Laser-catapulting additive manufacturing can be used to fabricate a microlens array directly on top of a photodetector. [Image: Martí Duocastella / Istituto Italiano di Tecnologia]
Researchers from the Istituto Italiano di Tecnologia in Italy report developing and testing a laser-based additive manufacturing technique for creating individual and arrayed microlenses with different geometries and optics (Opt. Mater. Express, doi: 10.1364/OME.9.002892). The technique uses a pulsed laser to punch out—or catapult—the shape of the lens from a thin polymer sheet onto a substrate. Once on the substrate, the polymer is heated and shaped into a convex lens via thermal reflow.
Using this laser-catapulting technique, the researchers demonstrated their ability to create high-quality single microlenses and arrays of tightly-packed circular, triangular or rectangular microlenses with radii spanning 50 µm to 250 µm. The researchers believe their simple laser-additive method for creating microlenses could bring micro-optics to a wider range of applications, including smartphone cameras, photovoltaic cells and 3-D microscopes.
A Better Way to Make Microlenses?
The need for micro-optics is growing because of their ability to boost the performance of light-based systems. For example, microlenses can be used to concentrate light for smartphone cameras, microscopes and solar cells, improving their performance even in low-light environments. While microlenses are currently available on the market, existing manufacturing methods can be complicated and pricey. Additionally, these methods often cannot produce lenses in tightly packed arrays or in specific shapes with specific focusing properties.
Team leader Martí Duocastella and his colleagues may have found an easier, more affordable way to make customizable microlenses by combining a fairly new technology called laser catapulting with additive manufacturing.
The team’s technique uses an ultraviolet pulsed nanosecond laser to punch out microlens shapes from a sheet of highly absorbing polymer. The polymer’s mechanical qualities allow it to absorb the laser pulse’s energy without causing it to liquify before it hits the substrate. Once on the substrate—which can be anything from a temporary sheet to direct deposition onto a camera or solar cell—the polymer shape is heated above its glass-transition temperature, causing it to melt slightly. This thermal flow allows capillary forces to reform the material into a convex microlens.
To create different microlens shapes, the researchers placed a binary mask over the laser beam. With the mask in place, they were able to create standard circular microlenses, as well as rectangular and triangular microlenses. To the researchers’ knowledge, this is the first time laser-catapulting additive manufacturing has been used to create microlenses in shapes other than circular. The laser-catapulting method also allowed the researchers to very accurately define where the punched-out polymer shapes landed on the substrate, resulting in densely packed and organized microlens arrays. (Click here to see a video of a microlens array created using the laser-catapulting additive manufacturing method.)
Experimental demonstration
Results from reproducibility, quality and precision testing were encouraging. The laser-catapulting technique produced microlenses with nearly perfect optical quality (surface roughness < 5 nm), and the calculated point spread function (PSF)—a test of an optical system’s quality—for an array of 15 microlenses showed a deviation of the average below 3 percent laterally and 6 percent axially. The low PSF values indicate the high reproducibility and precision of the laser-catapulting technique.
In a press release for The Optical Society, Duocastella expressed his optimism for the laser-catapulting method: “In addition to completely new applications, this method could lead to new cameras that acquire video under low-light conditions, solar cells with improved efficiency and microscopes that are better at capturing fast processes.”
The team is currently working on a way to integrate laser-catapulted microlens arrays into high-speed 3-D microscopy systems for capturing nerve cell communications and how viruses move intracellularly.