focal length. The refraction angle — more at the edges than in the middle — is controlled by the antennas' shape, size, and orientation. It could focus only a single near-infrared wavelength. Nanoantennas introduce a radial distribution of phase discontinuities, thereby generating respectively spherical wavefronts and nondiffracting Bessel beams. Simulations show that such aberration-free designs are applicable to high-numerical aperture lenses such as flat microscope objectives. In 2015 a refined version used an achromatic metasurface to focus different wavelengths of light at the same point, employing a dielectric material rather than a metal. This improves efficiency and can produce a consistent effect by focusing red, blue and green wavelengths at the same point to achieve instant color correction, yielding a color image. This lens does not suffer from the chromatic aberrations, or color fringing, that plague refractive lenses. As such, it does not require the additional lens elements traditionally used to compensate for this chromatic dispersion. Bi-metallic sandwich A bi-metallic flat lens is made of a sandwich of alternating nanometer-thick layers of silver and titanium dioxide. It consists of a stack of strongly-coupled plasmonic waveguides sustaining backward waves. It exhibits a negative index of refraction regardless of the incoming light's angle of travel. The waveguides yield an omnidirectional left-handed response for transverse magnetic polarization. Transmission through the metamaterial can be turned on and off using higher frequency light as a switch, allowing the lens to act as a shutter with no moving parts. Membrane Membrane optics employ plastic in place of glass to diffract rather than refract or reflect light. Concentric microscopic grooves etched into the plastic provide the diffraction. Glass transmits light with 90% efficiency, while membrane efficiencies range from 30-55%. Membrane thickness is on the order of that of plastic wrap. Holographic lenses Holographic lenses are made from a hologram of a conventional lens. It is flat, and present any drawbacks of the original lens (aberrations), plus the drawbacks of the hologram (diffraction). The hologram of a mathematical lens is flat, and it has the properties of the mathematical lens, but it has the drawbacks of the hologram (diffraction). Geometric-phase lenses Geometric phase lenses, also known as polarization-directed flat lenses are made by depositing liquid crystal polymer in a pattern to make a "holographically recorded wavefront profile". They exhibit a positive focal length for circularly polarized light of one direction, and a negative focal length for circularly polarized