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(a) Schematic drawing of recirculating reflective pulse shaper, called Deathstar Compact, for evenly spaced pulse sequences with adjustment of a single delay line to change the repetition rate. The output sequence (perspective change indicated by waved spacer lines) was focused onto the sample (b) where it illuminated an aluminum transducer and generated an acoustic pulse train. The acoustic wave packet propagated into and through variable thickness layers before being detected interferometrically at the air interface of a second aluminum film. (c) Calculated power spectrum of the photo-acoustic response in a polycrystalline Al transducer (parameters from Ref. 21) to single 200 fs pulse excitation and multiple pulse sequences with a 165 GHz repetition rate.
Raw data for different thicknesses with pulse shaper frequencies set to 165 GHz (a), 50 GHz (b), and 300 GHz (c). Smooth curves show interferometrically measured displacement while strongly modulated curves show corresponding strain (time derivative). Data have been normalized and vertically shifted. Inset in (a) shows the Fourier spectrum of the transmitted acoustic signal.
Results of present measurements of frequency-dependent acoustic attenuation in at room T and results from other spectroscopic techniques.
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