Femtosecond Laser-Induced Coherent Acoustic Phonon Echo Pulses in Layered PtBi2

Layered quantum materials, such as trigonal PtBi₂ (t-PtBi₂), are ideal for discovering new electronic and optical properties. When a femtosecond laser pulse is directed onto t-PtBi2, the resulting transient reflectivity reveals resonances of coherent acoustic phonon echoes, arising from the interference between the diffracted light and light reflected at the t-PtBi2/SiO2 interface. The phonon echo 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 C33, 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 C33 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 temperature difference and possible point defects in t-PtBi2, 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-PtBi2 due to its low thermal conductivity and high optical absorption.