Interferometry for Plasma Density Measurements of Ultra-short Terawatt Laser-Induced Plasmas in Low Pressure Gas

Multi-terawatt lasers can self-transform as they propagate in a medium resulting in strong spectral broadening and spatial breakup of the beam due to the highly nonlinear interaction. The resulting broad spectrum, high power beam can be useful in long range remote sensing and detection applications and in probing ultrafast phenomena. In past experiments, we have characterized this spectral broadening and spatial quality for different gases at low pressures (<200mbar). However, the plasma characteristics of these interactions were not explicitly determined. To correlate the spectral broadening and spatial quality of the beam with the photoionization rate at different gas pressures and pulse energies, we used an interferometry method. In our experiments, we use an 800nm central wavelength, 18mm diameter laser with terawatt peak powers that is focused with a 200mm focal length spherical mirror in a vacuum chamber held at a constant, low pressure ranging from 1e-2mbar to 200 mbar of laboratory air. A probe beam is picked off from the main beam and passes through the laser-induced plasma. Then, it is sent to an interferometer to directly measure plasma density. From these measurements, we have determined the plasma characteristics associated with the spectral broadening of the beam. As a result, we can utilize future forward spectrum measurements of this interaction to estimate the plasma density and ionization conditions of the laser-induced plasma at low pressures.