High-Energy-Density Plasma Nanorod by Collisionless Absorption of Ultrafast Laser Pulse Inside Dielectrics.

The generation of energetic and dense plasmas by femtosecond laser pulses within the bulk of solids can pave the way to study warm dense matter, shocks, extreme UV radiation, or the synthesis of new material phases. However, this has remained elusive because of the intrinsic dynamical effects of defocusing by the laser-generated plasma. Here, we demonstrate the generation of overcritical plasma densities inside transparent solids over long distances. We identify with experiments in bulk sapphire and first-principles simulations that femtosecond conical interference via a Bessel beam creates a dense plasma rod of typically less than 400 nm diameter. We show that collisionless resonance absorption plays a primary role in the energy deposition process, yielding a plasma with an energy density on the order of MJ/cm^{3}, whose length can reach several cm using only tabletop femtosecond lasers. This opens new avenues for developing high-energy-density physics within solids.