Fourier ptychography provides high-resolution and quantitative phase imaging of transparent biological specimens. However, it requires multiple measurements of the same scene, limiting the imaging speed. We proposed single-shot Fourier ptychography using polarization-encoded illumination multiplexing. We showcased our method by capturing fast-moving microorganisms at a frame rate limited only by the sensor's readout speed.

• H. Yoon, H. Chae, K. C. Lee, K. Lee, and S. A. Lee*, “Single-shot Fourier ptychography using polarization-encoded illumination,” Applied Physics Letters 126, 123702 (2025). [Paper] 

Optical diffraction tomography, also known as holotomography, is a 3D quantitative phase imaging technique that uses refractive index (RI) distribution as intrinsic imaging contrasts for label-free imaging. I have contributed crucially to launching HT-X1, the world's first holotomography system using LED illumination. My main contribution was developing a regularized reconstruction algorithm to solve the "missing cone problem" in 3D RI imaging.

• W. Lee, H. Yoon, S. Na, S. Lee, T. Kim, H. Hugonnet and Y.K. Park*, “Low-coherence holotomography for speckle noise-free live cell imaging,” SPIE Photonics West BiOS Jan. 2023. [Abstract] [Poster]

Due to the diffraction-limited frequency coverage of holotomography, RI is underestimated and cells appear elongated in the RI tomogram. This problem can be mitigated using regularized reconstruction algorithms for filling in the missing information. However, conventional algorithms have limitations for low-NA tomograms because the amount of unknown information to be inferred is too large compared to that of the known information. In order to solve this problem, we developed a deep-learning algorithm to transform low-NA tomograms into high-NA tomograms.

• H.-S. Min, D. Ryu, W. Lee and H. Yoon, “System and method for improving image resolution of 3-D refractive index microscope based on AI technology,” US Patent (Appl. No. 17/818,987), Aug 11, 2022. [Patent]

We have expanded holotomography's application to non-destructively inspect cracks in industrial glass samples. We were able to image RI tomograms of micro-cracks and their propagation inside the glass. Moreover, we were able to capture slight differences in RI near the cracks. We launched an industrial HT system (HT-X1i) with this technology HT-X1i) for defect inspection.

Wavefront shaping precompensates the input wavefront in order to deal with multiple scattering. With the help of my experiences in phase conjugation and time reversal in medical ultrasound, I participated in a review paper that thoroughly covers the principles and recent advances of wavefront shaping in the optical realm.

• Z. Yu, H. Li, T. Zhong, J.-H. Park, S. Cheng, C. M. Woo, Q. Zhao, J. Yao, Y. Zhou, X. Huang, W. Pang, L. Peng, H. Yoon, Y. Shen, H. Liu, Y. Zheng, Y.K. Park*, L. V. Wang* and P. Lai*, “Wavefront shaping: A versatile tool to conquer multiple scattering in multidisciplinary fields,” The Innovation 3, 100292 (2022). [Paper]

Ultrasound harmonic imaging is widely used to obtain images with high contrast and resolution. However, the prevalent pulse-inversion technique for harmonic imaging requires two Tx/Rx sequences in order to separate the nonlinear harmonic signal. We developed an adaptive harmonic-separation technique, which requires only one Tx/Rx sequence, exploiting expectation-maximization source separation. 

• H. Yoon and T.-K. Song*, “An adaptive harmonic separation technique for ultrasound harmonic imaging,” Ultrasound in Medicine & Biology 50, 743–750 (2024). [Paper]

• T. -K. Song and H. Yoon, “Ultrasound imaging apparatus and controlling method for the same,” Korea Patent (Appl. No. 10-2020-0084674), Jul. 9, 2020. [Patent]

To achieve artifact-free 3D imaging, densely packed 2D arrays are needed. However, the hardware and computational costs are massive with dense arrays. We designed generalized models for sparse 2D arrays that have minimal grating-lobe artifacts.

• H. Yoon and T.-K. Song*, “Sparse rectangular and spiral array designs for 3D medical ultrasound imaging,” Sensors 20, 173 (2020). [Paper]

• H. Yoon, S. Lee, J. Kim and T. -K. Song*, “Analytic design of sparse rectangular arrays for 3D medical ultrasound imaging,” in Proc. IEEE Ultrason. Symp., Oct. 2019, pp. 1785–1788. [Paper] [Poster]

Ultrasound imaging is a cost-effective tool for monitoring high-intensity focused ultrasound (HIFU) treatment in real time. However, because of the reflected HIFU signal, images are corrupted with HIFU interference. In order to eliminate the HIFU interference in the monitoring images, we developed an imaging algorithm using a sequence of four different transmission signals to cancel out the HIFU noise and its harmonics.

• H. Yoon, P. Kim and T. -K. Song*, “HIFU noise elimination technique based on phase and power modulation for ultrasound guided HIFU monitoring system,” The 18th International Symposium of International Society for Therapeutic Ultrasound (ISTU), May 2018. [Poster]

Ultrasound allows us to noninvasively deliver high enough energy for a targeted therapy inside the brain. As localized transmission is crucial for brain therapy, we proposed a hemispherical transducer array design with low grating-lobe artifacts by optimizing the array pattern, element size, and curvature. Based on the optimal design, an MR-compatible transducer array was manufactured.

In addition, to compensate for the phase aberration induced by the highly heterogeneous skull, we developed time-reversal simulators with k-wave toolbox and COMSOL Multiphysics.  

• H. Yoon, P. Kim and T. -K. Song*, “Design of 1024-element hemispherical arrays for ultrasonic brain therapy,” The Joint Meeting Between the 19th International Symposium of International Society for Therapeutic Ultrasound (ISTU) and the 5th European Symposium of European Focused Ultrasound Charitable Society (EUFUS), Jun. 2019 (Student Award Winner). [Poster]

Metal 3D printing suffers from anisotropic grain structure and porosity defects. In order to solve these problems, a pulsed-laser technique for reducing porosity defects and refining the grain structure during 3D printing was proposed and verified experimentally and theoretically. By focusing the pulsed laser onto the melt pool, accelerated Marangoni flow, shock waves, and cavitation were generated. With these pulsed laser-induced effects, gas traps were pushed away and a favorable environment for the crystallization of equiaxed grains was formed.

• H. Yoon, P. Liu, Y. Park, G. Choi, P.-P. Choi and H. Sohn*, “Pulsed laser-assisted additive manufacturing of Ti-6Al-4V for in-situ grain refinement,” Scientific Reports 12, 22247 (2022). [Paper]

• H. Sohn, P. Liu*, H. Yoon, K. Yi, L. Yang and S. Kim, “Real-time porosity reduction during directed energy deposition using a pulse laser,” Journal of Materials Science & Technology 116, 214–223 (2021). [Paper]