## 2 Materials and Methods
L\({}_{1}\), L\({}_{2}\), and L\({}_{3}\) are lenses projecting (with the required scale) into the center of the sample a Gaussian–like transverse field distribution at the output of a heating ytterbium laser operating in pulse-periodic regime; L\({}_{4}\) and L\({}_{5}\) are lenses projecting (with the required scale) into the center of the sample a Gaussian–like transverse field distribution in the far zone of a probe helium–neon laser (\(\lambda\) = 633 nm), L\({}_{6}\) is a lens projecting (with the required scale) the transverse distribution of the probe beam field onto the diaphragm (this distribution is selected in the cross section where the amplitude of the time-varying component of the probe radiation power is maximum); mirrors M\({}_{2}\), M\({}_{3}\), M\({}_{5}\), and M\({}_{6}\) have a reflection coefficient R\({}_{1071}\approx 100\%\) at a wavelength of 1071 nm, mirrors M\({}_{1}\), M\({}_{4}\), and M\({}_{7}\) have a reflection coefficient R\({}_{633}\approx 100\%\) at a wavelength of 633 nm, and mirrors M\({}_{4}\) and M\({}_{7}\) have a reflection coefficient R\({}_{633}\approx 0\%\) at a wavelength of 633 nm; E\({}_{\rm heat}\) and E\({}_{\rm probe}\) are the fields of the heating and probe beams, respectively. Mirror M\({}_{7}\) has a selective function, also due to the cutting of heating radiation by its edge. In this case, the heating radiation is absorbed into the beam trap and is not incident on the photodiode diaphragm. The directions of the heating (gray) and probe (red) beams are shown by arrows, and their waists are schematically shown in the sample area. amplified pulse-time-varying component was fed to the input of a digital oscilloscope operating in the averaging regime. In a real experiment, averaging was performed by \(\leq\)10\({}^{4}\) pulses, which made it possible to significantly (by more than 100 times) reduce the maximum amplitude of the noise component. Samples of ultrapure quartz glass (UQG) Suprasil 311 (\"Heraeus\") [24] and synthetic crystalline quartz (SCQ) (\"Quartz technologies\") [25] grown with the hydrothermal method were used for experimental research. Suprasil 311 quartz glass is made by the soot outside vapor deposition method. It is one of the most transparent quartz glasses on the world market and differs from other glasses of the Suprasil family by having a relatively high content of hydroxyl groups (\(\sim\)250 ppm), which increases its absorption for wavelengths around 1064 nm. In addition, the time-varying component, which was observed in the absence of samples when the probe and heating laser beams propagated in the ambient air, was measured. All the presented samples had significantly different absorption coefficients and thermo-optical parameters P [26] (an analog of dn/dT, which takes into account the stresses arising in the sample during local heating [17]), as well as the speed of spreading of the temperature profile when the medium is heated by a spatially inhomogeneous laser beam. This speed can be characterized quantitatively by using the mentioned parameter \(\tau_{\rm diff}\), derived from dimension considerations. Above, we conventionally called this quantity the heat diffusion time since the expression for \(\tau_{\rm diff}\) includes physical constants that affect the heat diffusion rate. The values of the constants for the samples used in the experiment are given in Table 1. \begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Sample**} & **Specific Heat** & **Density** & **Heat Conductivity** & \multirow{2}{*}{\(\tau_{\rm diff}\) = \(\rm{C\rho_{a}}^{2}/\Lambda\), **s**} \\  & **C. J/g\(\cdot\)K** & \(\rho_{\rm g}\)/cm\({}^{3}\) & \(\Lambda\), J/cm \(\cdot\)s \(\cdot\)K & \\ \hline Suprasil 311 & 0.728 & 2.2 & 0.0138 & \(2.56\times 10^{-3}\) \\ SCQ & 0.710 & 2.65 & 0.0845 & \(0.49\times 10^{-3}\) \\ Air & 1.02 & \(1.22\times 10^{-3}\) & \(2.53\times 10^{-4}\) & \(0.11\times 10^{-3}\) \\ \hline \hline \end{tabular}
\end{table}