micrograph of nanorods

SEM image of nanostructures on the surface of one of the microlenses in the artificial compound eye developed by a research group at Xi’an Jiaotong University, China. [Image: American Chemical Society]

Researchers in China have developed what they suggest is a simple, low-cost method for creating bio-inspired compound eyes—ranks of tiny lenses for imaging that mimic the natural lens arrays found in insects and crustaceans (ACS Nano, doi: 10.1021/acsnano.8b04047). The approach, which uses laser-driven expansion to puff up individual microlenses that are then decorated with nanostructures for antireflective and water-repelling properties, could find use in a range of imaging applications, according to the team.

Multi-lens advantages

Compound eyes consist of thousands of individual optical units, called ommatidia, arrayed across a curved surface. While vertebrate humans, whose eyes each have a single lens, are apt to turn up their noses at invertebrate compound eyes as a “primitive” vision solution, the approach of using multiple lenses to pull in separate images has a number of advantages. For example, compound eyes tend to be better than single-aperture eyes in field of view, light sensitivity and motion detection.

Those advantages have piqued the interest of optical engineers, who have devised a number of methods for creating arrays of optical elements with the natural compound eye as a model. But many of the methods for creating artificial compound eyes, relying on methods such as reactive-ion etching and nano-imprint lithography, have carried high cost and haven’t looked particularly scalable.

Further, according to the Chinese research team, many artificial compound eyes demonstrated thus far have sported inconsistent performance owing to deformations and distortions. In particular, the process of transferring the lens arrays to a curved surface can degrade the nanostructures at the surface, limiting the antireflective and hydrophobic characteristics required for good real-world performance.

Laser inflation

To get past the hurdles in both cost and performance, the research team at Xi’an Jiaotong University, China, developed a different approach. The researchers’ method begins with a dual layer of acrylic glass, poly(methyl methacrylate) [PMMA], placed on an off-the-shelf motorized stage. As the stage moves, the PMMA surface is irradiated at regular intervals with highly focused femtosecond pulses from an 800-nm laser.

The laser, penetrating into the middle of the glass, heats and softens the matrix, and causes doped material within the matrix to vaporize. As a result, the surface above the laser spot blows up like a balloon, creating a convex, lenslike bump in the acrylic. The spacing between individual bumps can be set by adjusting the speed of the motorized stage, and the researchers found that they could precisely control the diameter and height of the surface bumps by tweaking the laser power and irradiation time.

The resulting 2-D array of convex structures—each 11 microns high and 40 microns in diameter, and separated by a distance of 60 microns—is then used to create a mold of concave structures in a 500-micron-thick slab of elastomer. That flexible mold serves as a template that is used to line a larger, 5-mm-diameter curved chamber, and is held in place with negative air pressure.

The researchers then fill the chamber with a UV-curable epoxy resin, to create a curved surface dotted with an array of convex microlenses. Finally, leveraging crystal-growth techniques developed several years back by another research group in China for depositing well-aligned zinc-oxide nanorods on curved surfaces, the researchers decorated the convex microlenses with nanostructures to reduce their surface reflectance and make them super-hydrophobic.

Optical testbed

To gauge the optical properties of the artificial eye, the team set up an optical testbed consisting of a tungsten light source that could illuminate the eye from various angles in front, and a CCD camera in back to capture confocal images.

The researchers found that, in addition to reduced surface reflectance and high water repellence, the artificial compound eyes scored generally well in optical properties, with high uniformity and relatively low distortion across a variety of x/y coordinates. And a comparison of the point-spread functions of the artificial compound eyes with those of single-lens eyes suggested that the compound eyes actually did better in focusing and imaging ability. (Score one for the invertebrates.)

The authors believe that, in light of its relatively low cost and its ability to circumvent the quality problems in some earlier approaches, their fabrication method “has diverse potential optical applications such as surveillance imaging, light-field photography, and wide-angle communication antenna[s].”