High-sensitivity plasma density characterization by cross-polarized temporal interferometry

Absolute density measurements of low-ionization-degree or low-density plasmas ionized by lasers are very important for understanding strong-field physics, atmospheric propagation of intense laser pulses, Lidar etc. A cross-polarized common-path temporal interferometer using balanced detection was developed for measuring plasma density with a sensitivity of ~0.6 mrad, equivalent to a plasma density-length product of if using an 800 nm probe laser. Using this interferometer, we were able to measure plasma density over 5 orders of magnitude, which allowed us to verify different photoionization models in tunneling and multiphoton regimes. We have investigated strong-field ionization yield versus intensity for various noble gases (Ar, Kr, and Xe) at different wavelengths (267-800 nm) with both linear and circular polarizations. The experimental results were compared to ADK and PPT models as well as TDSE simulations. Since our method is sensitive to the refractive index, we can also measure the population of Rydberg states, which have similar susceptibility as free electrons. In some special cases, our experimental results showed a significant population of Rydberg states excited by multi-photon resonances, which was confirmed by TDSE simulations.