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Silicon scaffolds support complex microlenses

14 December 2020

A new laser-writing technique produces refractive-index gradients in microscale photonic elements.

The human eye’s lens has a refractive index that varies radially from about 1.386 at the outer edge to 1.406 at the center. The gradient reduces aberration, thereby enabling the eye to form clearer images than it would with a uniform lens. Now for the first time, three-dimensional gradient-index optics are available for fabricated microscopic devices thanks to a technique developed by Christian Ocier, Corey Richards, and coworkers at the University of Illinois at Urbana-Champaign, in collaboration with researchers at Stanford University.

The new technique relies on multiphoton direct laser writing (DLW), a lithographic process in which a focused laser beam prints 3D optical components in a volume of light-sensitive polymer photoresist. The beam chemically alters the illuminated polymer as it traces out the desired object’s shape. The untreated polymer is removed, leaving behind a lens, waveguide, or other component. Conventional DLW produces a single refractive index—that of the processed photoresist.

Instead of starting with a uniform layer of photoresist, Ocier, Richards, and colleagues infused it into scaffolds of either porous silicon or porous silica. The average pore size was about 60 nm—small enough that the material was effectively uniform to visible and IR light—and the scaffolds were transparent at the wavelength used for writing. Each scaffold gave the researchers access to a range of refractive indices: Increasing the power of the laser left increasing amounts of polymer in the pores after the untreated material was washed away. With the silicon scaffold, achievable indices ranged from 1.28, corresponding to the empty scaffold, to 1.85 at maximal filling; those values were lower for the silica scaffolds. The technique, dubbed SCRIBE (subsurface controllable refractive index via beam exposure), has a resolution of a few hundred nanometers, which is determined by the extent of the focused laser’s point-spread function.

Luneberg lens
Credit: Adapted from C. R. Ocier et al., Light Sci. Appl. 9, 196 (2020)

Using SCRIBE, the researchers fabricated a range of optical components, including cylinders, prisms, waveguides, and various lenses. In particular, they made the smallest-ever Luneburg lens, shown in the figure. The first two panels are a schematic of the lens’s refractive-index gradient and a fluorescent image in which the signal corresponds to the amount of photoresist polymer. Because of its spherical symmetry and radially varying refractive index, a Luneburg lens can focus a plane wave to a single point, shown in the third panel, regardless of the wave’s incident angle. Although larger versions are often used with microwaves and radio waves, the SCRIBE-made lens is the first with features small enough to focus visible light. (C. R. Ocier et al., Light Sci. Appl. 9, 196, 2020.)

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