Rise Time Decreases Research Articles

Thermoelastic response of pulsed photothermal deformation of thin plates

A theoretical investigation of the dynamic thermoelastic response of pulsed photothermal deformation (PTD) deflection detections for some Q-switch laser pulses and finite thickness samples has been presented. The results show that signals can be characterized by a quasistatic process when the laser pulse rise time is on the order of 1 μs and the sample thickness is in submillimeter range (typically for semiconductor wafers). However, as the pulse rise time decreases or the sample thickness increases, the dynamic wave behavior gradually becomes apparent, and the quasistatic approximation gives only a contour curve of the dynamic time evolution. When the rise time decreases on the order of 10 ns or less for the same kind of the samples, the PTD deflection signal reflects a totally dynamic wave behavior.

Effects of chronic spinalization on ankle extensor motoneurons. I. Composite monosynaptic Ia EPSPs in four motoneuron pools.

1. We examined the effects of 6-wk chronic spinalization at the L1-L2 level on composite monosynaptic Ia excitatory postsynaptic potentials (EPSPs) recorded in medial gastrocnemius (MG), lateral gastrocnemius (LG), soleus (SOL), and plantaris (PL) motoneurons. Amplitudes, rise times, and half-widths of composite monosynaptic Ia EPSPs evoked by low-strength electrical stimulation of peripheral nerves were measured in barbiturate-anesthetized cats and compared between unlesioned and chronic spinal preparations. 2. The mean amplitude of homonymous composite Ia EPSPs evoked by 1.2 times threshold (1.2T) stimulation and recorded in all four ankle extensor motoneuron pools increased 26% in chronic spinal animals compared with unlesioned controls. There was also an increased incidence of large-amplitude, short-rise time EPSPs. When the same data were separated according to individual motoneuron species, homonymous EPSP amplitudes in MG motoneurons were found to be unchanged. EPSPs recorded in LG motoneurons and evoked by stimulation of the combined LG and SOL nerve were increased by 46%. Mean EPSP amplitudes recorded in both SOL and PL motoneurons were larger after spinalization but statistical significance was only achieved when values from SOL and PL were combined to produce a larger sample size. 3. In LG motoneurons from chronic spinal animals, all EPSPs evoked by 1.2T stimulation of the LGS nerve were > or = 0.5 mV in amplitude. In unlesioned preparations, one fourth of the LG cells had EPSPs that were < or = 0.2 mV. 4. The mean amplitude of heteronymous EPSPs evoked by 2T stimulation of LGS and MG nerves and recorded in MG and LG motoneurons, respectively, doubled in size after chronic spinalization. Because homonymous EPSP amplitudes were unchanged in MG motoneurons, synaptic mechanisms and not passive membrane properties are likely responsible for increased heteronymous EPSP amplitudes in MG. 5. The mean 10-90% rise time of homonymous composite Ia EPSPs in pooled data from all motoneurons decreased 21% in 6-wk chronic spinal animals. Unlike EPSP amplitude, significant rise time decreases were found in all four motoneuron pools. Compared with the other motoneuron species, the mean homonymous rise time recorded in MG motoneurons was shortest and decreased the least in chronic spinal animals. Rise times of heteronymous Ia EPSPs in MG and LG motoneurons also decreased. The maximum rate of rise of homonymous EPSPs increased in all four motoneuron species. 6. The mean half-widths of Ia composite EPSPs decreased in 6-wk spinalized preparations in all motoneuron species.(ABSTRACT TRUNCATED AT 400 WORDS)

Mechanism of early anoxia-induced suppression of the GABAA-mediated inhibitory postsynaptic current.

1. We investigated the mechanism of hypoxia-induced depression of gamma-aminobutyric acid-A (GABAA)-mediated inhibitory postsynaptic currents (IPSCs) in CA1 neurons of hippocampal slices from 21- to 28-day-old rats. Cells were examined by whole-cell patch-clamp recording and hypoxia was induced by switching perfusion of the slice from oxygenated artificial cerebral spinal fluid (ACSF) to ACSF saturated with 95% N2-5% CO2. 2. Synaptic responses evoked by stimulation of the Schaffer collateral-commissural projection at a fixed holding potential (VH = -60 mV) during anoxia revealed that the IPSC appeared more sensitive than the excitatory postsynaptic current to anoxia-induced depression. All subsequent studies examined the GABAA-mediated IPSC synaptic responses in isolation by direct stimulation of GABA interneurons in the stratum radiatum in the presence of extracellular 3-(2-carboxypiperazine-4-yl)propyl-1-phosphonic acid (CPP) (20 microM) and 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) (50 microM) to block glutamatergic currents and intracellular QX-314 (lidocaine N-ethyl bromide, 1 mM) to block GABAB-mediated currents. When studied in this manner (VH = -60 mV) the GABAA-mediated IPSC appeared to change from an outward to inward current after exposure to anoxia. 3. The current-voltage relationship of GABAA-mediated IPSCs revealed that these changes resulted from a positive shift in the IPSC reversal potential without a significant change in the conductance. Thus under patch clamp apparent IPSC inhibition may result from a decrease in the extracellular concentration of chloride ions. Similar findings were observed with micropipettes that contained high intracellular chloride concentrations. 4. Miniature spontaneous IPSCs were examined in the presence of tetrodotoxin (1 microM) with micropipettes containing high intracellular chloride concentrations. The miniature IPSCs (mIPSCs) appeared as spontaneous transient inward currents. Consistent with an anoxia-induced decrease in extracellular chloride, the mean amplitude of the mIPSCs increased after the onset of anoxia. A significant decrease in rise and decay time was also noted during anoxia. The frequency of the mIPSCs also increased by approximately 300%. 5. The resting input resistance of the cells was examined by measuring the current resulting from a 20-mV hyperpolarizing pulse. A significant reduction in resistance was observed 2 min after the onset of anoxia. This still occurred, although to a lesser degree, in the presence of glutamatergic blockers (20 microM CPP plus 50 microM CNQX). In the presence of both GABAergic (picrotoxin, 100 microM) and glutamatergic blockers no significant reduction in resting input resistance was apparent after 2 min of anoxia.(ABSTRACT TRUNCATED AT 400 WORDS)

Modeling and measurement of bistable semiconductor lasers

A theoretical model of bistable semiconductor lasers (BSL) under optical triggering is proposed, which takes into account the amplification and absorption processes in the laser structure. It has been found that the main factor limiting the highspeed operation of BSLs is the carrier lifetime in the absorber. A novel solution has been proposed consisting of bombarding the absorber with a proton beam. This paper presents the first experimental and theoretical results on the transition time and the turn-on jitter of such a BSL with optical triggering. Measurements on bulk and multiple quantum well BSLs revealed the dependence law of transition time and turn-on jitter on the injection currents. It has been found that, for a constant current in one active section, the rise time decreases but the fall time increases with injection current in the other section. An optimal current value can then be found to minimize the overall transition time. The measured turn-on jitter and relaxation oscillation period decrease with increasing injection current, in good agreement with theoretical predictions. 2.5 Gb/s optical triggering with a bit-error-rate less than 10/sup -9/ is demonstrated, for the first time, by using such a BSL without electrical reset.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

High speed coherent switching below the Stoner-Wohlfarth limit

Measurements of the switching characteristics of high coercivity magnetic tapes for field pulse widths /spl ges/0.6 ns have been reported previously. In an effort to develop a simple theoretical framework in which to develop a simple magnetization dynamics, the well established Gilbert form of the Landau-Lifshitz equation, applied to a uniaxial, single domain particle with uniform magnetization was chosen. The results which were obtained using straightforward numerical methods show that when the damping constant /spl alpha/<1, coherent switching will sometimes occur well below the Stoner-Wohlfarth threshold. The amount of deviation which is significant over a wide range of experimentally observable parameters increases as /spl alpha/ and the pulse rise time decrease. The deviation is due to the nonequilibrium condition which arises for small values of /spl alpha/ pulse rise time. The final position is always the expected Stoner-Wohlfarth equilibrium position.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Transition time and turn-on jitter of optically triggered bistable lasers incorporating a proton bombarded absorber

Recently, we have demonstrated 1 Gbit/s error free operation of an optically triggered bistable laser, which is the highest bit-rate operation ever reported without electrical reset. This is achieved by using proton bombardment of the absorber section which induces a reduction of the carrier lifetime in this section. This letter presents the first experimental results on the transition time and the turn-on jitter of an optically triggered bistable laser incorporating a proton bombarded absorber section. Current injection conditions that minimize the overall transition time have been derived. It has been found that for a constant current in one active section, the rise time decreases, but the fall time increases with injection current in the other section. An optimal current value can then be found to minimize the overall transition time. The measured turn-on jitter and relaxation oscillation period decrease with increasing injection current, in good agreement with theoretical predictions.

Acute effects of spinal transection on EPSPs produced by single homonymous Ia-fibers in soleus alpha-motoneurons in the cat.

1. Excitatory postsynaptic potentials (EPSPs) generated in soleus motoneurons by single homonymous Ia-fibers were measured using intracellular recording and the spike-triggered averaging technique. Two groups of barbiturate-anesthetized adult cats were studied: one with the spinal cord intact and the other with the spinal cord severed at thoracic segment 13 (T13) several hours prior to recording. 2. In cord-transected cats, single homonymous Ia-fibers produced EPSPs in soleus motoneurons that were, on average, larger and faster rising relative to normal, as they are for those produced in medial gastrocnemius (MG) motoneurons (8, 12, 13, 40). Specifically, mean EPSP amplitude and rise time were, respectively, 261 +/- 22 microV and 0.65 +/- 0.05 ms for the transected group vs. 160 +/- 21 microV and 0.96 +/- 0.08 ms for the intact group. The group means for each parameter were significantly different (P less than 0.005). 3. The group difference in EPSP amplitude was largely due to a decrease in number of small EPSPs in the transected group (11% less than 100 microV compared with the normal 41%) and not due to the occurrence of unusually large ones. Ratios of the largest to smallest amplitude EPSPs produced in the same motoneuron were similarly distributed for intact and transected groups, implying that the effect of transection on EPSP size was uniform across different Ia-fiber synapses made with the same motoneuron. Mean EPSP amplitude for each transected cat (n = 5) was larger than normal, but in some cases the increase took greater than 10 h to express itself. 4. The normal tendency for EPSP rise time to decline on average with amplitude was absent in the transected group, wherein rise time was reduced to similar average values in all amplitude categories. This suggests that the decrease in rise time occurred independently of the increase in amplitude. In contrast, EPSP half-width, which tended tow ward lower than normal values [5.63 +/- 0.36 (SE) ms vs. 6.51 +/- 0.44 ms; P greater than 0.10], decreased in proportion with rise time as evidenced by the preservation of the normal relation between those parameters in transected cats. Normalizing EPSPs by motoneuron time constant (tau) reduced the group differences in rise time and half-width, suggesting that a fall in tau contributes to the abbreviation of EPSP time course. 5. The condition of the spinal cord best accounted for differences in synaptic strength between groups.(ABSTRACT TRUNCATED AT 400 WORDS)

Synaptic strength as a function of motor unit size in the normal frog sartorius.

A wide range of motor unit sizes exists in each frog sartorius, with values of tetanus and twitch tensions extending over 62- and 400-fold ranges, respectively. These differences primarily represent differences in the number, rather than the size, of muscle fibres in each motor unit. Tetanus-to-twitch tension ratios varied markedly between different motor units, ranging from 1.5 to over 190. Because directly stimulated muscle fibres have tetanus/twitch ratios no larger than 3, it is concluded that high tetanus/twitch ratios arise from recruitment of muscle fibres during the tetanus which respond with only subthreshold depolarization during a twitch. Small motor units, as judged either by their twitch or tetanus tension, were associated with higher tetanus/twitch ratios, suggesting that the average safety margin of a motor unit increases with the motor unit's size. Indeed, when the tetanus/twitch ratio of a motor unit is used to determine the fraction of fibres in the unit with subthreshold neuromuscular junctions, it is observed that there is a direct linear relationship between the size of the motor unit and its over-all efficacy of synaptic transmission. Measurements of the effects of changes in calcium concentration on motor unit twitch tension confirmed the last conclusion. Furthermore, this analysis revealed that large motor units, although having a wide range of transmission safety margins, are largely comprised of junctions of uniformly high safety margin. In motor units of smaller size, synaptic strengths become more evenly distributed over a wide range of values. Small motor units had consistently longer twitch rise-times than did larger units. This decrease in rise-time with motor unit size paralleled the decrease in tetanus/twitch ratios, raising questions about the regulation of muscle fibre contraction kinetics.

Anomalous potassium channel-gating rates as functions of calcium and potassium ion concentrations

With near normal monovalent ionic concentrations, the rate of increase of the potassium conductance after a depolarizing voltage-clamp step is slowed when the external calcium concentration is increased. This trend in the rise-time with changes in calcium is reversed when the axointernal potassium concentration is reduced (150 mM) and the periaxonal concentration is increased (50 mM); that is, the rise-time decrease with increasing calcium concentration. Furthermore, the degree of sigmoidality of the K-conductance time-course always increase when the rise-times increase for a given test potential. In the case of calcium surface-charge screening, these effects may be caused by a shifted distribution of K-ions within the channels following the altered ion gradient, and by a consequent shift in the reciprocal electrostatic interactions between the ionic charges and channel-gate charges.

Rupture propagation and focusing of energy in a foam rubber model of a stick slip earthquake

Dynamic displacements and velocities measured at the surface of a block of foam rubber are analyzed for stick slip events on faults with three different geometries; elliptical, rectangular (aspect ratio of 3.33), and long thin (aspect ratio of 12.66). Over 100 separate events were recorded, each with up to eight simultaneous point measurements of particle displacement. The positions of the measurements were along the fault trace and to distances of a few fault lengths. Acceleration of the rupture front is observed as the rupture propagates out from its point of nucleation. Deceleration of the rupture is also observed when it enters a region of lower effective stress. In the direction of rupture propagation the static displacements along the fault trace increase, and the rise times decrease. The combined effect causes a rapid increase in particle velocities. For a predominantly unilateral rupture, increases in particle velocities by a factor of 2 or 3 over those recorded near the center of the fault trace are common. At higher rupture velocities, there is a larger increase in the particle velocity (or decrease in the rise time) for a given increase in the static displacement. The amplitude of the far‐field displacement is similarly strongly dependent on the nature of the rupture. Differences in far‐field displacements of an order of magnitude are seen for measurements at equal distances from a unilateral rupture. When the rupture initiates at depth, a much more uniform distribution of particle displacements and velocities is obtained. Afterslip, i.e., motion occurring significantly slower than the time required for a shear wave to travel the length of the fault, is also observed. Comparison of our model results with surface static displacements following major earthquakes suggests that observed variation in static displacements may be due in part to variations in the bedrock stress field. Such stress heterogeneity can lead to a division of a major earthquake into a series of several subruptures, each of which rupture coherently, but the event as a whole behaves incoherently.

Berechnung der Aufbauzeit eines doppeltresonanten optischen parametrischen Oszillators

AbstractThe rise time for the output power of a doubly resonant optical parametric oscillator with ring resonator configuration is calculated for the special case of subharmonic generation. It is shown that the rise time decreases by increasing the pumping level (for a constant threshold power) and, for a constant pumping level, by increasing the resonator loss.

Thermal and spectral sensitivities of discrete slow potentials in Limulus eye.

The discrete, subthreshold, slow potential fluctuations (SPF's) which can be recorded intracellularly in Limulus ommatidia are sensitive to temperature and light wavelength. SPF frequency increases with increasing temperature (Q(10) about 3.5) and light intensity. The effects are additive. SPF rise and decay time decrease with increasing temperature (Q(10) between 2 and 3). There is a peak, near 520 nm, in the spectral sensitivity of SPF frequency. This peak may correspond to the wavelength of maximum absorption by rhodopsin in the ommatidia. Hydroxylamine produces a rapid, irreversible reduction of SPF frequency and amplitude perhaps owing to its action on the photopigment. The cornea and crystalline cones fluoresce (peak about 445 nm) when excited by near-ultraviolet energy (380 nm peak) and this fluorescence may influence SPF spectral sensitivity measurements. These findings suggest that the SPF's are the results of photolytic and thermolytic reactions occurring in the ommatidial visual pigments and that they have a role in the mechanisms which transduce light to electrical activity in the visual receptors.

EFFECTS Of CULTURAL CHANGES ON MAKARA EXPERIMENTAL BASIN

Abstract The effects of oversowing and topdressing unimproved pastures on small catchments of a few acres each Is discussed. Analyses are Intended to determine hydrological tendencies and are of a preliminary nature only. Detailed studies to clarify hydrological processes are to follow. The land management change indicated has, together with a trebling of the production, had a. considerable hydrological effect. There has been some decrease in, annual run-off, a probable increase In Infiltration, an increase in surface detention, a reduction In the number of days on which flow occurred and a reasonably uniform decrease in the percentage occurrence of given daily run-offs over the greater part of the flow range. Individual hydrographs have shown no decrease in rise time, but an Increase in depletion time, decreased peak discharges and decreased run-off.

Thermal and Spectral Sensitivities of Discrete Slow Potentials in Limulus Eye

The discrete, subthreshold, slow potential fluctuations (SPF's) which can be recorded intracellularly in Limulus ommatidia are sensitive to temperature and light wavelength. SPF frequency increases with increasing temperature (Q(10) about 3.5) and light intensity. The effects are additive. SPF rise and decay time decrease with increasing temperature (Q(10) between 2 and 3). There is a peak, near 520 nm, in the spectral sensitivity of SPF frequency. This peak may correspond to the wavelength of maximum absorption by rhodopsin in the ommatidia. Hydroxylamine produces a rapid, irreversible reduction of SPF frequency and amplitude perhaps owing to its action on the photopigment. The cornea and crystalline cones fluoresce (peak about 445 nm) when excited by near-ultraviolet energy (380 nm peak) and this fluorescence may influence SPF spectral sensitivity measurements. These findings suggest that the SPF's are the results of photolytic and thermolytic reactions occurring in the ommatidial visual pigments and that they have a role in the mechanisms which transduce light to electrical activity in the visual receptors.