The development of metalenses—lenses based on metamaterials, either 2D or 3D, rather than on bulk glass—has been pursued in the lab for years. The resulting devices have largely been interesting curiosities, having the advantages of extreme thinness and lightness, and sometimes the ability to image objects at somewhat beyond the diffraction limit because of their inclusion of evanescent electromagnetic waves in the imaging process. They also have the disadvantages of high loss, difficulty in fabrication, and, in many cases, poor imaging from even slight imperfections. The optics industry, which is based on practicality (not only must things work, but they must also be reasonably fabricable), will very carefully vet any new technology before putting it on the production line. Now, researchers at Harvard University (Cambridge, MA) and the University of Waterloo (Waterloo, ON, Canada) have developed a metalens that may get more than a first or second glance from the optics world.
The metalens design, which is monochromatic and thus suited for laser light, is based on the conversion of right-handed, circularly polarized incident light to left-handed, circularly polarized light. Therefore, the nanostructure on the lens actually operates as a half-wave plate. The researchers fabricated an array of high-aspect-ratio titanium dioxide nanofins on a glass substrate, giving the nanofins a spatially varying rotation angle to produce the proper output phase profile. They created three metalenses with operating wavelengths of 660, 532, and 405 nm, respectively—all the lenses had a 240 μm diameter and a numerical aperture of 0.8 (focal length of 90 μm), resulting in three 170X objective lenses at the three respective wavelengths. When tested at their design wavelengths, all three metalenses produced diffraction-limited spots. Their full-width half-maximum (FWHM) focal spots were also compared to those produced by a state-of-the-art Nikon microscope objective; the metalens FWHMs were all smaller. Efficiencies of the metalenses at 405, 532, and 660 nm were high—86%, 73%, and 66%, respectively. Reference: M. Khorasaninejad et al., arXiv:1605.02248v1 [physics.optics] (May 7, 2016).
Maendeleo ya lensi za metalsens kulingana na metamaterials, ama 2D au 3D, badala ya kioo cha wingi, yamefuatwa katika maabara kwa miaka. Vifaa hivyo vimeonekana kuwa vya kuvutia, na vyenye faida ya kuwa nyembamba na nyepesi sana, na wakati mwingine uwezo wa kuunda picha za vitu kwa kiasi fulani zaidi ya kikomo cha diffraction kwa sababu ya kuingizwa kwa mawimbi ya umeme katika mchakato wa picha. Pia zina hasara ya hasara kubwa, ugumu wa utengenezaji, na, katika visa vingi, picha mbaya kutoka hata kasoro ndogo. Sekta ya macho, ambayo inategemea vitendo (si tu lazima vitu vifanye kazi, lakini pia lazima viwe vinavyoweza kutengenezwa kwa busara), itachunguza kwa uangalifu sana teknolojia yoyote mpya kabla ya kuiweka kwenye mstari wa uzalishaji. Watafiti wa Chuo Kikuu cha Harvard (Cambridge, MA) na Chuo Kikuu cha Waterloo (Waterloo, Ontario, Canada) wametengeneza lensi ya chuma ambayo inaweza kupata zaidi ya mtazamo wa kwanza au wa pili kutoka kwa ulimwengu wa macho. Lens ya chuma, ambayo ni monochromatic na hivyo inafaa kwa ajili ya laser mwanga, ni msingi juu ya uongofu wa haki-mkono, mviringo polarized incoming mwanga kwa kushoto-mkono, mviringo polarized mwanga. Kwa hiyo, muundo wa nanostructure kwenye lensi kwa kweli hufanya kazi kama sahani ya nusu-wimbi. Watafiti waliunda safu ya nanofins ya titani dioksidi ya kiwango cha juu kwenye substrate ya kioo, ikitoa nanofins pembe ya mzunguko inayobadilika kwa nafasi ili kutengeneza wasifu sahihi wa awamu ya pato. Lens hizo zilikuwa na urefu wa mawimbi ya 660, 532, na 405 nm, na zilikuwa na kipenyo cha 240 μm na aperture ya 0.8 (mrefu ya fikira ya 90 μm), na zilikuwa na lensi tatu za 170x. Wakati kupimwa katika urefu wa mawimbi yao ya kubuni, wote tatu metalenses zinazozalishwa diffraction-limited spots. Vipimo vya FWM (Full-width half-maximum) vilikuwa na uwezo wa kufanana na vipimo vya Nikon, lakini vilikuwa na uwezo mdogo zaidi. Ufanisi wa metalsens katika 405, 532 na 660 nm ulikuwa juu - 86%, 73% na 66%, kwa mtiririko huo. M. Khorasaninejad et al. (Arxiv 1605.02248v1) (Mwezi wa 5 2016).