Spectral Modulation of Self-Guided Laser Pulses

In this paper the experimental results of spectral modulation of a self-guided laser pulse in an underdense plasma will be presented. Experiments were conducted using an ultrashort laser pulse ($\sim $50 fs) generated from the UCLA Ti:Sapphire laser system capable of delivering up to 10 TW of power. A gas jet was used to create a dense column of helium gas which the laser pulse ionized and self-guided through. By varying the laser pulse width, laser energy, gas jet density and gas jet length, different physical mechanisms of self-guiding were explored. In these experiments the guided laser pulse was spectrally and spatially resolved using a .25 m imaging spectrograph with 1.2 nm spectral resolution and 13 $\mu $m spatial resolution. Evidence of photon acceleration / deceleration due to the laser pulse interacting with density oscillations of a plasma wakefield will be presented and compared to simulation results. Additionally using the imaging spectrograph the percentage of the laser energy that was self-guided was determined.