Plasma Generation by Ultrashort Multi-chromatic Pulses During Nonlinear Propagation

The use of femtosecond lasers in industrial, biomedical, and defense related applications during the last 15 years has necessitated a detailed understanding of pulse propagation coupled with ultrafast laser-material interactions. Current models of ultrashort pulse propagation in solids describe the pulse evolution of fields with broad spectra and are typically coupled to models of ionization and laser-plasma interaction that assume monochromatic laser fields. In this work we address some of the errors introduced by combining these inconsistent descriptions. In particular, we show that recently published experiments and simulations demonstrate how this contradiction can produce order-of-magnitude errors in calculating the ionization yield, and that this effect leads to altered dimensions and severity of optical breakdown and laser-induced modifications to dielectric solids. We introduce a comprehensive treatment of multi-chromatic non-equilibrium laser-material interaction in condensed matter and successfully couple this model to a unidirectional (frequency-resolved) pulse propagation equation for the field evolution. This approach, while more computationally intensive than the traditional single rate equation for the free electron density, reduces the number of adjustable phenomenological parameters typically used in current models. Our simulation results suggest that intentionally multi-chromatic fields (i.e. strongly chirped pulses or co-propagating pulses of different frequencies) can be arranged to control ionization yields and hence ultrafast laser induced material modifications.