Ultrafast laser-induced guided elastic waves in a freestanding aluminum membrane

Ultrafast laser-induced guided acoustic waves in thin, freely suspended films are important for many applications adopting the laser-ultrasonics technique. These waves show unique dispersion relations, leading to minimal propagation losses and acoustic energy confinement. While this principle has been known, the separation of various physical effects in the formation of measured signals involving these guided acoustic waves has not been clearly elaborated. Here, we present a combined experimental and theoretical study on all-optical excitation and detection of these waves in a thin, freestanding aluminum membrane. The acoustic dynamics is excited and measured by using a femtosecond time-resolved pump-probe technique with controlled probing position, enabling spatially resolved detection. The measured signals are compared with an advanced numerical model that we developed earlier [H. Zhang et al., Phys. Rev. Appl. 13, 014010 (2020)], showing excellent agreement. The combination of experiment and simulation allows us to decode various physical effects in the signal formation, including different acoustic field components. Unknown material properties, such as acoustic attenuation coefficients, and the two complex photoelastic constants are quantitatively retrieved by fitting the measured signals. Furthermore, we provide evidence of nonlinear propagation of the excited guided acoustic waves.