These light-gathering polymer lenses are 3.5 X more powerful than glass, and are the first commercial nanolayered product to come out of many years of R&D at Case Western Reserve University. Photo courtesy of Michael Ponting.
The crystalline lens of the human eye is capable of sophisticated focusing and aberration corrections that require multiple elements in most artificial optical systems. Now a team of U.S. researchers has created a synthetic lens with the same focusing mechanism as the human lens: a refractive index that changes as the incoming light passes through (Opt. Express 20, 26746).
Human eyes have the same type of gradient reflective index (GRIN) lens as those of many other animals. Water dwellers such as fish and squid have spherical lenses, while those of land dwellers have aspheric lenses. In humans, the lens has a refractive index that varies from about 1.42 in its protein-filled core to about 1.37 at its collagen surface.
Researchers at Case Western Reserve University, PolymerPlus LLC, Rose-Hulman Institute of Technology and the U.S. Naval Research Laboratory (U.S.A.) attempted to replicate the same size, aspheric shape and refractive index distribution as the lens from a human eye. However, they had to tweak the parameters a bit because their polymer materials differ slightly in refractive index from natural proteins and because their artificial lens would be tested in air, rather than the aqueous environment of an eyeball.
The scientists built up the lens from alternating layers of polymethyl methacrylate and styrene-acrylonitrile resin in a technique called nanolayered polymer coextrusion. Each stacked film was less than a quarter-wavelength of visible light in thickness. The resulting “sandwich” had a refractive index varying from 1.49 to 1.54. Team members used heat and pressure to shape the sandwich into an aspheric lens.
To verify the refractive index gradient in the finished lens, the researchers tested slices of lens material with attenuated total reflectance infrared spectroscopy. An ordinary digital camera took pictures through the lens to verify the singlet lens’s focusing ability.
In the future, specially designed GRIN lenses could reduce the number of separate components needed in complex optical systems such as zoom lenses for cameras. Other possibilities include lightweight imaging and surveillance systems or perhaps even a replacement lens for human cataract patients.