Second- and Third- Order Nonlinear Optical Materials

Transcription

1 Larry Dalto Philip ulliva Alex K.-Y. Je 7.1 Itroductio 7 ecod- ad Third- rder oliear ptical Materials 7.1 Itroductio ecod-rder oliear ptical Materials Electro-ptic Materials ptical Rectificatio Icludig Terahertz Radiatio ad Detectio 7.3 Third-rder oliear ptical Materials ummary Ackowledgmets Refereces ecod- ad third-order optical oliearity ca perhaps be best uderstood as the coefficiets of the secod ad third terms i the power series expasio of molecular ad macroscopic polarizatio i terms of applied electric fields. P i Z a ij E j Cb ijk E j E k Cg ijkl E j E k E l C. P i Z c ð1þ ije j Cc ð2þ ijke j E k Cc ð3þ ijkle j E k E l C. P i Z c ð1þ ije ju cosðutkkzþ Cð1=2Þc ð2þ ijje jju ½1 Ccosð2utK2kzÞŠ C. ð7:1þ ð7:2aþ ð7:2bþ The terms b ad g represet the first ad secod molecular hyperpolarizabilities, whereas the terms c (2) ad c (3) represet the first- ad secod-order oliear material (macroscopic) susceptibilities. Each term arises from the oliear iteractio of applied electric fields with the quasi-delocalized electro distributio of molecules ad materials. Moreover, each of these terms ca give rise to a variety of oliear resposes reflectig differet frequecy depedeces ecod-order terms give rise to secod harmoic geeratio (see Equatio 7.2b), differece frequecy (e.g., terahertz frequecy) geeratio, optical rectificatio (see Equatio 7.2b), ad electro-optic modulatio (the Pockels effect). Third-order optical oliearity gives rise to third harmoic geeratio, phase cojugatio, optical Chapter7 26/8/ :01 JAPPIYAR XML MDEL CRC12a pp

2 7-2 Hadbook of Photoics limitig, optical parametric effects, ad all-optical modulatio (the Kerr effect). As is typical for power series expasios, the secod-order coefficiets are larger tha the third-order coefficiets. Although commercial applicatios have bee realized for both secod- ad third-order iorgaic materials, such as lithium iobate electro-optic ad titaium/sapphire optical parametric materials, orgaic oliear optical materials are still strugglig to obtai a beachhead o the commercial ladscape. To the preset time, secod-order oliear optical orgaic materials appear closer to commercial applicatio ad will thus receive greater attetio i this chapter. It is very difficult to defie a uiversal figure-of-merit (M) for either secod- or third-order oliear optical materials, as practical device performace will ofte deped o the details of device desig as well as itrisic material properties. However, the most simplistic ad yet somewhat realistic figure-of-merit ca be expressed as MZc (2 or 3) /ta, where t is the respose time for the system reactig to a electric field perturbatio ad t is the optical loss. or p-electro orgaic materials, the respose time, t is the phase relaxatio time of the cojugated p-electros, which is typically o the order of tes of femtosecods. If device badwidths are determied by the itrisic material respose time, badwidths of tes of terahertz are possible. The optical loss, a, typically icludes both absorptio ad scatterig cotributios. ptical loss will, thus, be iflueced by material heterogeeity as well as molecular structure. f course, practical device applicatios may also require may additioal material properties icludig stability (e.g., thermal ad photochemical) ad processability (e.g., solubility i spi-castig solvet, appropriate sublimatio temperatures for vapor depositio, appropriate glass trasitio temperatures for aoimprit lithography). The M defied above is most commoly used i rakig third-order oliear optical materials. With secod-order oliear optical materials, other factors such as the resistivity of metal electrodes used i electro-optic modulator devices ca limit badwidths, so the material M is commoly simplified to c (2) /a or as c (2) /a3, where 3 is the material dielectric costat. or example, for electro-optic applicatios, it is importat to match the velocity of propagatig optical ad radiofrequecy waves. Velocity matchig is optimized whe 2 Z3, where is the material idex of refractio. Quatum mechaical calculatios have prove useful i ivestigatig the relatioship of b ad g to molecular (chromophore) structure or simple polyees, the variatio of molecular hyperpolarizability with the legth of the cojugated p-electro structure ad with bod legth alteratio is reasoably well predicted by theoretical calculatios. Third-order oliear optical activity ca be observed for both isotropic ad aisotropic materials. rom the above polarizatio equatios, it ca be see that the symmetry requiremet for secod-order optical oliearity requires chromophores to exhibit either dipolar 23,24 or octupolar symmetry. or materials to exhibit secod-order optical oliearity, macroscopic dipolar or octupolar symmetry must exist; such symmetry is frequetly itroduced by electric field polig or by sequetial sythesis/self assembly from a fuctioalized surface. Because of the additioal symmetry requiremet for secod-order oliear optical materials, statistical mechaical calculatios have prove useful i guidig the optimizatio of desired ao- ad mesoscopic order. I the followig discussio, greater attetio will be paid to secod-order oliear optical orgaic materials tha for third-order materials. The reaso for this disproportioate focus relates to the fact that more commercial attetio has bee focused o secod-order materials, whose desig issues have therefore received more itese research scrutiy. or example, very little attetio has bee give to the optical loss ad photostability of third-order oliear optical materials, whereas these properties have bee extesive studied for a umber of secod-order materials. 7.2 ecod-rder oliear ptical Materials The primary applicatios of secod-order oliear optical orgaic materials iclude electro-optic modulatio, secod harmoic geeratio, optical rectificatio, ad terahertz radiatio geeratio ad detectio. With recet success i the developmet of visible wavelegth lasers ad light emittig diodes, Chapter7 26/8/ :01 JAPPIYAR XML MDEL CRC12a pp

3 ecod- ad Third-rder oliear ptical Materials secod harmoic geeratio has received less attetio. Also, absorptio at visible wavelegths of secod harmoic light by orgaic materials is problematic ad has ihibited the practical utilizatio of materials for this applicatio. A excellet review of secod harmoic geeratio, icludig a extesive discussio of the cocept of phase-matchig, has bee give by tegema. 28 This review remais highly relevat. The most commo applicatio of orgaic secod-order oliear optical materials is electro-optic modulatio. Electro-optic modulatio ivolves the applicatio of a low (relative to optical frequecies) frequecy electrical field to a material. This low frequecy field is ofte referred to as a radiofrequecy field, although actual frequecies ca rage from DC to tes of terahertz. The applied field perturbs the p-electro distributio of the material, which i tur alters the velocity of light propagatig i the material. Thus, electro-optic activity ca be viewed as voltage cotrol of the refractio idex of a material. The primary focus of this chapter will be a review of orgaic electro-optic materials ad devices. A icreasigly popular applicatio of secod-order oliear optical materials is terahertz geeratio ad detectio. This pheomeo is relevat to a variety of sesig applicatios, ragig from homelad security to medical imagig. rgaic materials also have potetial for promotig the developmet ad icreased utilizatio of terahertz spectroscopy. Terahertz geeratio is a example of optical rectificatio or differece frequecy pheomea. Like secod harmoic geeratio, terahertz geeratio ivolves the iteractio of two optical fields with the charge desity of the material Electro-ptic Materials rgaic electro-optic materials iclude sigle crystal materials such as 4 0 -dimethylamio--methyl-4- stibazolium tosylate (DAT), 13,29 chromophore/polymer composite materials, polymeric materials cotaiig covaletly icorporated chromophores (icludig heavily crossliked materials), 30,31,34 40 sigle-chromophore-cotaiig dedrimers, 24,41 44 multichromophore-cotaiig dedrimers, 44,45 chromophore-cotaiig dedroized polymers, 44,46 51 doped chromophore materials (icludig biary chromophore systems), 52 ad materials prepared by sequetial sythesis/self-assembly of chromophores from a fuctioalized surface by Lagmuir Blodgett or modified Merrifield techiques. 13,53 57 The vast majority of systems studied ivolve dipolar chromophores; the reader is referred elsewhere for a itroductio to octupolar materials I like maer, most devices are curretly prepared by electric field polig of polymeric or dedritic materials. or materials prepared by electric field polig or by sequetial sythesis/self-assembly, oly two ozero electro-optic tesor elemets (r 33 ad r 13 ) exist. These are give approximately by: r 33 Z 2f ð0þb zzz!cos 3 q=ð 0e Þ 4 r 13 Z f ð0þb zzz!si 2 q cos q=ð 0o Þ 2 ð 0e Þ2; ð7:3aþ ð7:3bþ where is the chromophore umber desity (molecules/cm 3 ), f(u) are local field factors that accout for the dielectric ature of the media surroudig chromophores, 0o ad 0e are the ordiary ad extraordiary liear refractive idices, ad the order parameters,!cos 3 q ad!si 2 q cosq, defie the orietatioal distributio of chromophores. Equatio 7.3 eglects the mior elemets of the molecular first hyperpolarizability tesor ptimizig Electro-ptic Activity The process of optimizig electro-optic activity is typically a two-step process, i which quatum mechaical calculatios are used to guide the improvemet of molecular first hyperpolarizability,b ijk, values ad statistical mechaical calculatios are employed to optimize the product of the chromophore umber desity ad the order parameter Quatum mechaical calculatios have Chapter7 26/8/ :02 JAPPIYAR XML MDEL CRC12a pp

4 7-4 Hadbook of Photoics guided the developmet of a umber of chromophores, leadig to dramatic improvemets i molecular first hyperpolarizability, i.e., to b 0 values o the order of or greater tha 1000!10 K30 esu. The tricyaoviylfura (TC) acceptor moiety show i cheme 7.1 has facilitated the developmet of workhorse chromophores appropriate for prototype device developmet I recet years, the sythesis of chromophores has bee greatly aided by the utilizatio of microwave-assisted sythesis techiques. 73,74 f course, for a chromophore to be carried forward for developmet of deviceappropriate materials, it must exhibit appropriate thermal, chemical, ad photochemical stability, acceptably low levels of absorptio at aticipated device operatioal wavelegths, ad appropriate processability (e.g., solubility i spi castig solvets, etc.). These features will be addressed later i this chapter; ote that the commets that apply for electro-optic materials also apply to other secod-order materials ad, to some extet, to third-order materials. The theoretically-ispired developmet of chromophores with improved molecular first hyperpolarizability ca be divided ito two categories: (1) variatios of the fudametal door, bridge, ad acceptor blocks of modular dipolar chromophores; ad (2) ivestigatio of ovel chromophore architectures such as X-shaped 18,19,75,76 ad twisted 20,21 chromophores. The former strategy has prove to be very effective i the past ad sigificat future improvemet may be possible followig this strategy. The latter strategy is much ewer, but may afford dramatic improvemets i molecular first hyperpolarizability, while permittig desirable auxiliary properties such as high trasparecy at operatig wavelegths. Molecular first hyperpolarizability (b) values are commoly measured by hyper-rayleigh scatterig (HR) uch measuremets are complicated by two-photo fluorescece ad by molecular aggregatio. A variety of modificatios have bee made to he HR techique i efforts to circumvet these problems, icludig the use of femtosecod pulse techiques, measuremets at a umber of wavelegths (usig laser wavelegth agility afforded by optical parametric devices), ad measuremets as a fuctio of cocetratio with measured b values determied exploitig extrapolatios to zero cocetratio Moreover, HR measuremets are ormally carried out to determie relative (to a stadard solvet such as chloroform) b values. Absolute values are most frequetly defied usig a Br TBDM abh 4 ah MeH/TH 0 0 C-RT quat. CHEME 7.1 abh 4 ah MeH 0 0 C-RT quat. 4) K(t-bu) TH 95% TBDM C C H C R 1 R 2 ETH RT 12h 62% H Br TBDM HCI(coc) quat. -Buli DM TBDM TH 78 C - RT 70% CI Br P(Et) 3 heat 3days quat. -Buli (1-equiv.) TBDM DM Et 2 78 C Br 67% TBDM H C R2 R 1 C C H P The sythesis of a chromophore cotaiig the tricyaofura (TC) acceptor. 4) Br Chapter7 26/8/ :02 JAPPIYAR XML MDEL CRC12a pp

5 ecod- ad Third-rder oliear ptical Materials itegratig sphere approach. A problem arises i the compariso of b values betwee theory ad experimet. Theoretical b values are calculated for isolated particles i the log wavelegth or zero frequecy limit. the other had, experimetal b values are measured i solutios of varyig dielectric costats ad at fiite ifrared wavelegths such as 1.9 mm. To avoid comparig data correspodig to differet coditios, relative b values are frequetly compared, e.g., both theoretical ad experimetal b values refereced to a stadard such as paraitroailie. 23 The product of dipole momet, m, ad molecular first hyperpolarizability, b, is also a useful quatity, particularly as the slope of the curve of r 33 versus i the low cocetratio limit is give by 2f(0)[mb]E p /5kT 4, where for the sake of simplicity the subscripts o b ad have bee dropped. The product mb ca be measured by electric field iduced secod harmoic geeratio (EIH) techiques. Like HR measuremets, may factors ca complicate the measuremets ad great care must be exercised to obtai meaigful data. If molecular first hyperpolarizability values relevat to electro-optic respose are to be extracted from EIH data, the care must be exercised to avoid multiphoto resoace cotributios. Agai, like HR measuremets, EIH measuremets should ideally be made at a umber of wavelegths. The secod aspect of optimizig electro-optic activity ivolves optimizig the product!cos 3 q. It is ow well-appreciated that itermolecular electrostatic (e.g., dipole dipole) iteractios ivolvig prolate ellipsoid-shaped p-electro chromophores ca lead to serious atteuatio of electro-optic activity The Mote Carlo calculatios i igure 7.1 illustrate this poit. These calculatios were carried out with the restrictio that the chromophores maitai a uiform lattice distributio. Differet results are obtaied if a o-uiform chromophore distributio is permitted. This latter treatmet Loadig parameter Loadig parameter = <cos 3 q> α r 33 /b(costat) Isig lattice Regio of ehacemet 2:1 blate 1:2 Prolate Idepedet particle lattice pherical umber desity (molecules/cc) IGURE 7.1 Calculatio (employig pseudo-atomistic Mote Carlo methods) of!cos 3 q vs. for chromophore shapes ragig from prolate to oblate ellipsoidal. Calculatios are based o a o-lattice approximatio, which meas that chromophores are restricted to uiform spacig amog chromophores. Differet results, particularly at high loadig, are obtaied for off-lattice calculatios, i which chromophores are permitted to assume a o-uiform lattice distributio (e.g., to aggregate). or off-lattice calculatios, results are much more sesitive to the details of chromophore structure. Pseudo-atomistic calculatios mea that p-cojugated segmets are treated i the uited atom approximatio whereas s-boded molecular fragmets are treated fully atomistically Chapter7 26/8/ :02 JAPPIYAR XML MDEL CRC12a pp

6 7-6 Hadbook of Photoics YLD_124 TBDM C 3 -TC TBDM C C TBDM C C C C 3 C C 50 H i 2 Exact mass: TBDM 3 C C 45 H i 2 Exact mass: PLD_41 C C C 3 C C PLD_33 TBDM C C C 3 C C C 3 C C C 3 C C C C 3 C C C TBDM 3 C C 200 H Exact mass: C, 61.16; H, 3.46;, 18.87;, 4.28;, 9.78;, 2.45 C TBDM C C C 155 H i 3 Exact mass: IGURE 7.2 our chromophore structures related to the data preseted i igure Chapter7 26/8/ :02 JAPPIYAR XML MDEL CRC12a pp

7 ecod- ad Third-rder oliear ptical Materials permits aggregatio effects to be take ito accout. The discussio of this latter treatmet, which ca be critical for cosiderig very large umber desities (high chromophore loadig), is sufficietly complicated to be beyod the discussio preseted here. I the preset discussio, we restrict our cosideratio to the more simplistic case of a uiform chromophore distributio. As show i this figure, itermolecular electrostatic iteractios ca also augmet polig-iduced ocetrosymmetric order. It has bee suggested i at least two studies that chromophore-polymer iteractios ca also ifluece order. 80,81 Ideed, i additio to chromophore shape effects illustrated i igure 7.1, the spatial ad dyamical restrictios associated with covalet bods impact polig-iduced order. This is illustrated i studies of multichromophore-cotaiig dedrimers (MCCD), as depicted i igure 7.2 ad igure 7.3.Iigure 7.3, a sigificat ehacemet i electro-optic activity is observed for a MCCD, relative to the behavior observed for the same chromophore i a polymer composite material. Ideed, behavior for the MCCD lies betwee that expected for a chromophore i a spherically-symmetric eviromet ad the idepedet particle limit (see igure 7.1). Eve more itriguig behavior is observed whe a secod chromophore is doped ito the MCCD. The electro-optic activity, as show i igure 7.3, icreased early liearly with a slope more tha twice the iitial slope of the r 33 versus plot for the same chromophore i a amorphous polycarboate (APC) polymer host. 32,33 A simplistic aalysis suggests that such behavior may reflect itermolecular electrostatic iteractios, actig to icrease!cos 3 q i a maer aalogous to that see i igure 7.1. However, ulike the case for chromophore/polymer composite ad udoped dedrimer materials, this behavior has ot yet bee quatitatively reproduced by theoretical calculatios. Moreover, all ecessary cotrol experimets to rule out other potetial cotributios to the uusual behavior show i igure 7.3 have ot yet bee completed. evertheless, the realizatio of electro-optic activities greater tha 300 pm/v for doped sigle-chromophore-cotaiig dedrimers, MCCDs, ad dedroized polymers is a importat milestoe ad provides the potetial (providig further improvemet i molecular first hyperpolarizability is forthcomig) of realizig electro-optic activity o the order of 1000 pm/v. f course, for large electro-optic activity to be meaigful, it must be accompaied by acceptable optical trasparecy, r 33 / E p YLD-124 / PLD-41 Dedrimer oly C 3 -TC / APC (molec/cc) IGURE 7.3 Data for the four chromophore structures of igure 7.2. ice a liear depedece of electro-optic activity, r 33, o electric polig field stregth is observed i all cases, r 33 /E p vs. is plotted to facilitate compariso amog data sets. Data for composite materials cosistig of the chromophore C3-TC i amorphous polycarboate (APC, Aldrich Chemicals) are idicated by triagles. Data for the two dedrimer materials PLD 33 ad 41 are idicated by squares. Data for samples of YLD 124 doped ito PLD 41 are idicated by diamods Chapter7 26/8/ :02 JAPPIYAR XML MDEL CRC12a pp

8 7-8 Hadbook of Photoics thermal stability ad photostability. We ow tur attetio to discussio of these issues, after a few commets about the measuremet of electro-optic tesor compoets. Electro-optic activity of thi film samples is most commoly measured by the followig techiques: Teg-Ma simple reflectio atteuated total reflectio (ATR), abry Perot iterferometry, PI, 81,88,89 ad two-slit iterferece. 90 Electro-optic activity ca also be measured i a variety of waveguide (e.g., Mach Zehder iterferometry, MZI), rig microresoator, 66,72 ad etalo devices. Each techique has its particular advatages ad limitatios, ad i geeral it is desirable to obtai electro-optic tesor compoets from multiple measuremets usig differet techiques. It ca also be importat to defie the dispersio of tesor compoets. A variety of techiques, such as ATR, PI, ad MZI devices permit both r 33 ad r 13 to be determied. uch complete tesor determiatio permits a more defiitive characterizatio of chromophore orietatioal order. These various characterizatio techiques ca also be adapted by the itroductio of temperature cotrol ad DC voltage applicatio stages to provide i situ moitorig of the itroductio of ocetrosymmetric order by electric field polig ad the subsequet relaxatio of that order Miimizig ptical Loss This discussio will first focus o material loss. Throughout the 1990s, most of the chromophores beig ivestigated had a charge trasfer absorptio maxima, l max, of less tha 600 m. or such materials, optical absorptio loss at 1.3 ad 1.55 micro telecommuicatio operatig wavelegths was most frequetly domiated by hydroge vibratioal overtoe absorptios. or dedrimer materials, optical loss values as low as 0.2 db/cm have bee observed. 94 Whe optical loss of greater tha 2 db/cm was observed, it was ormally idicative of light scatterig arisig from material heterogeeity associated with various processig coditios. 35 More recetly, chromophores with iterbad absorptio maxima approachig 800 m have come ito use. or these materials, ad eve for some earlier materials, 32,33 absorptio loss at telecommuicatio wavelegths is domiated by electroic charge trasfer absorptio. Thus, icreasig c (2) may ot lead to a improvemet i M, because it is accompaied by a correspodig icrease i a. The appearace of excito bads may eve lead to a decrease i M. I desigig ew chromophores for improved optical oliearity, it is critical to cosider optical loss. ptical loss due to electroic absorptios (both iterbad charge trasfer ad excitoic absorptios associated with aggregatio) is strogly iflueced by solvatochromic ad lie broadeig effects, particularly as these effects ifluece the log wavelegth tails of absorptios. Lie broadeig is frequetly defied by the heterogeeity of the chromophore eviromet ad thus ca be iflueced by chromophore order ad by the mode of attachmet of the chromophore to its surroudig matrix. The dielectric properties of the surroudig matrix will, of course, have a profoud effect o solvatochromic shifts. Researchers at Lockheed Marti 32,33 appear to be the first to focus o a effort to cotrol absorptio cotributios to optical loss by a systematic cosideratio of the roles played by the structure of the chromophore ad of the surroudig matrix. Quite differet effects o absorptio loss ca be observed for differet types of materials at telecommuicatio wavelegths. I this regard, dedritic materials may afford sigificat advatages relative to chromophore/polymer composite materials due to the fact that they permit cotrol of the local chromophore eviromet ad chromophore solubility i the surroudig matrix. or example, chromophores icorporated i fluoriated dedrimers typically exhibit blue (hypsochromic) shifted absorptio maxima relative to the same chromophore i a polymer such as amorphous polycarboate (APC, Aldrich Chemical). Moreover, the solubility of the chromophore i dedrimers is cotrolled by covalet bod attachmet. f course, commets made regardig dedrimers ca also apply to chromophores covaletly icorporated ito polymers, provided that access of chromophores to each other is ihibited by the covalet icorporatio. A high cocetratio of chromophores does ot ecessarily imply disastrous optical loss, as illustrated i igure 7.4, which shows the same chromophore i a covalet-bodig-defied chromophore budle ad i APC. 95 The absorptio maximum of the chromophore i the budle is shifted to higher eergy ad o detectable lie broadeig is observed. The molecular hyperpolarizability of the chromophore budle is early three times that of the isolated Chapter7 26/8/ :03 JAPPIYAR XML MDEL CRC12a pp

9 ecod- ad Third-rder oliear ptical Materials H Extictio coefficiet / M 1 cm 1 DC/DMAP CH HC HC C pepridie/th 4 C 60 C, 8days H H C 5 H 1 C C H H C H H H H C 6 Wave umber / cm -1 H H Compoud l max (m, CHCl 3 ) b / (b chloroform ) b (10-30 esu) H C 237±14 97± ± ± EH / H 4 A DCC/DMAP C C C ±6 479 DR IGURE 7.4 The compariso of optical spectra ad molecular hyperpolarizability for the same chromophore i APC ad i a three-chromophore budle (right had side of figure). the left had side of the figure, the sytheses of the three-chromophore budle ad the aalogous sigle chromophore are show Chapter7 26/8/ :03 JAPPIYAR XML MDEL CRC12a pp

10 7-10 Hadbook of Photoics chromophore. The experimetal results show here are i good agreemet with theoretical calculatios carried out for the respective structures. Care should be exercised with respect to extrapolatig the results of igure 7.4 to other materials; i geeral, the exact optical behavior will be determied by the precise traslatioal ad orietatioal positioig of chromophores, which will vary from structure to structure. ptical loss is also iflueced by scatterig losses. These frequetly arise from material heterogeeities itroduced by spi castig, electric-field polig, crosslikig (lattice hardeig), ad by various device processig steps (e.g., reactive io etchig of waveguides ad depositio of claddig layers). The problem is typically worse (igorig for the momet issues associated with crosslikig or lattice hardeig) for chromophore/polymer composites tha for chromophores covaletly icorporated ito dedrimers or polymers. Agai, the major issue is cotrol of chromophore solubility. or chromophore/polymer composites, problems iclude: the differetial solubility of the chromophore ad polymer host i spi castig solvets; sublimatio of chromophores durig bakig (to drive off residual spi castig solvets); sublimatio of chromophores durig electric field polig; ad electrophoretic phase separatio durig polig. Covalet-tailorig of chromophores ad their icorporatio ito host materials is a very attractive meas of cotrollig chromophore solubility ad packig (void volumes) i the fial material. As we shall see shortly, crosslikig (or lattice hardeig) is typically ecessary to achieve adequate thermal stability. ome crosslikig chemistries ca lead to phase separatio ad lattice strai ad thus dramatic icreases i optical loss due to light scatterig. ew cycloadditio crosslikig chemistries ivolvig the soft free-radical chemistry of the fluoroviyl ether group or the Diels-Alder/retro-Diels- Alder reactio lead ot oly to lattice hardeig without atteuatio of polig efficiecy, but also to materials with low optical loss. ptical loss ca also arise from the process of fabricatig buried chael waveguides. Whe this is doe by techiques such as reactive io etchig, care must be exercise to avoid pittig (waveguide wall roughess) due to reactive ios with excess kietic eergy (i.e., a physical rather tha chemical etch). Whe care is employed, the excess waveguide loss ca be 0.01 db/cm or less. 96 Loss ca also be itroduced i depositio of claddig layers if the solvet used to deposit the claddig layer attacks the electro-optic material. or both reactive io etchig ad depositio of claddig layers, the lowest loss is typically observed whe very hard electro-optic materials are used. Loss ca also arise from material damage (dielectric breakdow) durig electric field polig. ptical loss ca be iflueced by the structure of the device, e.g., bedig loss for rig microresoators. f course, oe of the greatest cotributios to total device isertio loss is couplig loss. With orgaic electro-optic materials, the domiat cotributio to couplig loss arises from a mismatch i mode size ad shape betwee light propagatig i silica trasmissio fibers ad that i the orgaic E waveguides. The solutio to this problem is typically to employ a mode trasformer structure Mode trasformers permit per facet couplig losses to be kept to a few teths of a db. The overall objective is typically to achieve a total isertio loss of less tha 5 db (the curret stadard for lithium iobate devices). Thus, if device legths of 2 cm or less are to be used, the material loss must be kept to less tha 2 db/cm. hort device legths, of course, have the advatage of permittig greater operatioal badwidths by miimizig loss occurrig i metal drive electrodes. The calculated performace of a Mach Zehder device for typical E, claddig, ad electrode material coditios is show i igure Maximizig Thermal tability The stability of electro-optic activity followig the cessatio of electric field polig is critical for device applicatios. I the simplest sese, thermal stability relates to the temperature differece betwee the operatig temperature ad the material glass trasitio temperature, T g. With composite materials, Q1 icorporatio of chromophores results i plasticizatio ad a correspodig reductio of glass trasitio temperature with icreasig dopat (chromophore) cocetratio. Thus, to realize sufficiet thermal stability to satisfy Telcordia stadards, it is ecessary to use a host polymer material with a iitial glass trasitio temperature o the order of 1508C or greater. Polycarboates, polyquiolies, ad polyimides have bee the most commoly employed polymers for producig composite materials appropriate for Chapter7 26/8/ :03 JAPPIYAR XML MDEL CRC12a pp

11 ecod- ad Third-rder oliear ptical Materials requecy (GHz) iber to fiber isertio loss (db) Iteractio legth, L(cm) Iteractio legth, L(cm) prototype device fabricatio. e of the problems with very high-glass-trasitio-temperature composite materials, such as polyimides, is that very high polig temperatures are required. uch temperatures promote chromophore sublimatio ad the decompositio of chromophores, ad ca also be icompatible with processig methodologies such as aoimprit lithography. High glass trasitio materials also commoly exhibit poor solubility i solvets used for spi castig. A coveiet meas of avoidig this requiremet for high temperature processig at itermediate stages of electro-optic material productio is to achieve thermal stability by crosslikig (lattice hardeig) i the fial stages of electro-optic material productio. With crosslikig, thermal stability will be defied by the desity of crossliks ad the flexibility of iterveig segmets. Throughout the 1990s, crosslikig chemistries used to elevate fial material glass trasitios temperatures frequetly ivolved codesatio (e.g., urethae ad sol gel) chemistry. 34,35 uch reactios give off water as a elimiatio product ad are iflueced by atmospheric moisture. Moreover, the expulsio of gaseous elimiatio products ca result i lattice disruptio ad icreased light scatterig. As the codesatio reactio proceeds, icreased lattice strai (e.g., lattice cotractio) ca also be a problem, which ca be serious with sol-gel glasses. Both thermal ad photo-iduced crosslikig chemistries have bee explored. 30,31 The latter has bee plagued by competitio for light absorptio ivolvig the photo-iitiator ad the E chromophore. More recetly, cycloadditio chemistries 36 38,44,46 52,102 have become popular for realizig high glass trasitio temperature (e.g., to 2008C) materials without the atteuatio of electro-optic activity or a icrease i optical loss associated with earlier crosslikig reactios. These chemistries are pictorially illustrated i igure 7.6. The fluoroviyl ether crosslikig reactio illustrated i igure 7.6a is a soft free radical reactio that has the advatage of yieldig high glass trasitio materials that are also characterized by very low optical loss at telecommuicatio wavelegths due to low hydroge cotet i the fial material. The Diels-Alder/retro-Diels-Alder reactio of igure 7.6b has the added advatage V p (Volts) V p (Volts) Material parameters: R 33 = 300 pm/v D(thickess) = 8 mm µwave loss = 0.75 db(ghz)1/2/cm iber couplig = 0.8 db/couplig Material loss = 2 db/cm Electrode dimesios L 2µm Example L = 5 mm V p = 0.75V BW(3dBe) = 90 GHz Isert. Loss = 2.6 db IGURE 7.5 The variatio of critical Mach Zehder device parameters (badwidth, drive voltage, fiber-to-fiber isertio loss) with device (electrical/optical field iteractio) legth. A example of performace for a 5-mm legth Mach Zehder electro-optic modulator expected with curret materials is also show Chapter7 26/8/ :03 JAPPIYAR XML MDEL CRC12a pp

12 7-12 Hadbook of Photoics (a) R 1 R 2 (b) cycloadditio Cocerted cycloadditio "Diels-Alder" R 2 R 1 H R R H 2 1 Retro Diels-Alder Approx = 120 C IGURE 7.6 luoroviyl ether (6a) ad Diels-Alder/- retro-diels-alder (6b) cycloadditio crosslikig reactios. (i some cases) of reversibility. That is, the material is crossliked below the glass trasitio temperature but becomes ucrossliked at the T g, permittig polig to be effected without beig atteuated by crossliks. Choice of diee ad dieophile ca permit systematic tuig of the glass trasitio temperature, icludig for the purpose of matchig the polig temperature to the thermal iitiatio temperature of the fluoroviyl ether crosslikig reactio (whe both types of cycloadditio crosslikig are used together). bviously, for irreversible crosslikig reactios such as reactio of the fluoroviyl ether moiety, it is importat to match polig temperatures to the thermal iitiatio temperature of the crosslikig reactio so that effective crosslikig is achieved without uwated atteuatio of polig-iduced order. A fial material glass trasitio temperature o the order of 2008C is more tha adequate for satisfyig Telcordia stadards for thermal stability (log-term stability at a operatig temperature of 858C). Cycloadditio crosslikig has bee demostrated to be effective i providig such stability Maximizig Photostability Photostability has bee show to be largely a matter of avoidig siglet oxyge chemistry tegema ad coworkers have demostrated most of the critical features of photodecompositio of orgaic electro-optic materials, icludig the absece of cotributios from multiphoto absorptio. Ideed, they defied a sigle photostability figure-of-merit, B/s, where B K1 is the probability of photodecay from the LUM (lowest uoccupied molecular orbital) charge trasfer state ad s is the iterbad (charge trasfer) absorptio coefficiet. This defiitio has bee used by subsequet researchers, although the data aalysis of tegema ad coworkers may have bee somewhat overly simplistic, cosequetly over-estimatig photo-istability. or example, more detailed aalyses demostrate that the decay data caot be fit with a sigle expoetial ad that observer power i the tegema pump probe experimet may lead to artificially fast decay. Moreover, tegema ad coworkers failed to carry out measuremets at telecommuicatio wavelegths; this was addressed i subsequet work by researchers at Corig. 107,108 The Corig group demostrated that the photostability M for a give chromophore structure could vary over four orders of magitude depedig o coditios that ifluece siglet oxyge chemistry. Eve larger variatio has bee observed by other groups ad photostability has bee show to improve with the use of small quatities of siglet oxyge quechers. I additio to pump-probe experimets carried out by tegema ad coworkers, researchers at Corig, Guter ad coworkers, ad Dalto ad coworkers, photostability has also bee ivestigated i operatig Mach Zehder devices by teier ad coworkers 69 ad by Ashley ad coworkers. 109 Agai, i these studies, photo-istability could be attributed to siglet oxyge chemistry, with good photostability beig observed for materials ad devices where this chemistry was partially ihibited. I summary, it appears that good photostability ca be achieved with appropriate materials modificatio or with appropriate packagig of devices to miimize the presece of oxyge. I this latter regard, the problems faced with orgaic electro-optic materials are aalogous (but ot quite so severe) as those faced with orgaic light emittig device (LED) materials. It should be oted Chapter7 26/8/ :04 JAPPIYAR XML MDEL CRC12a pp

13 ecod- ad Third-rder oliear ptical Materials that dese crystals such as DAT exhibit excellet photostability, agai cosistet with the role played by oxyge Radiatio Hardess Very little work has bee carried out examiig the performace of orgaic electro-optic materials ad devices i the presece of high-eergy gamma rays ad protos associated with space eviromets. Part of the problem of evaluatig the impact of high-eergy radiatio o device performace is isolatig chages attributable to orgaic electro-optic materials from chages of performace iduced by evets urelated to the orgaic electro-optic materials (e.g., damage to silica iput fibers). evertheless, the oe published report 110 o this subject appears promisig with respect to good stability exhibited by orgaic electro-optic materials abricatio of Prototype Devices The most commo prototype device fabricatio ivolves striplie Mach Zehder modulators; however, rig microresoator, cascaded prism, ad etalo structures have also bee produced ad evaluated. A critical issue with the fabricatio of prototype devices is how to deal with the properties of claddig ad electrode materials. Devices are typically multilayer structures cosistig of bottom (groud) electrode/bottom claddig/electro-optic waveguide/top claddig/top (drive) electrode. If materials are poled through claddig layers, the polig field ca be atteuated due to the resistivity of the claddig layers. A potetial solutio to this problem would be to idetify claddig materials with sigificat coductivity. Ufortuately, claddig materials with prerequisite coductivity have also exhibited, to the preset time, uacceptably high levels of optical loss. The presece of claddig layers thus makes it difficult to realize the same electro-optic activity i device structures that have bee achieved i thi films. Aother issue i the fabricatio of prototype devices is the impact of electrode materials ad device structure o performace, icludig operatioal badwidth. or striplie devices, such as Mach Zehder iterferometers, resistive losses i metal electrodes typically defie operatioal badwidths. horter electrode structures (see igure 7.5) lead to higher badwidths, but at a price of icreased drive voltage (V p, the voltage required to produce a p phase shift). As already oted, shorter devices afford the advatage of reduced isertio loss ad are more appropriate for high-desity itegratio of may modulators o a sigle chip. or resoated devices, such as rig microresoators ad etalo devices, badwidth is limited by the optical lifetime i the resoated structure. This is defied by the quality factor, Q, of the resoat device. High quality factors have the advatage of affordig reduced drive voltage operatio but limit the badwidth of the device. Very high ceter operatioal frequecies ca still be obtaied, but the badwidth about the ceter frequecy is limited by the Q. Rig microresoators have the added advatage of reduced size, which ca facilitate high-desity itegratio. I discussig badwidth, oe eeds to distiguish betwee digital ad aalog sigals. This poit will be illustrated i the followig simplified discussio of badwidth ad drive voltage requiremet for resoat device structures. or a resoator, the 3-dB electrical modulatio badwidth (the detected voltage is dow by 3 db) is give by Df 3dBe Z c = lq Z Df WHM ; where c is the speed of light, l is the wavelegth of light, Df WHM ad is the full-width at half-maximum of the badpass of the resoat device. The case of digital modulatio is cosidered first. or a 10-dB cotrast i the digital pulses, the voltage iduced frequecy shift of the badpass must be Df Z 3D WHM = 2: ð7:4þ ð7:5þ Chapter7 26/8/ :04 JAPPIYAR XML MDEL CRC12a pp

14 7-14 Hadbook of Photoics The frequecy shift with voltage is where d is the electrode spacig. Combiig, ðdf = VÞ Z ð eff Þ 2 r 33 c = ð2dlþ; V 10dB Z ð3dldf 3dBe Þ = ½ð eff Þ 2 r 33 cš: I digital systems, the required badwidth depeds o the modulatio format. Coservatively assumig that Df 3dBe ZB (the bit rate), the V 10dB Z 3dlB = ½ð eff Þ 2 r 33 cš: or example, if BZ10 Gb/s, Z1.6, r 33 Z300 pm/v, dz6 mm, lz1.3 mm, ad QZ5!10 3, the V 10dB Z1 V (with the result obviously scalig liearly with the bit rate). ow cosider aalog sigal modulatio. The optical wavelegth is set to the poit of maximum slope of the resoator trasmissio (badpass) curve: p Dl Z Dl WHM = ð2 ffiffi 3 Þ ð7:9þ It is helpful to defie a Vp equiv so that it correspods to a equal Vp of a Mach Zehder modulator: p ðv p equiv = Df 3dBe Þ Z 4pdl = ½ð3 ffiffi 3 Þðeff Þ 2 r 33 cš: ð7:6þ ð7:7þ ð7:8þ ð7:10þ or example, if Z1.6, r 33 Z300 pm/v, dz6 pm, ad lz1.3 pm, the (Vp equiv /Df 3dBe )Z 0.08 V/GHz. It is clear from the above aalyses for digital ad aalog sigal processig that the simplest route to improvig the performace of resoat devices is to icrease the electro-optic coefficiet, r, of the material used to fabricate the device. The curret performace of orgaic electro-optic materials moves resoat devices close to practical applicatio. Device structures are most commoly fabricated usig reactive io etchig, although rig microresoator ad Mach Zehder devices structures have also bee made by aoimprit lithography. 68 aoimprit lithography has the potetial advatage of allowig complex electro-optic circuitry to be mass-produced i a cost-effective maer. The rage of applicatio of orgaic electro-optic materials has bee recetly broadeed by the icorporatio of these materials ito silico photoic device structures. 66 The small dimesios of these structures ad the obvious potetial for coveiet itegratio with silico electroics is very attractive. Moreover, the small (aoscopic) dimesios of silico photoic circuitry result i a optical field cocetratio, which has recetly bee exploited to achieve optical rectificatio of light at mw optical powers. 66 Thus, the same device structure ca be employed for electrical/optical ad optical/electrical sigal trasductio Applicatios Applicatios of electro-optic materials ad devices iclude electrical-to-optical sigal trasductio, optical switchig, optical beam steerig, radiofrequecy sigal geeratio, phased array radar (radiofrequecy beam steerig), optical gyroscopes, aalog-digital coversio, frequecy coversio (time stretchig), ad sesig (of both physical ad chemical pheomea). The term radiofrequecy describes frequecies i the rage 0 30 THz. Electro-optic techology is at the heart of R Photoics, or the delivery of radiofrequecy sigals via optical trasmissio. May applicatios of electro-optic materials lie i the area of defese ad homelad security, but iterest is growig i the areas of computer chip maufacture, trasportatio, telecommuicatios (both fiber ad wireless), civil egieerig, ad Chapter7 26/8/ :04 JAPPIYAR XML MDEL CRC12a pp

15 ecod- ad Third-rder oliear ptical Materials medicie. or example, i the developmet of ext-geeratio computer chips, photoics may be used to route high frequecy iformatio amog various compoets of the chip, avoidig the problems of sigal loss ad heatig associated with movig electros through metal coectors. The field of embedded etwork sesig combies sesors, computer processors, ad commuicatio compoets o a sigle chip. Electro-optic devices provide critical sigal trasductio ad routig o embedded etwork sesig platforms. Embedded etwork sesig is fidig icreasig applicatios i medicie ad ifrastructure moitorig (civil egieerig) ptical Rectificatio Icludig Terahertz Radiatio ad Detectio ptical rectificatio is the differece frequecy aalog of secod harmoic geeratio. Like secod harmoic geeratio, it will occur wheever the electric field compoet of the optical field is large eough to produce a oliear perturbatio of the charge distributio of the oliear optical material. ormally, itese laser powers are required, but the aoscopic dimesios of silico photoic circuitry ca cocetrate optical fields from milliwatt diode lasers to the poit of producig optical rectificatio. 66 Thus, orgaic E/silico hybrid devices ca act as both electrical-to-optical sigal trasducers ad optical-to-electrical sigal trasducers. I other words, they have the potetial to compete with photodiodes as photodetectors, providig that sufficietly large material secod-order optical oliearity ca be obtaied. Aother maifestatio of optical rectificatio/differece frequecy geeratio is terahertz sigal geeratio/detectio. Recetly, Hayde ad coworkers, 111,112 Guter ad coworkers, 113 ad researchers i Japa have pioeered the use of orgaic electro-optic materials for terahertz applicatios (imagig ad spectroscopy). A advatage of orgaic materials is that optical ad terahertz waves propagate with comparable velocities i orgaic materials, permittig phase matchig of the optical ad terahertz radiatio. Also, the secod-order optical oliearity of orgaic materials is orders of magitude greater tha iorgaic crystallie materials such as zic telluride (ZTe). A problem ecoutered with poled orgaic (polymer) materials is that it is difficult to effectively pole thick (millimeter) films. uch film thickess would be ideal for terahertz applicatios. Amog the promisig sesor applicatios of terahertz radiatio is the ability to image plastic weapos. It is a attractive alterative to magetic (metal detector) sesig i the area of homelad security. The possible applicatios i biomedical imagig are also attractive. 7.3 Third-rder oliear ptical Materials Third-order orgaic oliear optical materials have very little i commo with secod-order oliear optical materials, other tha the fact that both ivolve sigificat p-cojugatio. ice o symmetry requiremet exists for third-order activity, a much broader rage of materials ca give rise to third-order optical oliearity, c (3). Ideed, third-order materials rage from cojugated polymers such as polyacetylee, to molecules such as C 60 ad C 70, to metallomacrocyclic complexes. Whe charge trasfer molecules are ivestigated, molecules with quadrupolar 117 symmetry (e.g., door acceptor door or acceptor door acceptor) frequetly exhibit larger optical oliearity tha correspodig dipolar (door acceptor) molecules. Dedritic materials cotaiig coected p-electro segmets have also bee observed to give rise to large third-order optical oliearities. Although the third-order optical oliearity for orgaic materials has bee icreased over the past two decades, values are still too low for practical applicatios. A possible exceptio may be for materials icorporated ito silico photoic circuitry. cherer ad coworkers 120 have demostrated all-optical modulatio to greater tha 5 THz with mw pump powers derived from a diode laser operatig at telecommuicatio wavelegths. The cocetratio of optical power i waveguides of aometer Chapter7 26/8/ :04 JAPPIYAR XML MDEL CRC12a pp