3-D printed metasurface design can control optical fields
A team led by scientists at the University of Washington has designed and tested a 3D-printed metamaterial that can manipulate light with nanoscale precision, which could have a major effect on autonomous transportation as well as optical elements for displays and sensors in VR or AR headsets.
A team led by scientists at the University of Washington has designed and tested a 3D-printed metamaterial that can manipulate light with nanoscale precision. Their designed optical element focuses light to discrete points in a 3-D helical pattern.
The team’s design principles and experimental findings demonstrate that it is possible to model and construct metamaterial devices that can precisely manipulate optical fields with high spatial resolution in three dimensions. Though the team chose a helical pattern — a spiral helix — for their optical element to focus light, their approach could be used to design optical elements that control and focus light in other patterns.
Devices with this level of precision control over light could be used not only to miniaturize today’s optical elements, such as lenses or retroreflectors, but also to realize new varieties. In addition, designing optical fields in three dimensions could enable creation of ultra-compact depth sensors for autonomous transportation, as well as optical elements for displays and sensors in virtual- or augmented-reality headsets.
“This reported device really has no classical analog in refractive optics — the optics that we encounter in our day-to-day life,” said Arka Majumdar, a UW assistant professor of electrical and computer engineering and physics. “No one has really made a device like this before with this set of capabilities.”
The team, which includes researchers at the Air Force Research Laboratory and the University of Dayton Research Institute, took a lesser-used approach in the optical metamaterials field to design the optical element: inverse design. Using inverse design, they started with the type of optical field profile they wanted to generate — eight focused points of light in a helical pattern — and designed a metamaterial surface that would create that pattern.
“We do not always intuitively know the appropriate structure of an optical element given a specific functionality,” said Majumdar. “This is where the inverse design comes in: You let the algorithm design the optics.”
While this approach seems straightforward and avoids the drawbacks of trial-and-error design methods, inverse design isn’t widely used for optically active large-area metamaterials because it requires a large number of simulations, making inverse design computationally intensive.
Here, the team avoided this pitfall thanks to an insight by Alan Zhan, who recently graduated the UW with a doctoral degree in physics. Zhan realized the team could use Mie scattering theory to design the optical element. Mie scattering describes how light waves of a particular wavelength are scattered by spheres or cylinders that are similar in size to the optical wavelength. Mie scattering theory explains how metallic nanoparticles in stained glass can give certain church windows their bold colors, and how other stained glass artifacts change color in different wavelengths of light, according to Zhan.
“Our implementation of Mie scattering theory is specific to certain shapes — spheres— which meant we had to incorporate those shapes into the design of the optical element,” Zhan said. “But, relying on Mie scattering theory significantly simplified the design and simulation process because we could make very specific, very precise calculations about the properties of light when it interacts with the optical element.”
Their approach could be employed to include different geometries such as cylinders and ellipsoids. The optical element the team designed is essentially a surface covered in thousands of tiny spheres of different sizes, arranged in a periodic square lattice. Using spheres simplified the design, and the team used a commercially available 3-D printer to fabricate two prototype optical elements — the larger of the two with sides just 0.02 cm long. The optical elements were 3D-printed out of an ultraviolet epoxy on glass surfaces. One element was designed to focus light at 1,550 nm, the other at 3,000 nam.
The researchers visualized the optical elements under a microscope to see how well they performed as designed — focusing light of either 1,550 or 3,000 nm at eight specific points along a 3-D helical pattern. Under the microscope, most focused points of light were at the positions predicted by the team’s theoretical simulations. For example, for the 1,550-nm wavelength device, six of eight focal points were in the predicted position. The remaining two showed only minor deviations.
With the high performance of their prototypes, the team would like to improve the design process to reduce background levels of light and improve the accuracy of the placement of the focal points, and to incorporate other design elements compatible with Mie scattering theory.
“Now that we’ve shown the basic design principles work, there are lots of directions we can go with this level of precision in fabrication,” said Majumdar.
One particularly promising direction is to progress beyond a single-surface to create a true-volume, 3-D metamaterial. “3D-printing allows us to create a stack of these surfaces, which was not possible before,” Majumdar said.
University of Washington
– Edited by Chris Vavra, production editor, Control Engineering, CFE Media & Technology, email@example.com.
Timu inayoongozwa na wanasayansi katika Chuo Kikuu cha Washington imebuni na kupima metamaterial iliyochapishwa kwa 3D ambayo inaweza kudhibiti mwanga kwa usahihi wa nanoscale, ambayo inaweza kuwa na athari kubwa kwa usafirishaji wa kujitegemea na vile vile vipengele vya macho kwa maonyesho na sensorer katika vichwa vya VR au AR. Timu inayoongozwa na wanasayansi wa Chuo Kikuu cha Washington imebuni na kupima metamaterial iliyochapishwa kwa 3D ambayo inaweza kudhibiti mwanga kwa usahihi wa nanoscale. Vipengele vyao vya macho vinaelekeza nuru kwenye sehemu tofauti-tofauti katika muundo wa mviringo wa 3D. """Mafunzo ya muundo wa timu na matokeo ya majaribio yanaonyesha kuwa inawezekana kuunda na kuunda vifaa vya metamaterial vinavyoweza kudhibiti kwa usahihi mashamba ya macho na azimio la nafasi ya juu katika vipimo vitatu." Ingawa timu ilichagua muundo wa helical - spiral helix - kwa kipengele chao cha macho ili kukazia mwanga, mbinu yao inaweza kutumika kubuni vipengele vya macho ambavyo vinadhibiti na kukazia mwanga katika mifumo mingine. Vifaa vyenye kiwango hiki cha usimamizi wa usahihi juu ya mwanga vinaweza kutumika sio tu kupanua vipengele vya macho ya leo, kama vile lensi au retroreflectors, lakini pia kutambua aina mpya. Kwa kuongezea, kubuni mashamba ya macho katika vipimo vitatu inaweza kuwezesha uundaji wa sensorer za kina cha ultra-compact kwa usafirishaji wa kujitegemea, na vile vile vipengele vya macho kwa maonyesho na sensorer katika vichwa vya macho vya hali halisi au kuongezeka. "Kipimo hiki hakina analog ya kawaida katika optics ya refractive, optics ambayo tunakutana nayo katika maisha yetu ya kila siku,"" alisema Arka Majumdar, profesa msaidizi wa UW wa uhandisi wa umeme na kompyuta na fizikia." "Hakuna mtu aliyewahi kutengeneza kifaa kama hiki kabla na uwezo kama huu. ""Timu hiyo, ambayo inajumuisha watafiti wa Maabara ya Utafiti wa Jeshi la Anga na Chuo Kikuu cha Dayton Research Institute, ilichukua mbinu inayotumiwa kidogo katika uwanja wa metamaterials ya macho kubuni kipengele cha macho: kubuni kinyume." Kutumia muundo wa kinyume, walianza na aina ya wasifu wa uwanja wa macho waliotaka kuzalisha - pointi nane zilizolenga mwanga katika muundo wa helical - na kubuni uso wa metamaterial ambao ungeunda muundo huo. "Hatujui daima intuitively muundo sahihi wa kipengele cha macho kutokana na utendaji maalum", alisema Majumdar. "Hapa ndipo muundo wa kinyume unapoingia: Unaruhusu algorithm kubuni optics. "" Ingawa mbinu hii inaonekana moja kwa moja na inaepuka hasara za mbinu za muundo wa jaribio na makosa, muundo wa kinyume hautumiki sana kwa metamaterials kubwa ya eneo la macho kwa sababu inahitaji idadi kubwa ya simulations, na kufanya muundo wa kinyume kuwa na hesabu nyingi. """ Katika utafiti huo, watafiti walipata msaada wa Alan Zhan, ambaye hivi karibuni alihitimu kutoka Chuo Kikuu cha Washington na shahada ya udaktari katika fizikia. Zhan alitambua timu inaweza kutumia Mie kutawanyika nadharia ya kubuni kipengele macho. Mie scattering ni hali ya mwanga wa mawimbi ya mawimbi ya mwanga ambayo huenea kwa njia ya mviringo au silinda ambayo ni sawa na urefu wa mawimbi ya macho. Nadharia ya kutawanyika ya Mi inaelezea jinsi nanoparticles za chuma katika glasi iliyotiwa rangi zinaweza kutoa madirisha fulani ya kanisa rangi zao za ujasiri, na jinsi vitu vingine vya glasi iliyotiwa rangi vinabadilika rangi katika urefu tofauti wa mawimbi ya nuru. "Utekelezaji wetu wa nadharia ya kutawanyika kwa Mie ni maalum kwa maumbo fulani - mduara - ambayo ilimaanisha tulilazimika kuingiza maumbo hayo katika muundo wa kipengele cha macho", Zhan alisema. "Lakini, kutegemea nadharia ya kutawanyika kwa Mie ilifanya mchakato wa kubuni na uigizaji kuwa rahisi kwa sababu tunaweza kufanya hesabu maalum sana, sahihi sana juu ya mali ya mwanga wakati unaingiliana na kipengele cha macho. ""Mkakati wao unaweza kutumika kujumuisha jiometri tofauti kama vile silinda na ellipsoids." "Kipengele cha macho ambacho timu hiyo ilibuni ni ""upande uliofunikwa na maelfu ya mviringo mdogo wa ukubwa tofauti, uliopangwa katika lattice ya mraba ya mara kwa mara.""" Kwa kutumia spheres, timu hiyo ilitumia printer ya 3D ya kibiashara kutengeneza vipengele viwili vya macho, sehemu kubwa zaidi ikiwa na urefu wa pembe 0.02 cm. Vipengele vya macho vilichapishwa kwa 3D kutoka kwa epoxy ya ultraviolet kwenye nyuso za kioo. Mfano mmoja ulibuniwa ili kukaza mwanga kwa urefu wa nm 1,550 na mwingine kwa urefu wa nm 3,000. Watafiti waliangalia vipengele vya macho chini ya darubini ili kuona jinsi walivyofanya vizuri kama ilivyopangwa - kuzingatia mwanga wa 1,550 au 3,000 nm katika pointi nane maalum kando ya muundo wa 3D. Chini ya darubini, sehemu zenye mwangaza mwingi zilikuwa katika nafasi zilizotabiriwa na michoro ya nadharia ya timu hiyo. Kwa mfano, kwa kifaa cha urefu wa mawimbi ya 1,550nm, sita kati ya nane za focal zilikuwa katika nafasi iliyotarajiwa. Vipimo viwili vilivyobaki vilionyesha tofauti ndogo tu. Kwa sababu ya utendaji wa juu wa prototypes zao, timu hiyo inataka kuboresha mchakato wa kubuni ili kupunguza viwango vya nyuma vya mwanga na kuboresha usahihi wa uwekaji wa vituo vya kuangazia, na kuingiza vipengele vingine vya kubuni vinavyolingana na nadharia ya kutawanyika kwa Mie. "Sasa kwa kuwa tumeonyesha kanuni za msingi za kubuni zinafanya kazi, kuna mwelekeo mwingi tunaweza kwenda na kiwango hiki cha usahihi katika utengenezaji", alisema Majumdar. Mwelekeo mmoja unaotarajia hasa ni kusonga mbele zaidi ya uso mmoja ili kuunda kiasi halisi, metamaterial ya 3D. "Kuchapisha kwa 3D kunatuwezesha kuunda rundo la nyuso hizi, ambayo haikuwezekana hapo awali, "Majumdar alisema. Chuo Kikuu cha Washington <unk> Iliyorekebishwa na Chris Vavra, mhariri wa uzalishaji, Uhandisi wa Udhibiti, CFE Media & Teknolojia, email@example.com.