Detection of ultrasonic vibrations in self-supporting thin films using a laser spectrometer

A heterodyne laser spectrometer has been used to detect the vibrations normal to the plane of a metallic self-supporting thin film (on the order of 1-μm thick or thicker) with a sensitivity on the order of 0.01 Å. The film is excited by attaching it to an interdigital transducer. With continuous scanning of the laser probe across the surface of the film and measuring the amplitude and phase of the vibrations at a fixed frequency, the propagation vector of the modes excited can be accurately determined. This makes it possible to calculate the sound velocity of the antisymmetric Lamb mode and to deduce either the thickness of the film or its elastic flexural modulus