Flat lens

See also: Superlens

A flat lens is a lens whose flat shape allows it to provide distortion-free imaging, potentially with arbitrarily-large apertures.[1] The term is also used to refer to other lenses that provide a negative index of refraction.[2] Flat lenses require a refractive index close to −1 over a broad angular range.[3][4]

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.[3]

The first, near-infrared flat lens was announced in 2012 using nanostructured antennas.[2] It was followed in 2013 by an ultraviolet flat lens in 2013 that used a bi-metallic sandwich.[3]

In 2014 a flat lens was announced that combined composite metamaterials and transformation optics. The lens works over a broad frequency range.[5]

Traditional lenses

Traditional curved glass lenses can bend light coming from many angles to end up at the same focal point, on a slice of photographic film or an electric 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. To correct this, these lenses use extra pieces of glass, adding weight and mass.[2]

Metamaterials

Flat lenses employ metamaterials, that is, electromagnetic structures engineered on subwavelength scales to elicit tailored polarization responses.[3]

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.[3]

Metamaterials can image infrared, visible and most recently, ultraviolet wavelengths.[3]

Types

Nanoantennas

The first flat lens used a thin wafer of silicon 60 nanometers thick coated with concentric rings of v-shaped gold nanoantennas to produce photographic images. The antennas were systematically arranged on the silicon wafer and refract the light so that it all ends up on a single focal plane, a so-called artificial refraction process. The antennas were surrounded by an opaque silver/titanium mask that reflected all light that did not strike the antennas. Varying the arm lengths and angle provided the required range of amplitudes and phases. The distribution of the rings controls focal length.[6][4]

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[6] wavelength.[2]

The 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.[4]

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. The new flat lens does not suffer from the chromatic aberrations, or color fringing, that plague refractive lenses. As such, it will not require the additional bulky lens elements traditionally used to compensate for this chromatic dispersion.[7]

Bi-metallic sandwich

A later 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 and exhibits a negative index of refraction regardless of the incoming light's its angle of travel. The wavequides 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.[8]

Membrane

Membrane optics employ plastic in place of glass to diffract rather than refract or refract light. Concentric microscopic grooves etched into the plastic provide the diffraction.[9]

Glass transmits light with 90% efficiency, while membrane efficiencies range from 30-55%. Membrane thickness is on the order of that of plastic wrap.[9]

See also

References

  1. ↑ "Flat spray-on optical lens created". Sciencedaily.com. 2013-05-23. doi:10.1038/nature12158. Retrieved 2013-10-20.
  2. ↑ 2.0 2.1 2.2 2.3 Schiller, Jakob. "New Flat Lens Could Revolutionize Cameras as We Know Them | Raw File". Wired.com. Retrieved 2012-09-01.
  3. ↑ 3.0 3.1 3.2 3.3 3.4 3.5 Xu, T.; Agrawal, A.; Abashin, M.; Chau, K. J.; Lezec, H. J. (2013). "All-angle negative refraction and active flat lensing of ultraviolet light". Nature 497 (7450): 470–474. doi:10.1038/nature12158. PMID 23698446.
  4. ↑ 4.0 4.1 4.2 Aieta, F.; Genevet, P.; Kats, M. A.; Yu, N.; Blanchard, R.; Gaburro, Z.; Capasso, F. (2012). "Aberration-Free Ultrathin Flat Lenses and Axicons at Telecom Wavelengths Based on Plasmonic Metasurfaces". Nano Letters 12 (9): 4932–4936. doi:10.1021/nl302516v. PMID 22894542.
  5. ↑ Barry Dennis. "BAE Systems develops a flat lens that acts like it's curved". Gizmag.com. Retrieved 2014-04-22.
  6. ↑ 6.0 6.1 "Lightweight, distortion-free flat lens uses antennae, not glass, to focus light | Harvard Magazine Jan-Feb 2013". Harvardmagazine.com. Retrieved 2013-10-20.
  7. ↑ Crisp, Simon (February 23, 2015). "Researchers advance ultra-thin flat lens to capture perfect colors". Gizmag. Retrieved February 2015.
  8. ↑ "All-angle negative refraction and active flat lensing of ultraviolet light | FrogHeart". Frogheart.ca. 2013-05-27. doi:10.1038/nature12158. Retrieved 2013-10-20.
  9. ↑ 9.0 9.1 "DARPA developing giant folding space telescope". Gizmag.com. Retrieved 2013-12-10.

External links