Single Pulse Case Research Articles

Extension of the cut-off in high-harmonic generation using two delayed pulses of the same colour

We demonstrate a novel technique for an extension of the cut-off frequency in high-harmonic generation using a shaped field, consisting of two delayed pulses, identical in all parameters except a carrier envelope phase shift of π. We obtain high harmonics up to an energy of 5.5Up + Ip, surpassing the conventional limit of 3.17Up + Ip. For high intensities approaching the threshold for barrier suppression ionization, beyond which no harmonics are generated, this mechanism acquires practical importance as a means to overcome the limit imposed by saturation. We analyse the underlying mechanism using both classical and quantum approaches, and highlight the role of the shaped field in modifying the electron trajectories in the continuum to enhance the collection of kinetic energy from the driving field. Of greatest importance are the trajectories describing electrons ionized across the first peak of the laser field which receive the enhanced kinetic energies responsible for the cut-off extension above the conventional limit. Interestingly, the most energetic trajectories do not contribute to the harmonic cut-off in the single pulse case. We conclude with a discussion of the sub-cycle structure of the emitted harmonic radiation and possible applications of the technique.

Double pulse laser-induced breakdown spectroscopy with femtosecond laser pulses

This paper presents results obtained in a study of collinear geometry double pulse femtosecond LIBS analysis of solids in ambient environment. LIBS signal enhancement of 3–10 fold, accompanied by significant improvement of signal reproducibility, in comparison with the single pulse case, has been found in different samples such as brass, iron, silicon, barium sulfate and aluminum when an optimum temporal separation between the two ablating pulses is used. The influence of the delay between pulses in the LIBS signal intensity was investigated and two intervals of interaction were established. A first transient regime from 0 to 50 ps, in which the LIBS signal increases until reaching a maximum, and a second regime that ranges from 50 to 1000 ps (maximum inter-pulse delay investigated) in which the signal enhancement remains constant. Emissions from both ionized and neutral atoms show the same pattern of enhancement with a clear tendency of lines arising from higher energy emissive states to exhibit higher enhancement factors.

Electron acceleration by amplified wakefield generated by two copropagating laser pulses in plasma

A detailed analytical theory is developed for studying the phenomenon of generation of high amplitude wakefield using a seed laser pulse and another coaxially propagating, trailing pulse. The two laser pulses are separated by a fixed distance between them and have the same polarization, frequency, length, intensity, and profile. The study shows that the maximum energy gained by the test electron significantly depends upon the distance between the two pulses. Phase space analysis shows that a test electron of significantly lower energy, injected behind the trailing laser pulse, will be trapped and accelerated to higher energy as compared to the single pulse case.

Analysis of Clipping Noise and Tone-Reservation Algorithms for Peak Reduction in OFDM Systems

Orthogonal frequency division multiplexing (OFDM) suffers from a high peak-to-average power ratio (PAR). Tone reservation is a popular PAR reduction technique that uses a set of reserved tones to design a peak-canceling signal. In a previous paper by Krongold and Jones, an active-set approach was developed to efficiently compute the peak-canceling signal. In this paper, we consider the use of clipping noise, which is generated when the OFDM signal is clipped at a predefined threshold, to design the peak-canceling signal. To this end, the clipping noise is analyzed as a series of parabolic pulses under tone-reservation constraints. The single-pulse case and the multiple-pulse case are treated. The analysis explains peak regrowth and the constancy of the clipping noise power spectrum over the whole OFDM band. Moreover, the clipping noise at the end of several clipping and filtering iterations is shown to be approximately proportional to that generated in the first iteration. The constant of proportionality is estimated via the level-crossing theory for high clipping thresholds. Using this analysis, a constant-scaling algorithm and an adaptive-scaling algorithm are proposed for tone reservation. These algorithms scale the filtered first-iteration clipping noise to compensate for peaks that are above the threshold. The simulation results show that the proposed algorithms achieve a larger peak reduction and lower complexity than the active-set algorithm.

Double pulse laser ablation and laser induced breakdown spectroscopy: A modeling investigation

A numerical model, describing laser–solid interaction (i.e., metal target heating, melting and vaporization), vapor plume expansion, plasma formation and laser–plasma interaction, is applied to describe the effects of double pulse (DP) laser ablation and laser induced breakdown spectroscopy (LIBS). Because the model is limited to plume expansion times in the order of (a few) 100 ns in order to produce realistic results, the interpulse delay times are varied between 10 and 100 ns, and the results are compared to the behavior of a single pulse (SP) with the same total energy. It is found that the surface temperature at the maximum is a bit lower in the DP configuration, because of the lower irradiance of one laser pulse, but it remains high during a longer time, because it rises again upon the second laser pulse. Consequently, the target remains for a longer time in the molten state, which suggests that laser ablation in the DP configuration might be more efficient, through the mechanism of splashing of the molten target. The total laser absorption in the plasma is also calculated to be clearly lower in the DP configuration, so that more laser energy can reach the target and give rise to laser ablation. Finally, it is observed that the plume expansion dynamics is characterized by two separate waves, the first one originating from the first laser pulse, and the second (higher) one as a result of the second laser pulse. Initially, the plasma temperature and electron density are somewhat lower than in the SP case, due to the lower energy of one laser pulse. However, they rise again upon the second laser pulse, and after 200 ns, they are therefore somewhat higher than in the SP case. This is especially true for the longer interpulse delay times, and it is expected that these trends will be continued for longer delay times in the μs-range, which are most typically used in DP LIBS, resulting in more intense emission intensities.

Measurement of Stark broadening of Mn I and Mn II spectral lines in plasmas used for Laser-Induced Breakdown Spectroscopy

We present measurements of the Stark broadening of several Mn lines in the conditions of typical laser-induced plasmas. Single-and double-pulse Laser-Induced Breakdown spectroscopy (LIBS) configurations are studied on a series of Fe–Mn alloy samples with Mn concentration ranging from 6% to 30%. The effects of self-absorption on the measured line broadenings are discussed in detail. In particular, the experimental results evidence that self-absorption is much higher in laser-induced plasmas generated with double pulses, compared to the case of single pulse. After measurement of the electron density, the Stark coefficients of several neutral and ionic Mn lines are derived through the measure of the broadening in conditions of optically thin plasma. The results obtained for singly ionized Mn lines are compared with the theoretical and experimental data present in the literature. For the first time, experimental measurements of the Stark coefficient for several neutral Mn lines are also presented.

Double pulse laser wakefield accelerator

Two-dimensional simulation studies are performed for modified laser wakefield acceleration. After one laser pulse, another identical laser pulse is sent to the plasma to amplify the wake wave resonantly. The simulation results show that the number of injected electrons is bigger than that of the single pulse case and the beam energy is higher as well. In addition, increase of the transverse amplitude is noticed in the wake wave after the second laser pulse. This shows that the transverse motion of the wake wave enhances the wave breaking for strong injection and acceleration of electron beams.

Multi-pulse laser-induced plasma spectroscopy using a single laser source and a compact spectrometer

A portable laser induced breakdown spectrometer (LIBS), built around a microscope using a Q-switched Nd:GGG laser releasing multiple pulses and a non-intensified fiber optic spectrometer, was used to investigate the signal improvement of emission data achievable by the multiple-pulse approach. LIBS experiments were carried out in air and using Al, Cu, Si and Zn metallic targets. It was found that the use of 7 laser pulses, separated by ca. 25 μs time intervals, enhance line emissions by a factor of 2.5 to 129 with respect to the single-pulse case, and also decrease the relative standard deviation of the signal based on five consecutive measurements to 2–5%. Time-resolved emission data and the observation of ablation craters suggest that the signal enhancement is mainly attributable to the reheating of the plasma by multiple laser pulses.

Three-dimensional analysis of laser induced plasmas in single and double pulse configuration

In this work we studied the morphology of plasmas induced by single and double laser pulses, with the purpose to improve the understanding of the formation and evolution of the plume in the two cases and the reasons of the increase of sensitivity and detection limits in the double pulse configuration. Single and double pulse laser-induced breakdown spectroscopy experiments were carried out on a brass sample in air. Spectrally, temporally and spatially resolved measurements have been performed on the plume and a deconvolution algorithm has been applied on the line-of-sight integrated spectra in order to separate the spectral information coming from different regions of the plasma. While in single pulse configuration only the major elements in the matrix were observed, in double pulse case also the minor elements emission lines were measurable. The values of line emission intensity, plasma temperature and electron density have been evaluated at different positions in the plume in both the configurations. Appreciable differences in plume dimensions (three times wider in double pulse configuration), and electron density values (two times higher in single pulse case), have been found while the maximum temperature in the plasma core was similar in the two configurations. Beside the spectroscopic measurements, we used the shadowgraphic technique to observe the evolution of the plume in single and double pulse configuration. In double pulse configuration, the expansion of the plume induced by the second pulse is sensibly faster than the one induced by the first pulse and, in double pulse case, the plume tends to fill the region encompassed by the shock wave formed by the first pulse; on the other hand no shock wave produced by the second pulse was visible. The results obtained by the two different approaches were compared and an explanation of the physical processes occurring is proposed.

Pulse shaping in infrared thermography for nondestructive evaluation

Infrared thermography is a common technique for nondestructive evaluation. In active infrared thermography privileged here, a thermal stimulation is required to generate relevant thermal contrasts on specimens. Among stimulation techniques, pulse heating is one of the most common along with lock-in thermography. In this article, a technique is discussed that shows that by modifying the pulse heating shape, higher thermal contrasts are generated (for a given level of energy injection). In fact, it was found that ideally, it is better to use two pulses separated by a short Δ time interval. Variation of Δ directly influences the thermal contrast. By increasing Δ, the thermal contrast first improves up to a certain level before to reduce below the reference value (single pulse case). This was tested on several materials of low, medium, and high thermal conductivity. Moreover, this was also confirmed by an appropriate thermal model (not discussed in the article). In the text, a theory and experiments are provided.

Open-system density matrix description of femtosecond laser desorption of electronically and vibrationally relaxing adsorbates: Single- and two-pulse scenarios

The femtosecond laser pulse induced desorption of a molecule (NO) from a metal surface (Pt) is investigated by means of open-system density matrix theory. Single- and two-pulse scenarios in the so-called DIMET limit (Desorption Induced by Multiple Electronic Transitions) are considered. Special emphasis is given (a) to a realistic modeling of the temperature profiles of substrate degrees of freedom in response to a laser pulse, (b) to the role of vibrational relaxation of the adsorbate-surface bond, and (c) to a critical comparison of our anharmonic “excitation–deexcitation” approach to the simpler Truncated Harmonic Oscillator model with electronic friction. For the single-pulse case, nonlinear scaling of the desorption yield with laser fluence is demonstrated and found to be in good agreement with experiment. For the two-pulse case, time-resolved correlation diagrams are calculated and interpreted.

Amplitude Profiles of Solar Type IIId Radio Bursts with Echo Components and the Global Dislocation of Their Elementary Quasi-Monochromatic Sources

We prove experimentally the possibility of direct observations of natural radio echoes on the Sun. The generally simple echo hypothesis on the origin of the time splitting in solar type IIId radio bursts sustained the crucial experimental test. These conclusions are based on both the new positional data and previous information obtained using the UTR-2 antenna system in different years since 1973. In summer 1992, we were able to register for the first time a large number of decameter type IIId bursts with pronounced echo-like components using a radioheliograph at frequencies of 20 and 25 MHz. Elementary (quasi-monochromatic) sources of the short-lived bimodal type IIId bursts were observed especially often on July 6. In this case, the time delay of the second peak, which depends on the heliolongitude of the active region, reached a maximum of about 7 s. All these double-peak bursts were localized in the central region of the solar corona, and, similar to the case of single-pulse “limb” type IIId bursts at a given radio frequency f, the observed elementary sources of these events appeared to be nonstationary. Such a dynamic pulsating source can radically change, at least once, its location in the celestial sphere in the field of view of a two-dimensional radio heliograph during the burst lifetime τΣ. While decaying, an echo-like burst formed rather slowly may abruptly jump from one location to another. As far as its short (3- to 4-s) steep-front precursor is concerned, its visible source behaved usually as an ephemeral sub- or ultrarelativistic object moving at a subluminal velocity over a global-scale trajectory. To all appearances, a twice-pulsing, narrow-band (Δ f/f∼ 0.01) radio burst with an intensity-time profile of type IIId is generally a rather long-term (τΣ ≈ 10-20 s) transient response of the solar atmosphere to the short-term flare of harmonic radio emission generated in the real (probably motionless and compact) source. This complex double-peak profile is due not only to the existence of a reflecting surface deep in the corona near the corresponding plasma level but also, to a considerable extent, to some opaque or semi-transparent structures located at heights of the middle corona. In particular, due to the strong influence of such structures, a coronal primary source of a bimodal burst can be seen on the Earth in some indirect rays before the solar radio echo and has the form of an additional delayed pulse.

Pulsing a Low Pressure Radiofrequency Discharge.

A 1 D Particle-in-Cell(PIC-MCC)simulation has been used to study the effect of temporal modulation(pulsing)of an rf(13.56MHz)source voltage on the evolution of an argon plasma. Plasma breakdown occurs during the first on-time of the pulse and analytic expressions for the density evolution during and after breakdown are presented for the single pulse case. The effect of multiple pulsing on the plasma development is then examined and compared with the steady-state evolution. Experimental measurements of pulsing in a parallel plate rf system are also presented, which have a high qualitative agreement with the simulation results.

Numerical modeling of ozone production in a pulsed homogeneous discharge: a parameter study

The pulsed volume discharge is an alternative for the efficient generation of ozone in compact systems. This paper presents a parameter study of the reactions in this kind of homogeneous discharge by using a numerical model which solves plasma chemical kinetic rate and energy equations. Simulations are performed for 10/sup -9/-10/sup -5/ s single pulses and oxygen gas density in the range 1<n<10 amagat, initial gas temperature 100<T/sub 0/<300 K and normalized electric field of 100<E/n<200 Td. Results are presented of ozone generation efficiency versus ozone concentration for different parameter combinations. Two parameter regimes are identified and analyzed. In the plasma phase ozone formation regime, where significant amounts of ozone are produced during the discharge pulse, it is found that higher ozone concentrations can be obtained than in the neutral phase ozone formation regime, where most of the ozone is formed after the discharge pulse. In the two-step ozone formation process, the rate of conversion of atomic oxygen plays a key role. In both regimes the ozone generation efficiency increases as a is increased or T/sub 0/ decreased. The maximum concentration is 3% at 10 amagat and 100 K. The results on ozone accumulation in multiple pulse discharges are presented. In contrast to the single pulse case, higher efficiency is achieved at lower gas density. This scaling can be explained by losses due to ion currents.

Excitation of two-level atoms in mode-locked laser fields

The excitation of two-level atoms in a laser field comprising many equally spaced coupled laser modes corresponding to a coherent pulse train is examined. The atom-field interaction is analysed via the optical Bloch equations for a rotating wave. In the limit of weak excitation they can be solved analytically and the time-averaged atomic excitation turns out to be a linear superposition of the contributions from the individual laser modes. Thus excitation spectra simply reflect the mode structure of the laser spectrum. Excitation spectra for strong fields are obtained by numerical integration of the optical Bloch equations. They exhibit a saturation behaviour differing significantly from the well known single-mode case. The temporal evolution towards the steady state is calculated for several numerical examples to clarify the origin of this behavior. For achieving maximum excitation, the laser pulse area, as in the single-pulse case, should be an odd multiple of π, and the mode spacing (pulse repetition rate) should exceed the natural linewidth of the atomic transition considerably. Under these conditions the time-averaged excited-state population approaches 1/2 while saturation broadening ensures nearly frequency-independent excitation within an extended fraction of the laser bandwidth.

A two-compartment model describing the release and negative inotropic action of acetylcholine on the heart.

Single-pulse stimulation of the vagus nerve in turtles ( Pseudymys floridana ) produced the well-known negative inotropic effect on the atria. After stimulation, the peak amplitudes of the atrial contractions exhibited an envelope which appeared to be proportional to the concentration of acetylcholine at the receptor sites. Postulating a quantized release of acetylcholine with each stimulus, the envelope was modeled by a two-equal-compartment configuration. In about 63% of the cases this envelope behaved as if it described a critically damped system, in 18% of the cases it behaved as if the system were slightly overdamped, and in the remaining 19% it corresponded to a lightly underdamped system. The average washout time constant per compartment calculated from the single-pulse response was 4.7 ± 0.8 seconds ( SD ). Repetitive stimulation was provisionally equated with a constant release of acetylcholine; its mathematical description was derived from the time integral of the single-pulse case, i.e., from a direct application of indicator-dilution theory. However, the average time constant obtained from this second type of fit was only 0.84 ± 0.4 seconds. This lower value was interpreted as a dynamic modification (perhaps nonlinear) due to accumulation of acetylcholine and probably associated with an increase in acetylcholine esterase activity.