The secret lies in a multi-level diffractive lens, a mere 7 microns thick, whose surface is populated by millions of microscopic staircase-like structures. Each terrace is precisely calculated to manipulate broadband near-infrared light between 800 and 900 nanometers. Unlike conventional refractive optics that bend rays through curved glass, diffractive lenses exploit the wave nature of light, using interference and phase control to sculpt the emerging wavefront. By cascading these microstructures across multiple discrete height levels, the team achieved a degree of light-shaping finesse usually reserved for bulky assemblies of lenses. The result is a beam with an average lateral width of just 2.4 micrometers that stays tightly collimated for a staggering 2,640 micrometers, yielding a depth-to-width ratio of 1,100 to 1. To put that in perspective, it is akin to a thread of light stretching the length of a football field while remaining thinner than a human hair.

“What’s exciting is the performance gain we get from such a simple change,” said research team leader Haowen Liang. “With a straightforward lens replacement, we were able to image much deeper into tissue while still maintaining high resolution. That’s very difficult to achieve with conventional optical designs.” The simplicity is deceptive; packing multiple optical functions into a single flat element required sophisticated computational optimization. The design process juggled the competing demands of a narrow lateral focus and an extended focal depth across a broad spectrum, a feat that would have been practically impossible without modern inverse-design algorithms. These computational tools explore vast parameter spaces to find unintuitive surface profiles that generate the desired needle-like beam.

Fabricating the lens pushed the limits of 3D laser writing, a nanoprinting technique that uses tightly focused laser pulses to polymerize a photosensitive resin voxel by voxel. The precision required to carve out the diffractive steps — each a fraction of a wavelength of light — is on the order of tens of nanometers. Recent advances in this additive manufacturing approach made it feasible to translate the intricate phase maps from the design software directly into physical glassy polymer structures with high repeatability, opening a practical pathway from theoretical light needles to real-world devices.

Optical coherence tomography is a natural home for this innovation. OCT constructs cross-sectional images by interferometrically measuring the time delay of light backscattered from different depths within a sample. But standard systems face an inherent trade-off: a lens that tightly focuses light on the surface will rapidly defocus deeper inside tissue, missing the fine cellular details that lie just microns below. The optical needle sidesteps this limitation by essentially creating a long, invariant focal region. In experiments, the team swapped the objective lens of a spectral-domain OCT setup with their diffractive element and demonstrated that high-resolution imaging could be maintained far beyond the usual working range. For ophthalmology, this means the ability to simultaneously capture crisp details of the corneal epithelium and the deeper endothelial layers in a single scan, potentially flagging early signs of glaucoma, diabetic retinopathy, or age-related macular degeneration long before structural damage becomes obvious.

Beyond the clinic, the implications ripple into consumer technology. Smartphone cameras and wearable sensors have long struggled to miniaturize high-performance optics without sacrificing image quality. A flat, sub-micron-thick element that replaces a stack of lenses while offering extended depth of field could radically shrink camera bumps and enable new modes of computational photography. Liang’s group is now working on dynamic versions of the lens that can tune the optical needle’s shape and intensity profile in real time, adapting to different imaging scenarios on the fly. They hope these reconfigurable diffractive elements will simplify architectures in everything from endoscopes to low-cost portable diagnostics.

The study, published in Optics Letters, demonstrates that the age-old battle between resolution and depth of field is not a fundamental law but an engineering constraint — one that can be elegantly circumvented by redesigning light itself rather than the lenses that guide it. As these flat optical elements mature, they may fundamentally change how we think about imaging, pushing the boundaries of what can be seen without adding bulk, cost, or complexity.

Subject of Research: Multi-level diffractive lens enabling an optical needle for extended depth imaging in optical coherence tomography.
Article Title: Optical Needle with Narrow Lateral Focal Width and Extended Longitudinal Focal Depth Enabled by Multi-Level Diffractive Lens
News Publication Date: 7-Jul-2026
Web References: 10.1364/OL.597598
References: J. Huang, Z. Duan, P. Xiao, H. Liang, “Optical Needle with Narrow Lateral Focal Width and Extended Longitudinal Focal Depth Enabled by Multi-Level Diffractive Lens,” Opt. Lett., 51, DOI: 10.1364/OL.597598.
Image Credits: Haowen Liang, Sun Yat-sen University

Keywords

Optical coherence tomography, diffractive lens, optical needle, multi-level diffractive optics, high-resolution imaging, extended depth of field, computational imaging, 3D laser writing, biomedical optics, flat optics.