  A computational method based on a first-principles multiscale simulation has
been used for calculating the optical response and the ablation threshold of an
optical material irradiated with an ultrashort intense laser pulse. The method
employs Maxwell's equations to describe laser pulse propagation and
time-dependent density functional theory to describe the generation of
conduction band electrons in an optical medium. Optical properties, such as
reflectance and absorption, were investigated for laser intensities in the
range $10^{10} \, \mathrm{W/cm^{2}}$ to $2 \times 10^{15} \, \mathrm{W/cm^{2}}$
based on the theory of generation and spatial distribution of the conduction
band electrons. The method was applied to investigate the changes in the
optical reflectance of $\alpha$-quartz bulk, half-wavelength thin-film and
quarter-wavelength thin-film and to estimate their ablation thresholds. Despite
the adiabatic local density approximation used in calculating the
exchange--correlation potential, the reflectance and the ablation threshold
obtained from our method agree well with the previous theoretical and
experimental results. The method can be applied to estimate the ablation
thresholds for optical materials in general. The ablation threshold data can be
used to design ultra-broadband high-damage-threshold coating structures.
