Characterization of thin metal films by ultrafast laser induced ultrasound

Ultrafast laser pulses incident on metals can lead to the generation of coherent phonon wave packets with frequencies in the gigahertz to terahertz range [1,2]. Material characterization using such ultrafast laser-induced ultrasound pulses provides access to a frequency range that is inaccessible by any other means. Our objective is to study the optical and acoustic properties of materials by generating and detecting ultrasound waves with ultrafast laser pulses. To generate high frequency acoustic waves in a way that also optimizes their detection, we use a pair of crossed 40 fs femtosecond pump pulses at 400 nm wavelength to project interference fringes on the surface of thin metal films. Because ultrasound is only generated in the interference maxima, this approach produces a spatially periodic array of acoustic pulses. The acoustic pulses propagate through the film and are reflected at the back surface. A delayed probe pulse (30 fs, 800 nm) then detects the returning acoustic echo by detecting a change in the optical response that occurs every time an acoustic echo returns to the surface. Because a periodic array of ultrasound waves was produced, we can detect the first order diffraction of the probe beam by this 'acoustic grating'. By performing these measurements in thin free-standing metal membranes, the influence of substrate interfaces is eliminated, and acoustic attenuation is only caused by propagation in the metal, providing clean measurements of the metal parameters without external factors.