Interpulse Time Research Articles

Fractal dynamics analysis of the VHF radiation pulses during initial breakdown process of lightning

[1] This paper presents the first analysis of fractal dynamics of lightning initiation process with a coherent approach. Using experimental data obtained from a broadband interferometer system operating at a bandwidth of 25-100 MHz, the sequences of inter-pulse time interval of the VHF radiation pulses emitted during the lightning initial breakdown process were analyzed with the multifractal detrended fluctuation analysis (MF-DFA) method. For 63 lightning flashes analyzed, it was found that there is an apparent long-range time correlated and nonlinear cascade behavior within the initiation breakdown process of the lightning. This result suggests that the initiation of lightning may be associated with progressively building up of correlated strong electrical field regions by self-similar scaling up (inverse cascading) of discharges across scales from small to large. The result may be promising toward our understanding of lightning initiation.

Effect of anomalous pulse timing on call discrimination by females of the gray treefrog (Hyla versicolor): behavioral correlates of neurobiology

Research has demonstrated that certain midbrain neurons of anurans 'count' interpulse intervals (IPIs). Some neurons fire after exposure to fewer intervals than do others. Counting can be reset to zero if an IPI falls outside the cell's tolerance range. We tested female gray treefrogs for behavioral correlates of these neural response patterns using phonotaxis tests in order to gain a better understanding of the mechanistic bases of female responses to calls. For example, previous work demonstrated females often prefer longer to shorter pulsed advertisement calls, even when the former occur at lower rates. Call attractiveness can also be reduced when pulse duration and timing have been manipulated experimentally or disrupted by acoustic interference. In this study, female responses were consistent with neural data, emphasizing the importance of IPIs. Females discriminated in favor of calls with normal interpulse timing relative to those in which a single IPI was too long or too short. Our data suggest that neural resetting of interval counting by inappropriate intervals may more strongly influence females than reduced firing in response to such intervals on an individual basis. Data also suggest a transition point between 125 ms and 175 ms at which an interval between pulse strings is treated as an interval between calls.

Characteristics of solid aerosols produced by optical catapulting studied by laser-induced breakdown spectroscopy

Optical catapulting (OC) constitutes an effective method to transport small amounts of different materials in the form of a solid aerosol. In this report, laser-induced breakdown spectroscopy (LIBS) is used for the analysis of those aerosols produced by OC. For this purpose, materials were catapulted using a Q-switch Nd:YAG laser. A second Q-switch Nd:YAG laser was used for LIBS analysis of the ejected particles. Data processing of aerosols was conducted using conditional data analysis. Also, the standard deviation method was used for the qualitative identification of the ejected particles. Two modes of interaction in OC (OC with focused or defocused pulses) have been evaluated and discussed. LIBS demonstrates that the distribution (spreading) of the ejected particles along the propagation axis increased as a function of the interpulse delay time. The mass density and the thickness of the target also play an important role in OC-LIBS.

False-target image synthesizer for countering ISAR via inverse dechirping

For real-time jamming signal generation in deceiving synthetic aperture radar (TSAR), the target characteristics modulation is always processed in the expensive field programmable gate array (FPGA). Due to the large computational complexity of the traditional modulating operation, the size and structure of simulated false-target are limited. With regard to the principle of in range compression of linear frequency modulated (LFM) radar, a novel algorithm named inverse dechirping is proposed for target characteristics modulation. This algorithm only needs one complex multiplier in the FPGA to generate the jamming signal when the radar signal is intercepted, which can be obtained by multiplication of radar signal samplings and the equivalent dechirped target echo in the time domain. As the complex synthesis of dechirped target echo can be realized by cheap digital signal processor (DSP) within the interpulse time, the overall cost of the jamming equipment will be reduced and the false-target size will not be limited by the scale of FPGA. Numerical simulations are performed to verify the correctness and effectiveness of the proposed algorithm.

Double-pulse laser-induced breakdown spectroscopy with 1030 and 257.5 nm wavelength femtosecond laser pulses

Double-pulse laser-induced breakdown spectroscopy (DP-LIBS) studies and the comparison with the single-pulse experiments were performed on the steel sample. Two harmonics of the femtosecond Yb:KGW laser at 1030 and 257.5 nm were combined in the collinear beam geometry to carry out double-pulse LIBS experiments at atmospheric pressure in air. The influence of the delay between the two (NIR pulse was delayed in regard to UV pulse) laser pulses on the LIBS signal intensity was investigated. The inter-pulse delay times were in the range from 0 to 200 ps. In the delay range from 0 to 40 ps the LIBS plasma emission intensity increased steadily and in the range from 40 to 200 ps (maximum inter-pulse delay investigated) the plasma lines’ emission intensity remained almost constant. LIBS signal intensity increased 3–5 times in comparison with the single pulse case when an optimum inter-pulse delay between the two ablating pulses was used. Keywords: laser-induced breakdown spectroscopy (LIBS), double pulse, laser plasma, laser ablation by femtosecond pulses

Effects of differential pulse frequencies of chicken gonadotrophin-releasing hormone-I (cGnRH-I) on laying hen gonadotrope responses in vitro

The aim of this work was to determine the effects of cGnRH I pulse frequencies on FSH and LH release and the changes in features and number of cultured laying hen FSH-cells and LH-cells in vitro. Primary adenohypophyseal cell cultures taken from laying hens were stimulated by four 5 min pulses using 1 or 10 nM cGnRH, administered with interpulses between pulses at 15, 30 or 60 min. Pulse frequencies and dose dependent effects were examined in six separate experiments including two controls. After the last interpulse time, the supernatants were collected and stored at −70° C until the performance of an indirect enzyme-linked immunosorbent assay (ELISA) using chicken LH and chicken FSH antisera at 1:1000 and 1:2000 dilutions, respectively. Supernatants were coated in duplicate on the inner surface of Immulon 2 plates and later blocked with the optimal solutions. They were incubated with each antiserum and subsequently with isotype-specific peroxidase-labeled anti-rabbit antibodies. Hydrogen peroxide/o-phenylenediamine was added as substrate/chromogen and the optical density (OD) was determined at 492 nm. The ABC immunocytochemical method was performed to characterize and re-count the gonadotropes employing anti-chicken FSH and anti-chicken LH as primary antibodies. The number of FSH-LH cells was obtained using stereological analysis and the data were statistically processed. The ODs obtained for each anti-hormone were compared with the control groups and with each other. Significant differences were found in number of aggregated-positive LH cells, which decreased with 1 nM cGnRH-I, 15 vs. 30 min pulses, increased with 30 vs. 60 min pulses, and also with 10 nM cGnRH-I, 30 vs. 60 min pulses. Aggregated positive FSH cells, however, did not show significant differences in percentage at any GnRH dose or pulse frequencies, but did show activity at low pulse frequencies of 15 and 30 min. The results suggest that LH cells varied in percentage in a dose dependent manner at higher pulse frequency (15 min) and were dose independent at low pulse frequency (60 min) and showed inactive features; while FSH cell numbers were unaffected showing features of activity at low pulse frequencies. High and moderate pulse frequencies of cGnRH-I (15-30 min) increased the FSH release in dose independent manner without changes in features or percentage of FSH cells. Low pulse frequency (60 min) of cGnRH-I increased LH release dose independently disminished LH cell percentage and showed changes in cells’ features. These results in avian cells showed differences in responses to GnRH pulse frequencies from those reported earlier in mammals.

Calculation of Double-Quantum-Coherence Two-dimensional Spectra: Distance Measurements and Orientational Correlations

The double quantum coherence (DQC) echo signal for two coupled nitroxides separated by distances ≳10 Å, is calculated rigorously for the six-pulse sequence. Successive application of six pulses on the initial density matrix, with appropriate inter-pulse time evolution and coherence pathway selection leaves only the coherent pathways of interest. The amplitude of the echo signal following the last π pulse can be used to obtain a one-dimensional dipolar spectrum (Pake doublet), and the echo envelope can be used to construct the two-dimensional DQC spectrum. The calculations are carried out using the product space spanned by the two electron-spin magnetic quantum numbers m(1), m(2) and the two nuclear-spin magnetic quantum numbers M(1), M(2), describing e.g. two coupled nitroxides in bilabeled proteins. The density matrix is subjected to a cascade of unitary transformations taking into account dipolar and electron exchange interactions during each pulse and during the evolution in the absence of a pulse. The unitary transformations use the eigensystem of the effective spin-Hamiltonians obtained by numerical matrix diagonalization. Simulations are carried out for a range of dipolar interactions, D, and microwave magnetic field strength B for both fixed and random orientations of the two (14)N (and (15)N) nitroxides. Relaxation effects were not included. Several examples of one- and two-dimensional Fourier transforms of the time domain signals vs. dipolar evolution and spin-echo envelope time variables are shown for illustration. Comparisons are made between 1D rigorous simulations and analytical approximations. The rigorous simulations presented here provide insights into DQC ESR spectroscopy, they serve as a standard to evaluate the results of approximate theories, and they can be employed to plan future DQC experiments.

Laser-induced breakdown spectroscopy of solid aerosols produced by optical catapulting

Laser-induced breakdown spectroscopy of particles ejected by optical catapulting is discussed for the first time. For this purpose, materials deposited on a substrate were ejected and transported from the surface in the form of a solid aerosol by optical catapulting using a neodymium-doped yttrium aluminum garnet (Nd:YAG) laser at 1064 nm. A Q-switched Nd:YAG laser at 532 nm was used for chemical characterization of the particles by laser-induced breakdown spectroscopy. Both lasers were synchronized in order to perform suitable spectral detection. The optical catapulting was optimized and evaluated using aluminum silicate particles, nickel spheres, and quartz and stainless steel particles. Experimental parameters such as the interpulse delay time, the sampling distance, the laser fluence, the sampling rate and the particle size have been studied. A correlation between these parameters and the particle size is reported and discussed.

Neurometabolite Concentrations in Gray and White Matter in Mild Traumatic Brain Injury: An 1H–Magnetic Resonance Spectroscopy Study

Single-voxel proton magnetic resonance imaging ((1)H-MRS) and proton MR spectroscopic imaging ((1)H-MRSI) were used to compare brain metabolite levels in semi-acute mild traumatic brain injury (mTBI) patients (n = 10) and matched healthy controls (n = 9). The (1)H-MRS voxel was positioned in the splenium, a region known to be susceptible to axonal injury in TBI, and a single (1)H-MRSI slice was positioned above the lateral ventricles. To increase sensitivity to the glutamate (Glu) and the combined glutamate-glutamine (Glx) signal, an inter-pulse echo time shown to emphasize the major Glu signals was used along with an analysis method that reduces partial volume errors by using water as a concentration standard. Our preliminary findings indicate significantly lower levels of gray matter Glx and higher levels of white matter creatine-phosphocreatine (Cr) in mTBI subjects relative to healthy controls. Furthermore, Cr levels were predictive of executive function and emotional distress in the combined groups. These results suggest that perturbations in Cr, a critical component of the brain's energy metabolism, and Glu, the brain's major neurotransmitter, may occur following mTBI. Moreover, the different pattern of results for gray and white matter suggests tissue-specific metabolic responses to mTBI.

Triplet-relaxation microscopy with bunched pulsed excitation

Obtaining high signal levels in fluorescence microscopy is usually spoiled by the concomitant population of the dark (triplet) state of the marker, which is often followed by photobleaching. Recently, we introduced the triplet relaxation (T-Rex) modality in fluorescence microscopy which led to a major increase in total signal and dye photostability. The idea behind T-Rex is to avoid the illumination of fluorophores in the triplet state, e.g. by using pulsed excitation with interpulse time distances that are long enough for the triplet state to relax between two pulses. While our previous implementation came at the expense of extended recording, here we investigate pulsed excitation patterns for T-Rex illumination implying shorter total recording times. In particular, we balance signal enhancement and imaging speed by exciting with bunches of quickly succeeding pulses that are separated by dark periods for triplet relaxation. Taking the green fluorescent protein and the organic dye Atto532 as examples, we observe the dependence of photobleaching and total fluorescence gain on the number of pulses within a bunch. Reaching almost T-Rex conditions this excitation scheme mimics fast scanning of the illumination beam and has the potential to improve a whole range of analytical tools that suffer from photobleaching and low signal levels.

Proton spectroscopic imaging of the human prostate at 7 T

The sensitivity of proton MR Spectroscopic Imaging ((1)H-MRSI) of the prostate can be optimized by using the high magnetic field strength of 7 T in combination with an endorectal coil. In the work described in this paper we introduce an endorectal transceiver at 7 T, validate its safety for in vivo use and apply a pulse sequence, optimized for three-dimensional (3D) (1)H-MRSI of the human prostate at 7 T. A transmit/receive endorectal RF coil was adapted from a commercially available 3 T endorectal receive-only coil and validated to remain within safety guidelines for radiofrequency (RF) power deposition using numerical models, MR thermometry of phantoms, and in vivo temperature measurements. The (1)H-MRSI pulse sequence used adiabatic slice selective refocusing pulses and frequency-selective water and lipid suppression to selectively obtain the relevant metabolite signals from the prostate. Quantum mechanical simulations were used to adjust the inter-pulse timing for optimal detection of the strongly coupled spin system of citrate resulting in an echo time of 56 ms. Using this endorectal transceiver and pulse sequence with slice selective adiabatic refocusing pulses, 3D (1)H-MRSI of the human prostate is feasible at 7 T with a repetition time of 2 s. The optimized inter-pulse timing enables the absorptive detection of resonances of spins from spermine and citrate in phase with creatine and choline. These potential tumor markers may improve the in vivo detection, localization, and assessment of prostate cancer.

Ultrafast Defect Manipulation with Optical Anisotropy in Fused Silica

AbstractWe report the evidence that the oxygen defects induced by focusing an intense infrared femtosecond laser pulse in fused silica can be self-organized by the interference pattern between photon and electron plasma wave. Self-organized nanostructure with a sub-wavelength modulation in refractive index exhibits form birefringence which is rewritable and directionally-controllable. Intriguingly, such optical anisotropy, which indicates a remarkable non-reciprocity, has initially evolved from residual birefringence originated from internal stress distribution due to local heating followed by structural change, regardless of interpulse time. This anisotropic light-matter interaction could be interpreted in terms of an asymmetric relation between light polarization and pulse front tilt. Apart from fundamental understanding of self-organization mechanism, the direction of encoded birefringence can introduce an entirely new concept for rewritable optical storage beyond the diffraction limit of light.

Double pulse laser-induced breakdown spectroscopy of bulk aqueous solutions at oceanic pressures: interrelationship of gate delay, pulse energies, interpulse delay, and pressure

Laser-induced breakdown spectroscopy (LIBS) has been identified as an analytical chemistry technique suitable for field use. We use double pulse LIBS to detect five analytes (sodium, manganese, calcium, magnesium, and potassium) that are of key importance in understanding the chemistry of deep ocean hydrothermal vent fluids as well as mixtures of vent fluids and seawater. The high pressure aqueous environment of the deep ocean is simulated in the laboratory, and the key double pulse experimental parameters (laser pulse energies, gate delay time, and interpulse delay time) are studied at pressures up to 2.76x10(7) Pa. Each element is found to have a unique optimal set of parameters for detection. For all pressures and energies, a short (< or = 100 ns) gate delay is necessary. As pressure increases, a shorter interpulse delay is needed and the double pulse conditions effectively become single pulse for both the 1.38x10(7) Pa and the 2.76x10(7) Pa conditions tested. Calibration curves reveal the limits of detection of the elements (5000 ppm Mg, 500 ppm K, 500 ppm Ca, 1000 ppm Mn, and 50 ppm Na) in aqueous solutions at 2.76x10(7) Pa for the experimental setup used. When compared to our previous single pulse LIBS work for Ca, Mn, and Na, the use of double pulse LIBS for analyte detection in high pressure aqueous solutions did not improve the limits of detection.

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.

Spectral simplification for resolved glutamate and glutamine measurement using a standard STEAM sequence with optimized timing parameters at 3, 4, 4.7, 7, and 9.4T

The C4 multiplet proton resonances of glutamate (Glu) around 2.35 ppm and glutamine (Gln) around 2.45 ppm usually overlap in MR spectra, particularly at low- and mid-field strengths (1.5-4.7T). A spectral simplification approach is introduced that provides unobstructed Glu and Gln measurement using a standard STEAM localization sequence with optimized interpulse timings. The underlying idea is to exploit the dependence of response of a coupled spin system on the echo time (TE) and mixing time (TM) to find an optimum timing set (TE, TM), at which the outer-wings of C4 "pseudo-triplet" proton resonances of Glu and Gln are significantly suppressed while the central peaks are maintained. The spectral overlap is thus resolved as the overlap exists exclusively at the outer-wings and the central peaks are readily separated due to the approximate 0.1-ppm difference in chemical shift. Density matrix simulation for Glu, Gln, and other overlapping metabolites at 2.3-2.5 ppm was conducted to predict the optimum timing sets. The simulated, phantom, and in vivo results demonstrated that the C4 multiplet proton resonances of Glu and Gln can be resolved for unobstructed detection at 3T, 4T, and 4.7T. For simplicity, only simulated data are illustrated at 7T and 9.4T.

T2ρ and T1ρ Adiabatic Relaxations and Contrasts

Transverse relaxation in the rotating frame (T2ρ) is the dominant relaxation mechanism during a train of adiabatic full passage (AFP) radiofrequency (RF) pulses with no interpulse time intervals placed after the 90° excitation pulse. The magnetization components remain transverse to the time-dependent effective field and undergo relaxation with the time constant T2ρ. Longitudinal relaxation in the rotating frame (T1ρ) is the dominant relaxation mechanism during a train of AFP RF pulses placed prior to an excitation pulse. Here, magnetization is aligned along the time-dependent effective field during adiabatic rotation undergoes relaxation with the time constant T1ρ. A detailed description of rotating frame relaxations due to dipolar interactions and exchange during adiabatic pulses is presented herein. The exchange-induced and dipolar interaction contributions depend on the modulation functions of the adiabatic pulses used. The intrinsic rotating frame relaxation rate constant is sensitive to fluctuations at the effective frequencey (ωeff) in the rotating frame, and this is modulated differently during the two types of AFP pulses. This may lead to the possibility to assess T1ρ and T2ρ relaxation influenced by dipolar relaxation pathways and exchange in human brain tissue and provide a means to generate T1ρ and T2ρ contrasts in MRI.

Doorway–window description of sequential three-pulse photon echo signals

The doorway–window ( DW) approach is extended to the description of the three-pulse photon echo (3PPE) signals, provided we can neglect the transient terms decaying as ∼ E i( T) ( E i( T) are the envelopes of the pulses involved and T is the interpulse delay time during which the material system is in its electronic population state). A simplified version of the DW approach is also developed, which is valid if we can additionally drop the transient terms decaying as ∼ E i( τ), τ being the interpulse delay time during which the material system is in its electronic coherent state. The proof-of-the-principle calculations of the 3PPE signal for the displaced harmonic oscillator model demonstrate that the simplified DW approach performs well, sometimes even beyond its expected domain of validity.

Fs/ns dual-pulse LIBS analytic survey for copper-based alloys

The quantitative analytic capability of a fs/ns dual-pulse Laser-Induced Breakdown Spectroscopy technique, based on the orthogonal reheating of a fs-laser ablation plume by a ns-laser pulse, is presented. In this work, it is shown how the effect played by the delay times between the two laser beams can vary the analytical response of this dual-pulse LIBS configuration. In order to address this task, the Sn, Pb and Zn calibration curves of five certified copper-based samples have been investigated. These calibration curves have been obtained, in air at atmospheric pressure, by integrating the emission data collected in two different inter-pulse delay zones, one in the delay interval of 1–41 μs, the other within the range of 46–196 μs. For drawing the species calibration curves, the emission intensities of the considered Pb(I), Sn(I) and Zn(I) electronic transitions have been normalized with a non-resonant Cu(I) emission line. The experimental results have shown that, by varying the inter-pulse delay between the two laser beams, complementary analytical results can be induced. By considering at once all data acquired within the inter-pulse delay time of 1–196 μs, this hypothesis has been strengthened. The calibration curves obtained in this way are characterized by excellent linear regression coefficients (0.988–0.999) despite of the large Sn, Pb and Zn compositional variation of the targets employed. The results presented reveal, for the first time, that, by taking into account the role played by the inter-pulse delay time between the two laser beams, the fs/ns dual-pulse LIBS configuration here used can be improved and provide very good opportunities for performing quantitative analysis of copper-based alloys.

Optical emission spectroscopy investigation of an ultra-short laser induced titanium plasma reheated by a ns laser pulse

The dynamics of a titanium plasma species, induced in air by coupling a fs-ablating laser pulse with an orthogonal ns-reheating laser source placed at the fixed distance of 1.0 mm from the target surface, has been followed by temporally resolved emission spectroscopy. The temporal evolutions of plasma features such as excitation temperatures and electron densities have been evaluated by using two different laser energies of the first fs-ablating laser pulse (0.8 mJ and 3.0 mJ). Optimum inter-pulse delay times, experimentally determined, of 250 μs and 500 μs were used for the fs laser energy of 3.0 mJ and 0.8 mJ, respectively. By experimental inspections of the main plasma species electronic transitions so obtained, a strong enhancement was evaluated up to one and two orders of magnitude for Ti(I) and Ti(II), respectively. Independently from the fs laser energy employed, the plasma features showed the same temporal behaviour implying that the ns-reheating characteristics of this process belong to the reheating mechanism itself. The experimental results have been discussed and the excited species evolutions and elementary processes involved, as well as, the local thermodynamic equilibrium departures, have been outlined.

Stability of LD pumped passively Q-switched solid-state lasers

For the laser diode pumped Cr,Nd∶YAG passively Q-switched Nd:YVO4 laser, in order to improve the stability of the Q-switched pulses, a long cavity is adopted, and the gain medium is located at the middle of the cavity. In the experiments, stable pulses are obtained. At the maximum incident pump power of 17.5W, the repetition rate increases to 71.3 kHz and the pulse width (FWHM) is narrowed to 0.4μs. The pulse-to-pulse amplitude fluctuation is less than ±10%. The inter-pulse timing jitter of the Q-switched pulse train is found to be less than ±3.5%.