Acoustic and Acousto-Optic Characteristics of Silicon Nanofoam

Silicon nanofoam is a porous material with a nanometer structure produced through a sol-gel process, and is used as a heat insulator. it is expected that the nanofoam may work as a good acoustic matching layer of an airborne ultrasonic transducer for highly sensitive and wideband ultrasound transmission/detection since the nanofoam has an extremely low acoustic impedance. The nanofoam may also have a possibility as an acousto-optic device because of its very low sound speed and optical transparency. In this study, we have estimated the fundamental acoustic characteristics of the nanofoam through acousto-optic measurements. Sound speed and acoustic attenuation were measured in the frequency range from 130 to 444 kHz using rectangular samples attached to a piezoelectric transducer. The sound speed and acoustic attenuation constant were approximately in the 140-150 m/s range and 4.3 x 10(-11) f(1.9) dB/(mm.Hz(1.9)), respectively. It was observed that the change rate in the optical refractive index of the nanofoam owing to sound pressure was approximately in the range of (1.2-1.6) x 10(-8) 1/Pa. Raman-Nath diffraction occurred at a relatively low frequency since the sound speed is low. We also observed modulation in the polarization of the transmitted light owing to ultrasonic waves. (C) 2009 The Japan Society of Applied Physics