News › Fraun­ho­fer IOF • Thin meta­sur­faces ins­tead of thick lenses

Rese­ar­chers use elec­tron beam litho­gra­phy to pro­duce metastruc­tures on large sur­faces, set­ting a new record

Bye bye, lens. Hello meta­sur­face! So-cal­led meta­sur­faces can help to make opti­cal sys­tems thin­ner in the future, while at the same time incre­asing their func­tion­a­lity. The pro­blem: Until now, con­ven­tio­nal manu­fac­tu­ring pro­ces­ses have often only been able to rea­lize small meta­sur­faces, often smal­ler than one square mil­li­me­ter. Rese­ar­chers at Fraun­ho­fer IOF have now suc­cee­ded for the first time in pro­du­cing a meta­sur­face with a dia­me­ter of almost 30 cen­ti­me­ters using elec­tron beam litho­gra­phy – a world record. The sci­en­tists have now published their method in the »Jour­nal of Micro/Nanopatterning, Mate­ri­als, and Metro­logy«. Fur­ther, the novel metastruc­ture will be on public dis­play for the first time at LASER World of Pho­to­nics in Munich, Ger­many, June 27–30.

»After 500 years of len­ses and mir­rors, it is time to think ahead,« explains Dr. Falk Eilen­ber­ger, head of the Depart­ment of Micro- and Nanos­truc­tu­red Optics at the Fraun­ho­fer Insti­tute for Applied Optics and Pre­cis­ion Engi­nee­ring IOF. So-cal­led meta­sur­faces can be an alter­na­tive here. These are com­pon­ents that con­cen­trate their com­plete opti­cal func­tion in a sur­face and achieve this func­tion in the sur­face through nanos­truc­tures. Eilen­ber­ger explains the dif­fe­rence to the clas­sic lens as fol­lows: »In len­ses, the func­tion is defi­ned by the macro­sco­pic geo­me­try. That’s why the lens is thick and cur­ved. Now we have a meta­sur­face ins­tead. It’s thin and on sca­les smal­ler than the wave­length of light.«

Meta-sur­faces have been used in sci­ence and rese­arch for some time. Howe­ver, the com­pon­ents here are often only a few square mil­li­me­ters in size. This is suf­fi­ci­ent for aca­de­mic rese­arch, but not for many indus­trial appli­ca­ti­ons, and cer­tainly not to become a real alter­na­tive to the clas­sic lens in the future. Rese­ar­chers at Fraun­ho­fer IOF in Jena, Ger­many have the­r­e­fore devo­ted them­sel­ves to the ques­tion of how inno­va­tive meta­sur­faces can be rea­li­zed on a lar­ger scale. As a result, they are now pre­sen­ting a meta­sur­face with a dia­me­ter of 30 cen­ti­me­ters for the first time. »We are not the inven­tors of meta­sur­faces,« Eilen­ber­ger holds. »But we are the only ones who can demons­trate it on such a large scale.«

High-reso­lu­tion struc­tures with high pre­cis­ion and efficiency

But how did the rese­ar­chers achieve this mile­stone? The ans­wer: with the help of elec­tron beam litho­gra­phy. »To pro­duce our meta­sur­face, we used a spe­cial wri­ting stra­tegy of elec­tron beam litho­gra­phy cal­led cha­rac­ter pro­jec­tion,« explains Prof. Dr. Uwe Zeit­ner, a rese­ar­cher at Fraun­ho­fer IOF and mem­ber of the institute’s sci­en­ti­fic direc­to­rate. Cha­rac­ter pro­jec­tion is a method in which a pat­tern is divi­ded into smal­ler units. An elec­tron beam is then used to create each of these small pat­terns in turn on a sur­face. This enables the fabri­ca­tion of com­plex struc­tures with high pre­cis­ion and effi­ci­ency. »Using cha­rac­ter pro­jec­tion, very high-reso­lu­tion struc­tures can be expo­sed in par­al­lel at com­pa­ra­tively high speeds. This is unu­sual for elec­tron beam litho­gra­phy,« Zeit­ner con­ti­nues. Tog­e­ther with his Fraun­ho­fer col­le­agues Dr. Michael Banasch and Dr. Mar­cus Trost, Prof. Zeit­ner has out­lined the poten­tial of elec­tron beam litho­gra­phy for the fabri­ca­tion of micro- and nano-optics on large areas in a paper that has now been published in the »Jour­nal of Micro/Nanopatterning, Mate­ri­als, and Metrology«.

With their paper, the aut­hors show that con­ven­tio­nal litho­gra­phic tech­ni­ques often reach their limits for fabri­ca­ting lar­ger struc­tures. »Due to the small struc­ture dimen­si­ons below the wave­length, high-reso­lu­tion elec­tron beam litho­gra­phy is in prin­ci­ple very well sui­ted for the fabri­ca­tion of meta-struc­tures,« the rese­ar­cher said. »Howe­ver, this tech­no­logy is rela­tively slow. So far essen­ti­ally only ele­ments with rela­tively small areas have been rea­li­zed with it – pri­ma­rily on the order of a few square mil­li­me­ters. For lar­ger areas, the expo­sure time very quickly rea­ches unrea­li­sti­cally large values.« By using cha­rac­ter pro­jec­tion, the sci­en­tists were now able to address both the high reso­lu­tion of elec­tron beam litho­gra­phy and the large ele­ment area wit­hout the expo­sure time »explo­ding,« accor­ding to Uwe Zeit­ner. The aut­hors of the paper thus show that elec­tron beam litho­gra­phy can be a tech­ni­que for fabri­ca­ting micro- and nano-opti­cal struc­tures on large areas.

Reduc­tion in size with simul­ta­neous increase in functionality

The new manu­fac­tu­ring tech­no­logy can help build opti­cal sys­tems signi­fi­cantly thin­ner in the future. »This tech­no­logy can revo­lu­tio­nize ima­ging opti­cal sys­tems,« says Falk Eilen­ber­ger, for exam­ple. »Because it will make it pos­si­ble to reduce the size of sys­tems while simul­ta­neously incre­asing their opti­cal func­tion­a­lity.« Uwe Zeit­ner adds con­crete appli­ca­tion examp­les: »Such large meta­sur­faces are par­ti­cu­larly advan­ta­ge­ous for com­pact optics in which large deflec­tion angles are requi­red in a small space. This is the case, for exam­ple, in virtual/augmented rea­lity glas­ses. Advan­ta­ge­ous designs can also be rea­li­zed with such approa­ches for very small optics in smart­phones.« Other poten­tial appli­ca­ti­ons include high-reso­lu­tion spec­tro­scopy or com­pu­ter-gene­ra­ted holograms.

Pre­sen­ta­tion at LASER World of Photonics

The 30-cen­ti­me­ter meta­sur­face will be on public dis­play for the first time at LASER World of Pho­to­nics in Munich from June 27 to 30, 2023. The Fraun­ho­fer rese­ar­chers can be found at booth 415 in hall A2.

Ori­gi­nal publi­ca­tion: Uwe D. Zeit­ner, Michael Banasch, Mar­cus Trost, »Poten­tial of E‑beam litho­gra­phy for micro- and nano-optics fabri­ca­tion on large areas,« Jour­nal of Micro/Nanopatterning, Mate­ri­als, and Metro­logy, Vol. 22, Issue 4 (June 2023), DOI: 10.1117/1.JMM.22.4.041405, URL: