This article has been reviewed according to Science X's editorial process and policies. Editors have highlighted the following attributes while ensuring the content's credibility: 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 is 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 cell uses the same type of liquid 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, with each 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.
Extract — continue reading at the source.
