Fig.6.Comparisonofthetransmittanceof“moth-eye”structuredPDMSmembrane(solidline)andunstructuredreferencesample(dashedline)fordifferentstrainfactorsinawavelengthrangefrom400to800nm.Asignificanttransmittanceenhancementoverabroadspectralrangeupto2.5%becomesvisible.

arenotabsolutelycomparable.E.g.,preparationmethodsmaydeviateinbothexperiments,andin-ducewavelength-dependentcharacteristics,andalsotheexperimentalsetupsdiffer:Inopticaldampingex-periments,thetransmissionsfordifferentbulkthick-nesseshavebeenreferencedtooneanother.Inthedirecttransmittancemeasurement,theabsolutetransmittedintensityiscomparedtoanondisturbedreferencebeam.Forthequantitativedisplaypre-sentedinFig.6,theeffectofbacksidereflectionofthemembranehasbeenremovedmathematicallybysubtractingtheFresnellosses.Thisapproachcor-respondstotherealsituationinvariablelenseswherethedominantreflectionoccursattheair-membraneinterfaceand,duetonearlyequalrefrac-tiveindices,thetransitionbetweenmembraneandfillingmaterialofthelensplaysonlyaninsignificantroleinthesystem’stransmissionlosses.

TheresultsshowninFig.6demonstrateasignifi-cantincreaseinthetransmittanceofupto2.5%oftheAR-structuredelastomermembranesincompari-sontotheunstructuredsamples.Theimprovedtrans-missionbehaviorofthe“moth-eye”structuredmembraneisalsoobservableunderstrain.Withinthedifferentmeasurementsperformed,anincreaseofthetransmittanceofupto2.5%isachievedovertheentirevisiblespectrum,whichmeansthatmorethanhalfofthesurfacereflectionlossesarecompen-satedbytheARstructures[seeFigs.6(c)and6(d)].Intheunstretchedcase,thedifferencebetweenbothcurvesrevealsaminimumeffect.Withan

increaseofthestrain,thedifferencebetweenbothcurvesineachdiagrambecomesmorepronounced.Inparticular,weobservedanincreaseofthetrans-mittanceinthe“moth-eye”-structuredsamplewhileincreasingthestrainfactor,whereasthereferencesampleshowedaslightlyreducedtransmittance.Thereducedtransmittanceoftheunstructuredmembraneunderstrainisnotcompletelyunder-stood.Possibleexplanationsmaypresumablybeeitherduetoincreasingabsorptionasaconsequenceofstretchedpolymerchainsand/orincreasingreflec-tivity,whichoccurswhenthesurfaceroughnessdecreasesasthepolymerisstretched[14].

5.Conclusion

Insummarywepresentedaprocessthatreducesthesurfacereflectionlossesonopticalelastomermem-branesbymorethan50%.Thisisachievedbypatterning“moth-eye”structuresdirectlyontotheelastomermembrane.Themanufacturingprocessin-volvesmasterstructuringbyself-assembly(BCML)followedbyamoldingstepinwhichthesubwave-lengthstructuresaretransferredintotheflexiblesiliconesubstrate.Incomparisontounstructuredre-ferencesamples,itwasshownthatthe“moth-eye”structuredmembranesallowforasignificanttrans-mittanceenhancementoverabroadspectralrange,evenwhenstrainisapplied.Withthisapproach,ARsurfacesformembranesusedintunableopticalelementssuchasadaptivelensesbecome

available.

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WeexpressouracknowledgementstoJanBrossmannfromthecompanyOpticsBalzers(Jena)fortheexperimentalsupport.ThisworkwasfundedbytheGermanResearchFoundationDFGwithinthePriorityProgramActiveMicro-optics.

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