Pulse duration

About: Pulse duration is a research topic. Over the lifetime, 19429 publications have been published within this topic receiving 286507 citations.

What can short-pulse pump-probe spectroscopy tell us about Franck-Condon dynamics?

Abstract: We examine the signal from pump-probe spectroscopy of a model system—nonrotating I2—at short time delays and compare signals calculated without approximation (a full quantum calculation), with a semiclassical Franck-Condon approximation, and with a classical simulation of the nuclear wave packet. In order to assess the complications of simulation and interpretation when the probe window lies in the spectroscopically and dynamically important Franck-Condon region, we concentrate on a case where pump and probe resonances are at the same internuclear distance. We find that the common practice of ignoring the pump-truncation effects of pulse overlap leads to an overestimate of the signal at short times. Moreover, both classical simulations and semiclassical Franck-Condon treatments can deviate significantly in form from the actual signal even with proper treatment of pulse overlap. The sources of these deviations can be seen in the evolution of the excited-state nuclear distributions calculated classically and under the semiclassical Franck-Condon approximation. Specifically, the differences in evolution of the classical and full quantum excited-state nuclear distributions are due to differing initial momentum distributions. We introduce an efficient method for calculating the pump-probe signal that takes advantage of the brevity of ultrashort pulses and can include pulse characteristics such as chirp. This short-pulse expansion method aids in the proper treatment of pulse-overlap and nonzero pulse duration and promises to simplify the incorporation of relaxation processes.

Modeling of laser-induced breakdown in dielectrics with subpicosecond pulses

Abstract: Theoretical study of ultrafast laser induced damage by short pulses (τ<1 ps) is carried out on large-band-gap dielectric in an effort to understand the complex physical processes involved. The numerical method of solving a general time-dependent Fokker–Planck type equation for free electron production is discussed in detail. The calculation shows that the collisional avalanche ionization competes with the multiphoton ionization even for pulse length shorter than 25 fs. Sensitivity tests of all the rates in the equation are performed and the most critical ones are identified. From these tests we obtain valuable information in developing new materials that have the desired damage fluence for specific applications. To describe the relaxation of electron plasma, a three body recombination rate is included. Thus, the temporal behavior of the electron density due to a single pulse is treated, as well as the case of exposure to two laser pulses with a time delay between them. The model is only partially successful in reproducing the recent experimental data. Effect of the presence of a linear decay term and optical defects on the damage threshold is considered in the context of the rate equation input.

Micromachined thin film SnO2 gas sensors in temperature-pulsed operation mode

Abstract: Gas detection measurements based on a micromachined SnO 2 gas sensor with periodically pulsed heater voltage are presented. Additionally, the field-effect-induced changes in resistivity of the sensitive layer caused by the heater voltage were investigated. The combination of both results leads to an improved design for low power SnO 2 gas sensors. In temperature-pulsed mode, the sensor resistances were measured at constant delays after the pulse edges. The measurements were carried out with the common test gases carbon monoxide and nitric dioxide in synthetic air with 50% humidity. In the cold pulse phase, the CO sensor response is higher and shows only a slow decrease with increasing pulse duration. The sensor sensitivity is related to the pulsed heated mode, on the one hand, and the continuously heated, on the other. The comparison of the measurement results reveals that the temperature-pulsed operation mode (TPOM) caused a significant reduction of power consumption and higher sensitivity.