A picosecond ultrasound system for measuring the elastic stiffness of thin films at high temperatures is developed, and the elastic stiffness of Co films is measured up to 771 K during heating and cooling processes. The elastic stiffness of an as-deposited film is smaller than that of bulk Co. However, during heating, stiffening due to crystallization is observed, and in the cooling process, the elastic stiffness follows the temperature dependence of bulk Co. These results indicate that the as-deposited film is softer than the bulk and that once stiffening has occurred upon annealing, the Co thin film shows a similar elastic property to the bulk. The temperature dependence of acoustic attenuation is also discussed.

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 .

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 .