Ultrafast ellipsometric interferometry for direct detection of coherent phonon strain pulse profiles

Abstract

We describe an ultrafast optical technique to quantitatively detect picosecond ultrasonic displacements of solid surfaces, thus giving access to the longitudinal strain pulse shape. Transient optical reflectance changes recorded at oblique optical incidence with a common-path interferometric configuration based on ultrafast ellipsometry monitor gigahertz coherent phonon pulses. We demonstrate for a tungsten film the quantitative extraction of the strain pulse shape free of distortions arising from the photoelastic effect, and analyze the results with the two-temperature model to obtain the value g≈3×1017  Wm−3 K−1 for the electron–phonon coupling constant. Analysis of the data also reveals a thermo-optic contribution.