A sensitive electromagnetic acoustic transducer for picometer-scale ultrasonic displacement measurements

The paper presents a novel design of an electromagnetic acoustic transducer (EMAT) for non-contact ultrasonic measurements. An EMAT sensor has been miniaturized with an overall receiver size to 1.0 cm diameter. It has been combined with a specially designed low noise preamplifier. Calibration procedures with a Michelson interferometer revealed that the EMAT sensor detected small displacement amplitudes as low as 1.0 ± 0.2 pm in aluminium samples, with a signal bandwidth up to 7.5 MHz (−3 dB). Alternatively, the sensitivity figure can be expressed in terms of a noise-equivalent displacement spectrum as 4.2 × 10−17 mHz−1/2. The absolute sensitivity of the EMAT was 1.98 ± 0.50 V pm−1 for the detection of longitudinal ultrasonic waves. This sensitivity is compared with other forms of ultrasonic transducer.