Probing the Spatial Impulse Response of Ultrahigh-Frequency Ultrasonic Transducers with Photoacoustic Waves

Abstract

Characterizing the radiated or received acoustic field of ultrasonic transducers using the spatial impulse response (SIR) represents an important step in testing, design, and optimization of ultrasonic transducers. However, for the ultrahigh-frequency acoustic field, conventional methods, such as the hydrophone method and the small-ball reflection method, are limited by narrow bandwidth and poor spatial resolution. Here, we propose a method to obtain the transducer's SIR through its response to photoacoustic waves, which allows high-precision acoustic field measurements, with spatial resolution as fine as 1.7 µm. We subsequently measure the SIRs of two focused ultrasonic transducers, with 20 and 93 MHz center frequencies, and confirm that the three-dimensional acoustic fields can be accurately reconstructed using the angular spectrum approach. More importantly, this method is unique to receive-only ultrasonic detectors, the SIR of which could not be measured previously with conventional methods, and it could facilitate ultrasonic transducer design, as well as other related fields, such as nondestructive evaluation, biomedical imaging, and particle manipulation.

Recent developments in photoacoustic imaging and sensing for nondestructive testing and evaluation

“…As another advantage, PA transmitters can be tiny and integrated with optical fibers, which facilitate selected applications such as an endoscopic investigation of regions that are difficult to access with piezoelectric transducers. Note that the PA-generated US waves (after or without interrogating the sample) can be detected using an ultrasound transducer [ 46 , 49 , 51 ] or using optical ultrasound detection technology [ 47 , 48 , 50 ].…”

“…6( a)]. Lu et al [ 51 ] utilized PA-generated US waves to produce a point-like source with a broad bandwidth and small size, which enables a high-precision (resolution of 1.7 μm) acoustic field measurement. A measurement of the spatial impulse response (SIR) of ultra-high-frequency ultrasound transducers was demonstrated [Fig.…”

“…(i) Brightfield view of the painting (rural landscape), (ii) PA image of the underlying sketch of the painting over a region indicated by the white box in (i), (iii) NIR image of the underdrawing at 1200 nm, and (iv) Brightfield view of the original pencil sketch before overpainting. Scale bar = 1 cm; ( b ): SIR measurement of ultrasound transducers using PA method [ 51 ]. 2D acoustic intensity profile of a 93 MHz focused transducer on x - y plane at z = 6.2, 6.5, and 6.8 mm (top).…”

“…NDT/E with PA-generated US waves Apart from the PA waves generated by laser illumination on the sample, PA-generated US waves can also be utilized to interrogate the sample for NDT/E [46][47][48][49][50][51]. A schematic is shown in Fig.…”

“…Comparison of the PA-measured and calculated results (bottom), showing good agreement. The PA-measured results mean the acoustic intensity distribution measured by the PA method (denoted as "PA" in the legend); the calculated results mean the acoustic intensity distribution calculated by the angular spectrum approach (ASA) based on PA-measured SIR (denoted as "ASA PA " in the legend) Elastic properties and complex refractive index 2.1 [31] Underdrawings in paintings 2.3 [40] Precision acoustic field measurement 3 [51] a…”