Simultaneous extraction of photoelastic constants and strain profiles from picosecond ultrasonic echoes via spectral analysis

We present an echo spectral analysis method for picosecond ultrasonics that allows simultaneous determination of the complex photoelastic constant of an opaque isotropic thin film and its acoustic strain profile directly from optical pump–probe measurements. In contrast to previous approaches, which require separate evaluation of the photoelastic constant or iterative fitting procedures, the present method yields both the photoelastic constant and the strain profile from acoustic echoes through a purely analytical treatment. The analysis makes use of both the amplitude and phase of transient optical reflectance changes induced by acoustic echoes and requires only the complex refractive index and the longitudinal sound velocity. To illustrate the method, we apply it to simulated echo waveforms for representative opaque films and show that reliable reconstruction is maintained even in the presence of refractive-index errors, background variations, random noise, and frequency-dependent ultrasonic attenuation. • We extract photoelastic constants from picosecond ultrasonic echoes using an analytical spectral method. • We subsequently obtain the acoustic strain profile by Fourier analysis over a broad frequency spectrum. • We validate the method with calculated ultrafast reflectivity and phase changes for an opaque thin film.