If you think metamaterial has only “invisible cloak” and/or “negative index of refraction” in her hat, think again. Researchers from the school of engineering and applied science at Harvard utilize a powerful feature of the metamaterial to create ultra-thin and flat lens that is diffraction limited. They also create a flat axicon as another example when pioneering in this field (A quick glimpse on the Axicon: A lens with a flat surface on one side, and a conical shape on the other, has the ability to focus a Gaussian beam into a Bessel beam at the focal region, and create hollow ring beam shape in the far field).
To focus light, we need to create a converging spherical wavefront, or at least, a wavefront that is converging. To do so, we need to introduce different phase retardation on different portion of the incident light. A spherical lens does so by letting the light pass through different amount of material. For example, when light is passing through the center of the lens, it lags behind compared with that passes through the edge of the lens. As a result, there is a phase difference between them. This phase difference, or phase retardation between them, produces a converging wavefront. However, when a plane wave like light passes through a spherical lens, it suffers from spherical aberration. That is to say the spherical lens does not produce a perfectly converging spherical wavefront which is required for the light to focus tightly. The light exiting from the edge of the lens suffers stronger deviation. The best way to solve this so far is to use an aspherical lens to correct this imperfection.