Precise Observation of Ultrasonic Pulses Using an SPR Sensor

Surface plasmon resonance (SPR) type ultrasonic sensors enable local measurements and have a flat frequency response in a wide frequency range. They are expected to be used in photoacoustic microscopy (PAM) and other applications that require broadband ultrasonic detection. In this study, we focus on the precise measurement of ultrasound pressure waveforms via a Kretschmann-type SPR sensor. The noise equivalent pressure was estimated to be 52 Pa
$_{\text {p-p}}$
, and the maximum amplitude of the wave measured via the SPR sensor linearly responded to the pressure until 427 kPa
$_{\text {p-p}}$
. Furthermore, the observed waveform for each applied pressure agreed well with the waveforms measured via the calibrated ultrasonic transducer (UT) in the MHz range. Moreover, we focused on the effect of the sensing diameter on the frequency response of the SPR sensor. The results show that the beam diameter reduction improved the frequency response at high frequencies. Evidently, we found that the sensing diameter of the SPR sensor must be carefully selected in consideration of the measurement frequency.