Time-resolved Characterization of Ultrashort Pulse Propagation

Abstract

The propagation of ultrashort femtosecond laser pulses in linear dielectric materials is determined in the time, space, and frequency domains by linear Maxwell optics through dispersion and diffraction. For intense pulses, pulse propagation is additionally modified by nonlinearities in the medium such as the optical Kerr effect, plasma generation, and self-phase modulation.

In this work we report the results of several experiments on the propagation of ultrashort pulses. In the linear regime, we characterize the temporal evolution of an ultrashort pulse during propagation through a linear dielectric under anomalous dispersion. Under these conditions the pulse evolution departs from the group velocity and group delay dispersion approximations, which leads to the formation of optical precursors. We describe an experiment which observes the propagation of optical precursors in a bulk condensed-matter dielectric. We generate ultrashort laser pulses and propagate the pulses through a bulk dye with an absorption resonance turned to the center wavelength of the femotsecond pulse. The pulse is then characterized in the time domain before and after propagation. Through numerical simulation we verify that the behavior of the precursors in the temporal pulse profile corresponds with the classical model.

Under very high intensity laser pulses, the nonlinearities induced by the propagation of the intense ultrashort pulse produce changes in the complex refractive index of the nonlinear material. We report the results of experiments involving time- resolved imaging of the propagation of ultrashort pulses in dielectric materials. We experimentally observe and characterize these effects through a weak-probe imaging effect which directly measures the nonlinearity in a time-resolved manner. In these experiments an intense femtosecond laser pulse is propagated in a nonlinear intensity regime while an unfocused low-intensity femtosecond pulse is used as to probe the nonlinear pulse. We use this technique to characterize femtosecond pulses in air and liquid, especially in the regime of optical filamentation. We subsequently calculate parameters such as the plasma density, the transverse extent, and the instantaneous refractive index within the femtosecond laser filament under conditions which are not accessible through most standard pulse measurement techniques.