In a recent study, researchers have developed a compound metalens that enables distortion-free imaging. The study, published in Engineering, presents a novel approach to on-demand distortion engineering using compound metalenses.
Metalenses have emerged as a promising technology with applications in beam steering, imaging, depth sensing, and display projection. However, optical distortion, a crucial factor in optical design, has been relatively unexplored in the context of meta-optics. The researchers addressed this gap by demonstrating a generic method for controlling distortion.
The compound metalens architecture consists of a doublet metasurface, which provides additional degrees of freedom compared to a single-layer metalens. This allows for custom-tailoring of the angle-dependent image height relations, enabling distortion control while minimizing other monochromatic aberrations.
The team experimentally validated their design by fabricating a compound fisheye metalens. The metalens exhibited diffraction-limited performance across a wide field of view of 140° and a remarkably low barrel distortion of less than 2%. In contrast, a reference metalens without compensation had a distortion of up to 22%.
The metasurface of the metalens comprises amorphous silicon nanopillars on a glass substrate, encapsulated in a polymethyl methacrylate (PMMA) coating. The researchers used numerical simulation and an analytically derived design, followed by ray tracing optimization, to design the metalens. The device was fabricated using laser direct writing for the field aperture stop and electron beam lithography for patterning the metasurfaces.
Characterization of the metalens involved measuring the point-spread function (PSF) using a customized setup with a rotatable optical axis. The Strehl ratio and modulation transfer function (MTF) were calculated to evaluate the focusing quality. The results showed excellent agreement with the design, indicating good fabrication fidelity and diffraction-limited performance across the entire field of view.
The optical distortion was evaluated by recording the image height versus the angle of incidence (AOI). The compound metalens demonstrated a linear dependence of image height on AOI, validating the design target. To showcase the wide-FOV, distortion-free imaging capability, a customized imaging setup was used to capture images of a cylindrical panoramic target. The images captured by the compound metalens showed minimal distortion compared to those of the singlet metalens.
The design principle and fabrication approach of the compound metalens have the potential to impact various fields, including consumer electronics, automotive and robotic sensing, medical imaging, and machine vision systems. This research opens up new possibilities for the development of high-performance optical devices with reduced distortion and improved imaging quality.
