Molecular Nonlinear Optics. Materials, Physics, and Devices PDF
Preview Molecular Nonlinear Optics. Materials, Physics, and Devices
QUANTUM ELECTRONICS—PRINCIPLES AND APPLICATIONS EDITED BY PAUL F. LIAO Bell Communications Research, Inc. Red Bank, New Jersey PAUL L. KELLEY Lincoln Laboratory Massachusetts Institute of Technology Lexington, Massachusetts A complete list of titles in this series appears at the end of this volume. MOLECULAR NONLINEAR OPTICS MATERIALS, PHYSICS, AND DEVICES Joseph Zyss Centre National d'Etudes des Telecommunications Laboratoire de Bagneux, Bagneux, France ACADEMIC PRESS, INC. Harcourt Brace & Company, Publishers Boston San Diego New York London Sydney Tokyo Toronto This book is printed on acid-free paper. @ Copyright © 1994 by Academic Press, Inc. All rights reserved. No part of this publication may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopy, recording, or any information storage and retrieval system, without permission in writing from the publisher. ACADEMIC PRESS, INC. 1250 Sixth Avenue, San Diego, CA 92101^1311 United Kingdom Edition published by ACADEMIC PRESS LIMITED 24-28 Oval Road, London NW1 7DX Library of Congress Cataloging-in-Publication Data: Zyss, J. Molecular nonlinear optics : materials, physics, and devices / Joseph Zyss. p. cm. — (Quantum electronics—principles and applications) Includes bibliographical references and index. ISBN 0-12-784450-3 1. Nonlinear optics. 2. Quantum electronics. 3. Optoelectronics— Materials. 4. Polymers. I. Title. II. Series. QC446.2.79 1993 621.36'9-dc20 92-42976 CIP Printed in the United States of America 93 94 95 96 BC 9 8 7 6 5 4 3 2 1 Contributors Numbers in parentheses indicate the pages on which the authors' contribu tions begin. J. H. Andrews (245), Case Western Reserve University, Department of Physics, Cleveland, OH 44106-7079 Gregory L. Baker (433), Michigan State University, East Lansing, Ml 48824 Shahab Etemad (433), Bell Communications Research, Red Bank, NJ 07701 Ryoichi Ito (201), Department of Applied Physics, Faculty of Engineering, The University of Tokyo, Bunkyo-ku, Tokyo 113 Japan Takayoshi Kobayashi (47), Department of Physics, University pf Tokyo, Hongo, Bunkyo, Tokyo 113, Japan Takashi Kondo (201), Department of Applied Physics, Faculty of Engineering, The University of Tokyo, Bunkyo-ku, Tokyo 113, Japan Mark G. Kuzyk (299), Department of Physics, Washington State University, Pullman, WA 99164-2814 Hilary S. Lackritz (339) (formerly Hilary L. Hampsch), Department of Chemical Engineering, Purdue University, West Lafayette, IN 47907 Pierre Le Barny (379), Thomson-CSF, Laboratoire Central de Recherches, Domaine de Corbeville, 91404, Orsay Cedex, France Isabelle Ledoux (129), Centre National d'Etudes des Télécommunications, Molecular Quantum Electronics Department, 196 avenue Henri Ravera, 92220, BP-107, Bagneux, France IX X Contributors Vincent Lemoine (379), Thomson-CSF, Laboratoire Central de Recherches, Domaine de Corbeville, 91404, Orsay Cedex, France Shaul Mukamel (1), Department of Chemistry, University of Rochester, Rochester, NY 14627 Jean-François Nicoud (129), Groupe des Matériaux Organiques, Institut de Physique et Chimie des Matériaux de Strasbourg (ICPMS), 6, rue Boussin- gault, 67083, Strasbourg Cedex, France Jean Paul Pocholle (379), Thomson-CSF, Laboratoire Central de Recherches, Domaine de Corbeville, 91404, Orsay Cedex, France Constantina Poga (299), Department of Physics, Washington State University, Pullman, WA 99164-2814 Philippe Robin (379), Thomson-CSF, Laboratoire Central de Recherches, Domaine de Corbeville, 91404, Orsay Cedex, France Y. R. Shen (101), Department of Physics, University of California and Materials Science Division, Lawrence Berkeley Laboratory, Berkeley, CA 94720 Kenneth D. Singer (245), Case Western Reserve University, Department of Physics, Cleveland, OH 44106-7079 Zoltan G. Soos (433), Department of Chemistry, Princeton University, Prince ton, NJ 08544 John M. Torkelson (339), Department of Chemical Engineering, Department of Materials Science and Engineering, Northwestern University, Evanston, IL 60208 Joseph Zyss (129), Centre National d'Etudes des Télécommunications, Molec ular Quantum Electronics Department, 196 avenue Henri Ravera, 92225, BP107-Bagneux, France Preface Seven years after the publication, in that same series, of two volumes dedicated to the quadratic and cubic nonlinear properties of organic materials, time has come for an update to help the reader keep up with an active level of worldwide research effort in both fundamental and applied directions. While some of the scientific foundations of the field, as outlined in the two previous volumes, had already been firmly established at the time and have remained relatively untouched, many important pending issues were recognized at this early stage but left unsettled. Among them were the relative perspectives opened up by χ{2) and χ(3) related phenomena toward application endgoals; the comparative assets of crystalline versus polymeric materials; and the technological potential of organics, be they crystalline, polymeric, or others for integration in waveguiding passive and active optoelectronic devices, such as modulators and switches. As a result of intensive work combining the contributions of physicists, chemists, and device engineers, these basic questions have now been clarified to a significant extent with some important technological bottlenecks identified and addressed in the realm of both crystalline and polymeric materials. Owing to recent remarkable developments that encompass microlithographic patterning and organic molecular beam epitaxy, current attitudes towards the viability of an "organic technology" is much less biased by prejudice than a decade ago. Electro-optic poled polymers, as reported mainly in Part III, have matured into patternable thin-film materials compatible with sophisticated multilayer XI XU Preface architectures for integrated optics. Furthermore, their compatibility with semiconductor technology should ensure a smooth transition from present purely semiconductor-based systems to hybrid solutions whereby polymers and semiconductors are adequately combined in view of their respective advantages. Such developments have been based, and will further depend, on a deeper understanding of the physical properties of polymers, in particular as they relate to the quality of interfaces and the thermal relaxation dynamics of the induced polar order in view of its subsequent stabilization. Fundamental as well as application-related aspects have thus been closely associated in this book, with successive sections, such as 3-1, 3-3 and 3-4, jointly addressing closely related physical and technological issues from different perspectives. As far as single crystals are concerned, various demonstrations in bulk formats have confirmed their potential for frequency doubling and optical parametric oscillation based on the availability of an increasing variety of molecular and lattice structures. Furthermore, the possibility of shaping crystalline structures in waveguides, as reported in Chapter 2-3, will further enhance the prospects for single crystals in quadratic nonlinear optics with near infrared laser frequency doubling to the blue as one of the main strategic goals. Development and refinement of a molecular engineering approach toward quadratic nonlinear effects has helped point out new families of molecules more precisely targeted to satisfy precise transparency-efficiency require ments. While paranitroaniline has been serving as a dominant paradigm in the field since the mid-seventies, related dipolar intramolecular charge transfer systems may now be viewed as special cases of more general multipolar nonlinear systems. Organometallic and organomineral systems, self-assembling methods, and mesogenic nonlinear systems also appear as new and promising avenues with perspectives discussed in Chapter 2-2. Besides their intrinsic nonlinear behavior, molecules are very sensitive to environmental conditions that become dominant at interfaces; Chapter 2-1 reviews this increasingly important domain of surface science in the case of air-water interfaces. Cubic nonlinear properties are more elusive than quadratic ones, and fundamental work, of both theoretical and experimental nature, is still needed to deepen our understanding of the underlying mechanisms so as to point out relevant structural and electronic features. This important issue is dealt with in Part I (Chapters 1-1 and 1-2) and again in Chapters 3-2 and 3-5. In this context, the magnitudes of cubic nonlinear susceptibilities do not suffice to account for subtle transient processes; the position and absorption xiii Preface cross-section of two-photon peaks as well as the dynamical features of induced absorption from photoexcited states play a dominant role. Advances in theoretical approaches (Chapter 1-1) and femtosecond probing techniques (Chapter 1-2) have significantly contributed to these issues. While amorphous side-chain polymers qualify for quadratic nonlinear optics and conjugated ones for cubic effects, it would nevertheless seem erroneous to confine crystals to quadratic applications; results that have appeared in course of publishing this book tend to blur this barrier as they point out the relevance for χ(3), by way of cascading mechanisms, of molecular crystals initially conceived for χ(2). While it may still be premature to predict the nature and extent of application breakthroughs in this field, the last decade has undoubtedly seen molecular nonlinear optics acquire full-fledged scientific status with recog nition from both physicists and chemists. Few other fields have benefitted to such extent from a fruitful and convergent cooperation between the two communities, although motivations may be somewhat different: Nonlinear susceptibilities at both microscopic and macroscopic levels have now come to be widely adopted by chemists in view of their enhanced sensitivity to such features as conjugation, charge transfer, protonation, environment and local field effects, and crystalline or polymeric organization. As a consequence β and y susceptibilities are now increasingly finding their way in chemical data bases, alongside absorption, dipole moment, dielectric constant, and other more traditional physico-chemical data. Conversely, physicists are attracted by the virtually unlimited possibilities to implement and manipulate properties at the ultimate molecular level. Advanced "guided" organic synthesis and molecular assembling techniques, combined with newly de veloped experimental tools, capable of probing and manipulating molecules at ultimate spatial, spectral, and time resolutions, are bound to open up a new frontier in molecular optical sciences. This book will have fulfilled its dual goal when supplying state-of-the-art information to the benefit of currently committed scientists while helping renew the field by attracting needed future contributors. JOSEPH ZYSS Chapter 1 MANY-BODY EFFECTS IN NONLINEAR SUSCEPTIBILITIES; BEYOND THE LOCAL-FIELD APPROXIMATION Shaul Mukamel Department of Chemistry, University of Rochester, Rochester, New York 1. INTRODUCTION 2 2. MODEL HAMILTONIAN FOR MOLECULAR MATERIALS; FRENKEL-EXCITONS 5 3. EQUATIONS OF MOTION: THE ANHARMONIC-OSCILLATORS PICTURE 7 4. THE SINGLE-PARTICLE LEVEL AND THE LOCAL-FIELD APPROXIMATION 11 5. THE ROLE OF TWO-EXCITON VARIABLES: ENHANCED NONLINEAR SUSCEPTIBILITIES IN MOLECULAR AGGREGATES 13 6. EXCITON-POPULATÏON VARIABLES AND EXCITON TRANSPORT 20 6.1 Interaction-Induced Extra Resonances: Degenerate Four-Wave Mixing 20 6.2 Exciton Transport in Real Space: The Wigner Representation . . .. 24 6.3 Transient Grating: The Time-Domain Analogue of Degenerate Four-Wave Mixing 25 7. GREEN FUNCTION EXPRESSIONS FOR χ(3) IN MOLECULAR NANOSTRUCTURES WITH ARBITRARY GEOMETRY 26 8. DISCUSSION 39 ACKNOWLEDGMENTS 42 REFERENCES 42 All rights of reproduction in any form reserved. ISBN 0-12-784450-3 2 Shaul Mukamel 1. INTRODUCTION The systematic calculation of nonlinear susceptibilities of optical materials, and the precise relationship between individual molecular hyperpolarizablities and the macroscopic optical response, constitute a complex challenge that has drawn a considerable theoretical attention [1-5]. The design of new optical materials with specified characteristics (fast switching, large suscepti bilities) and the interpretation of nonlinear optical measurements in terms of molecular properties and intermolecular forces require the development of a suitable theoretical framework. The response of a medium to optical fields is most conveniently formulated in terms of wave vector and frequency-dependent optical susceptibilities, which are the expansion coefficients of the macroscopic polarization field in powers of the Maxwell electric field E [6-9]. The problem of incorporating intermolecular forces in the linear optical response (i.e., the dielectric function) has a long history [10-15]. The local-field approximation provides a simple way to relate the polarizabilities of isolated molecules to the macroscopic susceptibilities. In this approach, the effect of intermolecular forces is included in an effective local electric field. The problem of calculating the response of an interacting ensemble of molecules to the electromagnetic field is then reduced to the response of isolated molecules interacting with the local field E , through an interaction Hamiltonian — μ-Ε^ where μ denotes L the molecular dipole operator. The Lorentz relation between the local field and the Maxwell field E (Eq. 1.4.2) can then be used to calculate the dielectric function. This procedure, which reduces the complex many-body problem to a single-body problem, was subsequently generalized and applied also to the calculation of nonlinear susceptibilities [3,5,9,16-18]. The nonlinear susceptibilities at a given order are then given in terms of sums of products of molecular polarizabilities of that order and lower orders. This simple, back-of-the-envelope calculation of macroscopic susceptibilities is, however, not rigorous [5,19]. It fails to take properly into account the correlated dynamics of the interacting many-body system, i.e. correlations among the molecules, as well as correlations between the molecules and the radiation field. Short-range (e.g., exchange) forces are totally neglected in this proce dure. Moreover, even the dipole-dipole forces are not fully taken into account. In addition, the resulting susceptibilities do not depend on the wavevectors, apart from the local-field contribution, but just on the frequen cies. This indicates that processes such as exciton migration and energy transfer and transport (e.g., the Forster transfer) [20,21] are neglected. Such processes are often added phenomenologically in order to interpret transient
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