Table of contents

A two-dimensional formulation is presented for holographic recording in thin photorefractive films. The corresponding rate equations take into account two components (parallel and perpendicular) of both fields and current densities. The role of volume- and surface-charge gratings in determining the space-charge field profiles is stressed. As an illustration of the new approach, some features of the short-time solution of the rate equations are discussed for both parallel and perpendicular geometries. The influence of relevant physical and geometrical parameters of the films is investigated. Finally, further extensions of the work to include steady-state and full transient solutions are pointed out.

This paper reviews some of the authors' recent research aimed at obtaining an understanding of the physical processes that determine the linear and nonlinear optical properties of nanocomposite materials. One result of this research is the prediction and experimental verification that under proper conditions two materials can be combined in such a manner that the nonlinear susceptibility of the composite exceeds those of the constituent materials. This paper also presents a survey of the various geometrical structures of composite materials.

The orientational enhancement mechanism responsible, in part, for the photorefractive effect observed in polymeric materials in which an anisotropic chromophore is free to orient in the presence of an external poling electric field is described. This mechanism does not require that the chromophore have a substantial first hyperpolarizability. This fact has been demonstrated by the demonstration of a substantial photorefractive effect in a chromophore with a tiny hyperpolarizability but a large linear polarizability anisotropy. The use of photorefractive polymers in holographic data recording is assessed using a sophisticated holographic data storage tester. These experiments have demonstrated the critical dependence of the recording bit error rate on background scattering and the severe requirements that this dependence places on sample quality.

The influence of charge injection induced by an electric field on the optical absorption of -sexithiophene () thin films is studied. The analysis of this electro-modulated absorption (EMA) at variable incidence angle and wavelength enables us to determine the various contributions in the stratified structure (metal/insulator/semiconductor). EMA enhancement at 1.5 eV can be unambiguously assigned to the radical cation . Owing to the difference of energy level for aluminium and gold electrodes, charge injection occurs in the dark in the case of gold, while it needs to be photo-assisted in the case of aluminium.

The mechanism of photoassisted poling of photochromes in polymers is demonstrated by SHG. The technique which works at room temperature is studied here for photomerocyanines, the open form of spiropyrans and also for a dihydrobenzofuran derivative which is the cyclized form of a furylfulgide. Both coloured forms have dipole moments and high first-order hyperpolarizabilities. In the case of photomerocyanines, a synergistic effect of the electric field and the UV light is observed. With both systems, particularly with the second one whose coloured form is thermally stable, a quasi-reversible photoswitching of second-order NLO properties by UV - visible light irradiation is observed.

After a rapid presentation of generally quoted figures of merit of photorefractive materials, we show that these figures of merit do not allow us to select a `best material'. We illustrate this important point by accurately defining the materials' properties relevant to several applications taken in various domains such as optical storage, real time interferometry and pattern recognition.

Since the linear electro-optical or Pockels effect is one of the strongest and most useful effects in poled polymers, it can be employed for investigating the magnitude, the spatial distribution and the thermal and temporal stability of the dipole orientation. Some of the relevant ellipsometric and interferometric techniques are briefly described. Emphasis is given to frequency-dependent electro-optical measurements which permit a relaxation spectroscopy based on the probing of the dipole orientation by means of frequency scanning of the linear susceptibility (optical birefringence). Such a spectroscopy is of particular relevance for the recently introduced `orientational enhancement' in photorefractive polymers. In addition, scanning electro-optical microscopy and electro-optical phase-shift interferometry are described as useful methods for measuring spatial dipole-orientation patterns.

Exciton relaxation processes in J-aggregates of two thiacarbocyanine dyes (TDC and THIATS) in water/ethylene glycol low temperature glass have been studied by means of steady-state, site-selective and time-resolved spectroscopy. The results of different experiments can be rationalized within the framework of exciton self-trapping (ST) in a 1D molecular chain. The measured Stokes shift is found to be proportional to the exciton - phonon coupling strength squared that is in accordance with the model of large radius ST excitons. A negative fluorescence anisotropy was observed for J-aggregates of both investigated dyes upon excitation outside the J-band. The observed monotonic dependence of the fluorescence anisotropy on the excitation wavelength is interpreted quantitatively within the framework of intraband absorption and Davydov splitting.

The nonlinear third order parameters for AlGaAs have been measured at wavelengths near half the semiconductor bandgap, around 1550 nm. It was found in waveguides that the nonlinear refractive index coefficient was the same to within 10% for both TE and TM polarized modes, and that the ratio of the cross-phase to the self-phase modulation coefficient is approximately unity. Such fortuitous values, along with this material's low multiphoton absorption, have allowed us to study spatial solitons and their interactions under ideal conditions. An experimental geometry was found in which the four-wave-mixing term through which orthogonally polarized beams exchange energy was effectively zero. For orthogonally polarized beams, the deflection and trapping by a spatial soliton of either a weak beam or another spatial soliton have been investigated and all-optical switching demonstrated. Furthermore, Manakov solitons which consist of two orthogonally polarized components of arbitrary relative intensity were observed for the first time in slab waveguides.

We study the photo-induced alignment of a nematic liquid crystal in contact with an azo-dye-doped polyimide alignment layer. Using polarized light to illuminate a planar nematic cell with one active alignment layer results in the formation of twisted nematic domains. We measured the dynamics of domain growth resulting from light-induced changes in the surface anchoring. We interpret the results in terms of a simple model for the evalution of the orientational distribution function of the dye and the resulting anchoring potential. We discuss possible applications of optical alignment.

Metal complexes display interesting properties in terms of molecular optimization of quadratic nonlinear properties, combined with a good thermal stability and a large flexibility of geometrical and electronic properties. The role of dimensionality and intramolecular charge transfer of the pure ligand is investigated here, in particular for Schiff base derivatives. Also, the influence of metal electronic configurations on the quadratic hyperpolarizability is investigated in the series of Ni II, Co II and Cu II atoms, showing the interest of open-shell structures as compared to closed-shell complexes in the case of a weakly nonlinear ligand. Finally, strongly octupolar two-dimensional systems, subphthalocyanines, are shown to present very large second-order nonlinear polarizabilities, with static values as high as measured on a trinitro-substituted SubPc by the harmonic light scattering technique.

The main properties of polyphenylene-vinylene (PPV) in its neutral and doped states by means of resonance Raman scattering (RRS) and high resolution solid state NMR are described. These two resonance techniques are complementary for following and characterizing the main modifications which occur when the polymer undergoes a transition from its neutral to its conducting form in terms of electronic distribution on the polymeric backbone or electron density on carbon sites. In RRS, we have shown that the changes observed in the Raman spectra of doped PPV consist of additional bands with a different resonant behaviour. We have performed dynamical calculations which led to the conclusion that polaronic or bipolaronic segments of different lengths are present. In high resolution NMR, we have identified the carbon sites which are the most affected by the charge carriers, in agreement with the above RRS results. Furthermore, NMR spectra exhibit two components at ppm and ppm, characteristic of two different species, one being totally ionic and the other embedded in a more ordered PPV phase.

Gel-glass dispersed liquid crystal (GDLC) films may be used as electro-optic devices. Films scatter light according to the number of droplets and the relative refractive indices of the LC and the silica matrix. LCs are birefringent; therefore their refractive index depends on the LC orientation and the optical angle of incidence. If the film is coated with transparent electrodes, and an electric field is applied, a reorientation of the LC director in the droplet occurs, producing a variation of the LC refractive index as `seen' by the incoming light. If the refractive index of the sol - gel substrate matches the new LC index, the material changes from an opaque, scattering state to a transparent state. This feature can be used for preparing devices for visual presentation, i.e. displays. Unaltered GDLCs switch from white opaque to colourless transparent states. The contrast ratio, viewing angle and switching time of GDLC displays are presented. If these materials are to be used for displays, colour needs to be incorporated for many applications. The use of GDLCs associated to gel-glasses doped with fluorescent dyes for preparation of emissive `all sol - gel' displays is presented, with the GDLC acting as an optical shutter for the RGB emission of the fluorescent dyes located at the backplane. A number of fluorescent dyes with high quantum yield for the chosen UV excitation, and appropriate colour coordinates, have been investigated.

The mechanism of photorefractivity in a family of model composites based on PVK is discussed. Novel aspects of photorefractivity that are unique in polymers are described. The effect of plasticization on their photorefractive performance is investigated.

This paper is concerned with the interplay between electromagnetic resonances and the phase-matching condition in guided-wave nonlinear optics. Several examples are given: quasi phase-matched second-harmonic generation using a grating with a subwavelength periodicity, cascaded optical bistability, Kerr-type distributed couplers under pulse illumination with duration of the order of the cavity build-up time or shorter, surface plasmon-enhanced stimulated Raman scattering at a grating overcoated by a few monolayers of a Raman active medium.

In recent years a large amount of activity has been carried out searching for novel or advanced materials which exhibit large nonlinear third-order susceptibility . Emphasis has been put on materials suitable for integrated optical circuits, since the guided-wave format can provide additional routes to the conception and realization of photonic devices. The preparation of nanoparticle-doped glass by the sol - gel technique represents one of the most promising approaches: here we give a short overview of alternative processes to control the size of semiconductor clusters and we present experimental results from CdS- and PbS-doped silica - titania sol - gel films.

A salt-based approach for the development of frequency conversion materials is presented. Salts are generally rugged materials due to their strong ionic bonding. A high degree of chemical substitution (both of anion and cation) is possible, including highly nonlinear organic ions. This flexibility is consistent with the use of empirical survey techniques to identify promising candidate materials. We describe in detail our experimental approach and present examples of nonlinear optical crystals we have identified in our materials surveys.

Dark spatial solitons can be used for generating light-induced waveguides and guiding light with light. A negative (self-defocusing) third-order nonlinearity is necessary to generate dark solitons. We show that the negative electronic nonlinearity is possible at 800 nm in several classes of organics including squaraine dyes, -conjugated polymers of poly(p-phenylenevinylene) type and polyaniline.

The sol - gel process provides a method to blend, down to the molecular scale, organic and inorganic components which can be mixed in virtually any ratio to obtain the desired properties. These hybrid nanocomposites are extremely versatile in their composition, processing and optical and mechanical properties. Hybrid organic - inorganic materials offer a wide range of interesting applications especially in optics. A short review of the most striking examples of hybrid organic - inorganic material used to produce materials with second-order optical nonlinearities is presented.

Systematic studies of the nonlinear optical properties of organic materials have led to the development of promising new materials for optical limiting. Several heavy metal substituted phthalocyanines are effective materials for optical limiters in the visible and near IR. Nonlinear absorption is the principal mechanism near the limiting threshold. In fast optical systems, refractive contributions also contribute at higher energies. These refractive contributions are predominantly thermal. The spectral window over which the limiter operates can be engineered by altering both the main ring and peripheral substitution of the molecules.

Second harmonic generation of ultrashort pulses in first- and higher-order quasi-phase-matched nonlinear structures is investigated theoretically. It is shown that using such structures high conversion efficiency can be achieved while preserving the ultrashort pulse duration. Using periodically poled bulk lithium niobate as an example, it is found that nearly complete conversion of a 900 nm wavelength 100 fs pulse to the second harmonic may be possible in a first-order structure without any broadening of the pulse width.

In studies of nonlinear optical (NLO) polymers, besides obtaining a thorough understanding of their nonlinear optical behaviour, it is necessary to characterize their linear properties. These are the real and imaginary parts of the index of refraction as a function of wavelength. A procedure will be described for determining these indices from measured and known optical properties of the individual components, that is, the polymer and the chromophore.

We report on a novel nonlinear polarization-modulated spectroscopic method for an accurate measurement of the nonlinear change of both real and imaginary parts of the complex refractive index in isotropic materials having either scalar or tensor photoresponse. It is based on a vector two-wave-mixing interaction and heterodyne detection of dynamic change of optical polarization. New data on steady-state and transient nonlinear characteristics of bacteriorhodopsin-based materials (suspensions and polymer films) are obtained using this method.

The transition of thinking from the use of all-optical logic arrays, to optoelectronic smart-pixel arrays, within digital information processing is described. Detectors with low conversion energies and fast modulators, developed from devices originally intended as bistable elements, now provide one of the technologies being pursued for the optics - electronics interface. The key advantage provided by optics is the huge bandwidth off-chip (THz); electronics provides high density locally interconnected (on-chip) logic devices. Applications that exploit this combination are being sought. One possible area is in the sorting of data sets, where non-local interconnections between stages and modest logic functionality per stage, are required in order to implement fast algorithms. The expected performance of a smart-pixel sorting module, such as that under construction by the Scottish Collaborative Initiative in Optoelectronic Sciences (SCIOS) is summarized. The move from all-optical to hybrid technologies does not eradicate the need for further advances in materials and in the processing control of materials with nonlinear optical (electro-absorption and electro-optic) responses.

Thanks to Pierre Zaleski, the former administrator of Europtica Services IC (ESIC), EOS has received a donation from the capital remaining after the dissolution of ESIC. According to the wishes expressed by ESIC, this donation will be managed in such a way as to generate revenue to be used for an EOS prize.

The prize is intended to recognise and encourage the European element of research into optics, in both the pure and applied fields. It will usually be awarded annually to the authors of an outstanding research paper, with the proviso that suitable work is submitted. To be considered as eligible the work must have been undertaken in a European country and published under the auspices of the EOS. Thus all papers published in the Society's journals (such as Pure and Applied Optics and Quantum and Semiclassical Optics), or from an EOS topical meeting or Europto meeting which report on European work are suitable. Further details of the rules and conditions are given below.

René Dandliker EOS Past President

Rules and Conditions for the EOS Prize

1. The purpose of the prize is to encourage a European dimension in research into pure or applied optics.

• The prize will normally be awarded annually to the authors of a research paper in the field of any branch of optics, provided that work of adequate quality is submitted.

• The conditions for eligibility are;

• That the work was performed in Europe

• That the work is published under the auspices of the EOS (for example in an EOS journal, at an EOS Topical meeting or at a Europto meeting)

The criteria by which the work will be judged are;

• High professional/academic/technical quality

• Work which is a collaboration between more than one European country will be viewed favourably

• The EOS prize committee shall consist of the Past President, one member of the EOS Board and one other member of the EOS who is not a member of the Board. This third member shall be nominated by the Past President and agreed by the Board, or nominated by the Board if the former does not yield a suitable candidate.

• The committee shall solicit proposals, evaluate the material and propose a prizewinner. The proposal shall be confirmed by the EXECOM. If the EXECOM should fail to confirm the winner, no prize shall be awarded that year. Any comments from the EXECOM shall be taken into account for the following year.

• All eligible papers published in the Society's journals shall be considered for the prize.

Organisers of EOS topical meetings and Europto meetings shall be invited to nominate any paper presented at their meeting.

Any member of the EOS may make a nomination at any time.

• An EOS prize fund shall be established which shall be used exclusively for the financing of the prize and the direct administration costs. The basis of the fund shall be the capital remaining after the dissolution of Europtica Services IC plus any further donations received unconditionally for this purpose. The fund shall be invested in a manner proposed by the treasurer in order to secure favourable interest. In no year shall the administration costs and the sums presented as prizes exceed the interest received during that year.

• The financial year for the prize fund shall coincide with the calendar year. The committee shall consider nominations received or published during the year for presentation at the Society's Annual Meeting the following year.

• The prize shall consist of a certificate and a sum of money which shall be proposed by the prize committee and agreed by the EXECOM.

• The EOS prize shall be presented to the winners at a special session of the EOS Annual Meeting or other EOS meeting at which the prize committee shall introduce the winners and the winners shall present their work.

Nominations should be sent to;

Main Secretariat, EUROPEAN OPTICAL SOCIETY, B.P. 147, F-91403 Orsay Cedex, France. Fax (33) 1 69 85 35 65 E-mail: francoise.chavel@iota.u-psud.fr.

co-sponsered by Société Française d'Optique

Materials for Nonlinear Optics

Val Thorens, 14 - 18 January 1996

Organizing Committee

Meeting Chair

F Kajzar, CEA-LETI, Centre d'Etudes de Saclay, France

Programme Chair

J Zyss, France Télécom, CNET Bagneux, France

Treasurer

P A Chollet, CEA-LETI, Centre d'Etudes de Saclay, France

Secretary

F Charra, CEA-LETI, Centre d'Etudes de Saclay, France

Scientific Committee

V Agranovich, Institute of Spectroscopy, Troitsk, Russia F Agullo-Lopez, University of Madrid, Spain W Blau, Trinity College, Dublin, Ireland D Bloor, University of Durham, England J C Dubois, Thomson CSF, Orsay, France H A Ferwerda, University of Groningen, The Netherlands C Flytzanis, Ecole Polytechnique, Palaiseau, France R Friend, University of Cambridge, England P Günter, ETH Zürich, Switzerland G Hadziioannou, University of Groningen, The Netherlands I Ledoux, France Télécom, CNET Bagneux, France R Levy, University of Strasbourg, France P Micheron, Thomson TRT, Guyancourt, France J Mlynek, University of Constanz, Germany G Möhlmann, AKZO Nobel, The Netherlands A Pron, Academy of Mining and Metallurgy, Cracow, Poland G C Righini, IROE, Firenze, Italy E Rosencher, Thomson CSF, Orsay, France G Roosen, Institut d'Optique, Orsay, France W Sibbett, University of St Andrews, Scotland M Soskin, Ukrainian Academy of Sciences, Kiev, Ukraine C Taliani, ISM-CNR, Bologna, Italy

Conference Sponsors

Centre National de la Recherche Scientifique, Ultimatech Commissariat à l'Energie Atomique, LETI, DEIN/SPE Conseil Général de Savoie Direction Générale de l'Armement, Direction Scientifique European Commission, Directorate General XII France Télécom, Centre National d'Etudes des Télécommunications Ministère de l'Enseignement Supérieur, de Recherche et de l'Insertion Professionelle Office du Tourisme de Val Thorens SOPRA

The present issue contains selected invited papers presented at the topical meeting, organized by the European Optical Society (EOS) and co-sponsored by the French Optical Society (SFO), on Materials for Nonlinear Optics held in Val Thorens on 14 - 18 January 1996. This seventh topical meeting was devoted to producing a general overview of different problems connected with the preparation, characterization and practical use of materials for nonlinear optical applications. It consisted of invited and contributed papers outlining the latest developments and two poster sessions.

It is commonly considered that the coming 21st century will be dominated by information exchange and other connected information technologies. This will require the flow and treatment of a very large amount of data. Nonlinear optics, in this context, is considered to offer a major advance in being able to sustain very fast, large volume, parallel information flow and processing.

The aim of the meeting organized in Val Thorens was to establish the state of the art in this field in Europe as well as to create a dynamic cross-interaction between people working with different materials on one side and material scientists and applied researchers on the other. The importance of the field under debate was indicated by the high level of attendance: about 140 scientists, coming not only from Europe but also from the US, Japan and as far afield as Australia.

The opening address to the meeting was delivered by Denis Randet, director of LETI, who insisted on the complementarity between the application fields of microelectronics and photonics. For example, electrons show better parameters for medium rate, short distance signal transmission, whereas photons will offer better performances for high rate and long distance, parallel signal transmission. In his opinion, with respect to the challenges to come, both fields must not be considered as competing, but instead one should take advantage of their complementarity.

Numerous research fields such as the optical signal transmission, all optical information processing, optical limiting, electroluminescence and its application in light emitting diodes, frequency up-conversion, image recognotion, optical information storage have been mentioned. Amoung the most important achievements one can cite important progress noted in elaboration of the photorefractive polymers (IBM), frequency converters in near infrared (Rockwell), frequency doublers and electro-optic modulators made with semiconductor based multiple quantum wells (Thomson), organic light emitting diodes (Thomson) and hybrid semiconductor-organic integration as exemplified by direct coupling of III - V based lasers with polymer waveguides and couplers (France Télécom / CNET). An important progress has also been noted in the understanding of the origin of nonlinear optical phenomena and their relation with the structure as well as in the material elaboration itself. Let us note the original technique of all optical orientation of organic molecules invented by the CEA team.

The organization of such a meeting requires the devotion and help of a large number of people. We would like to acknowledge warmly all those whose help was a determining factor in the success of this meeting. Particular thanks are due to the organizing committee members: conference treasurer, Pierre-Alain Chollet and conference secretary, Fabrice Charra for their dedication and willing accessibility. Thanks are also due to the scientific committee members whose advice and help in the selection of invited papers as well as of oral and poster contributions to the meeting were vital in ensuring its scientific quality. We also benefited from help and assistance from hotel management and we would like to express our thanks in particular to Michel Royboz, hotel manager and Eric Turco, his deputy. Warm thanks are also due to Annie Crime for secretarial help as well as to all conference sponsors whose financial help was crucial in its organization.

François Kajzar (Conference Chair) Joseph Zyss (Programme Chair)