Essentially two-dimensional at just 60 nm thick, a non-distorting lens with focusing power approaching the ultimate physical limit promises interesting application for biomedicine.1 "The compactness of our lens is a big deal," says principal investigator Federico Capasso, Robert L. Wallace Professor of Applied Physics and Vinton Hayes Senior Research Fellow in Electrical Engineering at Harvard School of Engineering and Applied Sciences (SEAS). To compensate for aberrations, imaging systems traditionally use combinations of convex, concave, and aspheric lenses—adding cost, complexity, size, and weight, Capasso explains. For an objective, it also reduces working distance from the specimen, which adds inconvenience.
Indeed, Capasso told BioOptics World, a simple and very useful application to life sciences could be compact, high-numerical-aperture objectives for microscopes. "The large numerical aperture (meaning large maximum acceptance angle) of our flat lenses would allow one to observe finer details than regular lenses...without the aberrations of conventional lenses." The minimum size of an object observable by an objective—that is, the objective's spatial resolution—is the wavelength divided by twice the numerical aperture. "With our lens, we could almost double it compared to most objectives."
|Light is delayed by optical antennas made of nanogold (inset) patterned in rings on the surface of a flat lens (left; diameter ~1 mm). Different colors indicate the amount of delay corresponding to each ring of nanoantennae. (Image courtesy of Francesco Aieta)|
Capasso, whose recent awards include the 2010 Berthold Leibinger Zukunft Prize, notes that his team has in mind "a particularly neat imaging application" that would facilitate easy focusing of light remotely in difficult-to-reach places, including the body for remote surgery, or for in-vivo high-resolution imaging of whole animals and specimens. This goal would be achieved by patterning the lens directly onto the facet of an optical fiber—a capability his group has developed in collaboration with soft lithography pioneer George Whitesides.
Capasso offers a simple explanation for how the flat lens works: He notes that with an ordinary convex lens, photons hitting the center go straight whereas those hitting elsewhere are bent by refraction towards the axis—which focuses the beam. Achieving the same effect with a flat lens, he said, requires patterning it with nanoantennas that differ in terms of geometry, depending on location relative to the lens's center: The nanoantennas further from the center bend light more dramatically than do those closer in-and the net effect is that a beam of light is focused.
He explains that while conventional lenses in the visible and near-infrared ranges can be made from any number of transparent materials, in other spectral ranges—like the mid- (3–20 μm) and far-infrared (100 μm or more)—the choice of suitable refractive materials is quite limited. So, flat optics offers an important alternative for both imaging and spectroscopy.
1. F. Aieta et al., Nano Lett., 12, 9, 4932–4936 (2012).