  We demonstrate for the first time to our knowledge the generation of
overcritical plasma densities inside transparent solids over long distances
using femtosecond laser pulses. This opens new avenues for high energy density
physics in confined geometry such as warm dense matter study or the synthesis
of new material phases. We show both with experiments and first-principles
simulations, that femtosecond conical interference via a Bessel beam creates a
dense plasma rod with typically 100 nm diameter in sapphire. The interaction is
in ideal conditions to trigger collisionless resonance absorption. This
mechanism plays a primary role in the energy deposition process, yielding a
plasma with an energy density on the order of MJ/cm3 and a length that can
reach several cm using only tabletop femtosecond lasers.
