Function Of Pulse Duration Research Articles

Picosecond laser study of the quasicontinuum of C2F5Cl

The quasicontinuum of the vibrationally hot (∼10 CO2 laser photons) C2F5Cl molecule was investigated using a two-laser pulse arrangement. The first laser pulse preheated the molecules into the QC (quasicontinuum) while the second picosecond CO2 laser was employed to probe the absorption characteristics of the excited molecules. The second laser could be tuned either in frequency or in pulse duration. Several experiments were performed. In the first experiment, the small signal absorption spectrum of C2F5Cl was measured and compared with the dissociation spectrum obtained by Borsella et al.1 The same ν6+ν10 hot band was observed with a considerably narrower linewidth. In the second experiment, the saturation properties of the hot band were investigated. It was found that the absorption line could be characterized as inhomogeneously broadened with no evidence for any broadening due to intramolecular damping. In a third series of experiments, the absorption of C2F5Cl at low as well as high laser energy fluences was measured as a function of the laser pulse duration between 22 and 120 ps. It was found that (i) the absorption can be characterized as predominantly intensity dependent and (ii) the small signal absorption cross section increased considerably as the pulse duration was decreased. These observations were discussed using the multitier energy level classification scheme in the QC. It was concluded that within the laser pulse duration, individual states within the first tier which interacted with the laser were not coupled to each other. The change in the absorption cross section as a function of the pulse duration is consistent with the picture that under laser irradiation, a rapid equilibrium was achieved with the final state populations remaining relatively constant in time. The final populations were dependent almost entirely on the laser intensity. Increasing the laser pulse duration had little effect on the final state distribution and the total energy absorbed by the molecule.

Formation of AlSb by laser and capacity discharge annealing

The preparation of AlSb thin films by either pulsed laser or capacity discharge annealing of Al/Sb sandwiches is compared in order to shed light upon the nature of the transformation. Using 1000 Å thick multi-layer films supported by glass substrates, we investigate the energy threshold for complete transformation as a function of pulse duration from 10 −7 to 10 −1 s. We thence calculate the temperature effect directly induced by the energy pulse to be at least 900 K or approximately the melting points of the metals.

Phase transitions in compound semiconductor films triggered by laser irradiation

The preparation of AlSb thin films by pulsed laser annealing of Al-Sb sandwiches is studied in order to resolve some past uncertainty about the temperature rise induced by the laser pulse. Using 1000-Å-thick 2-layer films supported by transmission electron microscope grids, we investigate the energy threshold for transformation as a function of pulse duration (from 15 ns to 100 ms) and of ambient temperature up to ∼600 K. We thence calculate the temperature effect directly induced by the laser to be ∼930 K, whereas inert gas furnace anneals of identical films show transformation at this temperature occurring only in times ≳100 sec. We discuss the isoenergetic nature of the system for short-laser pulses and the role of the heat of formation, and propose a model which is then shown to work equally well for the systems CdTe, CdSe, and AlAs. We conclude that the reaction is thermally triggered by the laser pulse, but is to some extent self- sustaining via the heat of formation locally distributed.

Preparation of AlSb films by laser annealing

The preparation of AlSb thin films by the pulsed laser annealing of Al/Sb sandwiches was studied in order to resolve some past controversy about the temperature rise induced by the laser pulse. Using films 1000 Å thick supported by transmission electron microscope grids, we investigated the energy threshold for complete transformation into AlSb as a function of pulse duration from 35 ns to 100 ms and at film temperatures ranging from -100 to 350 °C. By extrapolation we obtained the temperature rise directly induced by the laser beam as about 650 °C, i.e. approximately the melting point of the two metals, whereas during anneals of comparable films in an inert gas furnace the transformation at this temperature only occurs in times exceeding 100 s. We discuss the isoenergetic nature of the system for short laser pulses and the role of the heat of transformation, and we conclude that the reaction is thermally triggered by the laser pulse but is to some extent self-sustained via the heat of transformation which is locally distributed. Further, the laser-induced crystal growth is shown to be influenced by the beam shape.

A critical examination of the dual system theory in Ostrinia nubilalis

Beck's dual system theory (DST) is examined theoretically and experimentally by investigating the oviposition rhythm of Ostrinia nubilalis and its entrainment by light cycles. Several well-known circadian phenomena are not accounted for by the DST. 1) It does not generate transient cycles when light pulses fall during the advance portion of the circadian cycle. This is also reflected in DST-predicted phase-response curves (PRC's) for both Drosophila pseudoobscura and O. nubilalis. Steady-state phase advances are predicted to occur on day 1 after the light pulses by the DST, not several cycles later as has been observed in many cases. 2) It does not account for the observation that the magnitude of a phase shift (delta phi) is often a function of pulse duration of both delays and advances. The DST predicts the same + delta phi, for example, for a 0.5-h and a 6.0-h light pulse beginning 5.0 h after dusk. 3) The DST does not accurately predict steady-state phase relationships between the light cycle and the gating oscillation (P-system) in non-24-h light cycles. 4) The driver (S-system) is given the property of being temperature sensitive whereas the driven rhythm (P-system) is temperature compensated. This is contrary to accumulated data suggesting that the circadian pacemaker is temperature compensated.

Temporal summation in the auditory system of the goldfish

Previous investigations of temporal integration in fishes have presented conflicting results. Popper [J. Acoust. Soc. Am. 52, 596–602 (1972)] found no evidence for integration for signals between 10 and 500 ms in duration, while Offutt [J. Acoust. Soc. Am. 41, 13–19 (1967)] clearly did. Since one of the differences between these studies was the level of a noise background, we determined to measure the effect of noise on the detectability of tone pulses as a function of pulse duration. Three goldfish (Carassius auratus) were classically conditioned to detect tone pulses ranging from 2.5 to 100 ms in duration for repetition periods of 100 and 1000 msec under both ambient (unmasked) and broadband noise conditions. The threshold function of signal duration under noise showed nearly perfect integration (3 dB decrease in threshold per doubling of duration). The function under ambient conditions showed no consistent effects of signal duration, and thus no evidence for the type of integration expected from simple models [R. Plomp, J. Acoust. Soc. Am. 33, 1561–1569 (1961)]. We conclude that the presence of masking noise may account for the previous conflicting results, and that the nature of the neural processes used in signal detection by the goldfish are fundamentally different under noisy and quiet conditions. [Work supported by N.I.H. grants to R.R.F.]

A Laser Triggered Self-Sustaining Metals-Compound Semiconductor Transition for AlSb

ABSTRACTThe preparation of AlSb thin films by pulsed laser annealing of Al/Sb sandwiches is studied in order to resolve some past controversy about the temperature rise induced by the laser pulse. Using 1000 Ȧ thick two layer films supported by TEM grids, we investigate the energy threshold for complete transformation as a function of pulse duration from 15 nsec to 100 msec, and of ambient temperature from −100°C to 250°C.We thence calculate the temperature effect directly induced by the laser to be about 930°C, or approximately the melting point of the metals, whereas inert gas furnace anneals of comparable films show transformation at this temperature occuring only in about 100 sec. We discuss the isoenergetic nature of the system for short laser pulses and the role of the heat of transformation, and thus conclude that the reaction is thermally triggered by the laser pulse but is to some extent self-sustaining via the heat of transformation locally distributed. This model is also shown to have equal validity for the systems CdTe, CdSe and AlAs.

Latencies for chromatic and achromatic visual mechanisms

Relative visual onset latency and offset latency (visual persistence) were measured (using perceived simultaneity methodsl for pulsed chromatic stimuli presented under two conditions: as luminance increments of 0.2 or 0.3 log unit above a prevailing achromatic background field, or in hue substitution, matched in luminance to the achromatic field so as to eliminate spatio-temporal luminance transients. In the first experiment, for the hue substitution condition, both onset and offset latencies were found to be slow relative to latencies for the increment condition, and varied with wavelength according to a function resembling trichromatic saturation discrimination (slowest latency at 570 nm). Latencies to increment pulses of equal luminance did not vary with wavelength. In a second experiment, offset latency was measured as a function of pulse duration and pulse luminance level. The observed decrease in offset latency with pulse duration was markedly steeper at short pulse durations for chromatic increments than for hue substitution stimuli. Offset latency decreased with increasing pulse luminance for incremental stimuli but not for hue substitution stimuli. These results are interpreted as reflecting the temporal response properties of chromatic neural mechanisms (the hue substitution data) and the achromatic mechanism (the increment data).

Visual sensitivity to disparity pulses: evidence for directional selectivity

When a target's retinal disparity changes with a pulsed waveform, the target appears to execute a pulsed movement in depth. Visual sensitivity to such disparity changes was plotted as a function of pulse duration. These curves resembled low-pass filter characteristics. For a given direction of movement in depth, different sensitivity curves were obtained for targets located in front of and behind the plane of binocular fixation. However, depth sensitivities were similar for pulses directed from either location towards the fixation plane or directed from either location away from the plane. This suggests that movements in depth directed towards and away from the fixation plane are handled by different neural mechanisms in man in accord with single-neuron evidence in cat and monkey.

Heating of a Theta-Pinch Plasma by a CO2 Laser

The heating of a theta-pinch deuterium plasma by a CO2 laser has been studied as a function of pulse duration and energy. A hydrodynamical approach was used with temperatures and densities assumed uniform in a fully ionized plasma. This model takes into account the dynamics of the magnetic field, the axial and radial magnetic confinement, the shape of the laser pulse, and energy transferred to the plasma by inverse bremsstrahlung. We discuss the results (dimensions, temperature, absorbed power) snowing that an increase in pulse duration results in higher electron and ion temperature.

Energy consumption for cardiac stimulation as a function of pulse duration

Studies were performed of the threshold of cardiac stimulation as a function of impulse duration in dogs, acutely, and in humans acutely and chronically. Impulse durations from 0.1 to 5.0 msec. were utilized. o.1.Myocardial impedance does not vary as a function of impulse duration, within the measured range. 2.Voltage needed at threshold falls progressively from the shortest to the longest impulse durations. 3.Current utilized falls progressively from shortest to longest impulse durations. 4.Energy consumed at thresholds, acute and chronic, is highest at short impulse durations, falls progressively to a minimum at impulse durations of 0.2 to 1.0 msec., then rises at longer impulse durations. 5.Total charge consumed remains low until impulse durations of 0.2 and 0.5 msec. are reached. With lengthening of impulse, total charge consumed rises. An impulse duration of 0.5 to 0.75 msec. is more economical for production of ventricular systole. Studies were performed of the threshold of cardiac stimulation as a function of impulse duration in dogs, acutely, and in humans acutely and chronically. Impulse durations from 0.1 to 5.0 msec. were utilized. o.1.Myocardial impedance does not vary as a function of impulse duration, within the measured range. 2.Voltage needed at threshold falls progressively from the shortest to the longest impulse durations. 3.Current utilized falls progressively from shortest to longest impulse durations. 4.Energy consumed at thresholds, acute and chronic, is highest at short impulse durations, falls progressively to a minimum at impulse durations of 0.2 to 1.0 msec., then rises at longer impulse durations. 5.Total charge consumed remains low until impulse durations of 0.2 and 0.5 msec. are reached. With lengthening of impulse, total charge consumed rises. An impulse duration of 0.5 to 0.75 msec. is more economical for production of ventricular systole.

Information Cells on Intensity-Modulated CRT Screens

The distribution of current within the beam of a cathode-ray tube has a Gaussian variation. Consequently, if the wave form of a pulse of beam current is known, it is possible to determine the excitation of an intensity-modulated screen surface. In this report the nature of the excitation is discussed for an intensity-modulated sweep for both rectangular and triangular current waveforms. The results are extended to indicate what may be expected of an actual mapping radar set. The theoretical constants which arise in the development are evaluated from data obtained by the shrinking raster method of measuring spot size, which is termed line width in this paper for purposes of clarity. This permits determination of the size of an information cell on the screen as a function of pulse duration, sweep speed and line width. Then the number of information cells available on a tube may be determined. As a consequence, we find that there is no significant difference in the number of information cells available on the standard 5FP7, 7BP7, or 12DP7. Furthermore, the reduction of the over-all receiving-system frequency response by some radar displays may be estimated. In such a case the video bandwidth may be reduced.