A metalens photoacoustic microscope that switches focus with voltage

Just as it is difficult for a camera to keep both a nearby flower and a distant mountain sharply focused at the same time, conventional optical microscopes face a similar challenge when imaging structures located at different depths inside the body. Tighter focusing of light improves spatial resolution, but at the same time reduces the depth range over which the image remains sharply focused.

 

A joint research team led by Professors Junsuk Rho and Chulhong Kim at POSTECH has developed a dual-focus metalens-based photoacoustic microscopy system that can switch between different imaging depths simply by controlling an applied voltage, without mechanically moving any optical components. The study was published in Light: Science & Applications.

 

Photoacoustic microscopy is an imaging technique in which laser light is delivered to biological tissue, where optical absorption generates ultrasonic waves that are subsequently detected to form an image. In particular, optical-resolution photoacoustic microscopy tightly focuses light to achieve micrometer-scale spatial resolution, enabling detailed visualization of fine vascular structures. 

 

However, tighter optical focusing inevitably results in a shorter depth of focus. Consequently, only a limited axial region can be sharply imaged, making it difficult to visualize three-dimensional biological structures distributed over a broad depth range.

 

Conventional approaches have attempted to extend the imaging depth using Bessel beams, which can maintain a narrow central focus over a longer axial range. However, Bessel beams inherently generate pronounced side lobes around the central beam, which can introduce background signals, reduce image contrast, and degrade imaging quality. 

 

The POSTECH team addressed this challenge by controlling the polarization state of light rather than physically moving optical components.

 

The key to the system lies in the integration of two optical components. The first is a dual-focus metalens designed to generate focal spots at different axial positions depending on whether the incident light is right- or left-circularly polarized. The second is an electrically driven liquid-crystal cell that changes the polarization state of the incident light in response to an applied voltage. - the same type of liauid crystal used in smartphone displays. 

 

By combining these two components, the researchers were able to electronically switch between two focal depths simply by adjusting the voltage applied to the liquid-crystal cell, witheath switch taking only about 0.25 seconds. The metalens was further integrated with a ring-shaped ultrasonic transducer, allowing the optical and acoustic axes to be aligned within a compact photoacoustic microscopy configuration.

 

By alternating between the two focal modes, the system achieved an overall axial imaging range of approximately 1.2 mm. The lateral spatial resolutions at the two focal positions were approximately 3.1 μm and 3.7 μm, respectively, providing sufficient resolution to visualize microscopic vascular structures.

 

The performance of the system was experimentally validated by imaging the eyes of mice with chemically induced corneal injury. In the shallower focal mode, newly formed blood vessels in the cornea were clearly visualized. In the deeper focal mode, blood vessels near the iris, located approximately 2.2 mm below the surface, were selectively resolved. These results demonstrate that vascular structures located at substantially different depths can be selectively visualized at high spatial resolution without mechanically moving any optical components.

 

The technology could be particularly useful for ophthalmic imaging, where clinically relevant blood vessels are distributed across different depths within the eye. Diseases such as diabetic retinopathy and other vascular disorders require accurate assessment of vascular structures throughout multiple regions of the eye.

 

Professor Junsuk Rho explained, “By utilizing polarization multiplexing in a metalens, we can switch a high-resolution optical focus along the depth direction without mechanically moving optical components. Eliminating mechanical motion can also enable more compact and stable imaging systems, providing opportunities for applications in a wide range of biomedical and industrial imaging platforms.”

 

The study was co-first-authored by Dr. Hongyoon Kim of POSTECH’s Department of Mechanical Engineering, Eunwoo Park, a Ph.D. candidate in POSTECH’s Department of Biomedical Engineering, and Professor Jungwoo Park of Kyungpook National University.

 

The research was supported by the POSCO-POSTECH-RIST Convergence Research Center, the Global Convergence Research Support Program of the Ministry of Science and ICT and the National Research Foundation of Korea, the BK21 FOUR Program, Glocal University 30, the Asan Foundation Biomedical Science Scholarship, and the Presidential Science Scholarship.

 

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