A flat lens is a lens whose flat shape allows it to provide distortion-free imaging, potentially with arbitrarily-large apertures. The term is also used to refer to other lenses that provide a negative index of refraction. Flat lenses require a refractive index close to −1 over a broad angular range. In recent years, flat lenses based on metasurfaces were also demonstrated. History Russian mathematician Victor Veselago predicted that a material with simultaneously negative electric and magnetic polarization responses would yield a negative refractive index (an isotropic refractive index of −1), a "left-handed" medium in which light propagates with opposite phase and energy velocities. The first, near-infrared, flat lens was announced in 2012 using nanostructured antennas. It was followed in 2013 by an ultraviolet flat lens that used a bi-metallic sandwich. In 2014 a flat lens was announced that combined composite metamaterials and transformation optics. The lens works over a broad frequency range. Traditional lenses Traditional curved glass lenses can bend light coming from many angles to end up at the same focal point on a piece of photographic film or an electronic sensor. Light captured at the very edges of a curved glass lens does not line up correctly with the rest of the light, creating a fuzzy image at the edge of the frame. (Petzval field curvature and other aberrations.) To correct this, lenses use extra pieces of glass, adding bulk, complexity, and mass. Metamaterials Flat lenses employ metamaterials, that is, electromagnetic structures engineered on subwavelength scales to elicit tailored polarization responses. Left-handed responses typically are implemented using resonant metamaterials composed of periodic arrays of unit cells containing inductive–capacitive resonators and conductive wires. Negative refractive indices that are isotropic in two and three dimensions at microwave frequencies have been achieved in resonant metamaterials with centimetre-scale features. Metamaterials can image infrared, visible, and, most recently, ultraviolet wavelengths. Types Graphene oxide With the advances in micro- and nanofabrication techniques, continued miniaturization of conventional optical lenses has been requested for applications such as communications, sensors, and data storage. Specifically, smaller and thinner micro lenses are needed for subwavelength optics or nano-optics with small structures, for visible and near-IR applications. As the distance scale for optical communications shrinks, the required feature sizes of micro lenses shrink. Graphene oxide provides solutions to advance planar focusing devices. Giant refractive index modification (as large as 10^-1 or one order of magnitude larger than earlier materials), between