ULTRASONIC WAVE GENERATION BY TIME-GATED MICROWAVES

This paper shows how the energy transfer between electromagnetic waves and elastic waves at the surface of fluid or solid materials could co nstitute a new way to generate ultrasonic waves without contact for th e purpose of nondestructive evaluation and control. When time-gated mi crowaves strike the surface of a material, there is a generation of ul trasonic waves. The times-of-flight of these elastic waves inside the material decrease when the power of the incident microwaves increases. Therefore, the diminution of the propagation path indicates that the energy transformation appears in a volume close to the surface, the de pth of which is increasing along with the power of the microwaves and depends on the material properties. Consequently, the elastic wave gen eration is attributed to the evolution of the power of the microwave d uring the gate width that governs the frequency content of the ultraso nic waves. The generation of ultrasound is checked in water and polyme rs. If the microwaves are not too attenuated in the material, the ultr asonic waves can be generated both at input (air-solid) and output (so lid-air) interfaces. These peculiarities can generate new applications in the nondestructive evaluation and control of material. (C) 1998 Ac oustical Society of America. [S0001-4966(98)01308-3]