Picosecond Ultrasound Spectroscopy with a Stable Fiber Laser for Ultrahigh-Frequency-Oscillator Applications: from Nanomechanics to Biosensors

We developed a stable picosecond-ultrasound-spectroscopy system with a fiber-laser light source. A linearly polarized light pulse with 532 nm wavelength is split into pump and probe light pulses using a polarized beam splitter (PBS). The ultrahigh-frequency acoustic waves excited by the pump light pulse are successfully detected using the delayed probe light pulse with a signal-to-noise ratio higher than that of the traditional titanium--sapphire pulse laser. The wavelength used allows deep inspection of silicon because of less light absorption. The developed system is also applied to a biosensor with an ultrathin Pt film resonator, which shows significantly improved stability at 100 GHz. The amount of frequency change caused by the adsorption of target molecules is of the order of $10^{-2}$, which is much higher than that detected with conventional oscillator biosensors by a factor of $10^{4}$.