Nonlinear Organic Materials For VLSI PhotonicsMojca Jazbinšek, Payam Rabiei, Christian Bosshard, and Peter Günter Citation: AIP Conf. Proc. 709, 187 (2004); doi: 10.1063/1.1764020 View online: http://dx.doi.org/10.1063/1.1764020 View Table of Contents: http://proceedings.aip.org/dbt/dbt.jsp?KEY=APCPCS&Volume=709&Issue=1 Published by the AIP Publishing LLC. Additional information on AIP Conf. Proc.Journal Homepage: http://proceedings.aip.org/ Journal Information: http://proceedings.aip.org/about/about_the_proceedings Top downloads: http://proceedings.aip.org/dbt/most_downloaded.jsp?KEY=APCPCS Information for Authors: http://proceedings.aip.org/authors/information_for_authors

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Nonlinear Organic Materials For VLSIPhotonics

Mojca Jazbinšek∗, Payam Rabiei∗, Christian Bosshard∗† and Peter Günter∗

∗Nonlinear Optics Laboratory, Swiss Federal Institute of Technology, ETH Hönggerberg, CH-8093Zurich, Switzerland

†Present address: CSEM Alpnach, Untere Gründlistrasse 1, CH-6055 Alpnach Dorf, Switzerland

Abstract. Nonlinear optical and electro-optic materials are potentially very interesting for VLSIphotonic applications. The required material parameters for devices such as active microring res-onators are discussed. Organic nonlinear materials are considered as especially attractive due torelatively high and fast nonlinearities compared to their inorganic counterparts. Design strategiesfor nonlinear organic materials are reviewed and the issue of improving their thermal and photosta-bility is discussed.

INTRODUCTION

Recently active electro-optic resonators were suggested and demonstrated for switchingof light in a compact structure [1, 2]. Ring resonators employing an electro-optic mate-rial can be used to achieve wavelength selective switches with size as small as 50µm. Torealize the required resonators one has to consider severalcriteria for the electro-opticmaterial. First to achieve small devices one requires a large index contrast technology.An index difference of 0.3 between core and cladding is required to achieve devices witha few tens of microns in size. A second requirement for the material is high nonlinearityor high electro-optic coefficient. Finally the required materials have to be stable and onemust be able to process them to make devices with nano-meter accuracy.

Several classes of materials can be considered for VLSI photonic devices. One classof the materials is that of the passive materials. In this class one can consider glass andalso passive polymers. Passive materials provide limited functionality. To achieve a realphotonic VLSI technology one requires considering active optical materials. One canconsider semiconductors and nonlinear optical materials in this class. Semiconductorshave been considered for ring resonators devices [3]. However they are not very suit-able for micro-resonators. First the refractive index of semiconductors is very high. Thiswill require extremely difficult fabrication process to achieve waveguides with molec-ular scale smoothness in the walls. Also normally semiconductors are not transparentat optical wavelengths unless they are pumped. This pumpingintroduces spontaneousemission noise and also in WDM systems, signals are mixed. Hence the semiconductorplatform is not a suitable technology for future photonic VLSI circuits. Another class ofmaterials that might be considered is that of the nonlinear optical materials. This classof materials is considered rarely in integrated photonic VLSI mainly due to difficultyin fabrication of high index contrast waveguides. However these materials have sev-

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eral inherent advantages, which make them ideal for photonic VLSI systems. First theseare active materials. This means that one can generate, amplify or switch optical signalthrough nonlinear mixing between either optical or opticaland electrical signals. Sec-ondly, the material is transparent. Hence one can make integrated optical circuits withoutrequirement to pump the material to transparency. Thirdly the refractive index is not ashigh as semiconductors. This relaxes the fabrication tolerances to a manageable level.Several electro-optic materials can be considered for application in micro-resonators.The most well known electro-optic materials are the oxides such as LiNbO3 or LiTaO3or KNbO3. They have excellent nonlinear properties however the fabrication of thinfilms of these materials, which is required for VLSI photonics, is very difficult. Alterna-tively organic nonlinear materials can be considered. Among organics one can considerorganic crystals, Langmuir-Blodgett films and polymers.

In this paper we review some of the organic crystals and polymer nonlinear materialsthat might be used for the fabrication of the VLSI photonic devices. We will discussseveral crystal engineering strategies towards the optimized highly nonlinear opticalorganic crystals and also approaches to obtain the desired waveguiding structures inorganic crystals. Nonlinear optical polymer systems will be discussed along with thepossibilities to improve their thermal and photostability.

MATERIAL REQUIREMENTS FOR VLSI PHOTONICTECHNOLOGY

Several criteria are required for a given material so that itcan be used for VLSI photonictechnology. These criteria must be met before one considersthe material for develop-ments of photonic VLSI systems.

• Fig 1 shows the required refractive index to achieve micro-resonator with a givenradius [1]. As it can be seen on this figure to achieve micro-resonators with sizes assmall as 50µm in diameter one requires a refractive index of 0.3.

• The micro-resonators for practical applications should achieveQ as high as 107.This will limit the losses in the waveguide to 0.1 dB/cm. The material losses mustbe below 0.1 dB/cm to achieve devices for practical applications.

• One must be able to tune the resonance wavelength of the resonator as high as 1-10 GHz/V to be able to achieve practical switches and modulators [1]. Hence theelectro-optic coefficient must be as high as 30 pm/V.

• Another criteria for the material are the photo-stability and thermal stability. Noticethat changes in the refractive index as low as 10−6 directly change the resonancewavelength in highQ micro-resonators. The material must be stable enough so thatthe refractive index change is less than 10−6 in many years to achieve practicaldevices.

These conditions require a fair amount of research on the material and technologyto achieve the required criteria. In the following the optical, nonlinear optical andelectro-optic effects are introduced and the relevant properties of various materials arecompared.

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FIGURE 1. Minimum radius required to achieve 1dB/cm bending loss and the corresponding FSRcalculated for 1.55µm as a function of the refractive index difference between core and cladding for thestructure shown in the inset.

NONLINEAR OPTICAL EFFECTS AND MATERIALS

The nonlinear optical effects can be described in terms of the linear polarizationPL andthe nonlinear polarizationPNL induced by the electric fieldE

Pi = P0i +PL

i +PNLi = P0

i + ε0χ(1)i j E j + ε0χ(2)

i jk E jEk + ε0χ(3)i jkl E jEkEl + ... (1)

with P0 the spontaneous polarization,ε0 the electric constant, andχ(n) the nth ordersusceptibility tensor. For symmetry reasons, the odd-order susceptibilities are present inany material, whereas the even-order ones only occur in noncentrosymmetric materials.The susceptibility tensorχ(n) contains all the information about the macroscopic opticalproperties of the respective material. The magnitudes of the nonlinear susceptibilitiesdepend on the definition of the electric field amplitude

E(r, t) =12 ∑

k,ω

(

E(k,ω)ei(kr−ωt) +c.c.)

. (2)

By substituting (2) into (1) we get a distorted profile of the polarizationP. Based onthe characteristics of the involved electric fields severalsecond order nonlinear opticaleffects can be distinguished (see Fig. 2).

While equation (1) governs nonlinear effects on a macroscopic scale, one can alsoconsider this problem on the molecular level. The dipole moment of the moleculep consists of its ground state dipole momentµg and the induced contribution. Thecorresponding expansion

pi = µg,i + ε0αi j E j + ε0βi jkE jEk + ... (3)

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Second-harmonic generation

Sum-frequency generation

Difference-frequency generation

ωd 2ωdω1 = ω2 = ωd → 2ωd = ω3

ω1ω3 ω2ω3 → ω1 + ω2

ω1 ω3ω2ω1 + ω2 → ω3

ω3 ω2ω1ω3 – ω1 → ω2

ω1ω3 ω2ω3 → ω1 + ω2

Optical parametric oscillation

V~

Electro-optic phase modulation

Optical parametric generation

FIGURE 2. Schematic representation of important nonlinear optical and electro-optic effects.

defines the molecular coefficients: linear polarizabilityαi j and first-order hyperpolariz-ability βi jk . The task of linking the macroscopic coefficients to the molecular ones isa nontrivial problem because of interactions between neighboring molecules. However,most often the macroscopic second-order nonlinearities oforganic materials can be wellexplained by the nonlinearities of the constituent molecules (oriented gas-model [4]).

The Linear Electro-Optic Effect

The electro-optic effect is defined as the deformation and rotation of the opticalindicatrix if an electric field is applied to a noncentrosymmetric sample [5]. The linearelectro-optic effect can be also expressed using the nonlinearχ(2) tensor as

Pωi = ε0χ(2)

i jk (−ω;ω,0)E j(ω)Ek(0) (4)

However, it is generally not considered as a nonlinear optical effect, because one ofthe involved fieldsEk(0) is not an optical but a static electric field. Typically the linearelectro-optic effect is described in terms of the change of the optical indicatrix

∆(

1n2

)

i j= r i jkEk. (5)

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The above equation is also the defining equation of the electro-optic tensorr i jk . Forsmall changes the linear refractive index change can be approximated by

∆ni∼= −

12

n3i r iikEk. (6)

The linear electro-optic effect is widely used in electro-optic modulators. These devicesexploit the induced phase change of an optical wave and convert it to a change in theintensity. Therefore the optical intensity can be controlled by an electrical signal, afrequent task in telecommunications.

Because of the trend to higher and higher data rates in optical communication tech-nology there is an increasing need for new materials with large and fast respondingnonlinearities, which can be used for all-optical or electro-optic switching and modula-tion. Today most of commercialized products use inorganic materials such as LiNbO3since these materials are well understood, have excellent mechanical and chemical sta-bility and sufficiently large nonlinear optical coefficients for many applications. On theorder hand, organic materials have two important advantages in terms of high frequencyelectro-optic applications. They are naturally suited fortraveling wave electro-opticmodulators. Due to the low dielectric constant at low frequenciesε ≈ n2, the electricwave travels at about the same speed as the optical wave, which is not the case for mostinorganic electro-optic materials (ε � n2). This kind of phase matching is importantwhen building high frequency modulators. The second advantage of organic over inor-ganic materials is the almost constant electro-optic coefficient over an extremely widefrequency range. This property is essential when building broad-band electro-optic mod-ulators and field detectors. Table 1 lists selected inorganic and organic materials, theirproperties and figures of merit relevant for electro-optics.

Organic Nonlinear Optical Materials

Organic materials are of great interest for nonlinear optics. They offer a large numberof design possibilities and large nonlinear effects can be reached.

The basic design of nonlinear optical molecules is based onπ bond systems.π bondsare regions of delocalized electronic charge distributionresulting from the overlap ofπ orbitals. This delocalization leads to a high mobility of the electron density. Theelectron distribution can be distorted by substituents at both sides of theπ bond system.The extent of the redistribution is measured by the dipole moment, and the ease ofredistribution in response to an applied field by the hyperpolarizability. The opticalnonlinearity of organic molecules can be increased by either increasing the conjugationlength or by using appropriate electron donor and electron acceptor groups (Fig. 3).The addition of appropriate functionality at the ends of theπ system can enhance theasymmetric electronic distribution in either or both the ground state and excited stateconfigurations.

Optimizing the molecular hyperpolarizabilityβ is only one part in designing anelectro-optic material. To show a macroscopic second-order nonlinearity, the arrange-ment of molecules plays an important role. If they are arranged arbitrarily, as in a liquid,

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TABLE 1. Selection of electro-optic materials and their parametersrelevant for theelectro-optic response,λ is the measured wavelength,r is the electro-optic coefficient(unclamped, if not specified),n the refractive index, andε the low-frequency dielectricconstant (unclamped, if not specified); semiconductor GaAs, inorganic ferroelectric crys-tals LiNbO3, KNbO3, KNb0.55Ta0.45O3, Sn2P2S6, organic crystal DAST, and polymersA-095.11 and CLD-1.

r n n3r ε r/ε λ Ref.(pm/V) (pm/V) (pm/V) (nm)

LiNbO3 31.5 2.2 340 28 1.1 633 [6]GaAs 1.2 3.5 51 13.2 0.09 1020 [7]

KNbO3 63.3 2.2 650 44 1.4 633 [8]35∗ 2.2 350∗ 24∗ 1.5∗ 633 [8]

KNb0.55Ta0.45O3 (1400) (2.2) (15000) (2500) (0.6) 633 †

Sn2P2S6 170 2.8 4000 230 0.74 1313 [9]

DAST∗∗ 92 2.5 1470 5.2 18 720 [10]53 2.2 530 5.2 10 1313 [10]

Polymers:A-095.11‡ 20 1.66 92 2.8 7.1 1313 [11, 12]CLD-1 § 130 1.65 584 3.5 37 1313 [13, 14]

∗ clamped value† Estimations based on preliminary results at room temperature for a compound with the para- toferroelectric phase transition at 65◦

∗∗ Organic crystal 4-N,N-dimethylamino-4’-N’-methyl stilbazolium tosylate‡ Polyimide side-chain polymer based on disperse red§ Phenyltetraene chromophore in a guest-host polymer system

R1

R2

π - bridgeD A

π - bridge ,

FIGURE 3. Typical organic molecules for second-order nonlinear effects. The electron donor group (D)is connected to the electron acceptor group (A) through aπ electron system. The most common systemsare those containing one benzene ring (benzene analogs) andthose containing two benzene rings (stilbeneanalogs). R1 and R2 are usually carbon or nitrogen.

the molecular second-order nonlinearities are averaged toa zero macroscopic effect.There exist basically four kinds of materials showing a macroscopic nonlinear opticalresponse:

a) Thin films of highly ordered organic chromophores can be produced by theLangmuir-Blodgett technique [15]. A monolayer of molecules on a water surfaceis transferred layer by layer to a substrate.

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b) By molecular beam epitaxy (MBE) thin films of organic molecules can be pro-duced [15]. A beam of molecules is produced in ultra high vacuum using a Knudsencell. This beam is directed onto a substrate where the molecules condensate and un-der certain circumstances a noncentrosymmetric ordering of them can be achieved.

c) The nonlinear optical chromophores can be incorporated into a polymer matrix.By heating the polymer over the glass temperature and by applying a strong electricfield at the same time, they can be partially oriented. By cooling down below theglass temperature, the polar arrangement of the chromophores gets frozen in. Thisis very promising technique, since thin films can be easily produced and also largeelectro-optic effects can be reached.

d) Growth of noncentrosymmetricorganic crystals is the fourth possibility. Withthis technique the highest possible chromophore density and the best long termorientational stability can be reached. A problem of this technique is the preferredantiparallel ordering of dipolar molecules but there existseveral techniques toovercome this preference and to enforce a polar crystallization, as discussed inthe following section.

Although the relation between macroscopic nonlinearity and molecular structure isnot yet completely understood and no control over the individual molecules can beobtained, the mechanisms leading to large microscopic effects are well understood.A straightforward connection between microscopic and macroscopic nonlinearities isnot always possible to be established as several parametersplay an important role. Themeasurement of molecular hyperpolarizabilities is, however, of great importance. Onone hand, a direct comparison of different molecules is possible and useful structure-property relations can be found. The effects of substituents, conjugation length andplanarity of the molecules to the first-order hyperpolarizability have been thoroughlyinvestigated [15]. On the other hand, the comparison between microscopic and macro-scopic nonlinearities can confirm, correct or reject modelsconnecting microscopic andmacroscopic nonlinear optical coefficients.

The nonlinear optical and electro-optic properties of materials prepared by the polingtechnique (nonlinear optical chromophores incorporated into a polymer matrix) are usu-ally interpreted in terms of the oriented gas model. This model [4] assumes that the in-teraction between the nonlinear molecules and the poling field is satisfactorily describedby the dipole approximation, which is true in rarefied gases where the intermoleculardistance is large compared with the molecular dimensions. The model applied to solidsand liquids, except for local field correctios, takes into account only intramolecular con-tributions to the optical nonlinearity. Based on this modelthe measured first-order hyper-polarizabilityβ values could be reasonably well compared with the measured nonlinear-opticald and electro-opticr coefficients.

The relations between the microscopic and macroscopic nonlinear optical and electro-optic response depend on the structural parameters, the number density of moleculesand the angleθ between the molecular charge transfer axis and the polar crystallineaxis. In order to optimize the diagonal electro-optic coefficient, θ should be close tozero. For nonlinear optical applications in bulk crystals,such as frequency doubling,phase matching considerations come into play, and theoretical considerations lead toan optimum angle ofθ = 54.7◦. Using optimal hyperpolarizabilities we calculated the

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upper limits of the electro-optic coefficients (and similarly also of the nonlinear opticcoefficients), with all the intermolecular contributions (except for local field corrections)neglected [16]. The upper limits of electro-optic coefficients are presented in Fig. 4.

300 400 500 600 700 80010

100

1000

Cut-off wavelength (nm)

r max

(p

m/V

)

5000

DAST (r11

)MNA (r11)

COANP (r33)PNP (r22)

DCNP (r33)

MNBA (r11)MNA (r11)

FIGURE 4. Upper limits of electro-optic coefficients. The graph showsmaximum values ofr i j vs cut-off wavelength calculated from measured values of hyperpolarizabilities assuming an optimum parallelalignment of all molecules.◦: donor-acceptor disubstituted benzene derivatives;•: donor-acceptor dis-ubstituted stilbene derivatives;�: extended donor-acceptor disubstituted thiophene derivatives. The solidline is a fit for the stilbene derivatives and the dashed line afit for benzene derivatives. The shaded areapresents values ofr i j that should be reachable by optimized crystalline structures. Measured values ofelectro-optic coefficients are also plotted.

ORGANIC SINGLE CRYSTALS

There have been significant advances in understanding and optimizing classicalπ-conjugated donor-acceptor chromophores with large first-order molecular hyperpolariz-abilities in the area of organic nonlinear optics in the lastfew years [11, 14, 15, 17–22]. However, there are only few chromophores with very large molecular hyper-polarizabilities such as donor-acceptor stilbenes and tolanes that form potentially usefulcrystalline materials. The interest in molecular crystalsstems from the fact that the po-tential upper limits of macroscopic nonlinearities and long-term orientational stabilityas well as the optical quality of molecular crystals are significantly superior to those ofpolymers.

The calculation of upper limits with regard to electro-optic and nonlinear optical co-efficients (Fig. 4) showed that the macroscopic susceptibilities of crystalline materi-als based on highly extendedπ-conjugated donor-acceptor chromophores, e.g. donor-acceptor disubstituted stilbenes, have by far not reached the upper limit yet.

Crystal growth is the prototype of self-assembly in nature [23]. However, crystalliza-tion of large organic molecules with desired optical properties is still a challenging topic.

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There are several routes to achieve optimized nonlinear optical organic crystals that arelisted below (see e.g. [24]).

a) Use of molecular asymmetry Molecules tend to undergo shape simplification dur-ing crystal growth which gives rise to dimers and then high order aggregates inorder to adapt a close-packing in the solid state [25]. The high tendency of achiralmolecules to crystallize centrosymmetrically could be dueto such a close-packingdriving force. Therefore, if the symmetry of the chromophores is reduced, dimer-ization and subsequent aggregation is no longer of advantage to the close packingand increases the probability of acentric crystallization. This symmetry reductioncan be accomplished by either the introduction of molecular(structural) asymme-try or the incorporation of steric (bulky) substituents into the chromophore. Thesetwo approaches were widely and successfully applied to benzenoid chromophores.Tsunekawa and co-workers found that an introduction of a substituent at the 3-position of 4’-nitrobenzylidene 4-donor-substituted-aniline can induce a favorablenon-centrosymmetric packing for large optical nonlinearities [26]. This led to thediscovery of 4’-nitrobenzylidene-3-acetamino-4-methoxyaniline, MNBA (Fig. 5a),which shows a large SHG efficiency that is 230 times of that of urea. Another exam-ple using this approach, developed by Tam and co-workers is 3-methyl-4-methoxy-4’-nitrostilbene, MMONS, which shows a SHG powder efficiency of 1250 timesthat of urea [27].

b) Use of strong Coulomb interactions can help to override the weak dipole-dipoleinteractions to induce a non-centrosymmetric packing. Meredith proved the va-lidity of this concept for the case of 4-dimethylamino-N-methylstilbazolium salts.This led to the discovery of 4-dimethylamino-N-methylstilbazolium methylsulfate,DMSM (Fig. 5b), which shows a SHG efficiency of 220 times of that of urea [28].Subsequently, Nakanishi and co-workers found that the 4-toluenesulfonate an-ion was an effective counter-ion to induce the non-centrosymmetric packingof stilbazolium chromophores which led to the development of 4-hydroxy-N-methylstilbazolium 4-toluene-sulfonate, MC-PTS, which exhibits a SHG signal of14 times of that of urea at 1.06µm [29].Marder and co-workers adopted the same strategy to perform an extensiveinvestigation by means of varying the counterions of various stilbazolium chro-mophores including 2-N-methylstilbazolium and 4-N-methylstilbazolium cations[30]. They found that whereas rod-shaped 4-N-methylstilbazolium cations canoften be forced to crystallize non-centrosymmetrically this is not true for non-rod-shaped 2-N-methylstilbazolium cations. 4-dimethylamino-N-methylstilbazolium4-toluene-sulfonate, DAST, was shown to exhibit the largest powder SHG effi-ciency (1000 times urea) at 1.9µm. As in several other stilbazolium based acentriccrystals, a polar ionic sheet packing motif was evidenced inthe crystal packingof DAST. However, we have found that by either incorporatinga non-planar orbulky donating group or replacing the phenyl ring with a heteroaromatic ring intothe skeleton of 4-N-methylstilbazolium cations, the probability of getting acentriccrystals went down significantly. This suggests that the polar ionic sheet is verysensitive to the structural change of the stilbazolium cation. The ease of twinningand cracking of ionic crystals such as DAST are often detrimental for growing a

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NO 2

NH3CO

HN

H3CO

MNBA

NO 2

H 3 CO

H3C

MMONS

HO

N

H3C SO3

MC-PTS

N+H3CN(CH3 )2

H3C SO3

DAST

SO3Me

N+H3CN(CH3 )2

DMSM

N NH

NO2

(H3C)2 N

DANPH

SH3C S

N NH

NO 2

MTTNPH

N NH

NO2

HO

HO

3,4-DHNPH

NO

OH

H

O

OH

HO

Mero-2-DBA

3

NO

OH

OCH

O

OH

HO

Mero-2-MDB

(a)

(b)

(c)

(d)

FIGURE 5. Engineering strategies for inducing a noncentrosymmetricpacking of nonlinear chro-mophores and examples: (a) use of molecular asymmetry, (b) use of strong Coulomb interactions, (c)use of non-rod-shapedπ-conjugated cores, (d) supramolecular synthetic approach.

large and good optical quality bulk crystal. Nevertheless we have accomplishedto grow such large, high quality bulk crystals of sizes up 20× 20× 5 mm3 bycontrolled temperature lowering technique [31, 32].

c) Use of non-rod-shaped π-conjugated cores In contrast to donor-acceptor dis-ubstituted stilbene derivatives, hydrazone derivatives generally adopt a bent,non-rod-shaped conformation in the solid state because of the non-rigid nitrogen-nitrogen single bond ( ). We have found that donor-substituted(hetero)-aromatic aldehyde-4-nitrophenylhydrazones show an overwhelminglyhigh propensity for a non-centrosymmetric packing [33]. Ofparticular importanceis that the majority of these acentric crystals exhibit verystrong SHG signals thatare at least two orders of magnitudes greater than that of urea. Furthermore, mostof the hydrazone crystals developed show very good crystallinity and high thermal

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stability. Note, however, that the flexibility of the hydrazone backbone poses aproblem of polymorphism; however, with a proper control of the growth conditionssuch as careful choices of solvent and method of crystal growth, the desirableacentric bulk crystal phase can be selectively grown [34].The best example in this class is 4-dimethylaminobenzaldehyde-4-nitrophenyl-hydrazone, DANPH, which exhibits a very strong SHG signal that is comparableto that of DAST. Another potential candidate is 5-(methylthio)thiophenecarbox-aldehyde-4-nitrophenylhydrazone, MTTNPH, which also shows the same order ofpowder efficiency as DANPH. A third example is 3,4-DHNPH withan excellentalignment of the chromophores in the crystal lattice and a molecular hyperpolariz-ability comparable to DANPH [35].

d) Supramolecular synthetic approach This is the design of molecular or ionicaggregates or assemblies to favour the desirable crystal packing. It offers moredesign feasibility as one or both molecules can be tailor-made or modified tofit one another to acquire the desirable molecular properties in the solid state.Furthermore, the physical properties such as melting pointand solubility as wellas the crystal properties such as crystallinity and ease of crystal growth of the co-crystals can usually be improved compared to those of their starting components.Etter and co-workers first demonstrated the induction of a net dipole moment witha complimentary host-guest pair of 4-aminobenzoic acid and3,5-dinitrobenzoicacid; however, the SHG signal generated by this co-crystal is in the order of theurea standard [36] .We have found that the co-crystals formed from the merocyanine dyes (Mero-1and Mero-2) and the class I phenolic derivatives in which theelectron acceptor ispara-related to the phenolic functionality together with a substituent either in theortho- or meta-position (Fig. 6) show the highest tendency of forming acentric co-crystals. In addition, a large fraction of acentric co-crystals (25%) based on Mero-2and the class I phenolic derivatives exhibit strong second-harmonic signals that areat least two orders of magnitudes larger than that of urea. Their packing motifs canbe distinctively divided into two groups.

NR

O

Mero-1: R=CH3

Mero-2: R=CH 2 CH2 OH

HO Acceptor

Substituent

Class I phenolic derivatives

FIGURE 6. Chemical structures of Mero-1, Mero-2 and Class I phenolic derivatives.

The type I co-crystal is generally characterized by anionicand cationic assembliesor arrays. A fascinating example in this class is the co-crystal Mero-2-DBA (DBA= 2,4-dihydroxy-benzaldehyde) [37]. Mero-2-DBA containsa water molecule andpacks non-centrosymmetrically with space group P1 and point group 1. The anionicassembly is constructed by the co-aggregation of two DBA molecules in whichone of the molecules gives up a proton and bonds to another by ahydrogen bond.

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Additionally, Mero-2 acquires a proton and co-aggregates in anti-parallel fashionwith another Mero-2 by a short hydrogen bond constituting a cationic assembly.The interesting fact is now that, although the net dipole moment almost vanishesin this arrangement, the Mero-2-DBA co-crystal exhibits a large second-harmonicsignal in the powder test. This can be explained by the asymmetric position of thehydrogen bonded proton between the two Mero-2 dyes, which results in a positivereinforcement of molecular hyperpolarizabilities withinthe cationic assembly sinceMero-2 has a negative sign of the hyperpolarizability and the protonated form ofMero-2 ([Mero-2-H]+) has a positive sign of the hyperpolarizability. As a conse-quence, the co-crystal Mero-2-DBA is a potential candidatefor linear electro-opticeffects because of its perfectly parallel alignment of molecular hyperpolarizabilitiesin the solid state.Type II co-crystals are formed by linear molecular aggregates. One of the repre-sentative examples in this class is the co-crystal Mero-2-DAP which exhibits avery strong SHG signal that is three orders of magnitudes larger than that of urea[38]. The molecular aggregate is assembled by the highly electronegative oxygenof Mero-2 and the acidic proton of the phenolic derivative through a short hydrogenbond. Then the rod-like aggregates connect laterally by hydrogen bonds resultingin a staircase-like polar chain. These polar chains align ina parallel fashion con-stituting a two-dimensional acentric layer which is found to be the common andkey feature of all the highly non-centrosymmetric co-crystals in this class. Sincethe charge-transfer axis of Mero-2 is inclined by an angle ofabout 70◦ to the polardirection of the crystal, this co-crystal is an attractive candidate for nonlinear op-tical effects such as frequency-doubling. In addition, we have found in this newlydeveloped system that the orientation of the merocyanine dye can be changed andtuned within the crystal lattice by a careful selection of a guest molecule–phenolicderivative, provided that the linear molecular aggregate and the acentric layer pack-ing motifs are maintained. Although Mero-2 only exists in anamorphous state byitself, both types of co-crystals formed show greatly improved crystal and physi-cal properties compared to its constituted components. Another interesting type IIcrystal shown in Fig. 7 is Mero-2-MDB that is optimized for electro-optic appli-cations due to the parallel alignment of the nonlinear optical chromophores [39].

5 mm

FIGURE 7. (a) An as grown crystal of Mero-2-MDB. (b) X-ray structure ofa hydrogen bond directed,acentric layer structure of the co-crystal Mero-2-MDB.

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Table 2 lists some properties of the organic crystals DAST and Mero-2-MDB that wehave found the most interesting for their nonlinear opticaland/or electro-optic response.

TABLE 2. Some properties of the organic crystals DAST [10, 40, 41] andMero-2-MDB [39],λc is the cutoff wavelength in the bulk,n the refractive index,r the electro-optic coefficient, andd the nonlinear optical coefficient.

λc (nm) n r (pm/V) d (pm/V)

DAST 700 n1 (720 nm)= 2.519 r11 (720 nm)= 92 d11 (1318 nm)= 1010n2 (720 nm)= 1.720 r11 (1313 nm)= 53 d11 (1542 nm)= 290n3 (720 nm)= 1.635 r11 (1535 nm)= 47 d26 (1542 nm)= 39

M2-MDBphase I: 615 n1 (700 nm)= 2.07 r11 (1318 nm)= 24 d11 (1318 nm)= 108phase II: 680 n1 (700 nm)= 2.20 r11 (1318 nm)= 34 d11 (1318 nm)= 267

DAST Single Crystals for Integrated Optics

We are currently working on the organic salt crystallization approach for DAST inwhich oppositely charged molecules form a crystal that is stabilized through strongCoulomb interactions. The reasons for the growing interestto obtain high-quality DASTcrystals are its extraordinary high nonlinearities, the high second-order nonlinear opticalcoefficient and electro-optical coefficient, being respectively, 10 times and twice aslarge as those of the inorganic standard LiNbO3 (see Tables 1 and 2). Bulk DASTcrystal growth has been tried by different groups with various successes. Supersaturatedmethanol solutions are used and the temperature of the solution is carefully reduced. Thegrowth starts from either spontaneous nucleation [42] or seed introduction [31, 43, 44].A very high thermal stability of the solution (better than 0.01◦C over several weeks) isrequired for high quality samples. Therefore crystals in the size range of 1-2 cm3 aredifficult to obtain and large cracks due to thermal and/or mechanical shocks are presentif special care is not taken.

The production of large size high optical quality bulk organic crystal is a challengingand difficult task. For application in integrated optics devices, however, waveguidingstructures should be fabricated. In the following we present various approaches that weconsider to obtain the desired structures.

DAST Thin Film Growth

Single crystalline thin films of the highly nonlinear optical material DAST are veryinteresting for photonics applications. Much effort has been devoted to thin film growthof DAST. As an example, 2D-organic crystal growth using a melt shear or a solutionshear method attracted much attention [45–48]. This methodleads to 3µm thin and30-40 mm2 large crystals. Nevertheless, reproduction of these results has not beenpossible to date. Other efforts concentrated in evaporation methods. DAST decomposeswhen evaporated. In order to overcome this problem one approach was to evaporate

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the two components of DAST separately and using a carrier gasflow to deposit themon a substrate [49–51]. The surface of the obtained film is rough, with a thicknessvarying between 1 and 5µm. This is due to a crystallite diameter of only about 200nm. Thin films of DAST have been also prepared by polishing down bulk crystals. Thethinnest obtained samples were around 20µm [52]. The main drawbacks of this methodare the cracks that may form during polishing in the ultimateslimming step since themechanical shear strength will proportionally decrease with the thickness.

So far work in this area has led to several breakthroughs [53]. We have developedseveral new exploratory planar solution growth methods fororganic materials, in par-ticular for the DAST/methanol system: the two-dimensional∆T method, the travelingcell technique, the capillary and∆T-aided capillary methods, the undercooled flow celltechnique, the growth by solution epitaxy, and the fabrication using laminar flow [53].The most promising of these methods are the capillary methodand solution epitaxialgrowth. Solution epitaxial growth showed good results, in particular with respect to thelarge area (4 mm2) of these as-grown monocrystals. The thickness is to be further con-trolled with the growth temperature and temperature gradients. The minimum thicknessobtained throughout the different experiments was 10µm (with the capillary growthmethod).

Photobleaching

The refractive index of DAST crystals can be reduced by photobleaching [54, 55].An illumination with a frequency doubled Nd:YVO4 laser (λ = 532 nm) having anintensity I = 0.52W/cm2 for a few hours reduced the refractive index from 2.55 to1.64 at the wavelength 633 nm and from 2.14 to 1.58 at 1550 nm [54, 55]. A refractiveindex measurement by the light prism-coupling method yielded a depth of the refractiveindex change of 2.25µm [54]. Based on this photobleaching effect grating structures onDAST crystal surfaces were fabricated by interfering two coherent beams. In this casethe depth of the refractive index grating was found to be between 5µm and 7µm [54, 55].Photobleaching was also used to produce channel waveguidesin thin DAST samples[56] where UV resin was used as undercladding. Thin DAST crystals were fixed on thecured UV resin and covered with a photo mask. After exposure under a Xe lamp for 65 hwith an intensity of 0.24 W/cm2 buried DAST waveguides were fabricated.

We have studied the linear optical properties and absorption in DAST within theabsorption band from 260 nm - 700 nm in order to determine the depth-range ofphotobleaching [57]. The results were obtained from reflectivity measurements andbleaching experiments. We have shown that the depth range ofphotobleaching can bevaried between 0.2µm and 2.6µm by selecting a suitable wavelength [57]. Thereforephotobleaching is a useful method for structuring DAST surfaces for integrated optics.The parameters we obtained [57] can be used to calculate the depth of photobleachedmaterial that can be achieved with available lasers of different wavelengths.

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Femtosecond Laser Ablation

The advantage of femtosecond (fs) laser ablation for damagefree material processingis well known [58] and has been applied for various photonic and organic materials [59–64]. This method is especially interesting for the fabrication of waveguides for integratednonlinear optics, where the crystalline structure of the waveguide should be maintained.Furthermore, fs laser ablation is well suited for 2-dimensional structuring as required foroptical processing devices that are fabricated on a planar surface.

We have successfully demonstrated that DAST surface can be patterned by the fem-tosecond laser ablation method [65]. The depths and widths of ablated grooves weredetermined as a function of the fluence of the structuring laser beam and the ablated sur-faces were examined by optical microscopy. From these measurements we determinedthe threshold fluence for ablation and an ideal fluence range for almost damage freeablation. This was done using fs-laser pulses at the wavelengths of 775 nm, 600 nm,and 550 nm with a pulse width of about 170 fs. Fig. 8 shows photographs of an ablatedridge "waveguide" structure on the surface of DAST in top view and in side view. Twogrooves were written with a mutual spacing of about 15µm. For the side view picturethe sample was turned by 90◦ and the surface observed perpendicularly to the ablatedgroove. The side view profile reveals that the ablation of ridge waveguides using fs lasersin DAST is very promising for the realization of low loss optical waveguides by fs abla-tion. The ablated structure has an average width of 10µm and a depth of about 7µm. Aridge waveguide for IR light at telecommunication wavelengths requires structures withdimensions of about 2µm, which seems to be a realistic aim for future work.

10 µm20 µm

a. b.

FIGURE 8. a) Top view and b) side view of a ridge "waveguide" structure ablated atλ = 775 nm.

DAST as an Overlay Material

To utilize the outstanding electro-optic properties of DAST in a waveguiding structureone can use it also as an overlay layer as depicted in Fig. 9. Ifthe effective index of thepropagating mode in the waveguide is close enough to the one of DAST, a large partof the optical field is propagating in the DAST overlay to allow efficient modulation.For the effective optical contact between DAST and waveguides, a high quality DAST

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surface polishing was developed in our laboratory (roughness less than 10 nm RMS). Inour preliminary experiments we could already observe phaseand amplitude modulationof the optical field coupled into Si3N4 waveguides with DAST overlay.

Si SiO2

Si3N4

Electrode

DASTx1

x2

x3

DAST(0.3 µm)

(8 µm)

FIGURE 9. Configuration of the Si3N4 waveguides with DAST overlay.

POLED POLYMERS

Polymers are an important class of nonlinear optical materials as they combine the non-linear optical properties of conjugatedπ-electron systems with the feasibility of creatingnew materials with appropriate optical and structural properties. The incorporation ofnonlinear optical molecules in polymers is comparatively easy and can be done in dif-ferent ways. The simplest one is the mixing of the active molecules in a polymer matrixforming a guest-host system. Alternatives are the covalentlinking of the molecules toa polymer backbone in the form of a side-chain, their cross-linking between two poly-mer chains, or their incorporation in the main-chain. Figure 10 depicts schematicallythese four types of nonlinear optical polymers and Table 3 gives a comparison of theiradvantages and disadvantages.

D π AD

A

π

( )

polymer chain

functional chromophore

linking functionality

( )

( )

( ) ( ) ( )

( )

guest-host side-chain cross-linked main-chain

D

A

π

D

A

π

D π A

FIGURE 10. Types of nonlinear optical polymers.

To show a second-order nonlinearity a material has to be noncentrosymmetric. In apolymer the molecules are randomly oriented leading to a centrosymmetric structure.

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TABLE 3. Advantages and disadvantages of the different types of polymers for nonlinear optics.

Polymer type Advantages Disadvantages

Guest-host • unlimited selection of desirednonlinear optical guests andpolymer hosts

• easy thin film processing• inexpensive mass production

• decay of nonlinear optical activ-ity due to orientational relaxation

• low nonlinear optical activitydue to limited solubility of non-linear optical molecules in poly-mer matrix

• scattering losses due to inhomo-geneity

• sublimation of nonlinear opticalmolecules at elevated tempera-tures

Side-chain • high concentration of nonlinearoptical molecules

• tailoring of nonlinear opticalproperties via chemical modifi-cations

• increased orientational stability• low scattering losses

Main-chain • high concentration of nonlinearoptical molecules

• tailoring of nonlinear opticalproperties via chemical modifi-cations

• increased orientational stability• low scattering losses

• molecules difficult to orient toexternally applied field

• solubility can be lower

Cross-linked • tailoring of nonlinear opticalproperties via chemical modifi-cations

• high orientational stability

• increased scattering losses• poor stability

The symmetry can be broken by aligning the molecules in the direction of an appliedstrong electric field. If the polymeric system is brought to aglassy state by raisingthe temperature while the electric field is still applied, the opposing internal molecularforces decrease and the desired dipolar alignment is secured. The temperature at whichthe polymer goes from the solid state to a glassy state is called the glass transitiontemperature,Tg. Note that in order to be poled the nonlinear optical chromophores in thepolymer have to possess a permanent dipole moment. The two most common methodsof chromophore alignment are the corona and the electrode poling.

To be successfully used in nonlinear optic applications, poled polymers should meetthe following requirements:

• high electro-optic coefficientr33 (> 35 pm/V) at 1.3µm and 1.5µm• glass transition temperatureTg > 250◦C

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• less than 5% orientational relaxation at 80◦C over a few years• less than 5% orientational relaxation at 200◦C over a short time• no physical/chemical degradation up to 300◦C• low optical loss at operating wavelength (absorption loss< 1 dB/cm, scattering loss

< 0.5 dB/cm)• compatibility with different substrates and solvents• good thin-film processability• broadband transparency• low fabrication costs

In the past years several families of nonlinear optical polymers have been developed(see some recent reviews with comparisons of various polymer systems for nonlinearoptical applications[11, 14, 66, 67]).

Polyimide Side-Chain Polymers for Electro-Optics

As an example we discuss the properties of the polyimide side-chain electro-opticpolymers based on disperse red (DR1) chromophores that weredeveloped in the earlynineties through our collaboration with Sandoz Optoelectronics Research [12]. Themolecular structure is shown in Fig. 11. The alkyl-amino-functionalized nonlinear op-

N

(CH2)n

O

O

N

R1

N

N

N

O -

O

+

R2

FIGURE 11. Molecular structure of electro-optical side-chain polyimides based on disperse red.

tical azo chromophores, with the various substitution patterns shown in Table 4, areattached via a two- or three-carbon spacer linkage to an alternating styrene-maleic-anhydride copolymer. The three polymers denoted by A-095.11, A-097.07, and A-148.02 with the chromophores indicated in Table 4 were characterized for the nonlinearoptical properties and stability.

TABLE 4. Azo dye substitution pat-terns and glass transition temperaturesTg of selected modified polyimide poly-mers.

n R1 R2 Tg (◦C)

A-095.11 3 CH3 H 137A-097.07 3 CH3 Cl 149A-148.02 2 H H 172

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The glass transition temperature and the nonlinear opticalproperties of nonlinearpolymers depend in a large extent on the chromophore concentration and also on thepoling field parameters. For example, polymer A-095.11 witha chromophore concen-tration of 56 wt.% resulted inTg = 137◦C, nonlinear optical coefficientd33 = 34 pm/Vat λ = 1.54µm, and electro-optic coefficient ofr33 = 20 pm/V atλ = 1.313µm [12].

Thermal Degradation

Thermal degradation, i.e. the loss of activity due to thermal effects, does not give riseto any problem when using organic crystalline materials. The chromophores are fixed intheir orientation within the crystal lattice and cannot move from their positions, i.e. theycannot reorient to an isotropic distribution if they were grown in a noncentrosymmetricfashion. On the other hand, understanding relaxational processes in nonlinear opticalpolymeric materials is of critical importance in order to evaluate the long-term stabilityof poled polymers that are in development for potential electro-optic applications. Theactual usefulness of these materials relies on sufficient stability of the poling-inducedorder of the nonlinear optical chromophores within these polymers. An essential re-quirement for stabilizing polymeric nonlinear optical materials is the formation of aglassy state at relatively high temperatures. Amorphous polymers, among other glasses,typically show an evidence of a phase transition from a liquid-like to a glassy state whencooled down from high temperatures. As already mentioned, the temperature at whichthis transition occurs, is known as the glass transition,Tg. The physical origin of the glasstransition is primarily associated with the cooperative motions of large scale molecularsegments of the polymer. The actual experimental observance of a glass transition ismost easily probed by measuring enthalpic changes in the polymer as a function of tem-perature via differential scanning calorimetry (DSC). Several phenomenological theoriesdescribe the primary aspects of the glass transition, at least as they are experimentallyobserved.

Relaxation processes in nonlinear optically active modified polyimide polymers withside-chain azo chromophores listed in Table 4, having glasstransition temperatures inthe range of 140◦C< Tg <170◦C, have been studied by differential scanning calorimetry,dielectric relaxation, and second-harmonic generation experiments. These experimentsrevealed important information on stability [68]. We have shown that it is possibleto model the relaxational behavior of nonlinear optical chromophores both above andbelow the glass transition over more than 15 orders of magnitude in time using theTool -Narayanaswamy procedure incorporating the appropriate Williams–Landel–Ferry(WLF) parameters for the nonlinear optical polymers [68]. This leads to a scalingprediction for relaxation times in the glassy state with thescaling parameter(Tg−T)/T(Fig. 12).

The relaxation time therefore only depends on one parameter: Tg. Fig 12 and theunderlying theory now allow to define the requirements ofTg for a certain requiredstability. As an example, for a polymer withTg = 172◦C the relaxation time, (1/e) at anoperating temperature of 80◦C, is about 100 years. With aTg of 200◦C this relaxationtime dramatically increases. Annealing of polymers at elevated temperatures further

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increases the orientational stability [69]. Finally, it should be noted that the relaxationresults of the polyimide side-chain polymers could be slightly altered for differentpolymer systems.

0.0 0.1 0.2 0.3 0.4 0.5 0.6

103

104

105

106

107

108

109

1010

1 hr

1 day

1 mo

1 yr

10 yrs

100 yrs

(Tg - T ) / T

τ (

sec)

A-095.11

A-097.07

A-148.02

20% Lophine 1 / Ultem

FIGURE 12. Temperature scaling of normalized dielectric loss and second-harmonic generation relax-ation times with respect to the scaling variable(Tg−T)/T of three side-chain polymers of Table 4 andone guest-host polymer (20% Lophine 1/Ultem). It is clearlyseen that the nonlinear optical moiety hasto be coupled to the polymer main chain for increased stability. The two thick vertical lines are related toexamples discussed in the text.

Photochemical Stability

Whereas the best chromophores available to date show impressive characteristicsconcerning nonlinearity and processability, they often lack sufficient photochemicalstability, i.e. they tend to degrade under the influence of light. In fact, sufficient resistanceagainst photodegradation is a major hurdle to be overcome for further establishingorganic chromophores as one of the key components for photonics technologies.

Any molecule exposed to light can undergo photo-excitation. The probability for exci-tation depends on the absorption coefficient for the wavelength of interest or vice versa.In the excited state, molecules usually have a much higher chemical reactivity, whichleads, if a reaction occurs, to a degradation of the molecules and its nonlinearity. Thereaction in the excited state occurs with a certain probability, i.e. a molecule can beexcited several times, the best molecules used for electro-optic applications even upto 107 times, before degradation occurs. However, even though theprobability for thephotodegradation seems to be small at the first sight, it becomes of the prime impor-tance at high photon fluences, as in the case of waveguide based devices. The fact thathigh photon fluences can give substantial rise to the polymeric device degradation wasfirst realized and investigated in devices using the chromophore 4-dimethylamino-4’-nitrostilbene (DANS) [70] in 1993. However, the first published quantitative investiga-tions have not been published until 1998 [71]. The model usedto analyze the obtaineddata quantitatively was based on a model developed for the photodegradation of laser

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dyes by Dubois et al. [72]. The same group elucidated that, based on the obtained data,the lifetime predictions are still far too short for most commercial applications [73, 74].

There are many different photochemical processes that can lead to a degradationof molecules and especially nonlinear optically active chromophores. The degradationpathways for the systems under investigation always have togo via excited states, i.e.molecules in their ground state cannot decay in contrast forradioactive species. Intheir excited states, molecules are much more chemically and structurally active, anddegradation processes are therefore mainly activated raising electrons from their groundto an excited state. If the molecule is in an electronically excited state, there are severalways in which it can react or otherwise use or lose its excess energy. The most importantroutes are represented schematically in Fig. 13.

AB*

Physical

ABquenching

+M

AB+ + e-

Ionization

A + B

Dissociation

BA

Isomerization

AE + B or ABE

Direct reaction

+E

AB + CD*

energy transfer

Intermolecular

AB†

energy transfer

Intramolecular

(radiationless

transition)

LuminescenceAB + hν

+CD

(i)

(ii)

(iii)(iv)

(v)

(vi)

(vii)

(viii)

i.e. photo-oxidation

i.e. cis-trans

FIGURE 13. The molecule with the chemical structure AB can lose its energy after having been excitedto the state AB* using different de-excitation pathways that are schematically presented [75]. The routesinvolving real chemical changes are (i), (ii), and (iii).

For polymers and chromophores used in electro-optic devices, the material is exposedto continuous illumination, i.e. a photon flux, which may have rather high energy den-sities, especially in waveguides. For certain wavelengths, the absorption of incomingphotons is very significant, i.e. many molecules become excited, which is the case in-side the absorption band, or the absorption can be very low e.g. near the telecommuni-cation wavelengths, i.e. at 1.3µm and 1.5µm. In the excited states, as presented in Fig.13, the molecules can either return to their ground state viamultiple possible pathways(like emitting a photon), or they can undergo degradation either by a chemical change(dissociation or direct reaction) or a geometric deformation (isomerization). Our results

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of photostability studies suggest that there are only two dominating processes: cis-transisomerization and photo-oxidation [76].

There are two basic parameters that affect the photochemical stability. The first onedetermines the number of times a molecule can undergo excitation to a higher energystate before it decomposes. This number, the so-called inverse quantum efficiency forphotodegradationB, is a main figure of merit to generally describe the photochemicalstability. It is wavelength independent. As it turns out, the wavelengths used for appli-cations can be of major importance since the absorption cross-sectionσ at the operatingwavelength determines the device lifetime to a large extent. For this reason, the secondfigure of meritB/σ is introduced that is directly related to the device lifetime.

The idea to experimentally determine the inverse quantum efficiency for photodegra-dationB is based on a pump-probe scheme [77]. A molecule that changesits molecularshape and/or composition alters its absorption spectrum, i.e. monitoring the change ofthe main absorption peak gives insight into the number of molecules that undergo degra-dation under a given incoming photon flux. Therefore the probe wavelength with verylow intensity during a short time interval measures the absorption at this wavelength,while a constant incoming pump wavelength leads to a degradation of the molecules.We are using different lasers for pumping and probing beams and the sample is placedin a chamber that can be evacuated and refilled with differentgases, enabling the exam-ination of the atmospheric dependence of the photodegradation.

Different chromophore-polymer guest-host systems have been investigated. Bithio-phene chromophore CC172 was synthesized and characterizedin our laboratory [78].Bithiophene molecules are soluble in low polarity solventsand exhibit molecular non-linearities which are among the largest for stable NLO chromophores. Furthermore, weinvestigated the standard chromophore for nonlinear optical applications, DR1, a side-chain polymer A-095.11 (see Table 4), and one of the best chromophores available todate, the CLD-1 chromophore (see Table 1), a phenyltetraene, that was used to fabricateMach-Zehnder electro-optic modulators exhibiting for thefirst time half-wave voltagesof less than one volt [79], and also to demonstrate the first polymer based resonant ringmodulator [1].

Fig. 14 depicts the wavelength dependence of the figure of merit B/σ for CC172,CLD-1, DR1, and A-095.11 as guest molecules in polymethylmethacrylate. The valuesfor DR1 reported in Ref. [80] are included since they differ significantly from thevalues we obtained. In all five cases, the figure of merit increases over several ordersof magnitude from within the absorption band towards the infrared wavelengths due tothe decreasing absorption cross-section. For the absolutevalues ofB/σ, the azobenzenechromophore DR1 in the guest host system with PMMA and in the side-chain polyimideA-095.11, is superior to the bithiophene CC172 and the phenyltetraene CLD-1. Thisstems mainly from the difference in the absorption maxima. The onset of the steepincrease of towards longer wavelengths just starts, of course, outside the absorptionof the molecules, sinceσ becomes small. Consequently, molecules with a red-shiftedabsorption maximum like CC172 and CLD-1 exhibit always a smaller photochemicalstability compared to molecules absorbing mainly around 500 nm like the azobenzenes.This last point reflects the whole dilemma often encounteredin material science orscience in general: usually, a trade-off occurs between theproperty that is optimized(molecules with a red-shifted absorption peak exhibit usually higher nonlinearity) and

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another important figure of merit that is decreased (here, this role is played by thephotochemical stability).

0.5 1.0 1.5 2.0 2.5

60080010001400

1024

1026

1028

1030

1032

Photon energy (eV)

B/σ

(m

-2)

Wavelength (nm)

DR1 (our meas.)

DR1 (Ref. [80])

CC172

CLD-1

A-95.11

FIGURE 14. Wavelength dependence of the figure of meritB/σ for guest-host polymers based on themolecules CC172, CLD-1, DR1, and side-chain polymer A-095.11.

To better understand the mechanisms involved in photodegradation, we also per-formed measurements in different atmospheres, i.e. in oxygen, air, nitrogen and vacuum.For the CC172 in PMMA for example, the change from air to pure nitrogen increasesthe stability by one order of magnitude over the whole wavelength range. This alreadyindicates the importance of photo-oxidative processes in photodegradation. It is very in-teresting that we gain an additional order of magnitude whenchanging from nitrogento vacuum atmosphere. This suggests that even in nitrogen atmosphere reactive pro-cesses lead to photodegradation, i.e. that the degradationprocesses are mainly triggeredby the surrounding atmosphere. Using the gas barrier coating, another order of mag-nitude higher figure of merits were obtained. The most likelyexplanation for this isresidual oxygen in the vacuum since the investigation took place at 10−2 mbar and notin ultra-high vacuum. This stresses once more the prime importance of photo-oxidativeprocesses in photodegradation.

Characteristic differences in the degradation behavior were observed comparing thedegradation processes of CC172 in air and nitrogen. This is illustrated in Fig. 15, wherethe two respective degradation curves are compared. Whereas the degradation in air isnot in accordance with the simple degradation model assuming only one degradationchannel, and has to be explained by the two process degradation model that we havedeveloped [76], the photobleaching in vacuum follows exactly the mathematical functiondescribed by the simple model. This can be interpreted as theexistence of only onedegradation channel that does not involve oxygen or nitrogen. Consequently, oxygenis the main aggressor triggering photodegradation via photo-oxidation. It has to bekept away from the system to achieve the photochemical stability required in opticalwaveguide devices using organic nonlinear optical materials. The phenyltetraene CLD-1 shows essentially a behavior similar to the bithiophene CC172 with photo-oxidationbeing the main degradation process.

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0 200 400 6000

20

40

60

80

100

Integrated Energy (Jm-2)

Tra

nsm

issi

on (

%)

Vacuum

Air

One degradation

channel

Tra

nsm

issi

on (

%)

0 4 8 12 160

20

40

60

80

100

Integrated Energy (Jm-2)

Air

One degradation

channel

Two degradation

channels

FIGURE 15. Comparison of the degradation processes of CC172 in air and vacuum atmosphere atλ = 633 nm. The degradation in vacuum is much slower and can be described by a single degradationprocess whereas the degradation in air occurs on a much shorter timescale and the curve describing thedegradation process can only be described accurately usingthe two process degradation model [76].

CONCLUSIONS

Several classes of materials have been considered for applications in VLSI photonics.For applications like microring resonators it is of crucialimportance to choose a materialwith low losses, appropriate refractive index, and a high level of thermal- and photo-stability. Nonlinear optical materials are especially attractive since they allow an activecontrol over device operation. Many new inorganic and organic nonlinear materialshave been developed to meet the required material properties. Well designed organicnonlinear optical materials may be much superior to their inorganic counterparts dueto relatively high and extremely fast nonlinearities, the consequence of almost purelyelectronic origin of the nonlinear optical effects. We havediscussed in more detailstwo important classes of organic materials that are being extensively studied in the lastyears. Polymers are potentially cheap, especially easy to process in thin films and offerrelatively large design possibilities and hence a high freedom for the optimization oftheir properties. Their main disadvantage is that their nonlinear optical properties arelimited by the maximum chromophore concentration, their distribution in the polymermatrix and orientational relaxation. On the other hand, organic single crystals havesuperior nonlinear optical properties and stability, but they are difficult to process,especially in thin films that are needed for the development of structures compatiblewith microelectronic processing.

Macroscopic physical properties of materials arise from the properties of the con-stituent molecules. Many new molecules with increased nonlinearities were developedin the last few years. The incorporation of these molecules into a crystal lattice with de-sired properties is a difficult task. We have presented several crystal engineering strate-gies towards the optimized nonlinear optical organic crystals. However, growing of largesize crystals sufficient for applications remains a challenging topic, and the upper po-tential limits of achievable nonlinearities are still by far not reached. On the other hand,for applications in integrated optics, waveguiding structures should be fabricated. We

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have presented various approaches to obtain the desired structures in the organic crystalDAST.

Polymer electro-optic materials have recently been developed that, when fabricatedinto devices such as Mach-Zehnder interferometers, permitdrive voltages of less than 1V to be realized at telecommunication wavelengths. However, their long-term stabilityremains a difficult problem to overcome, especially in highly nonlinear polymer com-pounds. As thermal degradation is not a problem when using nonlinear crystals, it iscritical in nonlinear optical polymers, since the nonlinear chromophores are not fixedin the lattice as in crystals. To meet the requirements for successful applications, theglass transition temperature of nonlinear optical polymers should exceed 250◦C. Theother critical issue of organic nonlinear materials is their photostability since they tendto degrade under the influence of light. Various mechanisms for photochemical degra-dation have been presented. Our results suggest that two photodegradation processesare dominant, cis-trans isomerization and photo-oxidation. Several ways to improve thephotostability have been proposed. Together with an improved molecular design, fab-rication of highly-stable highly-nonlinear high-speed optical devices is a challenge forfuture research and technology.

ACKNOWLEDGMENTS

This work was supported in part by the Swiss National ScienceFoundation and theEuropean Union 5th framework program (IST, NAIS).

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