Femtosecond laser-induced sub-wavelength plasma inside dielectrics: I. Field enhancement

Abstract

The creation of high energy density ( 10 6 joules per cm 3 ) over-critical plasmas in a large volume has essential applications in the study of warm dense matter, being present in the hot cores of stars and planets. It was recently shown that femtosecond Bessel beams enable creating over-critical plasmas inside sapphire with subwavelength radius and several tens of micrometers in length. Here, the dependence of field structure and absorption mechanism on the plasma density transverse profile are investigated by performing self-consistent Particle-In-Cell (PIC) simulations. Two limiting cases are considered: one is a homogeneous step-like profile, that can sustain plasmon formation, the second is an inhomogeneous Gaussian profile, where resonance absorption occurs. Comparing experimental absorption measures to analytical predictions allows determining the plasma parameters used in PIC simulations. The PIC simulation results are in good agreement with experimental diagnostics of total absorption, near-field fluence distribution, and far-field radiation pattern. We show that in each case an ambipolar field forms at the plasma surface due to the expansion of the hot electrons and that electron sound waves propagate into the over-critical region.

Electron heating and radiation in high aspect ratio sub-micron plasma generated by an ultrafast Bessel pulse within a solid dielectric

“…This arises from the efficient Landau damping. In the over-critical region, the field oscillations are due to electron sound waves [21,23]. They penetrate relatively deeply into the overcritical region and we observe that they are curved.…”

“…We assume that nonlinear ionization has produced early in the pulse a plasma and we model the interaction of the most intense part of the laser pulse with this pre-plasma. We used the numerical code EPOCH [20], without ionization, and the numerical scheme is detailed in reference [21]. We maintained the numerical heating at a negligible level over the duration of the simulation (320 fs).…”