Real-time collective electron oscillations in non-relativistic femtosecond laser-produced plasma channels

We have investigated real-time collective oscillations of plasma electron density in a femtosecond laser-induced helium plasma using time-resolved Mach–Zehnder interferometry with picosecond temporal resolution. Ultrashort laser pulses were focused on the helium gas jet, creating a peak intensity >1016 W cm−2 sufficient to ionize helium gas and generate a transient plasma channel longer than the Rayleigh range of the focusing optics. A weak plasma wake was formed behind the laser pulse due to the ponderomotive expulsion of electrons in the plasma. The Coulomb restoring force from the ions sets up collective plasma oscillations on picosecond time scales.