3. H. Winick and G.P. Williams, Synchrotron Radiation News 4(5), 23 (September-October 1991).

4. TELL-TERAS Activity Report (Electrotechnical Laboratory, Japan) (in English).

5. K. Onuki, N. Saito and T. Saito, Appl. Phys. Letters 52, 173 (1988); K. Onuki, N. Saito, T. Saito, and M. Habu, Rev. Sci. Instru. 60, 1838 (1988).

6. ETL News No. 496 (May 1991) (in Japanese).

7. K. Yagi, H. Onuki, S. Sugiyama, and T. Yamazaki, Bunkyo Kenkyu 40(5) 281-290 (1991) (in Japanese with English abstract).

8. T. Saito, K. Katori, M. Nishi, and H. Onuki, Rev. Sci. Instru. 60, 2303 (1989); T. Saito, K. Katori, and H. Onuki, Physica Scripta 41, 783 (1990).

9. ETL News No. 499 (August 1991) (in Japanese).

Victor Rehn is currently a liaison scientist with the Office of Naval Research Asian Office in Tokyo. He assumed this position in May 1991. Since 1965 Dr. Rehn has been a research physicist with the Naval Weapons Center, China Lake, California. He started there as a research physicist in the Semiconductor Physics Branch, then as a supervisory research physicist he headed the Electron Structure of Solids Branch and the Semiconductor and Surface Science Branch, both in the Physics Division, Research Department. Dr. Rehn received his B.A. in physics at the University of California, Berkeley in 1953 and his Ph.D. in physics from the University of Pittsburgh in 1962. After completing his thesis research in nuclear quadrupole resonance studies of para dichlorobenzene and related materials, Dr. Rehn studied magnetoacoustic attenuation in metals at the University of Chicago. Upon moving to China Lake, he undertook research in electroreflectance of widegap semiconductors and insulators. Beginning in 1973, he participated in the establishment of the Stanford Synchrotron Radiation Laboratory and continued with the application of synchrotron radiation in research in semiconductors and semiconductor surfaces. In 1976 he initiated a research program in liquid-phase epitaxy, followed in 1984 by research in molecular-beam epitaxial growth and characterization of semiconductor materials and heterostructures. In 1987 he initiated research in the production of yttrium barium copper oxide superconductive thin films using excimer-laser ablation.

INTRODUCTION

Nonlinear optical (NLO) materials are characterized by their nonlinear polarization response to a laser beam electric field. The field of nonlinear optics has gradually developed since the discovery of lasers. Earlier work concentrated on inorganic ferroelectric and semiconducting materials, which led to the development of a number of devices such as optical modulators, second harmonic generators, and switches. Some of the devices have been already commercialized. Recently, through the molecular engineering approach of optimizing nonlinear optical properties, a number of new organic materials possessing much larger nonlinear optical efficiencies have been reported. They have attracted much attention as possible key materials for information processing for the 21st century. Consequently, there is worldwide research activity to identify novel NLO organic molecular and polymeric materials as well as use them in optoelectronic devices.

To review the current state of the art and recent advances in this fertile

field, a workshop sponsored by the Army Research Office Far East (AROFE) was organized by the Organic Photonic Materials Research Group at the Research Institute of Economy and

by Iqbal Ahmad

Industry, Tokyo University of Agriculture and Technology. It was held on 11-12 October 1991 at the Shokuryo Keikan, Kojimachi, Tokyo. The chairman was Dr. Hiroyuki Sasabe of RIKEN. In addition to the attendees from Japan, active researchers from the United States, France, and Korea participated. In the following some of the highlights of the presentations and discussions are summarized. A Proceedings of the workshop will be published in the near future.

MATERIALS FOR SECOND HARMONIC GENERATION

In recent years there has been increasing emphasis on the theoretical and experimental studies of the nature of the highly conjugated л electron states in organic crystals and polymeric materials that are responsible for their exceptionally large, second order nonlinear optical susceptibilities X(2). Specifically in view of their application in second harmonic generation (SHG) and electro-optic light modulation, poled polymers (Ref 1) have received much attention. The large X(2) values of the poled polymers depend on both the poling electric field and the second order hyperpolarizability, B, of the dopant molecules or the pendant molecular units (Ref 2). The ẞ values

arise principally from the л electron conjugated systems substituted by electron donor and acceptor groups. The magnitude of these values depends on the electron affinity of the acceptor and ionization potential of the donor groups, as well as on the length and nature of the л electron conjugated backbone structure. These trends have been examined in the light of general phenomenological models such as the equivalent internal field model and the two level charge transfer model (Ref 3). The most striking advantage of the poled polymers is the primarily pure electronic origin of large ẞ values over the entire frequency range from dc to optical frequencies, unlike the inorganic ferroelectric crystals such as KH2PO (KDP) and LiNbO, (LN), where the nonlinear optical response includes acoustic and optical phonon contributions.

Dr. Yoshito Shuto of the NTT

Optoelectronic Laboratories, Ibaraki, reported the hyperpolarizabilities ẞ of both the conventional monoazo dye Disperse-Red 1 (DR 1) and some novel diazo dyes determined by the solvatochromic method. Nonlinear optical susceptibilities X(2) of the dye substituted, poled polymethyl methacrylate copolymerized with methacrylate esters of DR 1 and nitro and dicyanoterminated azo dye derivatives and their

relationship with microscopic ẞ values of the dye molecular units were evaluated. Each of the dye molecules studied had a nitro group or a dicyanovinyl group as an electron acceptor and a diethylamino group as an electron donor. Shuto proposed a molecular statistical model to describe the relationship of the bulk X(2) and ẞ values in the presence of a poling electric field, which explained the large X(2) value of 1 x 106 esu at 1.06 μm for corona poled polymer doped with dicyanovinyl terminated diazo dye derivatives.

3h

Dr. Joseph Zyss of CNET, France, reported a new molecular and material engineering route based on the compatibility of previously reported beta enhancing features and a strict cancellation of all vector-like observables including the ground state dipole moment (Ref 4). It was shown that the dramatic reduction of rotational spectra such as cancellation of all irreducible components of order lower than J (except for the J=0 totally symmetrical representation) would result from multipolar order 2' of the molecular or material symmetry group. This consideration supports the relevance of the octupolar (J=3) groups in the context of quadratic linear optics with the trigonal D group standing out. The first reported example of a molecule and subsequent crystalline lattice showing significant second harmonic generation efficiency and complying with these symmetry criteria is 1-3-5, triamino 2-4-6, trinitrobenzene (TATB) (Ref 5). In this presentation Zyss also discussed the guanidinium complexes and a few other molecules and suggested that owing to the absence of dipoles and a more rounded shape of these molecules, they would have the advantage over para nitroaniline-like molecules as they were expected to be easily crystallizable, have a more favorable statistical occurrence of noncentrosymmetric crystal lattices, and have an optimized ratio of off-diagonal versus diagonal beta tensor components and

subsequent implications for device operation. Zyss also critiqued the traditional two-level quantum model, which relies on ground and excited state dipoles and is thus unable to account for the observed nonlinearities of octupolar systems. He proposed a three-level system instead and discussed various quantum mechanical approaches of the problem. Further details of this work are reported in the Proceedings of the Society of Photo-Optical Instrumentation Engineers (SPIE) meeting held in San Diego in August 1991.

The molecular arrangement for crystal engineering containing bulky substituents of biphenyls or hindered phenol derivatives for SHG materials is the subject of the studies by Prof. Masaru Matsuoka of the Department of Chemistry, University of Osaka Prefecture, Sakai. He discussed the effects of substituents on SHG efficiency of twisted biphenyls and hindered phenols and reported that in systems with high SHG efficiency, the 3,5 di-t-butyl group played an important role in effective molecular packing of biphenyl and phenyl derivatives.

As a part of the Ministry of International Trade and Industry's (MITI) Frontier Research Program at RIKEN, Saitama, Dr. Tatsuo Wada has studied the poling dynamics of polymers. In his presentation Wada reported experimental results of the optical second harmonic generation and spectroscopic absorption measurements performed to examine the electric field induced polar alignment in polymethyl methacrylate (PMMA) doped with 10% azo dye (DR 1) and polyurethane containing nitrostilbene moiety in the side chain. The dynamics of molecular orientation were monitored during the corona poling process with a parallel wire electrode configuration by meawire electrode configuration by measurement of absorption and transient SHG. A decrease in the intensity of the absorption spectrum observed after corona poling was attributed to the alignment of dipolar chromophores in

the direction of the poling field. Measurements of in situ poling or depoling showed that covalently attached polymers had longer time constants than molecularly doped polymers.

A general problem of poled polymers is the decay of their nonlinear optical coefficients due to local relaxation processes of polymer side chains (Ref 6) and low orientation order parameters. Prof. Miyata and Dr. Watanabe presented a novel approach that forms SHG active p-nitroaniline (p-NA) crystals spontaneously in polymer matrices even in the absence of an electric field. Thin film from a solution of 10% to 50% p-NA and poly e-caprolactone (PCL) in benzene was formed on a transparent substrate by spin coating. Highly aligned SHG active p-NA crystals, so-called transcrystals, are formed by scratching the spin coated film before it crystallizes. The SHG intensity was found to increase with an increase of p-NA content. At 21.6% p-NA, the SHG intensity was three to four times larger than 2-methyl-4nitroaniline (MNA). Moreover, its nonlinear optical coefficient is quite stable at room temperature for more than 1 year.

Use of polymeric optical materials for second harmonic generation of diode lasers for optical storage was discussed by Dr. D.M. Burland of IBM Research Division, Almaden Research Center, San Jose, California. This application is important because it is clear that to move into the Gbit/cm2 storage regime, the effective operating wavelength of the laser system must be shorter than 500 nm. This wavelength can be obtained either by developing a blue diode laser or through frequency doubling of the existing infrared diode lasers. Burland explained that in order to design an optical storage device, it would be desirable to have on the order of 5 to 10 mW of blue light incident on the surface of the optical disc. Making allowances for the losses in the optical system between the light source and the disc,

10 mW appears to be a lower limit on the source power. A typical GaAlAs laser can generate on the order of 100 mW in a single spatial mode at 800 nm. One thus needs a frequency doubling device with at least 10% output conversion efficiency when pumped with 100 mW of input power. Considering a polymer slab waveguide for a frequency doubling device, it can be shown that a value of X(2) of 60 pm/V would be needed (Ref 7). For second harmonic generation a figure of merit X(2)/n3 and the transparency wavelength range are used to select candidate materials. On the basis of the figure of merit, Burland identified poly-p-nitroaniline polymer (PPNA) and organic crystal 2-methyl4-methoxy-4'-nitrostilbene (MMONS) as possible candidates. However, he stated that no materials were known that satisfy both transparency and figure of merit requirements. So the major problem to be solved for organic materials is the reduction in the absorption at 2 w. He also showed that no p-substituted benzenes and substituted stilbenes or tolanes, among the poled polymers, have qualified as yet as practical chromophores for frequency doubling. He suggested that the indirect link between wavelength of absorption maximum and ẞ, calculated by assuming the validity of the two-level model, makes it difficult to find chromophores that would simultaneously exhibit efficient frequency doubling without absorption at the second harmonic.

The work reported by Dr. Khanarian of the Hoechst Celanese Research Co., Summit, New Jersey, supports the research of Burland. He reviewed the underlying principles of organic dyes for frequency doubling, their poling in polymers, and the fabrication of waveguides. In the experimental work performed, the technique of quasi-phase matching applied by both periodic poling and bleaching was used. Waveguides in the form of thin (1-10μm) films were fabricated by spin coating and standard lithographic techniques. A pulsed dye

laser/H2 cell system tunable near 1.3 μm was end fired into the slab waveguide mounted on a rotating stage. Since the fabricated periodicity (6μm) was slightly below the phase matching coherence length (6.2 μm), the slab waveguide was rotated to lengthen the grating until phase matching was observed. From the angular dependence one can deduce that the phase matching had occurred over a distance of 5 mm and the efficiency of the slab doubler was about 0.01 %/W. In the case of periodically bleached samples, the waveguides were more lossy and the wave matching occurred over only a distance of 1 mm. Recently, great strides have been made in frequency doubling in potassium titanyl phosphate (KTP), lithium tantalate, and lithium niobate periodically domain inverted waveguides. Comparing the results with the organic polymer devices, it is clear that great improvements have to be made in polymer doublers in order to become competitive with the inorganic doublers.

The success of nonlinear optical devices depends on the availability of appropriate organic crystals. Prof. Sherwood of the Department of Pure and Applied Chemistry, University of Strathclyde, United Kingdom, described his work on 4-nitro-4' methylbenzylidene aniline (NMBA), which is composed of linear molecules of high intrinsic second order hyperpolarizability and which pack in the solid state in a noncentrosymmetric and well-aligned form. NMBA crystals, therefore, have excellent modulation properties and high potential for Pockels cells and other equivalent applications. However, to grow them in the form of large single crystals is not easy. Sherwood reported successful preparation of large (5 by 3 by 1 cm3) single crystal tablets of high perfection by the use of selective seeding techniques starting from long, fine, hairlike crystals developed from an n-hexane solution. The seeding was done in a solution of NMBA in ethyl acetate.

The reduced half wave voltages of cut and polished specimens were found to be 2.8, 1.3, and 1.1 kV at 632.8, 514.5, and 488.0 nm, respectively. The thermal variation of the birefringence was also investigated and the temperature variation of the refractive index difference d A n/dT was 15.8 x 10-5 K.

MATERIALS FOR THIRD HARMONIC GENERATION

In the area of third order nonlinearity, Mr. Y. Suda of Tokyo Ink Manufacturing Co. Ltd., Tsukuba, Ibaraki, discussed his company's work on the metallophthalocyanine derivatives with axial ligands, which are known to have at least one order of magnitude larger X(3) values than those without axial ligands. He reported that the loose aggregation of tetrakis-(alkylthio) phthalocyanine gave X(3) values several times larger than those of co-facially aggregated ones at the same resonant wavelength. This dimeric aggregation at room temperature is derived from the permanent dipole and the liquid crystalline state. Thus, the unsymmetrically substituted phthalocyanines with a lone alkyl chain must be a better candidate for third order nonlinear optics.

Although there has been considerable research activity on the third order nonlinear optical behavior of organic molecules with linear chain structures, there are only a few theoretical or experimental studies on twodimensionalл electron systems. Wada described the molecular design and assembly of macrocyclic conjugated systems such as annulenes with an 18-28 membered ring size and metallophthalocyanine systems and their third order nonlinear optical properties. He reported that the third order optical response of tetradehydromethano annulenes increased with increased size of the macrocyclic conjugated structure. Wada also reported enhancement of third harmonic susceptibility in