Two-plasmon-decay induced fast electrons in intense femtosecond laser–solid interactions

The nonlinear coupling of intense laser pulse with plasma leads to excitation of several parametric instabilities, featuring plasmon, acoustic or electromagnetic modes. Specifically, the two-plasmon decay (TPD) instability, relevant to the generation of fast electrons, originates at the quarter critical surface, where the incident photon decays into a pair of electron plasmon waves. Although well explored by nanosecond lasers, the TPD instability is rarely seen in femtosecond laser–plasmas, mainly due to steep plasma profiles and the ultrashort duration of the driving pulse. Our experiments show TPD boosting of fast electrons in the femtosecond regime using low intensity contrast and temporally stretched laser pulses. The fast electron spectrum and 3ω0/2 harmonic emission from plasma show significant enhancement for stretched pulses (120 fs) in comparison to transform-limited short pulses (30 fs). The generation of fast electrons is found to be linked with the growth of 3ω0/2 harmonic. The effect of longer femtosecond pulses on TPD growth is observed using several fast electron diagnostics such as electron kinetic energy spectra, hard x-ray emission, rear side plasma emission, Cherenkov emission, and time-space resolved shadowgrams up to laser intensity of 1.5 × 1018 W cm−2. We, thus, provide robust and unambiguous demonstration of TPD instability driven generation of fast electrons in femtosecond laser–plasma interactions.