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Nanocavities drilled by femtosecond lasers

publié le , mis à jour le

Nanocavities drilled by femtosecond lasers

Interaction of the ultrashort laser pulses with transparent
dielectrics is a fast growing domain of material processing.
Contrary to metals, where the laser interaction is localized in
a narrow surface layer, dielectrics open a possibility of
energy delivering inside the material so the interaction takes
place in a confined geometry. This could have various applications
such as hidden inscriptions, a high density information
storage, optical switches, etc.

In this work, we present the electromagnetic calculations
of the laser energy deposition in the two-dimensional 2D
plane geometry and we are using them as initial conditions
for the 2D axially symmetric hydrodynamic calculations of
the blast-wave generation and the void formation. This separation
of the whole problem in two subsequent parts is possible
because of a large difference in the time scales : the
laser pulse duration and the time of plasma formation are
much shorter than the electron-ion temperature equilibration
time and the time of ion response to the strong electron pressure.
The hydrodynamic simulations show that the ion motion
starts after several ps, long time after the end of the laser
pulse.

Compared with the previous models,
our model provides a reduced absorption coefficient and
a much broader, elliptically shaped energy deposition zone.
The total absorption coefficient is a growing function of the
laser pulse energy ; however, at high intensities a significant
part of the laser energy is absorbed before the focus and the
localized absorption in the focal zone achieves its maximum
at the level of 33%. This figure could be improved by correcting
the incident wave front and optimizing the focal position
inside the target.

This absorption model was used as an input for the subsequent
hydrodynamic 2D axially symmetric simulations describing
the blast-wave propagation and the cavity formation.
The latter process is shown to be very sensitive to
properties of the EOS in the domain of negative presures.
The data given in the Sesame tables provide a better agreement
with the experimental data and opens other ways for
control of the cavitation process and optimization of the energy
deposition in dielectric materials.