Optimal Pulse Parameters Research Articles

Spatial and temporal interference during the ionization of H by few-cycle XUV laser pulses

We have investigated the ionization of H atoms by few-cycle XUV laser pulses, in particular the interference between electronic wave packets emitted at different time moments (temporal interference), and the interference between wave packets emitted at the same time, but following different paths (spatial interference). During the spatial interference, under appropriate conditions, the holographic mapping (HM) of the target atom's state is achieved, which has the potential to become a powerful tool to study atomic structure. First, by using classical trajectory Monte Carlo calculations, we have confirmed the existence of two distinct types of electron trajectories, which contribute to the formation of the HM interference pattern. Then, by using ab initio quantum-mechanical calculations, we have studied how the shape of the HM pattern is influenced by the laser pulse parameters. Finally, we have identified the optimal laser pulse parameters for the observation of the HM interference in the case of atomic species with an ionization potential close to 0.5 a.u.

Chirped laser excitation of quantum dot excitons coupled to a phonon bath

Motivated by recent experimental results, we present a detailed account of adiabatic population transfer in InGaAs/GaAs quantum dots by strong chirped laser pulses. Specifically, numerical simulations in very good agreement with experimental results allow for a clear physical picture of this process. An analytical expression is developed, which is used to analyze the strong-field adiabatic population transfer in the presence of the phonon environment. Optimal pulse parameters to achieve robust population transfer at temperatures as high as 80 K are identified and are confirmed by numerical simulations.

Optimisation des paramètres fréquentiels du signal d’émission appliquée à l’imagerie de contraste ultrasonore du second harmonique

Many ultrasound contrast-imaging techniques use the nonlinear behavior of contrast agents to enhance the contrast of medical images. They are based on the generation of harmonic frequencies when microbubbles are insonified by ultrasound waves. The frequential parameters of the transmitted ultrasound signal maximize the harmonic power of microbubbles. We propose an adaptive method, which seeks automatically the optimal pulse parameters. These parameters allow us to maximise an energetic cost-function. The transmitted signal is composed of two different half-sines truncated, instead of a sinus wave composed of two identical half-sines truncated. With the used simulation model, the method can gives us a gain of 1,3 dB compared with the non-optimized system. Such results are interesting and encourage us to continue in this way.

Enhancement of cavitation activity and particle removal with pulsed high frequency ultrasound and supersaturation

Megasonic cleaning as applied in leading edge semiconductor device manufacturing strongly relies on the phenomenon of acoustic cavitation. As the occurrence of acoustic cavitation is incorporating a multitude of interdependent effects, the amount of cavitation activity in the cleaning liquid strongly depends on the sonication conditions. It is shown that cavitation activity as measured by means of ultraharmonic cavitation noise can be significantly enhanced when pulsed sonication is applied to a gas supersaturated liquid under traveling wave conditions. It is demonstrated that this enhancement coincides with a dramatic increase in particle removal and is therefore of great interest for megasonic cleaning applications. It is demonstrated that the optimal pulse parameters are determined by the dissolution time of the active bubbles, whereas the amount of cavitation activity depends on the ratio between pulse-off and pulse-on time as well as the applied acoustic power. The optimal pulse-off time is independent of the corresponding pulse-on time but increases significantly with increasing gas concentration. We show that on the other hand, supersaturation is needed to enable acoustic cavitation at aforementioned conditions, but has to be kept below values, for which active bubbles cannot dissolve anymore and are therefore lost during subsequent pulses. For the applicable range of gas contents between 100% and 130% saturation, the optimal pulse-off time reaches values between 150 and 340ms, respectively. Full particle removal of 78nm-diameter silica particles at a power density of 0.67W/cm2 is obtained for the optimal pulse-off times. The optimal pulse-off time values are derived from the dissolution time of bubbles with a radius of 3.3μm and verified experimentally. The bubble radius used in the calculations corresponds to the linear resonance size in a 928kHz sound field, which demonstrates that the recycling of active bubbles is the main enhancement mechanism. The optimal choice of the pulsing conditions however is constrained by the trade-off between the effective sonication time and the desire to have a sufficient amount of active bubbles at lower powers, which might be necessary if very delicate structures have to be cleaned.

Parameter Optimization for Quantitative Signal-Concentration Mapping Using Spoiled Gradient Echo MRI

Rationale and Objectives. Accurate signal to tracer concentration maps are critical to quantitative MRI. The purpose of this study was to evaluate and optimize spoiled gradient echo (SPGR) MR sequences for the use of gadolinium (Gd-DTPA) as a kinetic tracer. Methods. Water-gadolinium phantoms were constructed for a physiologic range of gadolinium concentrations. Observed and calculated SPGR signal to concentration curves were generated. Using a percentage error determination, optimal pulse parameters for signal to concentration mapping were obtained. Results. The accuracy of the SPGR equation is a function of the chosen MR pulse parameters, particularly the time to repetition (TR) and the flip angle (FA). At all experimental values of TR, increasing FA decreases the ratio between observed and calculated signals. Conversely, for a constant FA, increasing TR increases this ratio. Using optimized pulse parameter sets, it is possible to achieve excellent accuracy (approximately 5%) over a physiologic range of concentration tracer concentrations. Conclusion. Optimal pulse parameter sets exist and their use is essential for deriving accurate signal to concentration curves in quantitative MRI.

Polarization-selective vortex-core switching by tailored orthogonal Gaussian-pulse currents

We experimentally demonstrate low-power-consumption vortex-core switching in magnetic nanodisks using tailored rotating magnetic fields that are produced with orthogonal and unipolar Gaussian-pulse currents. Optimal width of the orthogonal pulses and their time delay are found to be determined only by the angular eigenfrequency {\omega}_D for a given vortex-state disk of its polarization p, such that {\sigma} = 1/{\omega}_D and {\Delta}t = {\pi}p/2{\omega}_D, as studied from analytical and micromagnetic numerical calculations. The estimated optimal pulse parameters are in good agreements with the experimentally found results. This work provides a foundation for energy-efficient information recording in vortex-core cross-point architecture.

Optimized nanosecond pulsed electric field therapy can cause murine malignant melanomas to self‐destruct with a single treatment

We have identified a new, nanosecond pulsed electric field (nsPEF) therapy capable of eliminating murine melanomas located in the skin with a single treatment. When these optimized parameters are used, nsPEFs initiate apoptosis without hyperthermia. We have developed new suction electrodes that are compatible with human skin and have applied them to a xenograft nude mouse melanoma model system to identify the optimal field strength, pulse frequency and pulse number for the treatment of murine melanomas. A single treatment using the optimal pulse parameters (2,000 pulses, 100 ns in duration, 30 kV/cm in amplitude at a pulse frequency of 5-7 pulses/sec) eliminated all 17 melanomas treated with those parameters in 4 mice. This was the highest pulse frequency that we could use without raising the treated skin tumor temperature above 40 degrees C. We also demonstrate that the effects of nsPEF therapy are highly localized to only cells located between electrodes and results in very little scarring of the nsPEF-treated skin.

Optimization of RF excitation to maximize signal and T2 contrast of tissues with rapid transverse relaxation

Ultrashort echo time MRI requires specialized pulse sequences to overcome the short T(2) of the MR signal encountered in tissues such as ligaments, tendon, or cortical bone. Theoretical work is presented, supported by simulations and experimental data on optimizing the radiofrequency excitation to maximize signal-to-noise ratio and contrast-to-noise ratio. The theoretical calculations and simulations are based on the classic Bloch equations and lead to a closed form expression for the optimal radiofrequency pulse parameters to maximize the MR signal in the presence of rapid T(2) decay. In the steady state, the spoiled gradient recalled echo signal amplitude in response to the radiofrequency excitation pulses is not maximized by the classic Ernst angle but by a more general criterion we call "generalized Ernst angle." Finally, it is shown that T(2) contrast is maximized by flipping the magnetization at the Ernst angle with a radiofrequency pulse duration proportional to the targeted T(2). Experimental studies on short T(2) phantoms confirm these optimization criteria for both signal-to-noise ratio and contrast-to-noise ratio.

Characterization of retinal ganglion cell activities evoked by temporally patterned electrical stimulation for the development of stimulus encoding strategies for retinal implants

For successful restoration of visual function by retinal implant, a method for electrical stimulation should be devised so that the evoked activities of retinal ganglion cells (RGCs) should convey sufficient information on visual input. By observing RGC activities under different stimulation constraints, it may be possible to determine optimal pulse parameters, such as pulse rate, intensity, and duration, for faithful transmission of visual information. To test the feasibility of this approach, we analyzed RGC spike trains evoked by temporally patterned stimulation from retinal patches mounted on a planar multielectrode array. Assuming that the intensity of uniform visual input is transformed to amplitudes of pulse trains, we attempted to determine optimal methods for modulating the pulse amplitude so that the information essential for the perception of intensity variation is properly represented in RGC responses. RGC firing rates could be modulated to track the temporal pattern of pulse amplitude variations, which implies that pulse amplitude modulation is a plausible means to enable perception of temporal visual patterns by retinal implants. As expected, specific pulse amplitude modulation parameters were crucial for proper encoding of visual input. RGC firing rates increased monotonically according to the pulse amplitude in a defined pulse amplitude range (20–60 μA). The similarity between the RGC firing rate and the temporal pulse intensity pattern was highest when the pulse amplitude was modulated within this range. The optimal pulse rate range could be similarly determined.

Nanoscale colour control: W–O graded coatings deposited by magnetron sputtering

A new design of decorative tungsten oxide coatings is presented. The coatings weredeposited with a graded refractive index by magnetron sputtering from a tungsten targetand pulsing the reactive gas. The controlled injection of the reactive gas can produce aconcentration profile gradient from pure tungsten to tungsten trioxide, determining thefinal apparent colour of the coating. A dynamic sputtering model was built to simulatethe growth of the coating during the reactive gas pulsing which was validatedby direct measurement of the gradient of the oxygen content in the depositedcoatings. Finally, these results were used for an optical model allowing the opticalproperties of the deposited tungsten oxide layer to be described, again validated byexperimental analysis. This procedure allows the deposition of coatings with the desiredcolour by using the models to finding the optimal oxygen pulse parameters. Thisproposed method can be easily applied to almost any metal/metal oxide system.

Transthoracic impedance study with large self-adhesive electrodes in two conventional positions for defibrillation

External defibrillation requires the application of high voltage electrical impulses via large external electrodes, placed on selected locations on the thorax surface. The position of the electrodes is one of the major determinants of the transthoracic impedance (TTI) which influences the intracardiac current flow during electric shock and defibrillation success. The variety of factors which influence TTI measurements raised our interest to investigate the range of TTI values and the temporal TTI variance during long-term application of defibrillation self-adhesive electrodes in two conventional positions on the patient's chest—position 1 (sub-clavicular/sub-axillar position) and position 2 (antero-posterior position). The prospective study included 86 randomly selected volunteers (39 male and 49 female, 67 patients with normal skin, 13 patients with dry skin and 6 patients with greasy skin, 16 patients with chest pilosity and 70 patients without chest pilosity). The TTI was measured according to the interelectrode voltage drop obtained by passage of a low-amplitude high-frequency current (32 kHz) between the two self-adhesive electrodes (active area about 92 cm2). For each patient, the TTI values were measured within 10 s, 1 min and 5 min after sticking the electrodes to the skin surface, independently for the two tested electrode positions. We found that the expected TTI range is between 58 Ω and 152 Ω for position 1 and between 55 Ω and 149 Ω for position 2. Although the two TTI ranges are comparable, we measured significantly higher TTI mean of about (107.2 ± 22.3) Ω for position 1 compared to (96.6 ± 19.2) Ω for position 2 (p = 0.001). This fact suggested that the antero-posterior position of the electrodes is favourable for defibrillation. Within the investigated time interval of 5 min, we observed a significant TTI reduction with about 6.9% (7.4 Ω/107.2 Ω) for position 1 and about 5.3% (5.1 Ω/96.6 Ω) for position 2. We suppose that the long-term application of self-adhesive electrodes would lead to improvement of the physical conditions for conduction of the defibrillation current and to diminution of energy loss in the electrode–skin contact impedance. We found that gender is important when position 1 is used because women have significantly higher TTI (111 ± 20.3) Ω compared to the TTI of men (102.6 ± 24) Ω (p = 0.0442). Although we found some specifics of the electrode–skin contact layer, we can conclude that because of the insignificant differences in TTI, the operator of the defibrillator paddles does not need to take into consideration the skin type and pilosity of the patients. Analysis of the correlations between TTI and the individual patient characteristics (chest size, weight, height, age) showed that these patient characteristics are unreliable factors for prediction of the TTI values and optimal defibrillation pulse parameters and energy.

Vibrational state-selectivity of product HI in photoassociation reaction I + H → HI

In the photoassociation reaction I + H → HI induced by the ultra-short laser pulse, the two collision atoms, I and H, can combine into a molecule with a selective vibrational state. The state-selective population of the product molecule HI depends on the laser pulse parameters and the relative collision momentum of the two collision atoms. The theoretical calculations show that the optimal laser pulse parameters, such as the duration, intensity and frequency, can be used to control the vibrational state-selective population of the product molecule.

Pulsed Electron Avalanche Knife (PEAK-fc) for Dissection of Retinal Tissue

To evaluate the effectiveness and precision of tractionless retinal tissue dissection by the advanced version of the pulsed electron avalanche knife for fine cutting (PEAK-fc; Carl Zeiss Meditec, Jena, Germany). Porcine retina (in vivo) and human retina (in vitro) were incised with the PEAK-fc using various pulse parameters. The globes were then processed for light microscopy. Evaluation of all specimens focused on depth of the retinal cuts and on the degree of collateral damage. Retinal cuts performed both in vivo on porcine eyes and on human donor eyes showed very sharp edges with only little collateral damage. With probes of 600 mum in length, the optimal pulse parameters for precise and reproducible cutting of the retina were an amplitude of 350 to 380 V, a repetition rate of 300 Hz, and 30 "minipulses" per pulse of 100-microsecond duration. With increasing voltage, cuts also affected the retinal pigment epithelium and the choroid, followed by intravitreal bleeding during in vivo application. We demonstrated that PEAK-fc is capable of precisely cutting retinal tissue in vivo and in vitro using optimal pulse parameters. Further in vivo studies will be necessary to determine the efficacy of this new tractionless cutting device in vitreoretinal surgery.

In vivo 5-fluorouracil and fluoronucleotide T1 relaxation time measurements using the variable nutation angle method.

19Fluorine NMRS has the potential to enable noninvasive predictions of tumor response to 5-fluorouracil (5FU) therapy based on tumor pharmacokinetics. Knowledge of the T1's of 5FU and its fluoronucleotide anabolites (FNuc) is required for quantitative spectral analysis and selection of optimal pulse parameters. We used the variable nutation angle (VNA) method to determine T1's of 5FU and FNuc in subcutaneous Walker 256 rat mammary carcinosarcoma tumors transfected with a cytosine deaminase/uracil phosphoribosyltransferase fusion gene. We calibrated in vivo NAs using methoxydifluoroacetate to ensure the accuracy of these measurements. The T1's were calculated based on signal intensities acquired with NAs of 20 degrees, 35 degrees, 45 degrees, 60 degrees, and 75 degrees. The acquisition order of these NAs was shuffled to reduce the effect of signal variations. The determined T1's for 5FU and FNuc (2.3 +/- 0.1 s and 1.3 +/- 0.1 s, respectively) represent the first reported in vivo measurements for these metabolites in tumor.

Molecular isomerization induced by ultrashort infrared pulses. I. Few-cycle to sub-one-cycle Gaussian pulses and the role of the carrier-envelope phase.

Using 550 previously calculated vibrational energy levels and dipole moments we performed simulations of the HCN-->HNC isomerization dynamics induced by sub-one-cycle and few-cycle IR pulses, which we represent as Gaussian pulses with 0.25-2 optical cycles in the pulse width. Starting from vibrationally pre-excited states, isomerization probabilities of up to 50% are obtained for optimized pulses. With decreasing number of optical cycles a strong dependence on the carrier-envelope phase (CEP) emerges. Although the optimized pulse parameters change significantly with the number of optical cycles, the distortion by the Gaussian envelope produces nearly equal fields, with a positive lobe followed by a negative one. The positions and areas of the lobes are also almost unchanged, irrespective of the number of cycles in the half-width. Isomerization proceeds via a pump-dumplike mechanism induced by the sequential lobes. The first lobe prepares a wave packet incorporating many delocalized states above the barrier. It is the motion of this wave packet across the barrier, which determines the timing of the pump and dump lobes. The role of the pulse parameters, and in particular of the CEP, is to produce the correct lobe sequence, size and timing within a continuous pulse.

Optimized Pulse Parameters for Reducing Quantitation Errors Due to Saturation Factor Changes in Magnetic Resonance Spectroscopy

We present an analysis of the effects of chemical exchange and changes in T 1 on metabolite quantitation for heart, skeletal muscle, and brain using the one-pulse experiment for a sample which is subject to temporal variation. We use an optimization algorithm to calculate interpulse delay times, TRs, and flip angles, θ, resulting in maximal root-mean-squared signal-to-noise per unit time ( S/ N) for all exchanging species under 5 and 10% constraints on quantitation errors. The optimization yields TR and θ pairs giving signal-to-noise per unit time close or superior to typical literature values. Additional simulations were performed to demonstrate explicitly the dependence of the quantitation errors on pulse parameters and variations in the properties of the sample, such as may occur after an intervention. We find that (i) correction for partial saturation in accordance with the usual analysis neglecting variations in metabolite concentrations and rate constants may readily result in quantitation errors of 15% or more; the exact degree of error depends upon the details of the system under consideration; (ii) if T 1's vary as well, significantly larger quantitation errors may occur; and (iii) optimal values of pulse parameters may minimize errors in quantitation with minimal S/ N loss.

Pulse-driven switching in one-dimensional nonlinear photonic band gap materials: a numerical study

We numerically examine the time-dependent properties of nonlinear bistable multilayer structures for constant wave illumination. We find that our system exhibits both steady-state and self-pulsing solutions. In the steady-state regime, we examine the dynamics of driving the system between different transmission states by injecting pulses, and we find optimal pulse parameters. We repeat this work for the case of a linear periodic system with a nonlinear impurity layer.

Highly efficient electro-gene therapy of solid tumor by using an expression plasmid for the herpes simplex virus thymidine kinase gene

We report successful electro-gene therapy (EGT) by using plasmid DNA for tumor-bearing mice. Subcutaneously inoculated CT26 tumor was subjected to EGT, which consists of intratumoral injection of a naked plasmid encoding a marker gene or a therapeutic gene, followed by in vivo electroporation (EP). When this treatment modality is carried out with the plasmid DNA for the green fluorescent protein gene, followed by in vivo EP with the optimized pulse parameters, numerous intensely bright green fluorescent signals appeared within the tumor. EGT, by using the "A" fragment of the diphtheria toxin gene significantly inhibited the growth of tumors, by about 30%, on the flank of mice. With the herpes simplex virus thymidine kinase gene, followed by systemic injection of ganciclovir, EGT was far more effective in retarding tumor growth, varying between 50% and 90%, compared with the other controls. Based on these results, it appears that EGT can be used successfully for treating murine solid tumors.

Feedback Optimization of Pulse Width in the SORC Sequence

A method for increasing the signal-to-noise ratio (SNR) of nuclear quadrupole resonance (NQR) measurements by automatically adjusting a pulse parameter in real-time is presented. This approach is useful in situations where the optimal pulse parameters cannot be chosen beforehand due to lack of knowledge regarding the system. For example, NQR provides a means for detecting explosives by revealing the presence of 14N. In this particular application, the distance between the search coil and the explosive, as well as the temperature of the explosive, is unknown. As a result, a fixed set of pulse parameters will not yield the largest SNR for all possible search applications. This paper describes a feedback algorithm that uses measurements of the NQR signal to automatically adjust the pulse width in the strong off-resonant comb sequence to maximize the SNR of the NQR measurement. Experimental results obtained using a sample of sodium nitrite are presented.

Selective detection of metal species in HPLC and FIA by means of pulsed amperometric detection (PAD).

Triple-step pulsed amperometry is used to detect selectively metal complex species after HPLC separation in the presence of coeluting matrix compounds. Optimized pulse parameters enable accurate and selective detection of species even in the presence of electroactive compounds such as phenols or sulfurcontaining peptides. Complexes of tin(IV) with rutin and of platinum(II) with glutathione are presented as examples for these classes of compounds. It is demonstrated that by changing the pulse waveform direct and indirect detection modes can be realized. The method is applied to detect platinum species in a grass extract and iron(II)-lactate in fruit juice. Furthermore it is shown, that pulsed amperometry can be used as a detector in FIA for determining the fraction of metal complex formed after mixing of metal and excess ligand without separation.