A team at Harvard University has designed a 'metalens', around 100,000 times thinner than an equivalent glass lens. Constructed from titanium dioxide 'nanofins', the lens is a mere 600 nanometers thick, and can focus features smaller than the wavelength of the light it is imaging, thanks to its very high numerical aperture that allows it to focus light into a spot smaller than the wavelength of light being imaged. Senior author Federico Capasso and his fellow researchers successfully imaged structures 400nm in size, which is on the order of the bluest light in the visible spectrum. That suggests the tech is essentially diffraction-limited: resolving small structures as well as one ever theoretically could using visible light.

An electron microscopic view of the titanium dioxide metamaterial lens, created by engineers at Harvard University. These TiO2 towers are only 600nm in length, yet can image features traditional microscopes require lenses 6cm in length - 100,000 longer - to image. And apparently do so 30% better.

John A. Paulson School of Engineering and Applied Sciences/Harvard University

This is the first planar lens that works in the visible spectrum. The nanofins (shown above) are constructed using similar lithographic processes used to fabricate computer chips, and mounted on a thin layer of transparent quartz. The fins are made of titanium dioxide because it is clear, and interacts strongly with visible light. Each fin is angled and positioned so that the ensemble effectively bends light just as the curved surface of a lens would. Different focus can be achieved simply by changing the size, spacing and orientation of the pillars - it's just a matter of doing the computer simulations and calculations to dial in the proper design. Different materials can be used to target different wavelengths of light: previously, the team has used silicon to focus infrared light, for example.

The practical potential of lenses made from these so-called 'metamaterials' are almost infinite, considering the drastic reductions in size and weight that the technology makes possible. A planar surface saves considerable space and volume compared to the typical curved surfaces traditional lenses necessitate. Furthermore, the non-reliance on a perfectly curved spherical or aspherical surface means images can be relatively aberration-free. And the nanostructures themselves can be manufactured with incredible precision, and in a cost-effective way as well, due to the compatibility with traditional microprocessor chip foundries.

Currently, these lenses can't handle full-color imaging because of the very narrow band of wavelengths that can currently be imaged with any particular TiO2 arrangement (camera sensor CFAs actually need broad absorptive properties per filter). However, the team hopes to broaden the wavelengths over which the technology works in the future. This might allow for small yet very high numerical aperture, aberration-free imaging in cameras and cell phones.

But even without spectral broadening advances, the potential applications are very exciting. Obvious applications include microscopy and medical instruments, where structures larger than the wavelength of light are more the exception than the rule. But another potentially huge application is display technology - particularly virtual/augmented reality and wearable optics in general, where the size and weight of today's headsets is a major obstacle to mass adoption. Display technology actually utilizes narrow wavelength R, G, and B primaries, mixing them in just the right ratios to trick your eye/brain into perceiving the intended color. Small, flat lenses that can focus these primary colors with very high precision, aberration-free, might just open up a world of high-resolution and practical VR devices.

Source: phys.org