PhotonicsResearchHighlights–July2016R.A.Norwood

My current research portfolio consists of work ranging from nanoscale electro‐opticmodulators to40 squaremeterhybrid solarenergy systems. A common theme that cutsacross this 10 order ofmagnitude change in length scale is the coupling of fundamentalopticalmaterialsanddevicephysicstoemergingapplicationswithsignificantimpact.ThePhotonicMaterialsandDeviceLab(PMDL)isconstantlyseekingoutnewopticalmaterialsandphotonicdeviceinnovationsthatcanimpactabroadrangeofapplications,rangingfrominformation technology, to renewable energy, to infrared optics. We now provide asummaryofsomeofourcurrentresearchprograms. FOCUSOurARPA‐E FOCUS program has the goal of improving the usefulness of solar energy topowercompaniesbycombiningphotovoltaicelectricityproductionwithconcentratedsolarpowerheatproductionasseenintheschematic.TheprogramisledbySharpLaboratoriesofAmerica,aCOSIndustrialAffiliate,andourgrouphasbeenresponsibleforopticaldesignandcharacterization.ThepictureshowsformergraduatestudentByronCocilovo(nowDr.CocilovoandsoontobeworkingatApple)characterizingasegmentoftheopticalsystem.Acriticalpartofthesystemisanoptimizedhyperbolicdichroicmirrorthatsitsnearthefocusof the parabolic reflectors, with the job of sending the 500‐850nm light down toconcentratedPVcellunitswhileallowing therestof the light topass through to theheattube.ThemirrorwasdesignedtooptimizephotovoltaicenergyoutputoverthefullyearinTucson,accountingforkeyaspectssuchasbalancingtheseparatephotovoltaiccells. Thiswork was recently published in AppliedOptics (A. Miles et al., AppliedOptics 55, 1849(2016)). A full scale prototype of the system is currently under construction on a siteownedbytheUACollegeofAgriculturesandLifeSciences(CALS)justwestofI‐10.

Characterizingtheopticaltroughsegment

Optimizeddichroicmirrorspectrum

MOSAICOurARPA‐EMOSAICprogram is focusedoncreatingmoreefficient (>30%)solarpanelsbycombiningstandardsiliconpanelsfordiffuselightcollection(labeledas1‐Sunsheetinthefigurebelow)andhighefficiencyconcentratedphotovoltaicarraysforthedirectsunlight(knowasdirectnormalinsolationorDNI).Keyopticaldesignelements includea cylindrical lensconcentrator inonedirectionandawaveguidesheetconcentrator in theotherdirectionasshownintheschematic. Thiswork isalso done under subcontract to Sharp Laboratories of America and started inJanuary2016.

AthermalOpticalAdd‐dropMultiplexersSiliconphotonicsisamajortechnologydevelopmentinintegratedoptics,wherethegoal is to take thesubstantialmanufacturingknowledge thatexists forprocessingsiliconforelectronicsandapplyittomakingintegratedopticaldevices. Therearemany major corporations, start‐up companies, and universities dedicated torealizing this vision andmany products are now in themarket place. One of thedifficultieswithsiliconasanintegratedphotonicmaterialisthatithasafairlyhighthermo‐optic coefficient,meaning that its refractive index changes significantly asthe temperature is changed. Ordinarily this problem is solved by providingtemperature control for the chip, so that its performance doesn’t change as theambient temperature drifts. However, in electrical power hungry data centerenvironments, this is not a viable option. We have developed an approach formakingsiliconphotonicslessdependentontemperature,byreplacingthestandardsilicondioxidetopcladdingwithasol‐gelmaterialthathassimilarrefractiveindexbut a large and negative thermo‐optic coefficient. This material essentiallycompensatestheeffectinsiliconleadingtomuchlesstemperaturesensitivityforanopticaladd‐dropmultiplexerasshowninthefigure(S.Namnabatetal.,OFC2016,paperTu2F.3).Wehaverecentlyachievedpracticallycompleteathermalityinthesedevicesaswillbereportedinaforthcomingmanuscript.

Schematicshowingbasicapproachtoahighefficiencysolarpanel

Electro‐opticModulatorsOur work on electro‐optic modulators now spans from high speed low insertiondeviceswithoptimizedfabricationprocessesforRFphotonicsapplicationstonovelplasmonicmodulators only30microns in length. All processes required tobothfabricate and package EO polymer modulators have been modified to improvemanufacturability. The figuresbelowshowa40GHzEOpolymermodulatorat anintermediate step in the fabrication process as well the process by which fiberferrulesarepigtailedtotheEOpolymermodulatorchips.State‐of‐the‐artEO

OADMsiliconphotonicchip Datashowingreducedthermalsensitivity

40GHzEOpolymermodulatorchip Opticalfiber– chipattachprocess

polymersarebeingused,withmaterial figuresofmerit forhighspeedmodulatorsmanytimesbiggerthanthoseoflithiumniobate(R.Himmelhuberetal.,Sensors15,18239 (2015)). Plasmonic modulators take advantage of the subwavelengthwaveguidespossiblewithplasmonicconfinementtomakeultracompactdevicesasshownintheimagesshownbelow.

InfraredOpticalPolymersDespite tremendous advances in optical materials for the ultraviolet, visible andnear‐infraredoverthelastfewdecades,relativelylittleprogresshasbeenmadeindevelopingmaterials for themid‐wave infrared(MWIR;3–5micronwavelength)andthelong‐waveinfrared(LWIR; 8‐12 micronwavelength). We haverecently collaborated withDr. Jeff Pyun’s group inUA’s Chemistry andBiochemistry departmentto demonstrate the firstoptical polymers that canbe suitable for MWIRapplications. This isachieved by thedevelopment of a sulfur‐based copolymer materialthathas relatively lowhydrogen content, leading to reducedabsorption fromC‐Hvibrations.Thematerialscanbemoldedintolensesandwindows;thefigureshowsimagingat1550nm(greypicture)aswellastransmissionthroughawindowinthe3‐5 micron region (labeled as “e”). Picture “d” shows what is seen through aconventional optical polymer, PMMA, through an identical thickness of material.Theseexcitingresultshavegarneredattentionfrommajorcorporations,resultedin

Silicontaperenteringplasmonicwaveguideregion–initialsiliconwidthis~400nm

Completemodulatorwithwirebondsmadetothegoldelectrodes

one issued US patent thus far (US 9,206,218) and led to several publications (J.Griebel et al.,AdvancedMaterials26, 3014 (2014) and J. Griebel et al.,ACSMacroLetters4,862(2015)).DiatomPhotonicsTherehasbeenrecentinterestinthedevelopmentofnewmicroscaledeviceswithmesoscale to nanoscale features for a variety of applications including use inbiosensors, solar cells, batteries, detectors, and photonic and plasmonic devicesCurrentmethodsoffabricationofsuchdevicestendtorelyon2Dphotolithographyorchemicaletching,andwhilethesetechniquescanbeverypreciseandreliablefor2Dpatterns,theyarenotwell‐suitedtoproducingcomplex3Dstructures.Diatoms,a type of single‐celled photosynthetic algae, offer an alternate route to theproduction of micro and nano‐structured materials. Diatoms produce rigid cellwalls (called frustules)made of amorphous silica with a wide variety of species‐dependent shapes and features. Ofparticular interest isthefactthat inmanyspecies,thesefrustuleshaveperiodicholes,or pores, ranging in size from tens ofnanometerstoafewmicrons;thefiguretothe right shows a scanning electronmicrograph of a Coscinodiscus wailesiidiatom shells at various resolutions.Withontheorderof105uniquediatomspecies,there are many frustule shapes toinvestigateaspotentiallyinterestingphotonicdevices.Thefirstfigurebelowshowsour use C. wailessi frustules as a tunable filter for a bright white lightsupercontinuum source. The diffraction from the diatoms periodic structuredetermineswhatcolorwilldominate in thetransmitted light(K.Kieuetal.,OpticsExpress22,15992(2014)).

Surface enhanced Raman scattering (SERS) can occurwhen a rough or patternedsurfaceiscoatedwithaplasmonicmetal,suchassilverorgold.ThedevelopmentofSERS substrates is an active area of research and usually involves the use oflithographyorvariousetchingprocesses.TheperiodicstructureofholesindiatomfrustulesmakesitpossibletoobservestrongSERSwhenasilvercoatedC.wailessifrustule is coated with the benchmark Raman scattering molecule thiophenol asshown in the figure below,where the top spectrum is for thiophenol on a planarsilver film (mostly noise) and the bottom spectrum is for thiophenol on a diatomfrustule coatedwith silver (clear thiophenolpeaks). The signal isenhancedbyatleastafactorof300forthediatomSERSsubstrates(manuscriptinrevision).