Measurement of elastic constant and refraction index of thin films at low temperatures using picosecond ultrasound

In this paper, a picosecond ultrasound measurement is conducted to evaluate the low-temperature elastic and optical properties of thin films and semiconductors. Specimens are cooled with liquid He through a heat exchanger in a cryostat, and an ultrahigh-frequency acoustic pulse is generated using a femtosecond light pulse, which propagates in the film-thickness direction. Pulse echoes of the longitudinal wave and Brillouin oscillation are observed by the changes in reflectivity of the time-delayed probe light, which depend on the material, and give the longitudinal-wave out-of-plane elastic constant. When the stiffness is known, the Brillouin oscillation provides the refractive index. We determined the stiffness of a Pt thin film and the refractive index of Si at 5 K. The methodology developed in this paper is useful for studing the elastic and optical properties of metallic thin films and transparent materials at cryogenic temperatures.Kenichi Tanigaki, Tatsuya Kusumoto, Hirotsugu Ogi, Nobutomo Nakamura and Masahiko Hirao. Measurement of elastic constant and refraction index of thin films at low temperatures using picosecond ultrasound. Japanese Journal of Applied Physics, 2010, 49(7S), 07HB01. https://doi.org/10.1143/JJAP.49.07HB01