  Ultrafast laser pulses spatially shaped as Bessel beams in dielectrics create
high aspect ratio plasma channels whose relaxation can lead to the formation of
nanochannels. We report a strong enhancement of the nanochannel drilling
efficiency with illumination by double pulses separated by a delay between 10
to 500 ps. This enables the formation of nanochannels with diameters down to
100 nm. Experimental absorption measurements demonstrate that the increase of
drilling efficiency is due to an increase of the confinement of the energy
deposition. Nanochannel formation corresponds to a drastic change in absorption
of the second pulse demonstrating the occurrence of a phase change produced by
the first pulse. This creates a highly absorbing long-living state. Our
measurements show that it is compatible with the semi-metallization of warm
dense glass which takes place within a timescale of <10 ps after the first
laser pulse illumination.
