Quantum materials with layered structures offer an excellent platform for exploring emerging electronic, mechanical, and optical properties with potential for technological applications. Trigonally structured $t\text{−}{\mathrm{PtBi}}_2$ is such a material. When a femtosecond laser pulse is directed onto $t\text{−}{\mathrm{PtBi}}_2$, the resulting transient reflectivity reveals resonances of coherent acoustic phonons, arising from the interference between the diffracted light and light reflected at the $t\text{−}{\mathrm{PtBi}}_2$/${\mathrm{SiO}}_2$ interface. The resonance frequency is found to decrease with increasing the sample thickness from 71.48 GHz for the 20-nm-thick sample to 2.62 GHz for the 276-nm-thick sample. By combining the resonance frequency and mass density, we determine the out-of-plane elastic constant ${\mathrm{C}}_{33}$, which decreases from 84 GPa for the 20-nm-thick sample to 27 GPa for the 276-nm-thick sample, indicating high sensitivity to the sample thickness. However, the theoretical ${\mathrm{C}}_{33}$ value of 60 GPa obtained from first-principles calculations is higher than the experimentally determined saturated value for bulk. The discrepancy may be explained by considering the temperature difference and possible point defects in $t\text{−}{\mathrm{PtBi}}_2$, which can cause softening through wave sliding. Our combined experimental and theoretical investigation demonstrates that the elastic properties of layered materials can be obtained via light-matter interaction, which is particularly powerful for $t\text{−}{\mathrm{PtBi}}_2$ due to its low thermal conductivity and high optical absorption.

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• Acoustic phonons
• Elastic modulus
• Mechanical & acoustical properties
• Topological materials
• Layered crystals