Let's examine now the physical phenomena involved in these experiments. The optical intensity, $I_0$, of the laser beam is defined as $I_0=\frac{1}{S}\frac{d E}{dt}$, where $S$ is the beam surface and $E$ is the beam energy. When emitted on the surface of a medium and in the absence of reflection, the laser beam is absorbed with an exponential decay as a function of the medium depth $z$: $I(z)=I_0 \exp(- \gamma z)$, where $\gamma$ is the absorption coefficient of the medium. We have experimentally estimated $\gamma$ by measuring the fraction of light transmitted through different slices of the medium (with thicknesses of 0, 30, 50, and 100 $\mu$m) with a laser beam energy-measurement device (QE50LP-S-MB-D0 energy detector, Gentec, Qu\'ebec, QC, Canada). We found respective transmitted powers of 100\%, 42\%, 28\%, and 11\%, which gives $\gamma^{-1} \approx$ 40 $\mu m$ in our sample as indicated by an exponential fit.
