Ultrafast optical study of surface acoustic waves up to 50 GHz on patterned layered nanostructures

We report ultrafast optical pump-probe measurements of surface acoustic waves (SAWs) on patterned layered nanostructures. These very high frequency SAWs ranging from 5 to 50 GHz were generated and detected on the following patterned film stack: 25 nm Al / 60-112 nm a-SiO2 / Si (100). The Al was etched to form lines of rectangular cross section with pitches ranging from 1000 nm down to 140 nm and the lines were oriented parallel to the [110] direction on the Si. The absorption of ultrafast pump pulses from a Ti:sapphire oscillator operating at 800 nm generated SAWs that were detected by time-delayed probe pulses at 800 nm or 400 nm via a reflectivity change (ΔR). We compared our results to an isotropic elastic calculation and a molecular dynamics simulation. In all cases we identified a Rayleigh-like SAW with wavelength equal to the pitch and frequency in the range of 5 GHz – 24 GHz. In some samples, we identified additional SAWs or independent modes of the Al lines with frequencies close to 50 GHz. We also describe the effects of probe beam polarization on the measurement’s sensitivity to the different surface modes.