Switchable dual-focus metalens extends depth of photoacoustic eye imaging

For decades, one of the most stubborn trade-offs in optical microscopy has been the relationship between resolution and depth. Focus light tightly enough to resolve individual capillaries, and the sharp zone of your image shrinks to a razor-thin sliver. Loosen the focus to cover more depth, and the fine detail dissolves. A research team at Pohang University of Science and Technology (POSTECH) in South Korea has now demonstrated an elegant way out of this dilemma, using a flat, electrically switchable metalens to give photoacoustic microscopy two diffraction-limited focal planes that can be swapped at the flick of a voltage. Writing in Light: Science & Applications, the team led by Junsuk Rho and Chulhong Kim shows that the approach can image blood vessels across nearly the entire depth of a living rat eye, tracking the abnormal vessel growth that follows a chemical corneal burn.

The technique at the heart of the work is photoacoustic microscopy, or PAM, a hybrid imaging modality that marries light and sound. A pulsed laser is delivered into tissue, where molecules such as hemoglobin absorb the energy and heat slightly, expanding and launching ultrasonic waves. Because ultrasound scatters far more weakly in tissue than light does, these waves can be detected and traced back to their origin, yielding images with optical contrast but acoustic penetration. Optical-resolution PAM, the variant used here, focuses the laser beam as tightly as diffraction allows, achieving micron-scale lateral resolution that can resolve the microvascular networks feeding the iris, choroid, and cornea. The catch is that this tight focusing produces a very shallow depth of field: structures even a fraction of a millimeter away from the focal plane blur dramatically, forcing researchers into slow, repeated refocusing scans to build a three-dimensional image.

Previous attempts to stretch the depth of field have relied on nondiffracting beams, such as Bessel beams and needle beams, which concentrate light into long, pencil-like axial profiles. These beams do maintain resolution over a longer range, but they carry a punishing penalty: their energy is distributed into prominent side lobes that generate spurious photoacoustic signals, and spreading the laser power over a larger volume weakens the signal at any single depth, degrading the signal-to-noise ratio. The POSTECH team took a fundamentally different route. Rather than smearing a single focus along the axis, they built a metalens that produces two separate, diffraction-limited Gaussian foci, each as strong and clean as a conventional lens focus, and switches between them electrically without moving a single mechanical part.

The metalens itself is a marvel of nanoscale engineering. It consists of silicon nitride nanostructures, each smaller than the wavelength of light, arranged on a flat glass substrate. Each nanostructure, or meta-atom, acts as a tiny waveguide that imparts a precise phase delay to passing light. By combining two phase-control mechanisms, the propagation phase and the geometric, or Pancharatnam-Berry, phase, the designers encoded two entirely independent lens phase profiles into the same surface, one for right-handed circularly polarized light and one for left-handed circularly polarized light. When the incident beam is right-circularly polarized, the metalens focuses at 4.5 millimeters; flip the polarization to left-handed, and the focus jumps to 5.2 millimeters. In water, where the device is designed to operate to minimize photoacoustic signal loss, this translates into two focal planes separated by roughly 0.9 millimeters.

Choosing the right material was critical, because photoacoustic imaging drives the lens with a high-power pulsed laser at 532 nanometers, the wavelength at which hemoglobin absorbs strongly. Any absorption in the lens material would both waste laser energy and generate unwanted photoacoustic signals from the lens itself. The team used plasma-enhanced chemical vapor deposition to grow silicon nitride films, carefully tuning the ratio of silane to nitrogen gas precursors to engineer the film’s bandgap. The optimal recipe, a silane-to-nitrogen ratio of 1.52, yielded a high refractive index of 2.4 with a near-zero extinction coefficient at 532 nanometers, and a wide optical bandgap of 5.17 electronvolts. A full micrometer-thick film of this material absorbs only about two percent of the green laser light, keeping the lens optically quiet while maximizing the focusing efficiency of its meta-atoms, each of which was optimized to act as a half-wave plate with conversion efficiencies exceeding seventy percent.

The electrical switching is performed by a liquid crystal cell placed in front of the metalens. The liquid crystal, a common material called 5CB, rotates its molecules in response to an applied alternating voltage, changing the polarization state of the transmitted light. At 0.87 volts, the output is right-circularly polarized and the metalens focuses at the near plane; at 1.03 volts, it becomes left-circularly polarized and the focus shifts to the far plane. The response time is about 250 milliseconds, fast enough to alternate between modes during a scan, and the optical contrast between the on- and off-state foci reaches 40:1 at the near focus and 20:1 at the far one. Crucially, both focal spots exhibit Strehl ratios close to unity, meaning each behaves as a nearly ideal diffraction-limited Gaussian focus rather than a compromised beam shape. The measured lateral resolutions, 3.1 and 3.7 micrometers for the two modes, match theoretical predictions almost exactly.

To turn this into an imaging system, the researchers mounted the metalens on a custom ring-shaped ultrasound transducer made from an 18-micrometer-thick polyvinylidene fluoride film, with the metalens sitting in a central aperture so that the optical and acoustic axes are perfectly coaxial. Bench tests with resolution targets and layered nylon threads confirmed that switching modes reliably shifted the sharp focal zone by about 0.9 millimeters, extending the usable axial imaging range to roughly 1.2 millimeters, a dramatic improvement over conventional single-focus PAM. Tests in a scattering phantom made of water, gelatin, and intralipid showed that the deeper focus mode genuinely improved contrast and resolution for objects buried about a millimeter deeper, confirming the effect survives realistic tissue-like scattering.

The demonstration that will interest clinicians most came in living animals. The team imaged the eyes of Sprague-Dawley rats, first healthy animals and then a model of corneal neovascularization induced by applying an alkali-soaked filter paper to the cornea, a standard model of chemical burn injury. In the healthy eye, the short-focus mode sharply resolved the root vessels and fine microvasculature of the iris, while the long-focus mode brought deeper structures into view, including the posterior ciliary vessel and pupil-associated vasculature that appeared blurred in the near mode. Fourteen days after the burn, the dual-focus system mapped the newly grown corneal vessels in detail while simultaneously imaging iris vessels as deep as 1.5 millimeters beneath the neovascular layer in one mode and about 2.2 millimeters in the other, a 0.7-millimeter extension in detectable depth. Notably, the images confirmed that the pathological new vessels remained strictly confined to the cornea, leaving the iris vasculature untouched.

The quantitative results underscore why this matters. Vessels imaged out of focus appear artificially fat: iris vessels measured through the near-focus mode showed an apparent average diameter of about 42 micrometers, but when the focus was shifted deeper, the same vessels measured a truer 16 micrometers, comparable to the roughly 18-micrometer diameter of the corneal vessels measured in their own focal plane. Meanwhile, the total area of the neovascular lesion came out nearly identical in both modes, at 5.55 and 5.65 square millimeters, showing that the dual-focus approach yields reliable lesion-scale measurements even as it trades resolution between depths. The 0.9-millimeter focal separation neatly spans the thickness of a typical cornea, around 0.5 to 0.6 millimeters, meaning the clinically relevant corneal layers fall comfortably within the high-resolution regime of the system.

The researchers are candid about the limitations and the road ahead. The current frame rate of about 0.42 frames per second could rise to 20 frames per second with faster optical scanning, though that would demand motion compensation to counter respiration-induced eye movement. Switching modes changes the numerical aperture slightly, which can be corrected by subpixel registration when merging the two image sets, and extending the design to multispectral imaging will require metasurfaces that correct for the wavelength dispersion of silicon nitride. Still, the concept reaches well beyond the eye: the same tunable dual-focus optics could be folded into optical coherence tomography, two-photon microscopy, or metalenses designed around the optical properties of skin and brain tissue. What the POSTECH team has delivered is more than a clever lens. It is a demonstration that flat, electrically reconfigurable optics can solve one of biomedical imaging’s oldest trade-offs, offering a compact, mechanically silent path to high-resolution, high-contrast, volumetric imaging of living tissue.

Subject of Research: An electrically tunable dual-focus metalens integrated into photoacoustic microscopy for depth-extended in vivo imaging of ophthalmic vascular disease

Article Title: Electrically switchable dual-focus metalens for depth-extended in vivo photoacoustic imaging of ophthalmic vascular disease

Article References: Kim, H., Park, E., Park, J., Oh, D. K., Shin, J., Heo, H., Kim, C., & Rho, J. (2026). Electrically switchable dual-focus metalens for depth-extended in vivo photoacoustic imaging of ophthalmic vascular disease. Light: Science & Applications, 15(1), Article 369. https://doi.org/10.1038/s41377-026-02365-8

Image Credits: AI Generated

DOI: 10.1038/s41377-026-02365-8

Keywords: metalens, metasurface, photoacoustic microscopy, silicon nitride, liquid crystal, corneal neovascularization, ophthalmic imaging, depth of field, nanophotonics, in vivo imaging, microvasculature, flat optics