Nonlinear Optical Properties of Organic Molecules and Crystals. Volume 1

481 Pages·1987·8.215 MB·English

by  D.S. Chemla (Eds.)

NONLINEAR OPTICAL
PROPERTIES OF
ORGANIC MOLECULES
AND CRYSTALS
Volume 1
Edited by
D. S. Chemla
AT&T Bell Laboratories
Crawford Hill Laboratory
Holmdel, New Jersey
J. Zyss
Centre National d'Etudes des Télécommunications
Laboratoire de Bagneux
Bagneux, France
1987
AKT
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ACADEMIC PRESS, INC.
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Library of Congress Cataloging in Publication Data
Nonlinear optical properties of organic molecules
and crystals.
(Quantum electronics—principles and applications)
Includes indexes.
1. Molecular crystals—Optical properties.
2. Chemistry, Physical organic. I. Chemla, D. S.
II. Zyss, J. III. Series.
QD941.N66 1986 547.Γ3 86-8070
ISBN 0-12-170611-7 (v. 1 : alk. paper)
PRNITED IN THE UNTIED STATES OF AMERCI A
86 87 88 89 9 8 7 6 5 4 3 2 1
Preface: New Materials for a New Age
I. Concepts, Purpose, and Scope
The postindustrial societies, sometimes referred to as information-based
societies, urgent demand for an always increasing volume of information
processing and communication is continuously challenging modern
technology. To date most of the materials used for devices are minerals,
i.e., metals or semiconductors. Yet in almost all the cases, the best
performing and most reliable systems are living organisms, and the drive of
science has always been, consciously or not, to imitate them as closely as
possible. The latest step of the physical sciences in this direction is the idea
of using artificial "neural networks" for massive parallel computation. On
the other hand, the latest developments of molecular biology show that the
concentration of microscopic responses at each structural level in a living
organism results from the specific properties of organic molecules as such.
This last remark leads us naturally to wonder why evolution has lead to the
selection of this class of compounds. What particular features bestow them
with such selective, sensitive, fast, and intense responses to all sort of exter-
nal stimulations?
In the wake of these interrogations, it is very tempting to envision a new
technology that would exploit the remarkable properties of organic
molecules. However, it is now clearly established that the development of a
modern technology is necessarily rooted in a detailed understanding of the
materials involved and thus requires surpassing a knowledge-base
threshold. In the four centuries of development of modern science, we have
only begun to understand even the simplest physical phenomena. Thus it is
natural that technologies based on simple metals and, later, on simple
semiconductors were implemented first. Considering the tremendous in-
crease in complexity when moving from simple siliconlike structures to
organics and further to biological systems, this knowledge-base threshold is
certainly not around the corner for the more complex materials. However,
it would be a naive and perhaps dangerous view to reject these two latter do-
mains in an oversimplified, short-sighted, and linear vision of the development
ix
χ Preface
of science and technology. This is particularly true since a closer look at
organic science demonstrates that it is rooted in ancient and vast, although
so far poorly exploited, layers of technological know-how.
The approach toward material optimization in the field of organics runs
surprisingly parallel with but in the opposite sense to that which currently
leads semiconductor-based electrophotonics from bulk materials to quan-
tum wells, quantum wires, quantum dots, or grade-gap microstructures.
These new sophisticated semiconductor-based architectures are the up-to-
date advances in the evolution from well-mastered bulk technologies of
limited flexibility toward microscopically scaled technologies where optical
and electronic properties can be manipulated and tailored for applications.
This latter stage is backed by the considerable amount of know-how
generated at earlier levels of development. We are forced to recognize that
however interesting was the theoretical evidence that semiconductor band
structure could be engineered at the microscopic scale, such predictions
would not of themselves have spurred the development ex nihilo of the ap-
propriate growth, fabrication, and preparation techniques. Moreover,
manipulations of semiconductor properties are based on the control of
composition or of impurity concentration. Thus they involve brutal
4'physical" methods for introducing these activating heteroatoms that tend
to disturb the host lattices significantly (e.g., through ion implantation,
reactive ion etching, or epitaxial growth). In contrast, the chemical methods
used for predictive manipulation at the molecular level have developed into
a variety of mild, well-mastered, industrially oriented techniques based on
the virtually unlimited possibilities of organic synthesis. They are also more
controllable and flexible than the injection of dopants or compositional
changes in semiconductors. However, the development of devices requires
the organization of the optimized molecules into larger structures meeting
symmetry and shaping constraints imposed by macroscopic physical and
operational requirements. The control of this coherent arrangement of
microscopic building units is much less developed, and the rich possibilities
of organic chemistry are just now starting to be mature enough to open
promising avenues in this direction.
For historical reasons a number of transistorlike electronic devices using
electric charge transport have developed quite independently from photonic
devices using absorption, emission, and modulation of light. It is evident
that the two approaches, far from being opposed, are complementary and
intimately related. In fact the ultimate goal of an information technology
based on electromagnetic forces is a control of the electron state (source and
detector of the electromagnetic field) and photon state (vector of the elec-
tromagnetic field) limited only by quantum mechanics. At their common
root stands the symmetric current field (J ·Α - ρ φ) interaction. The
ultimate electromagnetic device will fuse both concepts at the microscopic
Preface xi
level in the conceptual and technological senses. In semiconductors the doping
by impurities introduces a micrometer scale asymmetry that is responsible for
the operation of all diodelike devices. By analogy an inter- or intramolecular
charge transfer between the donor and acceptor ends of a conjugated molecule
introduces a nanometer-scaled asymmetry already used in optical parametric
amplifiers and soon to be extended to other functions.
The artificial distinction between linear and nonlinear optics is now ob-
solete. Optics is in essence nonlinear. Both absorption and emission processes
were recognized and analyzed as such before the advent of lasers. Also consis-
tent with this assertion is the early observation of such "nonlinear processes"
as the Pockels, Kerr, and Faraday effects at the turn of the century or the
theoretical evaluation of two-photon optical transitions at the very beginning
of the development of quantum mechanics. It was nevertheless the spectacular
observation of frequency-mixing processes in the visible and the further
demonstration of sizable quantities of multiphoton processes following the in-
vention of the laser that led pioneers to invent the term "nonlinear optics" in
order to make room for new concepts against the mainstream of optics at the
time. A quarter of a century afterward, the distinction between " nonlinear
optics" and "optics" does not make any more sense than the previous one be-
tween "optics" and "classical optics" or the similar one between "music" and
"classical music." Furthermore, this usage wrongly induces the nonspecialist
to consider this domain as a rather highly specialized if not obscure subfield of
optics.
Within this context, the need for a unified presentation of the nonlinear op-
tical effects in organic molecules and crystals emerged quite naturally. The
development of the subject matter covered in this book obviously results from
a cross-fertilization among weakly related disciplines: chemistry, optics,
crystallography, electronics, applied mathematics, system architecture, etc.
Thus multidisciplinarity appears in the book because it is actually intrinsic to
the subject. The book is meant to be self-consistent. For example, the chemist
should find a clear treatment of the physics he lacks just as the physicist should
fine the introduction to the chemistry he needs. Furthermore, the solid-state
physicist with a previous background in, for example, band structure and the
related description of opto-electronic processes in semiconductors should find
the relevant material to make the link with the formalism used to describe the
corresponding processes in organics. We have also purposely meant in our
choice of contents to treat on equal footing technology and fundamental
science, since it is well recognized that each benefits from the other to a point
that such a distinction has become obsolete.
II. Content Description
The editors had to face a difficult choice between equally relevant
organizations for this book either by classes of materials (polymers,
xii Preface
molecules, ordered or quasi-ordered phases, etc.) or by types of optical
phenomena. The final choice was rather arbitrarily set on the classical distinc-
tion between quadratic (i.e., three-wave mixing) and cubic (i.e., four-wave
mixing) processes. We are aware of all sorts of limitations linked to this
choice, such as the necessity for a nonperturbative and nonlocal description of
(nonlinear) optical processes at high excitation levels in strongly delocalized
systems. In practice, however, most of the subject matter covered in this book
fits readily into such a classification. The materials, the symmetry constraints,
and the experimental techniques differ considerably from one section to the
other, leading to a natural diversification of working groups following this
scheme.
Professor Silbey introduces the book with a general overview of the basic
properties of organic matter. The two following chapters by J. Zyss and D. S.
Chemla and by J. F. Nicoud and R. L. Twieg are meant to serve a double pur-
pose, bridging the gap between more chemically and more physically oriented
readers and orienting them in both fields, from basic notions to state-of-the-
art quadratic optics in molecules and molecular crystals. The first of these two
chapters emphasizes the physical aspects of the subject, whereas the second
concentrates on the chemical aspects. Although readers will understandably
tend to consider first the chapter that best fits their own backgrounds, it is
highly recommended to try to overcome natural inhibitions and discover a dif-
ferent approach from the usual one so as to experiment with the idea that the
benefits of stereoscopic vision originate from doubled vision, in science as well
as in real life. Professor J. Morley and Dr. D. Pugh demonstrate in Chapter
II-3 the benefits derived from quantum-chemistry-based models so as to pro-
vide the synthetic chemist with cost- and timesaving guidelines, which help
each chemist find the way within the unlimited but otherwise embarrassing
jungle of organic molecules. Chapter II-4 by Drs. J. Badan, R. Hierle, A.
Perigaud, and D. Vidakovic deals with the technologically crucial bottleneck
of growth and characterization of high quality, bulk organic crystals and
waveguided structures. By contrast, Chapter II-5 by A. Barraud and M.
Vandevyver introduces the growth of thin films by the Langmuir-Blodgett
technique. A case study focusing on a specific material, namely urea, which
exemplifies a situation in which transparency in the UV region has been pur-
posely traded for nonlinear efficiency, is presented in Chapter II-6 by Pro-
fessor C. L. Tang and Dr. J. M. Halbout. This chapter illustrates the suc-
cessful strategy of a team concentrating all of its efforts on a single
molecular crystal as the unique target, with efforts ranging from molecular
solution measurements to the first demonstration of optical parametric
oscillation in an organic material. In Chapter II-7, Dr. D. J. Williams
discusses another type of trade off between the unpredictable orientation of
molecules in crystalline media and the possibility of acting on the polarity of
liquid-crystalline structures. In the final chapter of Part II, Drs. K. D. Singer,
Preface xiii
S. L. Lalama, J.E. Sohn, andR. D. Small introduce and link electro-optic
to other effects second-order optical processes with special emphasis on
organics. They show the dominant electronic contribution to the electro-optic
effect in such media as opposed to inorganic compounds.
Section III in Volume II is dedicated to third-order processes. Here the
polydiacetylene family is singled out for the moment because of its many ap-
pealing physicochemical features. Foremost among these is its unique
topochemical polymerization potential. This family is introduced by Pro-
fessors M. Schott and G. Wegner in Chapter III-l. Experimental studies of
third-order processes in these compounds are presented and discussed in
Chapters III-2 and III-3 by Drs. F. Kajzar and J. Messier and by Drs. G. M.
Carter, Y. J. Chen, M. F. Rubner, M. K. Thakur, S. K. Tripathy, and D. J.
Sandman, respectively. Related theoretical aspects are discussed in Chapters
III-4 and III-5: Professor C. Flytzanis presents a general discussion of the
essential relation between system dimensionality and optical response. In a
complementary approach, Professors J. M. André and J. Delhalle present
detailed computations on specific oligomeric systems and resulting trends,
strongly suggesting that polymeric structures, beyond polydiacetylenes, can
also be "engineered" in very much the same way as was previously established
for molecular systems and quadratic effects. The important aspects of
multiphoton spectroscopy as a tool to disentangle some of the previously
nonaccessible features of molecular-matter-excited states is surveyed in
Chapter III-6 by Dr. B. Dick and Professors R. Hochstrasser and H. P.
Trommsdorff. The concluding comment by Professor J. M. Lehn points out
one of the many avenues to be explored in the near future by applying
previously reported, however unrelated, modern concepts of chemistry to op-
tics. The up-to-date appendixes at the end of Volume II, expertly compiled by
Drs. J. F. Nicoud and R. Twieg, will certainly provide a much needed and
previously missing experimental data bank on powder SHG tests and
molecular second-order hyperpolarizability of organics. It is the editors' hope,
in view of the large amount of work that remains to be done in organics, that
the publication of such data banks will avoid unnecessary and time-consuming
duplications.
We anticipate that in such a rapidly evolving field an update or extension of
this book will be needed within the next few years. It is, however, our belief
that most of the material covered herein will be at the root of these further
developments, although unpredictable breakthroughs are also to be expected.
We apologize to those research teams who rightfully consider that they should
have appeared in the book, but time and space were severe constraints to
which we had to yield. We hope that any future edition will correct any serious
omission.
NONLINEAR OPTICAL PROPERTIES OF ORGANIC MOLECULES AND CRYSTALS, VOL. 1
Chapter |-1
The Structure and Properties of the
Organic Solid State
ROBERT SILBEY
Department of Chemistry
Massachusetts Institute of Technology
Cambridge, Massachusetts 02139
I. INTRODUCTION
In most textbooks on the solid state, molecular solids are given at most a few
paragraphs, and organic solids are never mentioned at all (the solid noble
gases being the "best example of molecular solids" [1]). This is unfortunate
since organic solids have an enormous range of interesting properties that are
almost continuously "tunable." For the last 15 years there has been increasing
awareness of these systems as their more interesting properties have been
uncovered: for example, superconductivity [2], spinless conductivity in
"doped" conjugated polymers [3], and large nonlinear optical responses [4].
In this review, I discuss the structure and properties of organic solids in a
general and, hence, incomplete way with examples taken from the recent
literature. It is clearly impossible to cover this field adequately in a few pages; I
hope only to give the flavor of its vastness and variations, and, since my own
3
Copyright © 1987 by AT&T Bell Laboratories, Incorporated.
All rights of reproduction in any form reserved.
4 Robert Silbey
field of research is the theory of optical properties, conductivity, and
relaxation, the review is undoubtedly skewed in these directions. However, I
hope the reader will come away with the feeling that understanding the
properties of organic materials is an interesting and exciting challenge.
II. STRUCTURE
A. Crystal Structures
Most organic solids are molecular solids; that is, the molecules retain their
geometric shape and properties to a large degree upon condensation from the
vapor. Thus, benzene, for example, is (almost) hexagonal in the crystal with
only small changes from the gas-phase molecule. In addition, the infrared and
low-energy optical spectra of the solid are very similar to those of the gas. To
be sure, there are changes; these are, however, small compared to those
occurring in typical ionic crystals or metals. Finally, organic solids have small
cohesive energies. Therefore, most organic solids are collections of molecules
held together by weak intermolecular interactions, usually described by van
der Waals forces. Because the molecules are non-simply shaped and the forces
are weak and anisotropic, organic solids (1) often have more complicated
structures than solids normally studied in elementary books on solid state
physics, (2) exhibit polymorphism and many phase transitions, and (3) often
have many structural defects.
Although a first-principles quantum-mechanical calculation of the equili-
brium geometric structure of a typical organic crystal is impossible at present,
phenomenological atom-atom two-body potentials to compute intermole-
cular interactions are available and useful. That is, assuming the molecules
have a certain geometry (given perhaps by the gas-phase data), we can com-
pute intermolecular forces by adding atom-atom (central force) interactions.
This gives a highly anisotropic molecule-molecule interaction. The atom-
atom interactions are usually assumed to be [5,6] either Lennard-Jones or
Buckingham potentials:
(la)
u(r) (lb)
B
where the parameters for carbon-carbon, carbon-hydrogen, and hyrdogen-
hydrogen interactions are fixed by fitting a few model compounds (typically
anthracene, naphthalene, etc.). Using these potentials, it is possible to compute
1-1. The Structure and Properties of the Organic Solid State 5
crystal structures, thermodynamic functions, phonon frequencies, and others
in reasonable agreement with experimental data. Although various authors
have chosen different atom-atom parameters [5,6], the final results of their
calculations do not differ markedly. Since the mid 1970s, these methods have
also been applied to the calculation of structural defects [7].
When Robertson [8] reviewed the organic crystals studied by X rays in the
early 1950s, there were 102 structures known; when Dunitz [9] reviewed the
field in the late 1970s, there were 10 4 structures known; in the late 1980s, the
number is probably > 105. The ability to study complex crystals of complex
molecules using automated equipment and sophisticated computer software
and hardware has led to an explosion in our knowledge of the static structure
of these systems. This does not mean, of course, that our understanding of the
other properties of these systems has increased in the same proportion.
Since 1960, the polyacenes have been the model organic molecular crystals
for studies of static structure, optical spectra, dynamic studies, etc. Anthracene,
naphthalene, and tetracene are relatively simple systems and exhibit many of
the typical behaviors of organic crystals. Anthracene is a planar conjugated
system with D symmetry in the gas phase. The normal crystal structure was
2h
determined by Robertson [8] and Cruikshank [10], and the molecule in the
solid is remarkably similar to the gas-phase structure (found by electron
diffraction) [11]. The crystal structure is monoclinic (space group Pljd) with
two molecules in the unit cell, related to each other by a screw axis (parallel to
the b crystal axis). Naphthalene has an almost identical structure to
anthracene, while tetracene and pentacene are very similar; however, the two
molecules in the unit cell in the latter two cases are not related by screw axis
symmetry as in naphthalene and anthracene. The crystal structures of pyrene
and a-perylene are different from these by the fact that a dimer of pyrene takes
the place of a single anthracene molecule [11]. This structure is quite
interesting because the dimer then has considerable π-electron overlap, which
is undoubtedly the reason for excimer formation [12] in these systems.
As already mentioned, these crystals often have phase transitions to other,
more closely packed structures. (In addition, metastable forms of anthracene
crystals [13] have been found under high pressure.)
Although the structure of the polyacenes is quite common in aromatic
crystals, another structure is also often found. This is a linear stacked structure
often found in halogenated aromatics, as in tetrachlorobenzene and 1,4-
dibromonaphthalene. In this case, the closest molecules form a stack of
aromatic planes tilted (i.e., nonperpendicular) in general to the stack axis.
There may be one or more molecules in the unit cell, either crystallographi-
cally equivalent or not. In contrast, the benzene crystal is orthorhombic with
four molecules in the unit cell related by screw axes parallel to the three crystal
axes.