Ultrathin Lens Has No Chromatic Aberrations

For all the optics fans out there, I found this article pretty interesting...

Ultrathin lens is free of chromatic aberrations

A new type of flat, ultrathin lens designed to be free of chromatic aberrations has been developed by researchers in the US. The device has a variety of potential applications, from ultralight imaging systems for drone aircraft to more compact lenses for mobile-phone cameras.Lenses for cameras, eyeglasses and other applications are traditionally based on refractive optics, which involves using curved lenses to bend light rays. The "optical power" of a conventional lens - how strongly it bends light - is proportional to its thickness, which means that a conventional refractive lens cannot be very thin. Refractive lenses also suffer from chromatic dispersion, wherein blue light bends more than red, and therefore multiple images are produced over a range of focal lengths. Multiple lenses can cancel out this dispersion, but this adds further to the weight, thickness and cost of a lens system. Diffractive lenses offer a route to ultrathin lenses by redirecting light using the interference between light waves as they pass through a series of slits in a thin opaque material. Such lenses can be effectively flat, and therefore much lighter and thinner than refractive optics. However, diffractive lenses suffer from much larger dispersion. And to further complicate matters, this dispersion is anomalous with red light bending more than blue.

Thin metasurface

In 2015 Federico Capasso of Harvard University and colleagues showed that a flat lens can be made that focuses all of the colours of broadband light in the same plane. The team then unveiled a device that focused broadband infrared light onto a single line using a thin metasurface. This device used dielectric resonators that interact directly with the electromagnetic field of light waves to impart any desired phase shift. However, this new technology brings its own challenges. Making the metasurfaces requires precision engineering because the resonators have to be smaller than the wavelength of the focussed light. Metasurfaces are also inherently polarization-sensitive, whereas a general-purpose camera lens needs to focus unpolarized light. Now, Rajesh Menon of the University of Utah and colleagues have focused broadband visible light using a different approach that involves creating a series of grooves in a soda-lime glass surface. The height and width of each groove was selected using a computer algorithm that optimized focusing across the entire visible spectrum. This involved using the conventional dispersion of the glass material to compensate for the anomalous dispersion of the grating, such that waves would be focused on the same line irrespective of wavelength.

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