Noncontact measurement of sub-micrometer ultrasonic vibration by near-field microwave

Abstract

In this article, we proposed a noncontact testing scheme for micrometer to sub-micrometer level ultrasonic vibration mainly generated by laser ultrasound upon the novel utilization of near-field microwaves. The measurement was performed based on a Ku-band near-field resonator-probe, which was optimized to perceive the low amplitude out-of-plane vibration in the ultrasonic range. A near-field electromagnetic measurement theory was established to help analyze the sensitivity and responsivity of the resonator-probe outfit. The electric field parameters were extracted with respect to the port based on a lumped-circuit model, which was further validated using a finite-element model. Experiments were carried out with respect to a piezoelectric ceramic disk designed to generate vibrations of desired amplitude, which is compatible to that in laser ultrasonic testing. The accuracy of the technique was assessed by comparing with the laser vibrometer measurement results. It was verified that the proposed near-field microwave probe can achieve considerable accuracy in perceiving sub-micrometer ultrasonic vibration up to 180 kHz.