Time-resolved thermal lens measurements of the thermo-optical properties of glasses at low temperature down to 20 K

In this work the time resolved thermal lens spectrometry was applied to measure the absolute values of the thermo-optical properties of low silica calcium aluminosilicate and soda lime glasses at low temperatures, in the range between 20 and 300 K. The thermal relaxation calorimetry was used as a complementary technique to determine the specific heat. The results showed a marked decrease of the thermal diffusivity with the temperature rise, with a dependence similar to that of the mean free path $(\sim T^{-1})$ in the interval between 20 and 70 K, while in the range between 70 and 300 K the dependence was $T^{-(0.33\pm 0.02)}$. The marked variation of the temperature coefficient of the optical path length change with the temperature rise was attributed to the increase in the coefficient of the electronic polarizability. The results also showed that for the aluminosilicate glass the excess in the specific heat correlated to the so-called boson peak occurred at about 17 K, higher than that of soda lime, which occurs at about 12 K. In conclusion, our results showed the ability of the time resolved thermal lens to determine the thermo-optical properties of glasses at low temperatures, bringing possibilities for experiments in a wide range of optical materials.

Figure 1

Thermal lens experimental setup. The excitation and probe beams are provided by an ${\mathrm{Ar}}^{+}$ ion laser $(\lambda =488\mathrm{nm})$ and a He-Ne laser $(\lambda =632.8\mathrm{nm})$ respectively. $M_i$, $L_i$, and $P_i$ correspond to mirrors, lenses, and photodiodes, respectively. The windows are made of quartz disks.

Figure 2

Normalized thermal lens signal $[I\left(\mathit{t}\right)∕I\left(0\right)]$ for LSCAS glass at 20 and at 50 K.

Figure 3

Thermal diffusivity $\left(D\right)$ as a function of the sample temperature for LSCAS and soda lime glasses. The inset presents the same data in logarithmic scale.

Figure 4

Specific heat as a function of the sample temperature for LSCAS measured in this work and for soda lime glasses retrieved from Ref. 18.

Figure 5

Thermal conductivity as a function of the sample temperature for LSCAS (a) and soda lime (b), measured in this work, and for silica vitreous (c) obtained from Ref. 18. The inset shows the data at low temperature in an expanded scale.

Figure 6

The values of $ds∕dT$ as a function of temperature for LSCAS (a) and soda lime (b) glasses. The inset shows the data at low temperature in an expanded scale.