# Organic Polymeric and Non-Polymeric Materials with Large Optical Nonlinearities.

код для вставкиСкачатьOrganic Polymeric and Non-Polymeric Materials with Large Optical Nonlinearities By David J. Williams Nonlinear optical properties are a sensitive probe of the electronic and solid-state structure of organic compounds and as a consequence find various applications in many areas of optoelectronics including optical communications, laser scanning and control functions, and integrated optics technology. Because of their strongly delocalized n electronic systems, polymeric and non-polymeric aromatic compounds show highly nonlinear optical effects. Nowadays, polymer chemists are able to tailor specific materials properties for various applications. Some organic substances with n electronic systems exhibit the largest known nonlinear coefficients, often considerably larger than those of the more conventional inorganic dielectrics and semiconductors, and thus show promise for thin-film fabrication, allowing the enormous function and cost advantages of integrated electronic circuitry. The electronic origins of nonlinear optical effects in organic n electronic systems are reviewed, with special emphasis being given to second-order nonlinear optical effects. Methods for measuring nonlinear optical responses are outlined, and the critical relationships of the propagation characteristics of light to observed nonlinear optical effects and to solid-state structure are discussed. Finally, the synthesis and characterization of organic crystals and polymer films with large second-order optical nonlinearities are summarized. 1.1. Nonlinearity of a Molecular Dielectric Response 1. Introduction Interest in nonlinear optics has grown tremendously in recent years, primarily because of the telecommunications industry's need for high-bandwidth optical switching and processing devices to service the information and data transmission needs of the computer age, and of the proliferation of sophisticated laser tools, which has nurtured and necessitated research on new methods for tailoring individual laser pulses to perform specific functions or be readily detected in complex experiments. Nonlinear optics is concerned with the interactions of electromagnetic fields in various media to produce new fields altered in phase, frequency, amplitude, or other propagation characteristics from the incident fields. The media in which these effects occur are becoming the subject of intense interest, since their adjunct properties (stability, ease of preparation, compatibility with microelectronic processing methods, adhesion, mechanical, and other properties) as well as their nonlinear optical properties will ultimately determine the technological utility of the effect. The origin of nonlinear optical effects will be described here in a phenomenological manner, and several specific effects and their experimental detection and characterization will be described in some detail. Recent efforts in chemical research have been aimed at achieving large second-order nonlinear responses in organic and polymeric materials"]. and several of these will be reviewed. [*] Dr. D. J. Williams Research Laboratories, Eastman Kodak Company Rochester, NY 14650 (USA) 690 0 Verlag Chemie GmbH, 0-6940 Weinheim, 1984 In order to gain an insight into the origin of nonlinear optical effects, the polarization P induced in a molecule by a local electric field E is expanded in powers of the electric field [Eq. (l)]. For the moment, it will be assumed that the polarization is a scalar quantity (as opposed to a vector). The first term a in the expansion is the familiar linear polarization and is the origin of the refractive index if the field E is associated with an electromagnetic wave in the optical frequency range. The coefficients are actually complex numbers, the real part in a corresponding to the index of refraction and the imaginary part to absorption of a photon by the molecule. When an electromagnetic field interacts with a molecule or a medium consisting of many molecules, the field polarizes the molecules. These, in turn, act as oscillating dipoles broadcasting electromagnetic radiation, which can then be detected at some point in space outside of the medium. In a nonlinear medium the induced polarization is a nonlinear function of the applied field. This is illustrated in Figure 1 for a medium where the first nonlinear term makes a significant contribution to the induced-frequency components. In this example the medium exhibits an asymmetric nonlinear response to the applied field E(o).A medium exhibiting such a response might consist of a crystal composed of molecules with asymmetric charge distributions, as shown, arranged in the crystal in such a way that a polar orientation is maintained throughout the crystal. From chemical intuition it is clear that the molecules are more easily polarized in the direction from the elec- 0570-0833,/84/0909-0690 $. 02.50/0 Angew. Chem. Int. Ed. Engl. 23 (1984) 690-703 tron-rich substituent D (donor) to electron-deficient substituent A (acceptor). h Y quency w into light frequency 2 w , and optical rectification, i.e. the ability to induce a DC voltage between electrodes placed on the surface of the crystal when an intense laser beam is directed into the crystal. On the other hand, liquids such as benzene or vapors of a polarizable medium exhibit a light intensity dependent refractive index (w), third-harmonic generation ( 3 w), and fifth- or higher order harmonics for very intense laser beams in certain metal vapors. Fig. 2. Plot of the nonlinear polarization response P to an incident electromagnetic field in a centrosymmetric medium. Fig. 1. Top: Plot of the polarization response P to an incident electromagnetic wave of field strength E(w) at frequency w in a noncentrosymmetric medium. Bottom: Fourier components of P at frequencies w , 2 0 , and 0. See also text. An important general statement can now be made regardingp. If the molecule (or medium in the illustration) is centrosymmetric, then b= 0. This can be readily seen by the following argument. If a field + E is applied to the molecule (medium), Equation (1) predicts that the polarization induced by the first nonlinear term is +p E2. If a field - E is applied, the polarization is still predicted to be +p E2, yet if the medium is centrosymmetric the polarization should be -PEZ. This contradiction can be resolved only ifp=O in centrosymmetric media. If we use the same arguments for the next higher order term, + E produces polarization + y E 3 and - E produces - y E 3 so that y is the first nonzero nonlinear term in centrosymrnetric media. Referring to Figure 1 again, the Fourier theorem states that a nonsinusoidal periodic response such as P can be described by the sum of a series of sinusoidal functions with appropriate coefficients of harmonics of the fundamental frequency a[*]. If this response P is asymmetric, an appropriate summation of the even harmonics (the Fourier components consisting of sinusoidal functions of the harmonics 0, 2w, 4 w , 60, etc.) describe the function P. If the nonlinear response is symmetric, however, the odd terms w , 3 w , Sw, etc. describe the response. Figure 2 shows an example of a nonlinear response in a centrosymrnetric medium such as benzene, where the polarizable n system is responsible for the symmetric nonlinear polarization. The Fourier decomposition would consist of responses at W , 3w, 5 w , etc. with progressively decreasing amplitudes (coefficients) of the higher order components. As the incident field E(w) in either example becomes more intense, the nonlinear terms contribute more significantly. Noncentrosymmetric crystals can exhibit harmonic generation ( 2 w ) or frequency doubling (SHG, second harmonic generation), i.e. conversion of coherent light of freAngew. Chem. Inr. Ed. Engl. 23 (1984) 690-703 Another example of a second-order nonlinear optical effect is the linear electro-optic or Pockels effect. Here a DC field is applied to a medium through which an optical wave propagates. Once again this effect arises through 8, but the polarization now has a contribution arising from E(0)E(w) rather than E(w)*as in second-harmonic generation. The change in polarization due to the presence of these two interacting field components effectively alters the refractive index of the medium 7, giving it a field dependence q(E). 1.2. Microscopic Nonlinearities The intuitive description of nonlinear optical processes in Section 1.1 could be extended to additional nonlinear effects, but many of the important aspects and consequences of nonlinear optics cannot be discussed without putting the effects on a more rigorous physical basis. In this section the constitutive relationships will be precisely stated and the reader will be referred to some of the treatises on the subject for additional background. The polarization induced in a molecule by an applied electric field is where the subscripts i, j, k, 1 refer to the molecular coordietc. denote the components of the nate system and Ej,Ejkr applied fieldf3’.Rather than being scalar quantities as expressed in Equation (I), it is obvious that the induced polarization in a molecule is a vector and is related to the electric-field vectors through the components of the tensor coefficients. The third-rank tensor Pijkhas properties similar to a vector[41,but y, a fourth-rank tensor, has the properties of a scalar. This can be seen more readily by relating the values of /j’ and y obtained in experimental measurements to the components of the tensors [Eqs. ( 3 ) and (4)]. 69 1 Recalling that the indices indicate the projection of field components in the direction indicated by the second two indices,) ,@ on the molecular axis indicated by the first index QzXx), the vectorial nature of p for fixed molecular and field directions is clear. Referring to Figure 3, where the z axis of the molecule is designated as passing through the carbon atoms to which the electron-rich substituent D (donor) and electron-deficient substituent A (acceptor) are attached, it is clear that the components of the arbitrarily chosen field direction will polarize the molecule most strongly in the z direction. If we devise an experiment where the applied field is pointed in the direction of the molecular z axis, it is clear that pz,, will be the dominant contributor to the nonlinear response. y, on the other hand, has the characteristics of a scalar with field projections contributing in all molecular directions. To demonstrate these points, an analysis of the properties of evenand odd-rank tensors is requiredL4]. Y )kz D*6?i Fig. 3. Organic compound with electron-rich (donor) and -deficient (acceptor) substituents which provide the asymmetric charge distribution in the n system required for large& Also shown is a reference coordinate system with z parallel to the dipolar axis of the molecule. Since the molecule is most easily polarized for field components parallel to the z axis, pzZzis the largest component of the p,,ktensor. x(') even-order coefficients require a detailed knowledge of the projection of the molecular hyperpolarizability tensors onto the unit cell of the noncentrosymmetric crystal containing that molecule. Here, therefore, we will only state the relationship for the simplest possible case, a rigid lattice-oriented gas['] (all molecules pointing in the same direction and fixed in space). The field factors for nonassociating liquids which were determined by On~ager['~'~] are generally used in these expressions [Eq. (9)]. F(m,)=(&d,+2)&,/(&d, +2&,) (9) Zyss and Oudur[",'21 recently analyzed the relation between and x(2)for molecules in various space groups and showed that polar molecules that crystallize in the 1, 2, m, and mm2 point groups have the largest potential nonlinear coefficients and that within those point groups the orientation of the molecule is critical. The coefficientx") is used below to illustrate several further points about the nature of the induced nonlinear polarization. The coefficient is often written as where i, j, k refer to the principal axes of the medium and indicate the tensorial characteristics of x('). The frequency arguments indicate the frequency of the resultant field w3 for input frequencies w2, w , . The minus sign is a convention indicating momentum conservation given by the vector sum 1.3. Macroscopic Nonlinearities Nonlinear optical properties are measured on macroscopic samples that consist of many individual molecules. Although it is possible to infer the values of the molecular hyperpolarizabilities from measurements on macroscopic samples, great care must be taken to determine the values of the internal electric fields, and the propagation characteristics of the generated fields must be carefully consideredE5I. The polarization induced in a medium by an external electric field is given by Equation (5)[@. E is the applied field and the x(")have meanings similar to the molecular coefficients in Equation (2). The odd-order coefficientsx"), x ( ~.).., are readily related to the molecular properties by Equations (6) and (7)[']. where k, (i= 1, 2, 3) is the wave vector given by k,=2n/jl, and points in the direction of propagation of the wave. Thus, for second-harmonic generation the expression (10) would be written x!;$(-2o;w,w), and for the linear electro-optic (Pockels) effect x!;$( - w ;0, w). A variety of non), linear optical effects can occur through x(') and x ( ~ depending on the exact nature of the input frequencies, the proximity of molecular vibrational or electronic resonances to the input frequencies or frequency combinations, and the phase-matching conditions, as indicated by Equation (1 l) and similar equations for x ( ~ A ) .wide variety of conditions can lead to interesting nonlinear effects. Various susceptibility functions and frequency arguments are listed in Table 1 for x(') and x ( ~ ) . Several comments are warranted concerning the information in the table. Since the incident field components can be in or out of phase with the natural electronic oscillations of the molecules constituting the nonlinear medium, is a complex number. Most of the effects listed and all of those discussed thus far occur through the real part Rek). The phenomenon of two-photon absorption occurs through the imaginary part Imkc3)).It is beyond the scope of any single article to discuss the myriad of nonlinear effects occurring through x(') and^'^'['^-'^]. In the fol- x N is the number of molecules per unit volume and F is a local field factor at frequency w which determines the value of the electric field at the site of the molecule[71.The 692 Angew. Chem. Int. Ed. Engl. 23 (1984) 690-703 x"' Table 1. Electric susceptibility functions x'~', for various types of interacting field components (frequency arguments explained in text), terminology used for the effect, and known applications. ~- ~ Susceptibility Effect Application x'*'(O;w, - 0 ) optical rectification hybrid bistable device [13] modulators [14], variable phase retarders harmonic generation [151 parametric amplifiers [161, I. R. up convertors poling x'2'( -w ;0 ,0) electro-optic (Pockels) effect frequency doubling frequency mixing dielectric saturation second-order optical rectification quadratic electro-optic effect AC electro-optic effect AC Kerr effect Brillouin scattering Raman scattering two-photon adsorption AC electro-optic effect AC Kerr effect self-focusing degenerate four-wave mixing frequency tripling tected by a photomultiplier tube. The signal-to-noise ratio can be improved with a boxcar averager. The reference beam is directed into a crystal such as quartz, whose sec- I06pm Nd YAG Laser I I F - variable phase retardation, liquid-crystal displays high-speed optical gates [ 171 optical bistability [IS], phase conjugation [I91 (image processing), real-time holography, optical transistors deep UV conversion Recorder Fig. 4. Schematic diagram of an experimental system for measurement of by the EFISH (electric-field-induced second-harmonic-generation) technique [24, 251. M = mirror, BS = beam splitter, F = filters, PMT= photomultiplier tube. Lens and timing pulses are not shown. ond-order properties are well known, so that fluctuations in beam intensity can be readily corrected in the output data. The value of the nonlinear coefficient is obtained from the ratio of the signals of the sample cell and a reference material such as quartz or LiNbOI with known [Eq. x(*) lowing, we will merely concern ourselves with the origin and characterization of /3 and in organic materials. x(') 2. Experimental Methods l,=the coherence length (see Fig. 6). 2.1. Determination of /3 From a chemical point of view, one aim of research in nonlinear optics might be to synthesize materials with useful nonlinear optical and other properties for specific applications. Characterization of /3 of the constituent molecules is essential for a structure-property based understanding of molecules. A technique has been developed for measuring /3 without necessitating the incorporation of the molecule into noncentrosymmetric crystal structures. In this technique, called electric-field-induced second-harmonk generation (EFISH), a strong DC electric field is applied to a liquid or a solution of the molecules of interest in order to remove the orientational averaging by statistical alignment of molecular dipoles in the medium[41.The induced second-order nonlinearity can then produce a signal at 2 w , from which /3 can be e x t r a ~ t e d ~ ~ ~ - ~ ~ ~ . Figure 4 shows schematically the essential elements of an EFISH e ~ p e r i m e n t [ ' ~ ~ The ~ - ~ ~1.06 ] . pm output of a Nd'+: YAG laser is split and directed into a sample and a reference cell. The sample cell (Fig. 5) is translated by a stepper-motor-controlled stage across the beam. The laser pulse is synchronized with a high-voltage DC pulse to induce harmonic generation in the cell. The 0.53 pm radiation is separated from the 1.06 pm pump beam by filters and a monochromator, and the harmonic intensity is deAngew. Chem. Int. Ed. Engl. 23 (1984) 690-703 I ' I 1 1 n / w -c- AP = Ax ton a _-c + w , Fig. 5. Top view (above) and edge view (below) of a cell used for EFISH measurements. The glass is ca. 3 mm thick and ca. 1 cm long. The gap in which the liquid is confined is 1-2 mm, and the electrodes extend about five times the gap spacing to avoid nonuniform electric fields at the glass-liquid interface. The cell is translated in the x direction with respect to the beam to produce the fringes described in Figure 6. The geometry of the sample cell is shown in greater detail in Figure 5 . The cell consists of two glass rectangles set at an angle a with respect to each other (Fig. 5 , top). Electrodes are placed above and below the glass surfaces (Fig. 693 It t I Fig. 6 . Left: Schematic of an absorption band A (unbroken line) and dispersive refractive index (7, broken line) as a function of the wavelength J. for a typical aromatic ring system. The phase retardation (A@) for light at the fundamental frequency w and harmonic 2w is determined by the dispersion in the refractive index (AT) and the path length travelled (0 (c=speed of light). W l A#=-Aq C For A@=O, 1(2w)/I(w)is a maximum; for Aq4=n/2 the same expression is a minimum. Right: Ratio of harmonic to fundamental intensity when the path length refractive index discontinuities (boundaries of the cell in Fig. 5) is varied in the x direction. The harmonic conversion depends on the phases of the electric tieids a n d thus also o n the path length. The "nonphase-matched" wedge technique can be extended to all nonlinear media (liquids, films, crystals), as long as the sample can be shaped into a wedge or otherwise manipulated to produce a periodic phase-matched condition. The reference sample in Figure 4 could, for example, be a polished quartz crystal with a wedge cut perpendicular to the direction of largest nonlinearity. In order to relate the experimental data obtained from the EFISH measurement to D, it is necessary to consider the expressions for microscopic and macroscopic nonlinearities. Since the experiment generates a harmonic field at 2 0 from the interaction of three waves (two optical field components (E") and a DC' field component (EO)), the induced nonlinear polarization must arise from a third-order nonlinear process P2w= f,,,E" E" E" where fs~kl = y 3 ) ( - 2 w ;w,@,0) We can define an effective second-order coefficient 5, bottom), and the liquid to be measured is poured into the wedge-shaped gap between the glass pieces. Translation of the cell in the x direction produces a path length variation A1 depending on the distance traversed. The need for the wedge geometry and translation of the cell is best understood by referring to Figure 6. The left-hand side of the figure shows the wavelength dependence of the refractive index 77 and the absorption A for an aromatic molecule at the fundamental and harmonic frequencies. A convenient representation of the electric field associated with a light wave is given by Equation (13): Eo is the amplitude, w is the frequency, and #(z) is a phase angle. Since the difference in the refractive index A q at w and 2w generates a phase-angle difference A#, which is also a function of the distance traversed I(z), the fundamental and harmonic fields will change their relative phases over some characteristic distance 1,. Since the flow of energy from fundamental to harmonic field or harmonic to fundamental field is a phase-sensitive process, we would expect to see oscillations in the harmonic intensity as the beam traverses the wedged cell with a defined characteristic coherence length f, (Fig. 6, right). The intensity distribution between fundamental and harmonic is determined solely by the phase relationships that exist at the last liquid/glass boundary. Oudar'25' analyzed the propagation of the various waves in a nonlinear medium with the boundary conditions of the liquid cell. In the absence of optical absorption, the functional dependence 1(20)/Z(w) is expressed in terms of Equations (14a) and (14b). 4 2 w)/l(w)= D + K sin2 ty D and K are calibration f a ~ t o r s [ ~ ~ ~ ~ ~ ~ . 694 ( 14a) dijk = r,J kl E, so that pZ"=d ilk .E".E" resembles a second-order nonlinear optical effect. For a pure liquid we can define a microscopic hyperpolarizability where N is the density of molecules, and F is a local field correction EF(wi)= F (the internal field). The relationship of yo to the hyperpolarizabilities of the molecules Dijk and yiiklrequires a statistical analysis'261,and the result is where the z axis is aligned with the dipole moment of the kT% y, so molecule. For conjugated organic systems pLLDZ/S that with this analysis D has been obtained for a wide variety of organic molecules. 2.2. Kurtz Powder Technique The Kurtz powder techniq~e'~'1is a convenient method for screening large numbers of powdered materials for second-order nonlinear optical activity without needing to grow large single crystals. A laser is directed onto a powdered sample, and the emitted light is collected, filtered, and detected with a photomultiplier tube. The technique is crude in the sense that it detects a convolution of all the Anyew Chem. In[. Ed. Engl. 23 (1984) 690-703 tensor components of x(') and makes little attempt to account for the propagation characteristics of the beams. Nevertheless, general trends can be observed and candidates for single-crystal studies identified. I I phase matchable T ' I (2( +>/<ac> - Fig. 7. Dependence of the second-harmonic intf isity on the dimensionless quantity (r)/(Q,where (r)is the average particle size and (Q is the average coherence length according to the Kurtz powder method [27]. Figure 7 illustrates the application of this technique for phase-matchable and nonphase-matchable powders. Many crystals are birefringent; thus it is often possible to find a direction in a crystal in which a component of the nonlinear tensor ~ f will 2 produce a harmonic wave that propagates with the same effective refractive index as the fundamental beam. This can be seen more easily in Figure 8 (top), where the refractive index for a uniaxial crystal is an ellipse. A light wave propagating through the crystal in a direction s has allowed polarization components at the major and minor axes of the ellipse perpendicular to s. One of these components qo (ordinary component of the refractive index) is independent of the direction of s, and the other depends on the angle 8 between s and the y axis. A similar ellipse with different dimensions could be drawn at 2w. If the two ellipses intersect, the possibility of a phasematched tensor component exists. The lower plot indicates that an angle Q, exists for phase matching. It is possible to tell if a crystalline powder is phase matchable from a study of Z(2w) versus particle size (Fig. 7). Since phase-matched second harmonic generation is extremely efficient, the random particles which fortuitously sit at the phase-match angle provide most of the signal when the normal coherence length for nonphase-matched harmonic generation is exceeded. Great care must be used in applying this technique quantitatively, since the results are particle-size dependent. 3. Second-Order Nonlinear Optical Properties As was indicated earlier, in the design of materials with large second-order nonlinear optical coefficients and other useful properties, the molecular hyperpolarizability must be optimized and then oriented in a medium (polymer, liquid, crystal) so that the propagating field can be transmitted from the medium. The electronic contributions to /3 and various approaches for obtaining the required noncentrosymmetry in the medium are discussed in this section. 3.1. Molecular Hyperpolarizability Molecules containing conjugated 71 electronic systems with charge asymmetry exhibit extremely large values ofg. The largest values are obtained when the molecule contains substituents that lead to low-lying charge-transfer resonance states (Fig. 9). The polarization can be visualized 2 Fig. 9. Ground-state and lowest energy polar resonance forms for p - and osubstitution. Resonance is forbidden in the case of m-substitution. X I 2w for w AFig. 8. Top: Refractive index ellipsoid for a uniaxial medium. s indicates the direction of an incident beam of light, and the semimajor and semiminor axes of the ellipse formed by the plane perpendicular to s with the refractive index ellipsoid define the allowed polarization directions for the propagating polarization. The refractive indices corresponding to those directions are the qo (ordinary) and ~ ~ ((extraordinary) 0 ) components. The value of q,(8) depends on the angle between s and the x-y plane.-Bottom: If a direction in ( 0 type ) ~ ~of, phase matching the medium can be found where ~ ~ ( m ) - ~ ~ one IS possible and large conversion efficiencies can be expected: 8, is the phasematch angle. Angew. Chem. Int. Ed. Engl. 23 (1984) 690-703 as arising from contributions of the substituent-induced asymmetry of the n cloud and the o skeleton of the molecule, as well as from field-induced mixing of the excited state (polar character) into the ground electronic configuration. The contributions to /3 have been separated into two parts for n i t r o a n i l i n e ~ [ ~[ Eq. ~ ~ '(21)]. ~~ is an additive part due to substituent-induced asymmetry in the charge distribution and DcT. is the chargetransfer term. Table 2 lists experimentally determined values of for various nitroanilines and the monosubstituted parent corn pound^^'^^. Also listed are values for P a d & obtained by simple vector addition of the experimentally determined values of for nitrobenzene and aniline, and Pa, obtained by a quantum mechanical calculation based on a charge-resonance two-level model described below. Note Padd 69 5 Table 2. Experimental values of p for various nitroanilines as well as nitrobenzene and aniline. Also listed are the additive determined from summing the values for aniline and nitrobenzene and the charge-transfer contribution bCTdetermined by the two-level model. 34.5 3.4 19.6 8 10.2 1.7 10.9 6 3.3 4 2.2 - - PhOH 0.f 1.1 the general agreement between the experimentally determined values for the disubstituted systems and the results of the calculations. Also note the obvious importance of the charge resonance structure, which, according to theory, should make its strongest contribution to /I in the p-nitroaniline derivative. In order to understand the origin of the additive term, it is useful to consider briefly the equivalent internal field model (EIF) for p in monosubstituted organic molec u l e ~ [ ~In~ ]this . model the charge asymmetry in the 71 system arises from the inductive effect of the substituent on the 7c system, leading to a “mesomeric” dipole moment A ~ L in the aryl ring; Ap is proportional to an internal field Eo arising from the substituent [Eq. (22)]. some disagreement regarding the validity of the procedures for obtaining ApF4],the agreement between experiment and theory is good and is supported by more-refined estimates of the induced dipole moment in the 7c systemr3’]. The certainty of the procedure for obtaining the additive components of in Table 2 seems reasonable in view of the success of the E I F model for monosubstituted benzene derivatives. The vector-addition procedure assumes independent influences of the substituents. The second term in Equation (21), pcT, has been accounted for by various methods and degrees of sophistication ranging from Pariser- Parr-Pople c a I ~ u I a t i o n s ~to~ ~ ~ ~ ~ 1 CNDO(S) studies with configuration interaction[331and other SCF-LCAO but a particularly useful approach for the physical organic chemist is the perturbation approach employing a two-level modelrz9].In this model the pcr term can be described in terms of a ground and first excited state having charge-transfer character (Fig. 9) and is related to the energy of the optical transition W, its oscillator strength J and the difference between groundand excited-state dipole moments Apg,, by Equation (27). F(w) = + P=a Eo+ Y E : ( a + 3 y E i ) E + 3 YE,, E Z + y E 3 (24) Chemical substitution on the ring therefore slightly modifies the linear polarizability (term 3), with appearance of a quadratic response (term 4). p, therefore, is given by the fourth term as and by substitution from Eq. (22) to Fig. 10. L,og/log plot of ,O versus the “mesomeric” dipole moment for some monosubstituted benzene derivatives [25]. Thus when a n external field is applied, the total field is ET = E + Eo, and Equation (2) becomes and by substitution I 1 Ap I D 1 --t “ “ ~ N o 2 “N&No2 1 W [ w-(2h w)Z] [ w2- (h w )‘1 The term F(w) accounts for dispersion and enhances pcT as the fundamental frequency ( 0 ) and harmonic (2w) approach the energy of the charge-transfer function. Good agreement was obtained between the value of pcT predicted by this expression and penp(assuming Pexp=pcT)for a series of disubstituted benzene and stilbene derivatives’251 (Table 3). Morrell and A l b r e ~ h t [tested ~ ~ ] the two-level model for pnitroaniline by calculating fi from CNDO(S)-determined electronic states using limited configuration interaction. They found that on using only the ground and first excited charge-transfer state, 75% o f the value of pexpcould be accounted for. They obtained/Ic.T= -25.1 x lop3’ esu by this procedure. Inclusion of 60 excited configurations to more closely approximate the eigenstate associated with the first excited state gave -34.3 x esu, which is in excellent agreement with Pexp.For the more extensively conjugated systems, the charge-transfer state is lower in energy with respect to other excited configurations, and we would o= Figure 10 shows a plot of experimentally determined values of p for a variety of monosubstituted benzenes versus the “mesomeric” dipole moment A,u[’~].Although there is 696 Angew. Chem. Int. Ed. Engl. 23 (1984) 690-703 Table 3. Charge-transfer contribution BcT, calculated from the two-level model, and experimental values& for a series of benzene and stilbene derivatives. 19 02N%No2 34.5 227 260 383 450 217 220 715 650 ance contribution is evident from the table. The reason for the low value of ,up for 6 (Table 4) is not clear, but the S atom may perturb the system in such a way that neither resonance form is predominant. Based on the theoretical notions described above, the effects of substituent patterns on a given chromophore are readily correlated with the electron donating and accepting strength of the substitution. Table 5 presents data on the Table 5. Longest-wavelength absorption band and fl for a series of p-disubstituted benzene derivatives. expect the two-level scheme to agree even better with experimental data. The data in Table 3 appear to confirm this. The success of the two-level model makes chromophore selection simple in many cases. If a molecule can be viewed as a superposition of resonating electronic structures with charge migration in the same direction along the molecular axis, should be large. By this procedure, merocyanine dyes such as 1[351 were shown to have the largest CN 232 (2.92) CHO 241 (4.83) (5.67) COCH, NO2 269 (13.34) 280 (9.12) 3 I4 (17.35) 310 (2.40) 378 (47.67) 297 (14.24) 352 (19.5) 41 8 (52.75) effects of various substituents on /3 and ,Icv Except for paminoacetophenone, the trends in ,Im and p with the nature of the donor and acceptor are consistent with contribution of the charge-transfer state to p. known molecular hyperpolarizability, and a noncentrosymmetric crystal containing the related dimethylaminostilbazolium chromophore 2[5,361 was shown to have the largest known value of by the powder method. The product ,ub was measured for a series of merocyanine-like compounds (Table 4). The importance of the charge reson- Table 4. Product of the dipole moment andp for molecules related to the merocyanine structure with the wavelength of the long wave absorption band an. Dulcic et a1.[37,381 investigated the dependence of p on the length of the conjugated 71-system for the compounds 9a 9c; they found it obeyed an empirical law [Eq. (28)) k = 2, n =number of conjugated double bonds in 9 . Although there appears to be a discrepancy in the sign ofp, later experimental work reports the sign as being (-); the data in Table 3 therefore confirm this. The two-level model has generally been successful in accounting for the features of various classes of molecules, but caution should be ercercised when it is resorted to for predictions, since fortuitous combinations of ground- and excited-state electronic properties could exist where 0. This should , has not only a magnitude but also a be apparent, since B sign. 4 760 570 6 50 560 3.2. Crystals and Polymers with Large x(*) 7 370 408 From the regularities observed in p for various chromophore types, substituent patterns, and conjugation lengths, it is clear that a wide variety of molecular structures would exhibit extremely large second-order susceptibilities if they Angew. Chem. Int. Ed. Engl. 23 (1984) 690-703 697 could be incorporated into noncentrosymmetric structures. In this section various approaches to achieving this will be discussed. x ‘“[esu 10-9 I - KDP -LiNbO, - -GaAs LiNbO, (E.0.) -1nSb nents have been studied in detail for the five crystals indicated by the arrows closest to the scale. The principal tensor component was determined for p-dimethylaminonitrostilbene doped into a liquid-crystalline polymer, and the remaining materials were studied as powders. Several points should be noted regarding Figure 11: Organic systems exhibit nonlinear coefficients which can exceed the best values for inorganic dielectrics and semiconductors. Furthermore, they are generally more useful because of increased transparency and resistance to laser damage. Although a number of crystalline organic materials have sufficiently large values ofX(’) to be useful in integrated optics and for other applications, so far no polymer or other thin-film has been shown to have values of x‘”(E0) approaching that of LiNb03. Considerable research on polymeric and non-polymeric organic materials remains to be done if technological utility is to be realized. Finally, the largest known value of (estimated by powder measurements) has been realized in an organic material. The significance of the magnitude of x(‘) immediately becomes apparent in equation (29), i.e. the equation for the efficiency of harmonic conversion by a plane wave propagating through a nonlinear material in the limit of low conversion[43! x(’) x(*) 10-5 where Fig. 11. x‘’’ scale for various inorganic crystals (right) and organic crystals and films (left). (a) R* = methoxycarbonylethyl. (b) Dye aggregate formed from an indolinobenzospiropyran and its ring-opened merocyanine form [39, 401. p-Dimethylamino(nitro)stilbene was investigated in a poled liquid crystalline polymer. The arrows to the left and the right indicate the values of the substances. Figure 11 depicts ax(’) scale with values of the susceptibility, generally obtained from second-harmonic-generation measurements for various organic and inorganic materials (for comparison). InSb has one of the largest known value of x”),but, being a semiconductor, it is highly absorbing in the visible region of the spectrum and is therefore not useful for many applications. LiNb03 is the most extensively studied material for device applications involving the linear electto-optic effect. It suffers, however, from degradative photorefractive effects, which can cause history-dependent performance in various device applications. Potassium dihydrogen phosphate (KDP) has been used widely for phase-matched S H G of high-powdered near-IR lasers. In LiNb03, x(”(SHG) results from electronic polarization, whereas x(”(E0) results from polarization of the positions of the nuclei. In organic n electronic systems, x(’)(SHG) and x(”(EO) both result from polarization of the TI cloud, resulting in nearly identical values for the two effects. The measured values of ~ ( ’ ) ( - ~ o ; w , w ) ~and ~’~ ,y(’)( -@;LO, O)f4’)for 2-methyl-4-nitroaniline (MNA), which are 540 x lo-’* and 500 x lo-’‘ esu, respectively, verify that the n electronic system is the primary contributor to both effects. O n the left side of Figure 11 are values for various organic materials. The nonlinear optical tensor compo- x(’) 698 P= (x‘”)’W’Z(W) ?73C3Eo Here, I is the path length, w is the fundamental frequency, q is the refractive index, c is the speed of light, is the dielectric constant of a vacuum, and Ak (the phase mismatch) is w/cAq. The sinc(x) function is defined as sin x -= X sinc x and sincx approaches 1 as x approaches zero. Since the conversion efficiency is proportional to &(*))’,a crystal of the type shown at the bottom of Figure 11 could b e ca. lo7 times more efficient than KDP or urea, which are normally used for megawatt lasers. A material of this type could provide large conversion efficiencies for solid-state diode lasers (= 10 mW power) if the appropriate nonlinear optical characteristics could be incorporated into the crystal. Thus, nonlinear optics could be extended to very lowpower lasers. In designing technical devices many other advantages could be realized with coefficients in the esu range. Another important consideration is brought out in Eq. (29): the implications for phase matching. At the phase-match condition Ak = 0 or p20 - qm= 0, sinc2(x)= 1, and the conversion efficiency is simply related to x‘”1’. (Other beam propagation characteristics, however, may lead to “walk o f f ’ or spatial divergence of the interacting fields, thus limiting the harmonic conversion.) If Ak # 0 the function oscillates as indicated previously. For Gaussian Angew. Chem. lnl. Ed. Engl. 23 (1984) 690-703 beams and real crystals the situation is somewhat more complicated, but the same basic behavior, as illustrated by the plane-wave case, is followed. In the following sections the properties of several of the materials in Figure 11 that illustrate key aspects or problems in second-order nonlinear materials will becommented on in more detail. 3.2.1. 2-Methyl-4-Nitroaniline (MNA) The crystal growth and properties of MNA have been studied in detail by Leuine et aLC4'],who determined from harmonic generation studies, and by Lipscomb et who measured the electro-optic coefficients. Earlier studies of B for p-nitroaniline[''' indicated that a noncentrosymmetric crystal containing the chromophore in an appropriate geometry could have very large x('). It was hypothesized that a bulky substituent creating a sterically asymmetric ring could lead to the required crystal structure, and this was indeed verified by examination of a variety of monosubstituted p-nitroaniline derivatives. From powder measurements, MNA was identified as a candidate for single-crystal growth, and vapor transport and solvent growth techniques were used to obtain the crystals. The unit cell structure is illustrated in Figure 12. The Cc space group, which is contained in the m point group (monoclinic), has the unit cell symmetry elements indicated. Note that the polar axis P has a projection of the dipole moment onto the P axis determined by 0 and $. various nonlinear coefficients on the geometry of the molecule in the unit cell. For MNA the coefficient should be maximized when the angle 0 is 90". This should be apparent, since for a small dihedral angle $ the projection of the polar charge-transfer axis will be maximum when it is parallel to the crystal polar axis. therefore gives a very large electro-optic but since the polarization and field directions are colinear, this could not possibly be a phase-matchable coefficient for SHG. xzyy, on the other hand, is a phase-matchable coefficient and for this space group is a maximum for 0=54", $ = O : Oudar by a facand Zyss's analysis['21predicts an increase in tor of 3.5 and more than an order of magnitude greater conversion efficiency for optimum orientation of an MNA type molecule over that actually observed [41, 421. Although the relationship between crystal structure and second-order nonlinear properties is complex, the details have been elucidated and summarized['21.The maximum achievable value xtzax)as a fraction of what one would predict from the oriented rigid gas model described earlier is given in Table 6. x"' Table 6. Polar point groups and maximum fractional value of the rigid oriented gasX(') for a phase-matched interaction and for optimum molecular orientation in the unit cell. Point group X li,',,, 1,2, m, mm2 62M, 6 , 3 , 3 M , 32 222,6mm, 6,4mm, 42M, 23,43M 4 0.38 0.25 0.19 (no possible phase interaction) 3.2.2. trans-4'-Dimethylamino-N-Methylstilbazolium Methyl Sulfate (DSMS) The title chromophore, DSMS 2, X=H3COS0F, was selected according to the principles outlined at the end of Section 3.1. It contains extremely electron-rich and electron-deficient groups linked in a trans-stilbene fashion, leading to an expanded conjugated n-system. The possibility of metastasis of the counterion suggests investigating a variety of steric and coulombic influences on the crystal structure of a common chromophore. SHG harmonic intensity measurements are shown in Table 7 for a variety of r n a t e r i a l ~ [ ~The . ~ ~caution ~. with which data of this type must be viewed is illustrated by comparison of m-nitroanil- Fig. 12. View of the unit cell of 2-methyl-4-nitroaniline (MNA). The space group Cc is isomorphous with the C, point group, which has a polar axis P and mirror plane m. B and @ are 70 and 8",respectively: x, y, z are the dielectric principal axes, m is a pseudo-mirror plane. The large projection of the molecular polar z axis onto P makes this an ideal candidate for electro-optic applications. Structural data from [12]. Oudar and Z ~ S S [have ' ~ ] analyzed the relationship between the various components of the hyperpolarizability tensor Pijkand the macroscopic nonlinear coefficients xfd for various crystal symmetries. Their objective was to determine the space groups that would give the largest achievable nonlinearities as well as the dependence of the Angew. Chem. Int. Ed. Engl. 23 (1984) 690-703 Table 7 . Harmonic intensity (relative to m-nitroaniline) determined by the Kurtz powder method. Crystalline powder a-Quarz m-Nitroaniline M NA 1 "( +)-Campher sulfonate' 2" 10 2 @lop 2' NO: 2" C6HsCH=CHCOF 2" c10: 2" BF," 2" ReO: 2 " CH,OSO$ Relative harmonic intensity 0.008 I 3 0.007 0 0.01 0.5 1.5 5 10 18 30 699 ine and MNA. Their coefficients x(') differ by a factor of about 3, yet their harmonic intensities differ by a factor of only about 3 rather and not-as we might expect from Equation (29)-by a factor of 9. Since no attempt was made to classify particle sizes, and the powder technique is known to be very dependent on this and other factors, variances of this sort might be expected. Nevertheless, the DSMS powder showed an order of magnitude higher conversion efficiency than MNA, which has the largest known value reported to date for a crystal. This constituted justification for growing DSMS single crystals and characterizing the linear and nonlinear properties of the crystal. Preliminary crystal structure studies have shown that the crystal is described in the mm2 point group, for which a large value of is expected (Table 6). Furthermore, the molecular dipolar axis is tilted at an angle of 34" with respect to the twofold screw (polar) c axis. Since a tilt axis of 54" 74' would be optimum for phase-matched SHG and 0" for the linear electro-optic coefficient["], the crystal structure is a compromise and is expected to give large values for both effects. Table 8. Harmonic intensity (relative to that-of urea) for a series of powders. The numbers in parentheses are relative to m-nitroaniline and are obtained by comparing the MNA values in Tables 7 and 8 and applying the appropriate multiplier to the values in Table 8. Crystalline powder Rel. harmonic intensity 1.0 (0.13) 10 8.8 (1.20) 11 7.5 (1.02) 12 10.0 (1.36) 13 32.0 (4.36) 14 22.0 (3.0) 15 80.0 (10.9) 16 115.0 (15.6) 17 80.0 (10.9) 18 70.0 (9.55) x(') x'*) 3.2.3. Materials Optimized for SHG with Semiconductor Lasers Twieg and exhaustively studied molecules containing the nitroaniline and related chromophores for SHG activity by the powder technique. Their objective was to identify powders with large SHG conversions and transparency suitable for use with 0.80-1.5 pm lasers. Table 8 lists harmonic efficiency measurements for powders of various classes of substances. Generally, only the compound showing the largest efficiency for a particular class is shown. For instance, 10 is a substituted urea derivative, 11 a push-pull alkene, 13 a MAP analogue, etc. Given the uncertainties of the powder technique as applied here, any of these compounds would be reasonable candidates for further study, depending on factors such as the ease of crystal growth, oxidative stability, toxicity, etc. Compound 19 is particularly interesting because its powder efficiency is about half that of the stilbazolium salt mentioned previously, yet it is very transparent in the visible region owing to the blue shift attributed to the nitrogen heteroatom (cut off sz 430 nm). 19 (xx) to the z polar axis. A film with these properties would have an electro-optic coefficient proportional to ziti, and phase matching for SHG could be achieved through the smaller Alternatively, harmonic generation could be achieved by wave guiding, which would impart additional flexibility in phase matching. The advantages of wave-guided SHG[471will not be discussed here, but a tremendous potential advantage of polymer films over crystals lies in the fabrication of thin films for wave-guided nonlinear optical devices. - Paled film Unp o Ie d rigid film pmnnnrnm ' L 4'1 1 3.2.4. Molecularly Doped and Aligned Polymers A possible route to preparing polymeric films with large second-order nonlinear coefficients is to remove the orientational averaging of a dopant molecule with large /?by application of an external DC electric field to a softened film. This might be accomplished by heating the film above the host polymer glass-transition temperature T,, then cooling the film below T, in the presence of the external field. The poling would provide the alignment predicted by the Boltzmann distribution law and impart C," symmetry to the film. Such a process is illustrated schematically in Figure 13. For this particular symmetry and i.e. there are two nonvanishing coefficients, incident beam polarized parallel (zz) and perpendicular x!:; 700 160.0 (21.8) xi;;, 7 I Polar axis 2 Fig. 13. Schematic of imparting C," symmetry (polar axis, isotropic perpendicular to the polar axis) to an isotropic medium containing permanent dipoles with an external electric field /:. To predict the value of x'') after poling, one needs to evaluate the expressions (32) and (33). (32) (33) Angew. Chem. Int. Ed. Engl. 23 (1984) 690-703 These expressions resemble that for the oriented rigid gas [Eq. (7)], where F is the total local field-factor correction, 0 is the angle between the molecular polar z axis and the laboratory (field-induced) z axis, and $ is the projection of the molecular z axis onto the laboratory x axis. Since these projections are induced by an external field, which competes with thermal randomizing effects, the Boltzmann thermal averages of these projections must be computed [Eqs. (34) and (35)][“‘]]. (34) (35) ,u is the dipole moment of the dopant molecule and E is the applied DC field. The ((cos~))is isotropic, since no polar alignment is induced orthogonal to the field direction. culations and the experimental complications. Nevertheless, the experiments demonstrate that electric-field-induced alignments that can be retained in the film upon removal of the field can give rise to large second-order nonlinear coefficients. 3.2.6. Field-Induced Alignment of Quasicrystalline Dye Aggregates A final example of a noncrystalline but highly ordered system where SHG can be induced by an external field is provided by “quasicrystals” obtained from the photolysis of indolinobenzospiropyrans in nonpolar solvents in the presence of an external field[”]. Upon irradiation, the spiro form (SP-A) undergoes a sequence of photochemical steps[511 leading to the formation of complexes A,B, which in turn form polar aggregates. The insoluble aggregates 0” L 230 DANS f C H z c=o - F j OR’ * * * f C H 2 -c=o r t rn OR‘ R’ = fCy),-O-@COO-@N Rz= fCH2),-0-@COO-@OCH3 U 0 I c=o I H,c/‘+c Hp SP-A I c=o I 2% . CH2 H3C SP-B m ~ 0 . 5 n = 0.5 Fig. 14. Components of a molecularly doped liquid-crystalline polymer film oriented by an external field and which exhibited large SHG [46]. The principles outlined above were used to induce a second-order nonlinearity in p,p‘-dimethylaminonitrostilbene (DANS) doped at 2 wt% into a liquid-crystalline polymer host (Fig. 14). The host polymer belongs to a class of polymers in which mesogenic chromophores are attached via a flexible (CH2),-spacer to the rigid backbone[481.It was shown that polymers of this general structure, which exhibit a nematic mesophase above T,, could be aligned by an external field, eliminating the light-scattering nematic texture, and that the alignment was retained upon cooling beshowed a low Tg DANS, for which 8=4.5 x lo-*’ es~[~’], pleochroic effect in the polymer host; the molecular z axis tended to align with the long axis of the mesogens, causing it to exhibit linear dichroism in its absorption band. Under these circumstances the electric-field-induced polar alignment, as opposed to the axial alignment associated with pleochroism, requires the thermal averages discussed above. If Ising model statistics are assumed to describe polar alignment in an axial environment, Equation (32) must be multiplied by a factor of 5 . Equation (36) then holds. was measured as 6 x lo-’ esu for an alignment field of 0.6 V/pm, while xL:L(Ising) was calculated to be 3 x l o p 9 esu. This is remarkable and perhaps coincidentally good agreement in view of the approximations made in the calAngew. Chem. Int. Ed. Engl. 23 (1984) 690-703 can be isolated between the electrodes used to apply the fields and show threadlike morphologies along the field lines. In SHG studies on threads formed in the presence and absence of external the following observations were made: Threads formed in the absence of a field exhibit no SHG. SHG could be induced in these samples when DC external fields were applied. Threads formed in the presence of electric fields exhibit strong SHG when measured with radiation polarized parallel to the threads, but the SHG was considerably weakened with light perpendicular to the threads. In each case these measurements were carried out with the field removed. A larger SHG response was detected when a secondary field was applied parallel to the threads. The details of these observations led to the proposal of a model consisting of stacks (Fig. 15) of near-neighbor interacting dipoles described by the following parameters: 8 is the angle made by the dipoles with respect to the stack axis, r is the distance between dipoles, U ( t l ) and U ( t t )are the interaction energies between antiparallel and parallel neighbors, respectively, and U ( t ) and U ( l ) are the dipolar interaction energies in the direction of and against the field, respectively. The model predicts that for 8< 54”44‘ the parallel configuration will dominate. The spectral red shift of the aggregates is consistent with the parallel coupling of the induced dipoles[501;if they are almost parallel 701 I / / 'J Fig. 15. Top: Parameters for a stack of molecular dipoles separated by r and making an angle 0 with respect to the stack axis. Only the near-neighbor interaction energies U ( f t ) = U ( l l ) and U ( t l ) = U(i(lt) and dipolar energies against ( U ( l ) )and in ( U ( t ) ) the direction of the applied field are considered. Bottom: For 8<54"44', long sequences of parallel dipoles are favored. A field increases the relative number of dipoles in the direction of the field ( W denotes the appropriate probability distribution [39]). to the permanent molecular dipoles, the red shift is a strong indicator of a parallel arrangement of molecular dipoles. The model uses the classical partition function to calculate the probability distributions W ( t t ) , W(lT),W ( t ) , W(1). For aggregates formed in the absence of a field and with O < 54" 44', randomly oriented aggregates consisting of arrays of parallel dipoles with occasional changes in phase would be expected. This is consistent with the lack of SHG observed for samples formed in the absence of a field. When a field is subsequently applied, a signal with a quadratic field dependence due to movement of the phase boundary is expected and indeed observed. For aggregates formed in the presence of a field we would expect not only a much larger signal, owing to preferential orientation of the aggregates as they are formed, but also a quadratic field-dependent signal ; these features are in fact observed. The overall agreement between experiment and theory is gratifying for such a complex system. Owing to the particulate nature and size of the aggregates, however, it was not possible to extract a value for x(') for this system. 4. Outlook Although considerable progress has been made, both in understanding the electronic origins of molecular nonlinearities in organic TC systems, as well as in our ability to explain quantitatively the relationship of molecular arrangements in a crystal or polymer to the observed nonlinearities a posteriori, the design and fabrication of new systems with optimized nonlinear optical properties, linear optical properties, and adjunct properties remains a major challenge for chemical research. The major areas of solidstate chemical research that will affect this field are crystal 702 design and growth, crystal thin-film technology, and polymer thin-film technology. The design of crystalline materials that lend themselves to the growth of high-quality, large single crystals at a reasonable cost is a major challenge. Since the categories of application for such materials span a wide variety of requirements in terms of device design (frequency doublers, frequency mixers, parametric oscillators, etc.), available laser power, transparency, and cost, no attempt will be made to enumerate a specific set of requirements as a target for future research. One special case worth mentioning, however, is the use of urea crystals to extend the range of usability of dye lasers to 240 nm and double the output of the popular bluelgreen argon ion laser at 480 nm via SHGi5']. Due to its phase-matching characteristics, urea exhibits high SHG efficiency relative to the popular inorganic crystals; furthermore, it has a high nonlinear coefficient, and it is possible to grow large high-quality crystals that have high damage threshold and can be cleaved and polished. These characteristics have spawned commercial development efforts for nonlinear crystals of urea. In contrast, dimethylurea crystals[531,although they extend the pbasematching range even further into the UV and have other desirable optical properties, nevertheless have poor mechanical properties, which make them unsuitable for manufacturing. In addition to the applicability of single-crystal technologies, a variety of opportunities exist for thin-film structures where planar processing techniques might permit deposition onto Si o r other substrates. LargeX") or^'^) values might permit the fabrication of the active components for optical circuitry, making use of the nonlinear refractive index of such materials. Three promising approaches have emerged for obtaining the required asymmetry for large x('): 1) Molecularly doped polymers with electric field alignment, 2 ) topochemical polymerization of asymmetric diacetylene monomers[541,and 3) Langmuir-Blodgett multilayer techniques[541.The first approach offers considerable flexibility in terms of design and selection of materials, but requires that permanent alignment be imparted by application of an external electric field. The second approach is still beset with problems regarding single-crystal growth and the restrictions of the highly conjugated polymerized diacetylene chain. Langmuir-Blodgett techniques may offer the ultimate in chemical design flexibility but require extensive resources and expertise. Many of the electro-optic functions occurring through could also be achieved through the quadratic electrooptic effect if materials with sufficiently large x(3)could be developed without the stringent symmetry requirements of the linear electro-optic materials. Thin films containing the polydiacetylene chain formed by Langmuir-Blodgett techniques have one of the largest known values of x(3"551 (-lo-" esu), which may be useful for certain applications. An additional advantage of Langmuir-Blodgett films is that radiation-induced polymerization leads to a convenient method of patterning that might be used for device fabrication. Although considerable progress has been made in this multidisciplinary field over the past several years, much creative chemistry still remains to be done if broad tech- x(') Angew. U i e m . In[. Ed. Engl. 23 (1984) 690-703 nological utility of organic/polymeric, nonlinear optical materials is to be brought to fruition. 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