All-Optical Poling of Polymers for Phase-Matched Frequency Doubling

Chapter 18

All-Optical Poling of Polymers for Phase-Matched Frequency Doubling

Downloaded by UCSF LIB CKM RSCS MGMT on November 21, 2014 | http://pubs.acs.org Publication Date: August 11, 1995 | doi: 10.1021/bk-1995-0601.ch018

J. M . Nunzi, C. Fiorini, F. Charra, F. Kajzar, and P. Raimond Groupe Composants Organiques, Laboratoires d'Electronique et de Technologies de l'Information, (Commissariat a l'Energie Atomique—Technologies Avancées), Centre d'Etudes Saclay, F91191 Gif sur Yvette, France

The resonant nonlinear optical process which permits the all-optical preparation of noncentrosymmetric materials for frequency doubling is described in terms of a polar orientational photoinduced effect occuring into the centrosymmetric distribution of molecules. A permanent optical poling of acrylic copolymers is efficiently achieved by seeding -type preparation with a dualfrequencylaser. Nonlinear chromophores undergo a net light-induced rotation resulting from reversible izomerization cycles. Optimisation of the preparation conditions in thin films yields afrequencydoubling second-order susceptibility as high as using the corona-poling technique. The polarization lies in the film plane. This opens a new direction towards the molecular engineering of transparent phase-matched oriented micro-structures for efficient frequency conversion.

The multi-functionality of organic materials which is offered by organic synthesis diversity as well as by physico-chemical tailoring yields an infinity of possibilities for the optimisation of material properties from the molecule to the device. However, the practical realisation of devices forfrequencyconversion, with an aim at building a compact blue laser source, is still a major technical problem. The main routes explored for the realization of compact blue-light sources are electro-luminescence, fluorescence up-conversion andfrequencydoubling (SHG), but up-to now, only frequency doubling has proved useful for an efficient blue light generation. In this respect, a large collection of non-centrosymmetric molecules has been engineered and tested [7], but there still remains major difficulties withfrequencydoubling. 1. A polar orientation of molecules is essential in order to achieve noncentrosymmetric assemblies of molecules with a non-zero %(). 2. The second harmonic wavelets generated at different places along the propagation direction of the fundamental beam must all interfere constructively. 2

0097-6156/95/0601-0240S1ZOOA) © 1995 American Chemical Society In Polymers for Second-Order Nonlinear Optics; Lindsay, G., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1995.

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Polar ordering is not a universal tendency of molecules. Molecules indeed generally arrange into centrosymmetric structures which minimise the electrostatic energy of dipoles. Polar ordering is naturally achieved by means of the short range interactions between molecules which take place in some specific supra-molecular assemblies such as crystals, Langmuir-Blodgett films, liquid crystals and ferroelectrics. Those effects have already been reviewed with the aim at optimising the related nonlinear optical properties. This led for instance to ideas such as using so called octupolar molecules which, by symmetry, have zero-permanent dipole moment [2]. For the sake of processability, compatibility with other technologies and ease of tailoring, it is interesting to use polymers for nonlinear optics [3]. However, polar orientation is not spontaneous in dispersed media such as liquid and polymer solutions. It is usually achieved by using the so called electric field poling method. It can be performed with molecules having a strong permanent dipole moment. Molecules align their dipole moment parallel to an applied static-electric field EJX: in order to minimise electrostatic energy. Starting from an isotropic solution, this leads to a net molecular orientation such as the one pictured in Figure 1 in which noncentrosymmetry manifests simply as the physical distinction between upward and downward orientations. Orientation

UP

hole-burning:

DOWN

Figure 1: Polar orientation resulting from an oriented selection among an isotropic set of arrows. Thin and bold arrows represent molecules with different second order hyperpolarizabilities p. An interesting question is related to the possibility of using light-matter interactions in order to orient molecules. For instance, light beams can be used to draw wave guides using their bleaching effect, as well as to promote local heating in order to pole locally using the electric field poling technique [4]. However, no polar orientation is achieved by monochromatic optical fields E^. They change sign periodically at rates as high as 10 Hz. Either polarized, they only induce optical anisotropy in molecules which have an anisotropic polarizability (Fig. 2). This leads to photo-induced dichroism or birefringence. No polar orientation is achieved because the net effect is quadratic in field. Indeed, the field itself has no polarity. The excitation process is called polarized hole-burning [5]. Another question now is: how can we perform an orientation holeburning such as in Figure 1 ? 15

In Polymers for Second-Order Nonlinear Optics; Lindsay, G., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1995.

242

POLYMERS FOR SECOND-ORDER NONLINEAR OPTICS

Polarized hole-burning: ^ —

Ground-state molecule


^mmm Excited molecule

Polarized light:

Figure 2: Photo-induced anisotropy using vertically or horizontally-polarized monochromatic light-beams. The discovery that combinations of optical fields could exhibit polarity opens up new possibilities for optical handling of molecules [6]. A standard situation in which an optical field exhibits polarity is the coherent superposition of a field at a fundamental frequency co with its harmonic E at frequency 2oo. It is pictured in Figure 3. Polarity of the cubic interferences appears in the non-zero average cube t where the subscript t holds for time average. 2(Q

The sign of the polar interference is determined by the relative phase between co and 2© beams. The fact that in condensed materials fundamental and harmonic frequency propagation-speeds b different make the optical polarity alternate between positive and negative extrema. In a dispersive material with index AI(GO), the periodicity of this alternance is )J2(n((Q)-n(2(Q)) were X is fundamental wavelength. This is precisely the periodicity which is in principle necessary in order to achieve a quasi phase-matching forfrequencydoubling [7]. The observation of a second-harmonic generation in optical fibres prepared by an intense light at 1064 nm [8] or with a simultaneous seeding light at double frequency (532 nm) [9] revealed the possibility of inducing a noncentrosymmetry using light in a centrosymmetric material. It is the first reported application of a mechanism involving cubic interferences with * 0. In order to build the molecular engineering rules which permit a control of this effect, it is necessary to identify efficient processes by which condensed materials record . 3



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Figure 3: Polar cubic interference between fields at fundamental and harmonic frequencies. 3

effects find a simple description in the case of a solution of molecules excited under resonant conditions. The origin of the nonlinearity can then be pictured in terms of a hole burning into the distribution of molecular orientations. Let us consider a two-level molecule. Under one-photon excitation at 2CD, the projection of its wave function on the excited state is proportional to (p .£ ) where p is the transition dipole-moment between ground and excited states. Under two-photon excitation at co, the projection of its wave function on the excited state is proportional to (p .£ )(8p.£ )/^co where 8p is the dipole-moment difference between ground and excited states. A molecule which is irradiated at resonance with fields E and 2S2GO> respectively at fundamental and harmonic frequencies, experiences a transition probability to the excited state which contains the one and two-photon interference contribution: (p .£ )(p* . E*)(8p.£*)/â) [10]. In this expression, brackets represent scalar products between vectors. This interference term which is proportional to 7s depends on the polar orientation of the molecule via 8p. The process involves the microscopic polarizability tensor p of the molecule. The average value of this cubic interference over molecule orientations is zero in isotropic mixtures. That means that the cubic interference is not associated with an index grating. However, absorption is preferentially directed towards molecules oriented either upward or 01

01

a)

2(0

01

(B

m

oi

2m

i J

3

downward, as in Figure 1, depending on the phase of the product 7s 7s* . There results a net noncentrosymmetry. The overall process of second harmonic generation which is pictured in Figure 4 can be viewed as a six-wave mixing processes [77]. The 2a)


244


2


process consists in the record and readout of a %() grating caused by the interference [12]. The grating is a quasi phase-matched one and unlike with usual quasi phase-matching techniques, there is no simultaneous induction of an index grating [13]. Hence, using dual-frequency optical excitation, both polar orientation and phase matching conditions can be achieved in polymers. Six-wave mixing effects are usually assumed to be small but it can be proved that they are the dominant contributions in degenearte wave-mixing experiments performed with polymers excited under two-photon absorption conditions [14-16].

Preparation process:

Figure 4: Preparation process for all-optical poling of materials using orientational hole-burning. The first experimental observation of an optical selection of molecular orientations is the transient induction of non-centrosymmetry recorded using phasematched frequency doubling in a solution of the polar molecule 4-diethylamino- 4'nitrostilbene in tetrahydrofuran [77]. The same procedure has been applied to the orientation of non-polar molecules such as the triphenyl-methane dye ethyl-violet which exhibits octupolar symmetry [77]. It has also been applied to ionic molecules and used to study their dipole moments [18]. Unlike electric field poling in which molecules undergo an electric field assisted rotation, the optical selection of orientations corresponds to a bleaching of molecules [19]. A cause of relaxation of the induced non-centrosymmetry is thus recovery of the bleaching. However, in liquid solutions, loss of orientation has been identified as the fastest relaxation process. Loss of orientation is related to viscosity; it completes within a nanosecond, leading to isotropy. Such a fast relaxation effect is removed in glassy polymers [10].


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The experimental set-up for the optical preparation of plastic materials is pictured in Figure 5. It consists in a pulsed picosecond Nd: YAG laser delivering both fundamental (1064 nm) and harmonic (532 nm) wavelengths at 10 Hz repetition-rate. Energies are 500 pJ and 2.5 uJ, respectively at 1064 nm and 532 nm. Beam diameter is 2 mm at the sample location.

L Figure 5: Experimental set-up. A fast photodiode synchronizes the sampler. SHG intensity is measured with the photo-multiplier tube (PMT). P: polarizers; F: interferential filter at 532 nm; SFS : spatial filtering system; R: dielectric mirror for fundamental rejection; S: shutter synchronised with insertion of the green blocking RG670-Schott filter. BK7 glass plate is fixed on a rotating stage for phase adjustment between writing beams. Poling is performed using dual excitation with fundamental and harmonic beams. During preparation time in Figure 6, the shutter infrontof the photomultiplier-tube is closed. Each minute, a test of the second-harmonic efficiency is performed. A red-cutting filter (Schott RG-670) is then inserted in front of the sample in figure 6 and the shutter is opened. The experiment is fully automated. Preparation:

Test

Red Filter Shutter 532 ran 1064 nm Sample

3PMT

Figure 6: Beam arrangement for seeding-type preparation and test experiments. Polymer samples where spin-coated films of a random methacrylic copolymer with nonlinear 4-nitro-4'-amino-diazobenzene chromophores grafted as pendant side groups. Molar concentration was n = 35% (Fig. 7).


246


QH

CH

3

3

-f i - C H a r ) — f i - C H r ) 0*

c v

c

n

0

1 _ n

0 * ^0-CH

3

HC 2

V "0" ^G^°

H

/N

N=N

N 2

r

CH3CH2


Figure 7: Side-chain 35/65-copolymer of DR1 in PMMA. The growth and decay dynamics of the efficient susceptibility = Yil/d^y induced in this polymer is displayed in Figure 8. The frequency doubling coefficient reaches c?33 = 45 pm/V after 2 hours of preparation. It is as high as what is currently obtained using corona electric-field poling [20,21]. After seeding-type preparation, the decay dynamics of the induced ^33 is the same as with the same polymer prepared using the corona-poling technique. This corresponds to an electrooptic coefficient r close to 10 pm/V at 1.3 um-wavelength. Molecules are oriented in the plane of thefilm.Order parameter is close to 0-2. 33

3

-150

-100

-50

0

50

100

Time (min) Figure 8: Growth and decay dynamics of the efficient susceptibility ^33. The permanent orientation achieved in Figure 8 may be attributed to a net light-induced rotation effect [22]. It is related to trans-cis izomerization (Fig. 9) in the same way as the one which promotes the light-assisted electric-field poling effect [23].


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102

Figure 9: Dlustration of the photo-induced molecular-axis rotation by trans/cis izomerization. Spontaneous reverse cis to trans reactions are thermally activated. We have studied the spatial profile of the induced second-harmonic efficiency. It is pictured in Figure 10. Unlike what is currently observed in seeded bulk glasses [2426], the spatial profile along the polymer-film plane is uniform. This confirms that the optical poling of polymers results in a local effect. Our preparation technique thus permits patterning of micro structures using the laser spot.

Figure 10: Spatial profile of seeded second-harmonic efficiency alongfilmplane. In this respect, an interesting question concerns the effect of accumulated space-charge on the nonlinear optical coefficient. Space-charge fields are indeed known to dominate SHG in seeded optical glass-fibres [27,28]. Considering that we get the same nonlinearity as using corona-poling, we can expect internalfieldsas large as 1 MV/cm, the usual polingfield,in the optically poled region. We thus prepared a polymerfilmusing various angles of incidence (Fig. 11). It experimentally results that the SHG signal does not depend on the angle of incidence, proving that space-charge field effects are not the dominant effects with this polymer.



248


small incidence angle: Charge separation ~ beam diameter (-mm)

large incidence angle: Charge separation ~ active-layer thickness (~pm) Figure 11: Probe of space-charge field effects using different angles of incidence during preparation. The polarization profile of the induced second-harmonic coefficient in the film-plane is reported in Figure 12. For this measurement, the film was prepared using parallel polarizations for fundamental and harmonic beams. The second-harmonic signal was detected through an analyser parallel to the reading fundamental beam. Figure 12 represents the square-root of the signal as a function of the film angle 6 around the laser-beam axis. Thefittingfunction is a cos(6). That means that we get a good dipolar alignment of molecules with = 3%^ [79]. (

2

2

The optically induced %() is proportional to 7s 7s* . As the fundamental and harmonic beams have a relative phase AO, %() is proportional to C0S(AO + A/r.z) where Ak = 2co(n(to)-n(2cD))/c. The relative phase AO can be controlled by varying the tilt angle 6 of a dispersive BK7 glass which is inserted in front of the sample in Figure 5. The optically-induced second harmonic signal in thin films thus depends periodically on this phase, with a 180° period, as reported infigure13 [29]. 2a)

2



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Figure 12: Polarization dependence of the seeded SHG coefficient. Angle 0 is measured relative to the polarization axis around the beam-propagation direction.

Phase A $ (degree)

Figure 13: Experimental dependence of the SHG intensity, induced after a 20minutes preparation-time, with the relative phase AO of the co and 2© writing beams. Solid line corresponds to a theoretical dependence with An = 0.3-index mismatch. Sample was 0.1 pm thick with 0.3-optical density at 532 nm.


250


While the sample-thickness is increased, oscillations with the phase AO are clamped [30], It is what appears in Figure 14 in which the contrast of modulation is plotted with respect to the sample thickness. It represents the transitionfromthe SHG in a thin film to the phase-matched SHG in a thick sample. Phase-matched SHGgrowth is illustrated with the optical-poling of centimetre-size copolymer rods in Figure 15. In these samples, DR1 loading is close to 310" % per monomer in order to maintain a low optical density.


3

0

0.2

0.4

0.6

0.8

1.0

Relative length l/l

c

Figure 14: Theoretical and experimental modulation-amplitude of the SHG signal with respect to the sample thickness / relative to the coherence length l - )J2An. c

1200 DR1M2:(e = 5 m m ) ; D O = 1 . 2 DR1M1 : ( e = 7 m m ) ; D O = 1.7 900

£

600

300

0*

3000

6000

9000

12000

Seeding time (sec)

Figure 15: Phase-matched SHG in thick PMMA rods with low DR1 doping-level.


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An optimised poling efficiency also requests a good interference between one and two-photon absorption effects, in order to maximise the polar ^-term. It requests equalisation of one and two-photon absorptions; that is ^ E o n c ^ =(8px/ /^Q)) . 2

(D

2

3

Obviously, the %()-initial growth-rate is proportional to £ which is also 1^ x/

1/2 2(D

.

saturates as ^ ^ ( B ^ / l X ^ o ^ ^ © + (A»^H/^®) )> when the poling becomes as 2

efficient as the loss of polar orientation resultingfromaxial excitation processes [23]. Such behaviour is observed in Figure 16 in which the / //^-ratio was varied while keeping the phase-AO constant. It is clear that optimisation of the optical-preparation conditions can lead to orders of magnitude gain on the poling efficiency.


2o)

20000 Seeding Time (s)

Figure 16: SHG-amplitude growth at different seed intensity-ratios (^Q/ZQÔOO). In an orientational hole-burning regime, the all-optical poling technique suffersfromthe lack of transparency of the doped polymer. A possibility in order to improve on this nonlinearity-transparency trade-off is to use photochromic molecules such as the spiropyran in Figure 17. Such molecule which can be oriented in its coloured merocyanine form which exhibits a large p is transparent in its bleached form [31]. The doped PMMA-polymer sample was irradiated with a UV-lamp during the optical preparation. Signal growth and decay are illustrated in Figure 18. Without any optimisation, we get a maximum nonlinearity ~ 0.6 pm/V. After preparation, the SHG signal drops as fast as the decoloration process which is illustrated in the inset, down to a quasi-constant« 0.3 pm/V-level. At this stage, the polymer is transparent.


252


Spiropyrane

Merocyanine


Figure 17: Reversible photochromic ring-opening and closure reactions in NitroBIPS spiropyran. 1.25x10

s

•

1.00x10

3

n

CD

~ /i

J [ • •• i

sap

•

0.75x10"

• 2(0 signal (l=2jim, OD=0.45, d =0.3pm/V)

•

.,

\ Time (s) COLORLESS

COLOURED

0.50x10

W

0.25x10 STOP SEED STOP UV LAMP

• • ]

-400

-800

400

800

1200

Time (sec)

Figure 18: SHG signal growth and decay in a spiropyran doped PMMA-polymer with 2 pm-thickness and 0.45-optical density in the coloured form. Inset represents UV-assisted coloration and bleaching dynamics at 545 nm. The possibility of inducing a long-lived optical orientation in organic materials opens a door to the microscopic control of organised structures suitable for frequency conversion. The ratio of oriented molecules in seeded plastics is larger than 0.2. The technique compares well in efficiency with the electric-field poling method and as optically-induced orientation and excitation have the same symmetries, polar orientation can in principle be improved with this technique (Table I). The magnitude of the induced %() is as large as achieved using other poling techniques (Table II), orders of magnitude larger than in seeded optical fiber materials. In addition to the possibility of patterning micro structures with the laser beam, a major breakthrough with this new technique is the natural possibility of inducing a quasi-phase-matched orientation over large distances. Experiments are now in progress in order to improve in this direction, with an aim at orienting molecules in a plastic wave guide. With the 2


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use of organic materials, we have the possibilities offered by molecular engineering to tailor their optical properties in order to optimise materials for frequency conversion devices. In addition, the rich tensor properties of B? permits the preparation of materials with new symmetries such as the octupoles [52]. Table! Comparison of the physical mechanisms involved into electric-field poling (EFP) and orientational hole-burning (OHB) techniques. Poling Technique EFP [ OHB /(3) Nonlinear process Efficiency 8p.8P F? Orientation effect B? Excitation effect kT kT Relaxation effect Table n. Nonlinear optical characteristics of some selected electrically poled polymers: a) calculated; b) see ref.[20]; c) polymer used in this study; i) initial value; f) final value after relaxation. ref. Polymer r (pm/V) d (pm/V) A,(pm) 33

33

wrr_ 19^9 HCC-1238 DANS DCV-MMA PPNA 3RDCVXY fPSÔNPP PLBP-73 PVCN-DR Bis A-NPDA Bis A-D03 Bis A-DIAM* Bis A-DIAM* DRGPMMA^ DRGPMMA^ Polvimide

^8 18.2 46f

20 28 15

lQf 80 18 13 15-30 14 25

40

42' 7Sf 74* Ttf 73

17* 45* 18* 18

1 ^ 1.3 1.34 1.58 1.06 1.5 1.06 0.63 1.06 1.06 1.32 1.32 1.32 1.32 1.32

1.31

ii 33 34 35 36 37 3X 39 40 41 42

43

References 1. 2. 3. 4. 5. 6. 7. 8.

Chemla, D.S.; Zyss, J.; Nonlinear Optical Properties of Molecules and Crystals; Academic press: Orlando, 1987. Zyss, J.; Nonlinear Optics, 1991, 1,3. Hornak, L . A . ; Polymers for Lightwave and Integrated Optics; Marcel Dekker: New York, 1992. Yilmaz, S.; Bauer, S.; Gerhard-Multhaupt, R.; Appl. Phys. Lett., 1994, 64, 2770. Moerner, W.E.; Persistent Spectral Hole-Burning: Science and Applications; Springer: Berlin, 1988. Baranova, N.B.; Zel'dovitch, B.Ya; JETP Lett., 1987, 45, 716. Armstrong, J.A.; Bloembergen, N.; Ducuing, J.; Pershan, P.S.; Phys. Rev., 1970, 127, 1918. Osterberg, U.; Margulis, W.; Opt. Lett., 1986, 11, 516.


254 9. 10. 11. 12.


13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26. 27. 28. 29. 30. 31. 32. 33. 34. 35. 36. 37. 38. 39. 40. 41. 42. 43.

POLYMERS F O R SECOND-ORDER NONLINEAR OPTICS

Stolen, R.H.; Tom, H.W.K.; Opt. Lett., 1987, 12, 585. Charra, F.; Kajzar, F.; Nunzi, J.M.; Raimond, P.; Idiart, E . ; Opt.Lett.,1993, 18, 941. Charra, F.; Devaux, F.; Nunzi, J.M.; Raimond, P.; Phys. Rev. Lett., 1992, 68, 2440. Yeh, P.; Introduction to Photorefractive Nonlinear Optics; Wiley: New York, 1993, 377. Khanarian, G.; Norwood, R.A; Hass, D.; Feuer, B.; Karim, D.; Appl. Phys. Lett, 1990, 57, 977. Nunzi, J.M.; Grec, D.; J. Appl. Phys., 1987, 62, 2198. Nunzi,J.M.; Charra, F.; Nonlinear Optics, 1991, 1, 19. Charra, F.; Nunzi, J.M.; J. Opt. Soc Am. B, 1991, 8, 570. Nunzi, J.M.; Charra, F.; Fiorini, C.; Zyss, J.; Chem. Phys. Lett., 1994, 219, 349. Markovitsi, D.; et al.; Chem. Phys., 1994, 69, 219. C.Fiorini, F.Chaira, J.M.Nunzi, J. Opt. Soc Am. B, 1995, 12 (1), in press. Chollet, P.A.; Gadret, G.; Kajzar, F.; Raimond, P.; Zagorska, M . ; SPIE Procs., 1992, 1775, 121. Broussoux, D.; et al.; Rev. Tech. Thomson-CSF, 1989, 20-21, 151. Jones, C.; Day, S.; Nature, 1991, 351, 15. Sekkat, Z.; Dumont, M . ; Appl. Phys. B, 1992, 54, 486. Dianov, E . M . ; et al; Sov. Lightwave Com., 1992, 2, 83. Dominic, V.; Feinberg, J.; Phys. Rev. Lett., 1993, 71, 3446. Driscoll, T.J.; Lawandy, N.; J. Opt. Soc. Am. B, 1994, 11, 355. Ouellette, F.; Photoinduced Self-organization Effects in Optical Fibers, SPIE Procs., vol.1516, 1991. Ouellette, F.; Photosensitivity and Self-Organization in Optical Fibers and Waveguides; SPIE Procs., vol.2044, 1993. Fiorini, C.;Charra, F.; Nunzi, J.M.; Raimond, P.; Nonlinear Optics, 1994, 9, in press. Fiorini, C.;Charra, F.; Nunzi, J.M.; Raimond, P.; EQEC'94, Amsterdam, 1994, paper QThF5. Nakatani, K.; Atassi, Y.; Delair, J.A.; Guglielmetti, R.; Nonlinear Optics, 1994, 8, 33. Fiorini, C.;Charra, F.; Nunzi, J.M.; Zyss, J.; CLEO Europe'94, Amsterdam, 1994, paper C M K 3 . Haas, D.; et al.; SPIE Procs., 1990, 1147, 222. Mohlmann, G.R.; et al.; SPIE Procs., 1990, 1147, 245. Singer, K.D.; et al.; Appl. Phys. Lett., 1988, 53, 1800. Eich, M.;et al.; J. Appl. Phys., 1989, 66, 2559. Shuto, Y.; et al.; SPIE Procs., 1991, 1560 184. Ye, C.; et al.; Nonlinear Optical Effects in Organic Polymers, J.Messier et al. eds., N A T O ASI Series E , vol. 162, Kluwer: Dordrecht, 1989, p. 173. Broussoux, D.; et al.; Proc. Int. Conf. on Fine Chemicals for the Electronic Industry, Univ. of York, 1990, April 18-20. Mandal, B.K.; et al.; Makromol. Rapid Commun., 1989, 12, 63. Reck, B.; et al.; SPIE Procs., 1990, 1147, 74. Gadret, G.; et al.; SPIE Procs., 1991, 1560, 226. Zysset, B.;et al.; SPIE Procs., 1993, 2025, 70.

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All-Optical Poling of Polymers for Phase-Matched Frequency Doubling

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