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

Abstract

High-resolution three-dimensional imaging is essential for visualizing fine biological details. Unlike conventional optical imaging, photoacoustic (PA) imaging provides volumetric imaging by capturing time-of-flight ultrasound waves generated from optical absorption within tissues. Optical-resolution photoacoustic microscopy achieves micron-scale lateral resolution through tight optical focusing but suffers from a shallow depth of focus. To address this, non-diffracting beams (e.g., Bessel and needle beams) have been explored, but they introduce high side lobes that reduce the signal-to-noise ratio and distribute laser energy over a larger volume, reducing signal intensity. Here, we present an electrically modulated dual-focus metalens-based photoacoustic microscopy (meta-PAM) system designed to enhance axial coverage (1.2 mm) while preserving high lateral resolution (3.7 μm) in three-dimensional imaging. Our system features a silicon nitride nanostructured metalens that switches between two diffraction-limited focal modes at a visible wavelength without mechanical movement, with focal switching voltages of 0.87 V and 1.03 V and a response time of 250 ms. We demonstrate its effectiveness by imaging a chemically induced corneal burn in rat eyes, successfully monitoring neovascularization with high resolution across the entire eye depth range. Our compact yet effective system provides building blocks for future high-resolution volumetric imaging with an improved signal-to-noise ratio.