Research Interest

Elastic/Acoustic Metamaterial

Electromagnetic Acoustic Transducer (EMAT)

Ultrasonic Sensor/Actuator

Ultrasound Applications

Research Interests

My research interests have their roots in ultrasonic elastic waves. Spanning from elastic ultrasonic sensors and actuators, including electromagnetic acoustic transducers (EMAT), elastic metamaterials, and their applications. EMATs enable the generation and recording of longitudinal and transverse ultrasonic waves in the medium. Elastic metamaterials exploiting resonances inside the unit structure realize unprecedented wave phenomena including extraordinary wave transmission

Barrier-through Ultrasonic Imaging

Imaging technologies that provide detailed information on intricate shapes and states of an object play critical roles in nanoscale dynamics, bio-organ and cell studies, medical diagnostics, and underwater detection. However, ultrasonic imaging of an object hidden by a nearly impenetrable metal barrier remains intractable. Here, we present the experimental results of ultrasonic imaging of an object in water behind a metal barrier of a high impedance mismatch. In comparison to direct ultrasonic images, our method yields sufficient object information on the shapes and locations with minimal errors. While our imaging principle is based on the Fabry-Perot (FP) resonance, our strategy for reducing attenuation in our experiments focuses on customising the resonance at any desired frequency. To tailor the resonance frequency, we placed an elaborately engineered panel of a specific material and thickness, called the FP resonance-tailoring panel (RTP), and installed the panel in front of a barrier at a controlled distance. Since our RTP-based imaging technique is readily compatible with conventional ultrasound devices, it can realise underwater barrier-through imaging and communication and enhance skull-through ultrasonic brain imaging.

C.I. Park et al., Nature Communications, 14, 7818 (2023).

Elastic Metamaterial

Efficient ultrasound transmission is crucial in vast areas including noninvasive surgery, structural health monitoring, and underwater detection. However, extreme impedance contrast of interfering media spanning from solid to liquid and even gas seriously harms ultrasound transmission efficiency. Here, we propose a complementary meta-layer (CML) to “virtually cancel out” not only dissimilar solid but also liquid and gas barriers for highly enhanced transmission through the barriers in wave propagation perspective. Our proposition is apparently the first elastic CML and it is uniquely designed in a monolayer. The meta-atom forming a single meta-layer consists of spatially separated dipolar and monopolar resonators with completely independent tunability of effective negative properties. The proposed CML is capable of enhancing the ultrasound transmission by 593%, 1426%, and 3280% respectively for the polymer, water, and air barriers by numerical simulations. The proposed methodology to “cancel out” diverse barriers of highly dissimilar impedances can be groundbreaking in various ultrasound applications.

C.I. Park et al., Int. J. Mech. Sci., 206, 106619 (2021).

Electromagnetic Acoustic Acoustic Transducer (EMAT)

Despite the fact that omnidirectional shear-horizontal (SH) waves have recently received much attention, there is no systematic design method with which to develop the corresponding transducers with enhanced output and power concentration levels at a target frequency. Here, we propose a method which can be used to design an optimal annular-array magnetostrictive patch transducer (MPT) that fulfills the above-mentioned objectives. The proposed transducer consists of optimally configured multiple magnetostrictive patches, consistently placed permanent magnets, and toroid coils. For the design, the transducer output signals are theoretically predicted using Green's function of guided waves in a plate and the analysis results are then used to develop new high-power annular-array MPTs generating omnidirectional SH waves in a plate. The validity of the analysis and the effectiveness of the design method were confirmed with numerical simulations and pitch-catch wave experiments performed on an aluminum plate. In the experiments, the mode selectivity and omnidirectivity of the designed transducer were also checked. While the proposed method is mainly developed for MPTs, it should be equally useful for the design of similar annular-type piezoelectric and electromagnetic acoustic guided wave transducers.

C.I. Park et al., Smart Mater. Struct., 8, 075005 (2019).

The use of a specific wave mode is critical in ultrasonic non-destructive evaluations but it is difficult to generate a specific mode, especially a higher mode at a frequency where there exist multiple wave modes. Here, we propose a compact omnidirectional shear-horizontal wave MPT (magnetostrictive patch transducer) having two annular magnetostrictive patches for the generation of a nearly pure SH1 (second shear-horizontal) mode in a plate for frequencies above the first cutoff frequency. While a common wavelength-matching approach would typically require the use of several patches and does not appear completely to eliminate unwanted omnidirectional wave modes, the proposed MPT, with only two annular patches, generates the desired SH1 mode predominantly with the unwanted SH0 (first shear-horizontal) mode nearly eliminated. For the design, the geometries of the annular patches are optimally configured to maximize the ratio of the SH1 mode to the SH0 mode. Numerical simulations and experiments confirm the effectiveness of the proposed bi-annular shear-horizontal wave MPT. Because the SH1 mode near the first cutoff frequency is highly dispersive, the developed transducer is expected to be critically useful in various applications, such as ultrasonic inspections of wall thinning.

C.I. Park et al., Ultrasonics, 99, 105958 (2019).

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