Chirp Dependent Plasma Generation by Ultrashort Pulse Lasers*

The ionization dynamics of plasma generated by high-intensity, ultrashort pulse lasers (USPL) are experimentally demonstrated to be dependent on the chirp sign, or frequency ordering within the pulse envelope. Chirp is often used to control the onset location of USPL-generated filaments, or low density (~10¹⁶ cm^-3) plasma channels, so it is necessary to consider how temporal pulse structure affects ionization. In filamentation, multi-photon ionization dominates the initial ionization, with tunneling ionization and collisional ionization with neutrals contributions typically ignored. Many USPL plasma generation models assume a constant ionization rate based on a laser pulse defined by the central frequency and envelope duration. We will report ongoing investigations on the effects of intra-envelope temporal structure on ionization dynamics. Specifically, spectroscopy of the USPL-generated plasma was conducted for air pressures from 760 to 1 Torr and for pulse durations from 45 to 370 fs. Equal pulse durations with opposite chirps were chosen, meaning that the positive chirp produced a pulse with lower frequencies temporally at the front of the envelope, while negative chirp results in higher frequencies temporally at the front of the pulse envelope. Noticeable variations were observed in the relative intensity and spatial distribution of atomic and molecular spectral lines for equal pulse durations with opposite chirps. The decrease in pressure accentuated these differences and provided insight into pressure dependent ionization dynamics. This work will inform modeling efforts in better understanding how USPL induced ionization processes change with initial environmental and laser pulse conditions.