Availableonlineatwww.sciencedirect.com
Journal of Applied Research and Technology
www.jart.ccadet.unam.mx JournalofAppliedResearchandTechnology13(2015)561–565
Original
Norland Optical Adhesive 72 ® as phase holographic material
Mauricio Ortiz-Gutiérrez
a,∗, Arturo Olivares-Pérez
b, Edgar Alvarado-Méndez
c, Mónica Trejo-Durán
c, Marco Antonio Salgado-Verduzco
aaFacultaddeCienciasFísicoMatemáticas,UniversidadMichoacanadeSanNicolásdeHidalgo,Morelia,Mich.,Mexico
bInstitutoNacionaldeAstrofísicaÓpticayElectrónica,A.P.51y216,72000Puebla,Pue,Mexico
cDivisióndeIngenierías,CampusIrapuato-Salamanca(DICIS),UniversidaddeGuanajuato,Mexico Received31March2015;accepted14October2015
Availableonline19November2015
Abstract
CharacterizationoftheholographicmaterialcomposedbyadhesivepolymerNorlandOpticalAdhesive72®(NOA72®)wasstudied.Witha wavelengthof457nmfromanArlaser,realtimephaseholographicgratingsunderdifferentparameterssuchasenergy,frequencyandthickness wererecorded.Thediffractionefficiencyoftherecordedholographicgratingswasmeasuredandsomeexperimentalresultsareshown.Furthermore, thematerialwasusedtorecordFourierholograms.
AllRightsReserved©2015UniversidadNacionalAutónomadeMéxico,CentrodeCienciasAplicadasyDesarrolloTecnológico.Thisisan openaccessitemdistributedundertheCreativeCommonsCCLicenseBY-NC-ND4.0.
Keywords:Holographicrecordingmaterials;Holographicgratings;Photosensitivefilm;Fourierholograms
1. Introduction
The researchof photosensitivematerials is an active field wherethe maingoalis tofind materialswithdesirable char- acteristics for optical data storage. Some of these special characteristics are highsensibility, highresolution andwidth spectralrange, lowcost,betweensomeothers(Smith, 1969).
Therearemanykindsofmaterialsthathavebeenusedforthis purpose,suchassilverhalide,photoresist,dichromatedgelatin, photopolymers,thermalrecordingmaterials,photothermoplas- tics,photochromics,andphotorefractivecrystals(Bjelkhagen, 1996;Hariharan,1980;Kang,Kim,&Bae,2004;Kostuk,1999) andrecentlythemostusedarephotopolymers.
Thephotopolymershasexcellentholographiccharacteristics, suchas highrefractionindexmodulation,realtimerecording and low cost. The response of these materials depends of parameters such as incident beam intensity, monomers con- centration,polymerizationvelocity,humidity,temperatureand thickness(Adhami,Lanteigne,&Gregory,1991;Gallegosetal.,
∗Correspondingauthor.
E-mailaddress:[email protected](M.Ortiz-Gutiérrez).
PeerReviewundertheresponsibilityofUniversidadNacionalAutónomade México.
2005; Gleeson,Kelly,O’Neill, &Sheridan, 2005).There are some papers where the thickness of the photopolymer film is of great importance (Neipp et al., 2003; Ortu˜no, Gallego, García, Neipp, & Pascual, 2003). The spectral sensibility of these materials can be easily modified if the photopolymers aremixedwithdyes(Luna-Moreno,Olivares-Peréz,&Berriel- Valdos,1997;Luna-Moreno,Olivares-Pérez,Berriel-Valdos,&
Osorio-Alarcón,1998;Ortiz-Gutiérrez,Alemán,Pérez-Cortés, Ibarra-Torres,&Olivares-Perez,2007).
Someofthephotopolymeremployedinopticalstorageare giveninBudinskiandBudinski(1999),Fernándezetal.(2006), Ibarra and Olivares-Pérez (2002), Martin, Leclere, Renotte, Toal,andLion(1994),Naydenova,Sherif,Mintova,Martin,and Toal(2006).OneofthesepolymersisanadhesivecalledNorland OpticalAdhesiveNo.65®(NOA65®).Pinto-Iguaneroandco- workersreportstheuseofNOA65®initsnaturalformtorecord computer generated Fourier holograms by microlithography techniques (Pinto-Iguanero, Olivares-Pérez,& Fuentes-Tapia, 2002).
In this paper the use of Norland Optical Adhesive 72® (NOA72®)asphotosensitivematerialisproposed.Thisiscured withultravioletlightbetween315and400nmandvisiblelight between400and450nm.Thewavelengthsofmaximumabsorp- tion are 320, 365 and 420nm. The complete curing process
http://dx.doi.org/10.1016/j.jart.2015.10.014
1665-6423/AllRightsReserved©2015UniversidadNacionalAutónomadeMéxico,CentrodeCienciasAplicadasyDesarrolloTecnológico.Thisisanopenaccess itemdistributedundertheCreativeCommonsCCLicenseBY-NC-ND4.0.
requires5Jofenergyintherangefrom315to450nm(Norland Products, 2015). To characterize the photosensitive material phaseholographicgratings(PHG)usingalightbeamwithan argonlaser wavelength of 457nmwere recorded. Diffraction efficiency(DE)ofholographicgratingsrecordedusingabeam of He–Ne laserwas measured.The materialdoes notrequire preparation processes or developingprocesses. An important property of the adhesive is its transmittance spectrum inthe range400–1200nm,whichmakesthismaterialidealforman- ufacturing diffractive phase elements. Also in thispaper, the results obtained when recordingFourier hologramsof binary objectsarealsopresented.
2. NOA72®properties
Thetypicalapplicationsofthispolymerisforputtinglenses inmetalmounts,boundingplastictoglassandcoldblockingby meansofthecuredprocess.SomepropertiesofpolymerNOA 72®aredescribedinNorlandProductsIncorporate(1999).
Thepolymer-curedprocessdependsoftheintensityandthe wavelengthofUVradiation.BeforeexposurethepolymertoUV radiation,theadhesiveisinliquidstatebecausethemonomers and photo initiators will not react with each other. When is exposedtoUVthephotoinitiatorsundergoachangecreating freeradicalsthatreactwithmonomerscreatingmonomerchains.
Inthecuredstate,themonomerchainsconverttocross-linked polymerchains.InNorlandProductsIncorporate(1999)appear typicalanswersofpre-cureandfullcureprocessofthe NOA 72®.
3. Photosensitivefilmfabrication 3.1. Preparationofthephotosensitivecells
The NOA 72® is deposited in cells manufactured by two glassplatesseparatedbytwoacetateplasticfilmsplacedatits sideends.Different cellsweremanufacturedbychangingthe thicknessoftheseparatorfilm.Thethicknessoftheplasticfilm usedwas110,220and330mandweremeasuredwithdigital micrometerfromMitutoyomodelIP65.Cellsareclampedby twoofitslateralendsleavingopentopandbottom.Thecellis positionedvertically toallowthepolymertobeplacedatthe top,andbygravity,thepolymerfillsthecell.
3.2. Experimentalsetup
TorecordthePHGinphotosensitivecell,theexperimental arrangementshowninFigure1isusedand,as noted,abeam of lightfromanArlaser passesthroughabeam splitter(BS) andsent two beams of light to the mirrorsM1 andM2.The M1 andM2 mirrors deflectthe beamstoward themirror M3. Thismirrorplacedat45◦fromthehorizontaldeflectsthebeams towardthephotosensitivecellwheretheyoverlapandgenerate aninterferencepatternthatisrecorded.Thephotosensitivecell isplacedhorizontallytoavoiddamagefrompossiblerunoffof thepolymer.
Diffraction orders
–1 0 +1
Ar laser λ = 457 nm
BS
He-Ne laser λ = 632.8nm
Photosensitive cell
M1
M3
M2
θ
Figure1.Opticalsetupemployedtorecordholographicgrating.Abeamfrom Arlasercreatesaninterferencepatternonthephotosensitivefilm.Thebeamfrom He–Nelaserreadsthegratinganddiffractsthreelightspotscalleddiffraction ordersandwiththephotodetectortheDEismeasured.
TomeasuretheDEofPHGrecordedabeamoflightfroma He–Nelaserisused.Figure1showsthatthelaserbeamincident ontheareainwhichthegratingisrecordedproducesdiffraction patternwhichconsistsofbrightspotscalleddiffractionorders.
The central brightspotis calledzero-order, side brightspots are called diffractionorders +1 and−1. DE, expressed as a percentage,isdefinedastheratiooftheintensityofthe+1(or
−1)diffractionorderandtheintensityoftheincidentbeamas showninthefollowingequation.
DE=100 I1
Ii+Ir (1)
whereI1isthe+1diffractedorderintensity,Iitheincidentbeam intensityandIristhereflectedbeamintensity.Inthisequation weareconsideringtheFresnellossesbecauseofthereflection onthephotosensitivecell.
3.3. ModulationoftherefractionindexbyKogelnik’stheory BasedontheDEmeasuredvalues,themodulationamplitude oftherefractionindexncanbecalculatedusingtheKogelnik theory(Kogelnik,1969)accordingtothenextequation
η= λcosθarcsin√ DE
πd (2)
wheredisthegratingthickness,λisthereadingbeamwave- lengthandθistheincidentangleofthereadingbeam.
3.4. Fourierholograms
AlsoFourierhologramswererecordedinthismaterialwith goodresults.Figure2showstheexperimentalsetupforrecor- dingFourierhologramsthatareformedbytheinterferenceof two light beams, one of them called reference beam comes directlyfromalaser,andtheotherbeamcalledobjectbeam,is
Ar laser λ = 457 nm
BS BE BO
Object beam Reference beam
Photosensitive cell
M1
M3
M2 L1
L2
Figure2.Schematicrepresentationoftheexperimentalsetupusedforrecording Fourierhologramsofbinaryobjects.
theFouriertransformofabinaryobjectilluminatedbyaplane wave.ThelightbeamreflectedinthemirrorM2passesthrough abeamexpanderBEtogeneratesphericalwave.Thiswaveis collimatedbythelensL1 placedatitsfocallength oftheBE andgenerates aplane wave.This plane waveilluminates the objectlocatedinthefocalplaneofthelensL2andperformsthe Fouriertransformontheplanewherethephotosensitivecellis placedandgeneratestheinterferencepatternwiththereference beam.
ToreconstructtheimagerecordedintheFourierhologram, itisilluminatedwiththesamelightbeamwasusedasreference beam.InFigure3aschemeforreconstructionofFourierholo- gramsisshown.InthisschemealightbeamfromaHe–Nelaser isincidentonthephotosensitivecellwhereFourierhologram wasrecorded.Thebeam isdiffractedas itpassesthrough the materialandinaremotedisplaytherealandconjugateimages areformed.
Screen
UMSN H
UMSNH
Fourier hologram
He-Ne laser
Conjugate image Real image
Central bright
spot
Figure3.Representationoftheexperimentalsetupusedforthereconstruction ofaFourierhologram.
18
16
14
12
10
8
6
4
2
0
2 4 6 8 10 12
Energy (J/cm2)
DE (%)
14 16 18
Grating frequency:
153 lines/mm Sample thickness:
110 μm 220 μm 330 μm
Figure4.Graphofthediffractionefficiency(DE)vs.exposureenergy.The maximumDEisobtainedwiththe330mthicknesscellandis17.5%.
4. Results
TorecordthePHGinNOA72®alightbeamwasusedwith λ=457nmofamultilineArlaser.Thislineisusedbecauseitis theclosesttothewavelengthofmaximumabsorbance(420nm) (NorlandProducts,2015).Tomeasurethediffractionefficiency ofholographicgratingslaserHe–Neisusedwithλ=632.8nm.
ThisemissionlineisusedbecausetheNOA72®isnotsensi- tivetothiswavelengthandthusdoesnotaffecttheregistration process.ThetwolightsourcesarefromCoherent;Arlaserhas avariableoutputpowerandHe–Nelaserhasanoutputpower of5mW.
PHGwasrecordedineachofthecellsatroomtemperature;
theangle betweenbeamswas2◦sothatthespatialfrequency of theholographicgratingsis153lines/mm, andaccordingto our measurements, the estimated frequencyerror is 3%. The DEwasmeasuredwithanopticalpowermeterNewportModel 2930C.Cellthicknesswasmeasuredwithadigitalmicrometer fromMitutoyomodelIP65.
TheDEoftherecordedPHGisshowninFigure4andasis shownthevaluesofDEaredifferentforeachPHG.ThePHG recordedinthecellof330mhaslargeDEandreaches17.5%
whilethePHGrecordedonthephotosensitivecellwith220m thicknesshasamaximumDEof4.3%.LowerDEwasobtained withthecell110mandislessthan1%.
Thicknessisanimportantparameterinthisadhesiveasshown inFigure4.Thematerialhasanonlinearbehaviorwithrespect toenergyexposure.Theenergyrequiredfortheseresultsisvery high because the material was exposed tothe wavelength of 457nmandthepeakabsorbancenearestisat420nmandisof theorderof1%.
AfterthepolymerhasbeenexposedandthePHGisrecorded, changesitsphasefromliquidtosolidandremainsinthisstate withoutreturningtoitsliquidphase.
Using the results of the diffraction efficiency shown in Figure 4the refractiveindexmodulation accordingtoKogel- nik theory is calculated. The refractive indexmodulation for
0.00000 0.00003 0.00006 0.00009 0.00012 0.00015 0.00018 0.00021 0.00024 0.00027
2 4 6 8 10 12
Energy (J/cm2)
Δn
14 16 18
Sample thickness 110 μm 220 μm 330 μm
Figure5. Refractionindexmodulationforthedifferentsamplethicknesses.The valuesareobtainedfromDEresultsaccordingto Kogelnikformulaforthe 153lines/mmgrating.
holographicgratingsrecordedinthedifferentcellsisshownin Figure5.
PHGis recorded bymodulation of the refractiveindexso thatmaterialshowsphasemodulation.Figure5showsthatthe photosensitivecellswithdifferentthicknesswhichwereexposed tothesameenergyhavedifferentmodulation.Cellswithhigher volumehavemorefreeradicalsthatinteractinthecross-linking processproducinghighermodulationoftherefractiveindex.
Anotherimportantparameterforholographicgratingsisthe spatialfrequencythatcanbemodifiediftheinterferenceangleis changed.ThischangecangeneratePHGofdifferentfrequency.
Theanglebetweenthebeamsischangedsothattheinterference patternsgeneratedhavedifferentspatialfrequency.Intheexper- imentalarrangementshowninFigure1theanglewaschanged toproduceholographicgratingswithspatialfrequencyof153, 191,382and570lines/mm.
Accordingtothe resultsofthediffractionefficiencyofthe holographic grating recorded in the cell of 330m, one can observethatitisthebestcellforholographicrecording.Inthis
18
16
14
12
10
8
6
4
2
0
2 4 6 8 10 12
Energy (J/cm2)
DE (%)
14 16 18
330 μm sample thickness Grating frequency:
153 lin/mm 191 lin/mm 382 lin/mm 571 lin/mm
Figure6.Graphofthediffractionefficiency(DE)vs.exposureenergy.The maximumDEisobtainedforthespatialfrequency153lines/mm.
0.00000 0.00005 0.00010 0.00015 0.00020 0.00025 0.00030
2 4 6 8 10 12
Energy (J/cm2)
Δn
14 16 18
Grating frequency 153 lin/mm 191 lin/mm 382 lin/mm 571 lin/mm
Figure7.Graphoftherefractiveindexmodulationforallgratingswithdifferent spatialfrequency.
sense,thiscellisusedtorecordtheholographicgratingswith differentspatialfrequencyandtheresultsareshowninFigure6.
Thehighestdiffractionefficiencyisobtainedfortheholographic gratingof153lines/mmand,asalreadymentioned,hasavalue of17.5%.Forthegratingswithspatialfrequencyof191,382, and571lines/mmDEisverylow.Theseresultsindicatethatthe materialhasalowresolution.
Consideringtheresultsofthediffractionefficiencyshownin Figure6,inFigure7therefractiveindexmodulationisshown.
The refraction index variation (n) by Kogelnik’s theory wereobtainedbythedataofFigures5and7,asafunctionof energy,whichisimplicitlyrelatedtodiffractionefficienciesand the optimumspecificthicknessesfromcellemulsionsformed withNOA72®.
ForrecordingtheFourierhologram,arrangementshownin Figure2wasused.Theangleof2◦andthephotosensitivecell with thicknessof 330mwas used becausethese conditions were the best results obtained. Once Fourier holograms are recorded,thearrangementshowninFigure3wasusedtorecon- structtheimageswithaHe–Nelaser.Theimageswerecaptured withaCCDcameraandareshowninTable1.
IntheleftcolumnofTable1picturesofsomeofthebinary objects used to make its Fourier hologram are shown. Right
Table1
ListofbinaryobjectsandrealimagesreconstructedfromFourierholograms.
AcronymandlogooftheUniversidadMichoacanadeSanNicolásdeHidalgo wereusedasbinaryobjects.
Binaryobjects Realimages
column of Table 1 shows the photograph of the real image obtainedinthereconstructionoftheFourierhologramfromone ofthediffractedordersoftheconjugatepair.
5. Conclusions
The polymeradhesiveNOA 72® is agood photosensitive materialsuitableforrecordingholographicphasegratings.Grat- ingsrecording areperformed by modulationof the refractive indexandreachmaximumdiffractionefficiencyof17.5%with acellthicknessof330m.
Thephaseholographicgratingthatisrecordedinthismate- rialhaslowresolutionwith153lines/mm.Thewavelengthused isλ=457nmfromanArlaser.Tomeasurethediffractioneffi- ciencyalightbeam of λ=632.8nm fromaHe–Ne laserwas used. Beforeregistration process the polymer is inits liquid phaseandattheendoftheregistrationprocessbecomessolid.
Besidestherecordedholographicgratingsisalsopossibleto recordFourierhologramsofbinaryobjects.Notrequirecomplex proceduresforpreparationofsamplesandrequiresnodevelop- mentprocesses.
Conflictofinterest
Theauthorshavenoconflictsofinteresttodeclare.
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