic field suppresses the spin coherence signals for a fixed duration of pump and/or probe pulses, and this suppression occurs for smaller fields the longer the pulse duration is. The reason for suppression is the varying spin orientation due to precession during pulse action.

KrF laser ablation of a polyethersulfone film: Effect of pulse duration on structure formation

Polyethersulfone (PES) films were processed with KrF laser irradiation of different pulse durations ( τ). Scanning electron microscopy (SEM) and Raman spectroscopy were employed for the examination of the morphology and chemical composition of the irradiated surfaces, respectively. During ablation with 500 fs and 5 ps pulses, localized deformations (beads), micro-ripple and conical structures were observed on the surface depending on the irradiation fluence ( F) and the number of pulses ( N). In addition, the number density of the structures is affected by the irradiation parameters ( τ, F, N). Furthermore, at longer pulse durations ( τ = 30 ns), conical structures appear at lower laser fluence values, which are converted into columnar structures upon irradiation at higher fluences. The Raman spectra collected from the top of the structures following irradiation at different pulse durations revealed graphitization of the ns laser treated areas, in contrast to those processed with ultra-short laser pulses.

Mechanisms of atrial-selective block of Na+ channels by ranolazine: I. Experimental analysis of the use-dependent block

Atrial-selective inhibition of cardiac Na(+) channel current (I(Na)) and I(Na)-dependent parameters has been shown to contribute to the safe and effective management of atrial fibrillation. The present study examined the basis for the atrial-selective actions of ranolazine. Whole cell I(Na) was recorded at 15°C in canine atrial and ventricular myocytes and in human embryonic kidney (HEK)-293 cells expressing SCN5A. Tonic block was negligible at holding potentials from -140 to -100 mV, suggesting minimal drug interactions with the closed state. Trains of 40 pulses were elicited over a range of holding potentials to determine use-dependent block. Guarded receptor formalism was used to analyze the development of block during pulse trains. Use-dependent block by ranolazine increased at more depolarized holding potentials, consistent with an interaction of the drug with either preopen or inactivated states, but was unaffected by longer pulse durations between 5 and 200 ms, suggesting a weak interaction with the inactivated state. Block was significantly increased at shorter diastolic intervals between 20 and 200 ms. Responses in atrial and ventricular myocytes and in HEK-293 cells displayed a similar pattern. Ranolazine is an open state blocker that unbinds from closed Na(+) channels unusually fast but is trapped in the inactivated state. Kinetic rates of ranolazine interactions with different states of atrial and ventricular Na(+) channels were similar. Our data suggest that the atrial selectivity of ranolazine is due to a more negative steady-state inactivation curve, less negative resting membrane potential, and shorter diastolic intervals in atrial cells compared with ventricular cells at rapid rates.

Observation of craters formed by single pulse discharge by stacking cross sectional shapes: comparison of craters in liquid and air

This paper describes the results of comparing craters formed by single pulse discharge in liquid and in air at comparatively short pulse durations. Investigations were conducted by stacking cross-sectional shapes that the authors developed to clarify the volumes of resolidified metal and molten metal at the crater. In liquid, most of the resolidified layer remained at the bottom of the crater or in its vicinity. In air, the resolidified layer existed only at the downstream side of the high-speed gas flow. The removal ratio was found to be minus in liquid as most of the molten material remained on the crater. In air, more than 70 per cent of the molten material was removed at pulse durations above 2.5 µs. Thermal analysis results indicate that about 26 per cent of the discharge energy was used for melting the workpiece regardless of the pulse duration in liquid. In air, the discharge energy was found to decrease with longer pulse durations (i.e. from 20 to 10 per cent for 2.5 to 2.8 µs) due to the decrease in the heat flux in the workpiece as a result of the arc column being moved at high speed by the high-speed gas flow in the discharge gap.

Discrete laser spot transformation hardening of AISI O1 tool steel using pulsed Nd:YAG laser

Discrete laser spot transformation hardening is a process that creates isolated laser hardening spots, usually distributed in a certain pattern, on a component surface, covering only a fraction of the surface region that is being treated. The process offers several unique advantages for tribological applications, including improved lubrication conditions and wear performance and increased productivity. However, very limited information is available on the appropriate selection of processing parameters to achieve optimal results. In this paper, discrete laser spot hardening of AISI O1 tool steel has been studied using a pulsed Nd:YAG laser. Effect of various laser processing parameters, including laser pulse energy, pulse duration and defocus distance, on characteristics of the laser treated spots are investigated. Maps are experimentally established for processing parameter selections in discrete laser spot transformation hardening of the AISI O1 tool steel. Results show that the maximum diameter and depth of transformation hardening zones with no surface melting increase with the increase of laser pulse energy. However, they are not markedly affected by laser pulse duration. On the other hand, longer pulse durations at a given pulse energy reduce the size of softening zone surrounding the central hardening zone and are thus more favourable for most practical applications. Short laser pulse durations below 8 ms tend to produce shallower hardening zones and are not recommended for wear applications.

Simulation of Heat Distribution and Thermal Damage Patterns of Diode Hair-Removal Lasers: An Applicable Method for Optimizing Treatment Parameters

We simulated the heat distribution and thermal damage patterns of diode hair-removal lasers for different spot sizes, pulse durations, and fluences as a guide for optimization. Recently, the concept of thermal damage time as a reference for pulse duration has become a subject of debate. Laser-Induced-Temperature-Calculation-In-Tissue (LITCIT) was used for the simulations. Skin was modeled as two homogenous layers of epidermis/dermis and two coaxial cylinders as the hair shaft/ follicle. Opto-thermal coefficients of the components and the radiant parameters of the laser (diode, 810 nm) were defined. At constant fluences and pulse durations, the damage occurred deeper when larger spot sizes were used. At constant pulse duration, high fluences caused significant damage to the hair follicle and epidermis. By using longer pulse durations (≤ 400 ms) at constant fluences, there was more effective damage to the hair follicle while sparing the adjacent epidermis and dermis. Because of the time-dependent temperature profiles, an increased pulse duration creates a moderate, gradual rise in the target's temperature. Pulse durations > 400 ms are accompanied by unwanted dermis damage. Our results show that using very long pulse durations near the tissue damage time (≤ 400 ms) creates better efficacy in treating unwanted hairs while avoiding unwanted damage.

Ultrafast demagnetization in ferromagnets and magnetic switching in nanoclusters when the number of photons is kept fixed

Different from thermally and magnetically induced demagnetization, the laser-induced demagnetization relies on the laser photon field. However, what has been unknown is how the spin moment reduction correlates with the number of photons. Here, our first-principles calculation in ferromagnetic nickel and cobalt clusters shows that the number of photons is not the sole decisive factor for the magnetization change, contrary to earlier belief. For the same number of photons absorbed, the shorter the laser pulse, the larger the induced spin moment reduction. Besides a simple decrease in the magnetic moment, a short pulse also excites a strong coherent spin oscillation, which disappears when using a longer pulse. The longest pulse duration where we observed this oscillation is about 20 fs. Future experiments can directly test our prediction. We show that for our generic ultrafast spin-switching Λ-process on metallic nanoclusters the electronic correlations constitute a key ingredient, which allows for spin and charge separation. By selectively removing correlational channels, we gradually inhibit the magnetic switching. The dynamics proceeds far from any electronic thermal equilibrium and thus no temperature can be attributed to the system or any subsystem.

Spatial and Spectral Imaging of Retinal Laser Photocoagulation Burns

To correlate in vivo spatial and spectral morphologic changes of short- to long-pulse 532 nm Nd:YAG retinal laser lesions using Fourier-domain optical coherence tomography (FD OCT), autofluorescence (AF), fluorescein angiography (FA), and multispectral imaging. Ten eyes with treatment-naive preproliferative or proliferative diabetic retinopathy were studied. A titration grid of laser burns at 20, 100, and 200 milliseconds was applied to the nasal retina and laser fluence titrated to produce four grades of laser lesion visibility: subvisible (SV), barely visible (BV, light-gray), threshold (TH, gray-white), and suprathreshold (ST, white). The AF, FA, FD-OCT, and multispectral imaging were performed 1 week before laser, and 1 hour, 4 weeks, and 3 and 6 months post-laser. Multispectral imaging measured relative tissue oxygen concentration. Laser burn visibility and lesion size increased in a linear relationship according to fixed fluence levels. At fixed pulse durations, there was a semilogarithmic increase in lesion size over 6 months. At 20 milliseconds, all grades of laser lesion were reduced significantly in size after 6 months: SV, 51%; BV, 54%; TH, 49%; and ST, 50% (P < 0.001), with retinal pigment epithelial proliferation and photoreceptor infilling. At 20 milliseconds, there was healing of photoreceptor inner segment/outer segment junction layers compared with 100- and 200-millisecond lesions. Significant increases in mean tissue oxygenation (range, four to six units) within the laser titration area and in oxygen concentration across the laser lesions (P < 0.01) were detected at 6 months. For patients undergoing therapeutic laser, there may be improved tissue oxygenation, higher predictability of burn morphology, and more spatial localization of healing responses of burns at 20 milliseconds compared with longer pulse durations over time.

Comparison of Diode Lasers in Soft Tissue Surgery using CW and Superpulsed Mode: An in vivo Study

In 1995, on the IDS, the first diode laser for dentistry was shown. It was an 810 nm diode laser with CW mode.133 Peak power was 6 W. In 2000, a short pulsed diode laser with 810 nm, 20 W, 50 μs pulse and an average power of 2, 0-6, 7 W was introduced which showed a faster cutting and only 50% of the necrosis zone of an CW mode diode laser.154 In 2002, the peak power reached 30 W with a pulse duration of 9 μs, a frequency of 20000 Hz and a limited average power of 10 W. Today frequencies up to 30000 Hz and peak powers of 50 W are possible. To gain a sufficient result, the peak power has to be more than 8 W in this case.141 During the first-year of diode laser treatment in dentistry, only CW mode was possible. Several studies at that time showed that CW mode and 1 W was enough to reach a bactericidal effect on and in roots and root canals so as on implant surfaces.13-16,51,88,106,118,120,154 Application of 3 to 4 W in CW mode led very fast to carbonization of the soft tissue; the carbonization caused higher absorption followed by a heavy thermal damage and necrosis of the tissue.71,148,149,154 To approach better results in soft tissue treatment without much carbonization, it was necessary to interrupt the CW mode. That was done by chopping the CW mode. Pulses down to several 100 μs were realized.141,97,145 The peak power of the pulses was in fact not higher than the peak power of the CW mode pulse, but the applied dose was decreased and the carbonization and thermal damage were reduced. A further progress were pulse durations of 9 μs, a frequency of 20000 Hz and a peak power of 50 W, which could only be generated by DPL (digital pulse technique). Using these parameters could generate a fast, sharply edged cut nearly without thermal damage of the surrounding tissue and only minimal carbonization at the cutting edges. Several studies in the recently past years have shown that short pulses with high peak powers could generate a much better result than lower powers with longer pulse duration.17,24,26,34,35,109,151,152 Therefore, it is a major task of this study to answer the question if highly pulsed diode lasers are more suitable for soft tissue surgery than CW mode diode lasers and how the achievable results differ.

Left Ventricular Pacing with Long Pulse Duration Can Avoid Phrenic Nerve Stimulation

Background: One of the major limitations of left ventricular pacing is phrenic nerve stimulation. We evaluated the characteristics of the strength-duration curves of the left phrenic nerve (PN) and epicardium of the left ventricle (LV). Methods: A total of 12 mongrel dogs were studied. Capture threshold was assessed during pacing with 180 bpm and various pulse durations (0.25–2.0 ms) in an open chest model. A commercially available LV lead was inserted through a small hole created in the pericardium. The tip electrode position was maintained at the course of the left PN with gentle manual pressure under visual inspection. The LV threshold was evaluated at the epicardium just below the PN. Results: The strength-duration curve of the PN had a higher rheobase and shorter chronaxie than that of the LV. The LV threshold was higher than that of the PN at pulse duration <1.0 ms in bipolar pacing, but significantly lower than that of the PN at a pulse duration of 2.0 ms, especially during unipolar pacing (1.3±0.21 vs. 2.0±0.56 V, LV vs. PN, p=0.012). Conclusion: LV pacing without PN stimulation could be achieved in pacing with long pulse duration. The longer pulse duration option can be considered in the development of devices for LV pacing such as cardiac resynchronization therapy.

Collisional lasing on a self-terminating transition of a helium atom

Laser on a self-contained transition of a helium atom is studied under excitation of the helium mixture with molecular gases by single long-duration (up to 700ns) or double nanosecond pulses. In He — H2O and He — NH3 mixtures, no limitations were found on the pulse repetition rate and the laser pulse duration obtained was equal to that of the pump pulse.

Pulse duration dependent nonlinear propagation of a focused femtosecond laser pulse in fused silica

The nonlinear propagation of a single focused femtosecond laser pulse in fused silica has been investigated both experimentally and by numerical simulations. In particular, the filamentation behavior was systematically studied by varying pulse duration. At low pulse energy, the peak plasma density inside the filament first increases to a maximum value with increasing pulse duration and then begins to decrease. At relatively high pulse energy, denser plasma can be induced around the geometrical focus with a certain longer pulse duration, where the peak power is already below the self-focusing critical power and no filament is formed. This pulse duration dependent behavior can be explained by different ionization mechanisms.

Threshold Suprachoroidal-Transretinal Stimulation Current Required by Different-Size Electrodes in Rabbit Eyes

Aims: To evaluate the threshold currents required by different-size electrodes under various stimulus parameters by suprachoroidal-transretinal stimulation (STS) in rabbit eyes. Methods: An electrode array with 4 differently sized electrode contacts (50, 150, 350 and 500 µm in diameter) was implanted into the suprachoroidal space of the rabbit eyes. The electrically evoked potential was recorded extradurally on the visual cortex, and the threshold was measured for each size electrode by a biphasic current pulse at the pulse durations of 0.25, 0.5, 0.75 and 1.0 ms. Stimuli were applied at 1, 2 and 4 Hz. Results: The current thresholds with the 50-µm electrode were 80 ± 21.2 and 41.4 ± 6.5 µA for 0.25- and 1.0-ms pulses, respectively. That obviously exceeded the limits for safe stimulation with platinum. Compared to 350- and 500-µm electrodes, a lower threshold can be obtained by the 150-µm electrode just at 1 Hz. And the most significant change in the threshold as the stimulus frequency increased to 4 Hz was obtained by the 150-µm electrode with short-duration pulse (0.25 ms). Conclusion: The large surface electrode with long-duration pulses may be preferable for the STS prosthesis. And stimuli with low frequency are suggested to be the safer choice.

THE TEMPORAL AND SPECTRAL CHARACTERISTICS OF “FAST RISE AND EXPONENTIAL DECAY” GAMMA-RAY BURST PULSES

In this paper, we have analyzed the temporal and spectral behavior of 52 fast rise and exponential decay (FRED) pulses in 48 long-duration gamma-ray bursts (GRBs) observed by the CGRO/BATSE, using a pulse model with two shape parameters and the Band model with three shape parameters, respectively. It is found that these FRED pulses are distinguished both temporally and spectrally from those in the long-lag pulses. In contrast to the long-lag pulses, only one parameter pair indicates an evident correlation among the five parameters, which suggests that at least four parameters are needed to model burst temporal and spectral behavior. In addition, our studies reveal that these FRED pulses have the following correlated properties: (1) long-duration pulses have harder spectra and are less luminous than short-duration pulses and (2) the more asymmetric the pulses are, the steeper are the evolutionary curves of the peak energy (Ep) in the νfν spectrum within the pulse decay phase. Our statistical results give some constraints on the current GRB models.

Passively Q-switched Yb^3+:NaY(WO_4)_2 laser with GaAs saturable absorber

Passively Q-switched Yb(3+):NaY(WO(4))(2) lasers have been demonstrated using a GaAs saturable absorber. Under continuous wave pump mode, significant pulse shortening effects have been observed at high pump powers. At a pump power of 12 W, stable Q-switched output has been obtained with a pulse duration of 5 ns, pulse repetition rate of 83 kHz and a pulse to pulse timing jitter of less than 2%. With pulsed pump mode, much longer pulse duration and reduced pulse stability have been observed. It is proposed that the heating of GaAs may play an important role in the Q-switched operations under CW pump conditions.

Alloying elemental change of SS-316 and Al-5754 during laser welding using real time laser induced breakdown spectroscopy (LIBS) accompanied by EDX and PIXE microanalysis

Experimental studies of pulsed laser welding of stainless steel 316 in keyhole mode was done to examine a vaporization model based on the kinetic theory of gases and the thermodynamic laws. A long pulsed Nd:YAG laser with variable duration of 1–12 ms and 9–17 Gw/cm2 was employed. The undesirable loss of volatile elements affects on the weld metal compositions and the alloy properties. The model predicts that the loss of alloying elements strongly takes place at higher peak powers and longer pulse durations. On the other hand, the model shows the rapid migration of Mn and Cr based on the pressure and concentration gradients from the molten pool. Accordingly, the concentrations of iron, chromium, nickel and manganese were determined in the weld pool by means of the energy dispersive x-ray analysis (EDX) and proton induced X ray characteristics (PIXE) microanalysis. The change of weld metal composition of aluminium alloy 5754 in keyhole mode laser welding, was investigated using the model and was supported by the successive measurements. The model predicts that the concentration of magnesium in the weld metal decreases, while the aluminium concentration increases. Moreover, the real time concentrations of aluminium and magnesium elements in the weld metal were determined by laser induced breakdown spectroscopy (LIBS) at different conditions. We conclude that variation of the Al to Mg concentration ratio is negligible with various laser power densities while it is strongly correlated to the pulse duration.

Real-Time Data Acquisition System for Control and Long-Pulse Operation in Tore Supra

The tokamak Tore Supra was initially designed to operate long pulses, though the control and data acquisition systems nevertheless had to be upgraded to effectively perform these long-duration pulses. Continuous data acquisition was required for controlling the thermal phenomena, and huge amounts of data had to be handled, especially with the development of video imaging for the safety of the actively cooled vessel. A distributed architecture had to be developed so that real-time feedback could be implemented to ensure safe operation and to optimize the long-duration plasma performance. This paper briefly describes the initial control and data acquisition system and then reviews the developments and enhancements that were made to perform long pulses in Tore Supra.

Pulse-echo interaction in free-flying horseshoe bats, Rhinolophus ferrumequinum nippon

Because horseshoe bats emit a long-duration pulse, the returning echo temporally overlaps with the emitted pulse during echolocation. Here, the pulse-echo interaction that horseshoe bats actually experience during flight was examined using onboard telemetry sound recordings. Doppler-shifted returning echoes produced beats in the amplitude patterns of constant-frequency components. Bats shortened the pulse duration with target distance, but the overlap duration was at least 8 ms within the approach phase. The computations suggest that the phase difference in slowly amplitude-modulated sound (the beat signal) provides a useful cue for target localization.

Nucleic Acids Electrotransfer-Based Gene Therapy (Electrogenetherapy): Past, Current, and Future

About 25 years after the publication of the first report on gene transfer in vitro in cultured cells by the means of electric pulses delivery, reversible cell electroporation for gene transfer and gene therapy (DNA electrotransfer) is at a cross in its development. Present knowledge on the effects of cell exposure to appropriate electric field pulses, particularly at the level of the cell membrane, is reported here. The importance of the models of electric field distribution in tissues and of the correct choice of electrodes and applied voltages is highlighted. The mechanisms involved in DNA electrotransfer, which include cell electropermeabilization and DNA electrophoresis, are also surveyed. This knowledge has allowed developing new nucleic acids electrotransfer conditions using combinations of permeabilizing pulses of high voltage and short duration, and of electrophoretic pulses of low voltage and long duration, which are very efficient and safer. Feasibility of electric pulses delivery for gene transfer in humans is discussed taking into account that electric pulses delivery is already regularly used for localized drug delivery in the treatment of cutaneous and subcutaneous solid tumors by electrochemotherapy. Because recent technological developments made DNA electrotransfer more and more efficient and safer, this non-viral gene therapy approach is now ready to reach the clinical stage. A good understanding of DNA electrotransfer principles and the respect of safe procedures will be key elements for a successful future transfer DNA electrotransfer into the clinics.

Investigation of stimulated Raman scattering using a short-pulse diffraction limited laser beam near the instability threshold

AbstractShort pulse laser plasma interaction experiments using diffraction limited beams provide an excellent platform to investigate the fundamental physics of stimulated Raman scattering. Detailed understanding of these laser plasma instabilities impacts the current inertial confinement fusion ignition designs and could potentially impact fast ignition when higher energy lasers are used with longer pulse durations (&gt;1 kJ and &gt;1 ps). Using short laser pulses, experiments can be modeled over the entire interaction time of the laser using particle-in-cell codes to validate our understanding quantitatively. Experiments have been conducted at the Trident laser facility and the Laboratoire pour l'Utilisation des Lasers Intenses (LULI) to investigate stimulated Raman scattering near the threshold of the instability using 527 nm and 1059 nm laser light, respectively, with 1.5–3.0 ps pulses. In both experiments, the interaction beam was focused into pre-ionized helium gas-jet plasma. Measurements of the reflectivity as a function of intensity and kλD were completed at the Trident laser facility, where k is the electron plasma wave number and λD is the plasma Debye length. At LULI, a 300 fs Thomson scattering probe is used to directly measure the density fluctuations of the driven electron plasma and ion acoustic waves. Work is currently underway comparing the results of the experiments with simulations using the VPIC particle-in-cell code. Details of the experimental results are presented in this manuscript.