## 2 Materials and Method  
Figure 1 shows the electron-optical scheme used for the measurements in our recent papers [14; 15]. In this paper, a single-mode ytterbium fiber laser (\(\lambda\) = 1071 nm) operating in pulse-periodic mode was employed as a heating laser, and a continuous-wave, single-mode, single-frequency, low-noise, stabilized He-Ne laser (\(\lambda\) = 633 nm) with an output power of 2 mW was employed as a probe laser. Using the lens system [14; 15], a Gaussian radiation profile of the heating and probe laser beams (coaxial) was formed in the sample, as well as the necessary transverse size of the beams in the waist (the beam radius at the 1/e level in the waist of the heating beam was 47 \(\upmu\)m, and that of the probe beam was 100 \(\upmu\)m). The centers of the beam waists were located in the center of the sample. The repetition rate of the heating radiation pulses (\(\approx\)30 Hz) and their duration (1000 \(\upmu\)s) were set using a driving pulse generator. The output energy of the heating laser radiation pulse (33 mJ) was set using the laser control unit. The heating pulse had a quasi-rectangular shape [11], and its power was 33 W. The average power of the heating radiation was about 0.9 W, which was three orders of magnitude greater than the power of the probe radiation. The principle of operation of the TPCI scheme can be summarized as follows. Pulsed heating radiation creates in the sample a time-varying, spatially inhomogeneous distribution of temperature deformations causing refractive index variations. The probe beam experiences diffraction on the refractive index inhomogeneities, resulting in a time-varying component of the radiation power. This is the result of interference between the perturbated and unperturbed parts of the probe beam or, put in another way, single-beam interference [11; 22]. The lens located after the sample transferred the image of the cross-section of the probe beam to the diaphragm of the photodiode. The area from which the image was transferred was located at a distance of 2 cm from the output surface of the sample. The power of the time-varying component was recorded using a photodiode (70% quantum efficiency at the probe radiation wavelength) with an aperture (acting as a spatial filter) and a diffuse plate (to smooth out the effect of inhomogeneity of the photodiode cathode and speckle structure of the field on the photocathode [38]). Then, an amplifier (40 dB gain in the frequency band from 2 Hz to 250 kHz) separated the time-varying component, and the 100-fold amplified signal from the photodiode was inputted to a digital oscilloscope operating in the averaging mode (typical value of the number of averaged pulses is \(\sim\)10\({}^{4}\)) to improve the signal-to-noise ratio. At the repetition rate used the averaging time was about 6 min. The principle of calculating the absorption coefficient in this scheme was described in detail in our previous papers [11; 12; 15; 35]. To compare the observed signals when measuring the absorption of samples of different lengths from different materials, the following problems were solved. First, there was the problem of thermoelasticity during heating by focused radiation [11]. As a result, a coefficient was determined that allowed one to take into account the influence of the thermoelastic properties of the material on the change in the observed signal associated with absorption. Secondly, the problem of diffraction of the probe radiation on spatially inhomogeneous deformations caused by focused heating radiation was solved [11]. As a result, a functional dependence of the time-varying component of the probe radiation power on the absorption coefficient was determined. This dependence took into account all possible geometric parameters of the scheme (the length of the constrictions, the length of the sample, the size of the diaphragm on the photodiode, etc.) [15].