By Paul Bragulla
A breakthrough by researchers at the Harvard School of Engineering and Applied Sciences (SEAS) has resulted in flat silicon lenses with a thickness of only tens of nanometers. The focusing power that can be achieved with this technique approaches the theoretical limit imposed by diffraction and it suffers from none of the fish-eye, coma, or astigmatism distortions inherent in other lenses. Additionally, they can be designed to work with any wavelength of light from terahertz to near-IR and possibly beyond. As lasers and other high-performance optical devices become ever more common in day-to-day life, the potential technological effects of this development are hard to overestimate. Similar work has been done at the NIST and the UBC, which developed ‘spray on’ flat lenses.
The key is a “plasmonic metasurface” of microscopic gold antennae that absorb and re-emit incident light. By modifying the delay before light is reemitted from various parts of the lens, its direction can be changed; a mechanism analogous to the result of varying material thickness in traditional lenses. The antennae are constructed by layering a nanometer-deep coating of gold onto the silicon substrate then removing all but a regularly spaced array of V-shapes with the desired properties. Electron Beam Lithography (EBL) was used by the researchers for this part of the process.
The initial prospects for this technology are in the realms of microscopy and infrared imaging. The aberration-free nature of flat lenses makes them attractive options for microscope objective lenses, especially those with high numerical aperture, and for obtaining super high-resolution images. One of the most exciting applications, though, is the potential to create lenses that operate with wavelengths of light for which there are few transparent refractive materials, such as the mid-infrared and terahertz regions.
The possible applications of the technology are greatly expanded if it becomes possible to make flat lenses for a wider variety of frequencies and if the focusing efficiency is increased from the current figure of ~10%. Flat lenses with an increased frequency range could replace the majority of the components in many optical systems, and efficiency on par with that of conventional lenses might allow them to be used in high-energy lasers. The researchers suggest several possible ways forwards in terms of increasing efficiency including the use of low-loss metals for the antennae structures and antireflective coatings. Improving the frequency range would likely depend on better micro-lithography techniques being developed for this application.