Organic Materials for Photonics. Science and Technology

407 Pages·1993·7.594 MB·English

by  G. Zerbi (Eds.)

EUROPEAN MATERIALS RESEARCH SOCIETY MONOGRAPHS, Volume 6
ORGANIC MATERIALS
FOR PHOTONICS
Science and Technology
Edited by
G. ZERBI
Dipartimento di Chimica Industriale e
Ingegneria Chimica Giulio Natta,
Politecnico di Milano, Italy
1993
NORTH-HOLLAND
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Library of Congress Cataloglng-ln-PublIcatIon Data
Organic materials for photonics ·. science and technology / edited by
G. Zerbi.
p. cm. — (European Materials Research Society monographs ;
v. 6)
"Summer School on Organic Materials for Photonics was held in
Obereggen, Italy"—Pref.
ISBN 0-444-89916-2 (acid-free paper)
1. Optoelectronic devices—Materials—Congresses. 2. Photonics-
-Materials—Congresses; 3. Polymers—Congresses. I. Zerbi,
Giuseppe. II. Series.
TA1750.073 1993
620. Γ 1795—dc20 93-1775:
CIP
ISBN 0-444-89916-2
© 1993 Elsevier Science Publishers B.V. All rights reserved.
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Preface
In August 1991 a "SUMMER SCHOOL ON ORGANIC MATERIALS FOR
PHOTONICS" was held in Obereggen, Italy, organized by the EMRS-NETWORK
ON POLYCONJUGATED POLYMERS with the sponsorship of the Max Planck Institut
für Polymerforschung of Mainz, Germany. The purpose of the School was to present
students with a new field of Material Science which is being developed and which may
have a relevant impact in future technologies. This book contains most of the notes of
the lectures given at the School.
The editing,of the book took longer than the time predicted and the final editing
was taken care of by the ESPRIT-NETWORK OF EXCELLENCE ON ORGANIC MA-
TERIALS FOR ELECTRONICS (NEOME) which has derived from the EMRS Network.
It is presently known that poly conjugated organic materials show peculiar electrical
and non-linear optical properties; this fact has opened a whole new field of Material
Science aimed at the development of new technologies. The non-linear optical responses
of this class of organic materials are presently an active field of research both in academic
and industrial laboratories.
Optical non-linearities represent a challenge to the theoretical physicists for the un-
derstanding of the interactions of light with matter, expecially in dense media.
For many years non linear optical properties were observed and studied mostly for
inorganic materials which are, by tradition, closer to the physicist's feeling. As soon
as it has been discovered that organic molecules may show larger and faster responses,
physical chemists and organic chemists stepped into the field with the wish to understand
the physical phenomena at the molecular level and with the hope to synthesize new and
better molecular systems. The dream to obtain new devices with outstanding properties
is the intellectual force which drives the efforts of many researchers.
The field has grown very rapidly in the past few years and is still developing thanks
to the interdisciplinary collaboration of many researchers who have decided to join their
efforts in a new field of the Science of Organic Materials.
Because of the variety of problems and techniques involved, students and beginners
with different backgrounds who approach poly conjugated materials do not find it easy to
enter the field and to become constructively productive.
The aim of this book is to introduce in a tutorial way the necessary concepts and
to quote the relevant references which may help the student to grasp the fundamental
concepts and to perceive the relations between:
1. The physics of each molecule taken first as an independent object when placed in a.
beam of light
2. The physics of the molecules organized in a tridimensional network when illuminated
vi Preface
by an intense electromagnetic wave
3. The use of the new optical properties for the inventions of new devices and new
technologies.
This book is divided in four sections:
In section 1, INTRODUCTION W.Frank presents the technological aspects of
Optronics defined as a new field of technology which uses light for transmitting, processing
and storing signals which carry any kind of information. Since technology in Optronics
is based on properties which are directly linked to the architecture and distribution of
electrons in each single molecule, the molecular aspects cannot be overlooked.
The case of ISOLATED MOLECULES (chapts. 2 through 7) is treated in section
2 which deals with the optical properties of molecules as isolated objects. In chapter 2
(M.Gussoni, C.Castiglioni, M.Del Zoppo and G.Zerbi) the theoretical principles of molec-
ular optics are presented in order to make the students understand the molecular bases
of the processes of light absorption, light scattering and light emission. After a detailed
presentation of the general principles of the theory of the interaction of light with matter
at the molecular level, the theory aims at those aspects directly related to non linear
responses of poly conjugated systems. In chapter 3 B.Kohler reviews the information that
can be obtained from optical absorption and emission experiments most relevant to con-
jugated organic molecules. Since optical excitation to excited electronic states and their
ultrafast relaxations are the basis for NLO properties in chapter 4 H.Bieter presents the
state of the art of our understanding in this field. The experimental problems for obtain-
ing and measuring pulses of light in the picosecond and femtosecond time domains to be
used as probes of the electronic excitations are presented in chapter 5 by Z. Valy Vardeny.
All experiments need interpretation or the results of the experiments should be predicted
by a quantum chemical approach capable to treat molecules as a whole and as realis-
tic objects without simplifications. This problem is faced by J. L. Bredas in chapter 6
where he presents a critical survey of the quantum chemical techniques used in the de-
scription of conjugated molecules and polymers. Quantum Mechanics is then used for the
prediction of microscopic (molecular) polarizabilities and hyperpolarizabilities. Since the
previous chapters have treated theories and have presented simple cases as paradigma for
more complex systems the problems of the chemistry of poly conjugated materials cannot
be overlooked. The paradigma cases of the chemistry of poly conjugated molecules are
presented in chapter 7 by W. J. Feast.
Section 3 , OPTICS OF CONDENSED MEDIA (chpts. 8 through 10) contains
extensive discussions of the theoretical and experimental aspects of the propagation and
coupling of electromagnetic waves in condensed media. The basic physics is presented in
chapter 8 by G. Leising while the problem associated with quadratic non linear responses
(χ(2)) are discussed by /. Ledoux in chapter 9. The experimental methods for the mea-
surements of non-linear optical susceptibilities in condensed media are reviewed by C.
Bubeck in chapter 10.
Preface vii
Section 4, APPLICATIONS AND DEVICES (chpts 11 through 18) represents
one of the first attempts to present the students with the state of the art in the field of
applications of the NLO properties of organic materials. Unfortunately the present views
of the politics of science in most of the developed countries which determine funding is
that Basic Science is successful and worth of support only if it carries (possibly in a short
time) to realistic technological achievements of interest to industry. Our approach is to
prefer a humble critical description on what has been already achieved, on what are the
hopes and the problems still to be solved. We wish to avoid the pompous fanfare on
glorious future accomplishments (generally defined as heuristic) which are certainly not
around the corner for a certain time. We believe, however, that the applications will
certainly be more numerous in the near future and will be of interest to many industries.
In section 4 experts from universities or from industries report on what has been
already achieved and on what is being presently done. While organic χ^ 2) materials
are more mature for device fabrication as discussed by P. Robin (chapter 11) and by
G. Mbhlmann (chapter 12), χ^3) materials are the center of intense activity, but are in
their infancy when applications are considered. The case of χ( 3) materials for all-optical
switching is discussed by S. Etemad in chapter 13.
Since optical waveguides are the basic structures in optical integrated circuits the
principles and the problems of optical waveguiding are discussed by A. P. Persoons in
chapter 14 and by W. Knoll in chapter 15. R. Rubner, S. Birkle, R. Leischner and M.
Sebald describe the various techniques for patterning functional layers using organic mate-
rials as photoresists. These techniques have been already widely used in microelectronics
and are becoming increasingly important for the fabrication of NLO devices.
The impact of organic materials in Optical Storage (G. H. Werumeus Buning, chapter 17)
and in Digital Optical Computing (Κ. H. Brenner, chapter 18) is clearly presented. New
classes of poly conjugated materials are seen to play a relevant role in these developing
modern technologies.
The organization of the Summer School and the editing of this book were made possible
by the hard work and the friendly and kind collaboration of my coworkers Dr. M. Gussoni,
Dr. C. Castiglioni, Dr. M. Del Zoppo, Dr. M. Veronelli, of Mrs. F. Ranucci and of my
daughter Carlotta. I am very grateful to all of them.
Giuseppe Zerbi
Chairman of NEOME
Dipartimento di chimica industriale, Politecnico di Milano
Milano, February 1993.
xi
Polarizabilities:Units of Measurement
1 MOLECULAR Ρ OL ARIZ ABILITIES
SI esu a.u.
[[μμ]] Cm statC cm bohr e
[[aa]] c2 m2 J"1 statC2 cm2 erg 1 bohr*
[β] c3 m2 J"2 statC* cm* erg _9 bohr** e-1
h) c4 m 4 J"3 statC* cm4 erg~* bohr7 e~2
2 BULK POLARIZABILITIES
SI esu
(«. = i)
p(n) η Ε η Cm-2 statV cm~l
= € ο χ
X" (m y-1)"-1 {cm statV~l)n-1
Ε y m"1 statV cm'1
3 CONVERSION FACTORS
[statC] = [cmigts \
[statV]
1 statC
— 2.998 υ ι °
1 statV = 2.998 · 102y
1 &o/ir = 0.52917 · 10"8cm
le = 4.803 · 1010es«
ORGANIC MATERIALS FOR PHOTONICS
Science and Technology
G. Zerbi (Editor) 3
© 1993 Elsevier Science Publishers B.V. All rights reserved.
Optronics
W. Frank
Forschungsinstitut der Deutschen Bundespost Telekom
Am Kavalleriesand 3, D-6100 Darmstadt, Germany
Abstract
The requirements for specific devices for guiding, routing, switching and multiplexing
of light in an optical telecommunications system are presented. Suitable materials for
these purposes inorganic as well as organic are discussed. The basic principles of influ-
encing optical behaviour of matter by electric fields are explained. Organic materials are
highly favorized for future applications when inexpensive mass production of devices will
be required.
1. INTRODUCTION
The use of optical as well as electrical signals in modern information technology has
created a great variety of words to describe the essentials of the new technology fields :
photonics, optical electronics, electro-optics, photo-electronics, Optronics etc.. The overlap
of these areas has grown to such an extend that we can consider them to be synonyms
for the same subject:
Technologies using "light" and electrons for transmitting, processing and storing signals
which carry any kind of information.
When talking of "light", may it be either visible or infrared, the idea is it should extend
its domain on expense of the use of electronics. We will discuss this new field using the
example of optical telecommunications, and begin with the requirements of present and
future technology for optical digital signal transmission. The needs for special devices
will be explained and suitable materials for manufacturing them will be discussed and
compared in respect of potentially low costs when produced in large amounts.
2. ANALYSIS OF THE PRESENT SITUATION
The common understanding of the topic "optical telecommunications" leads to the
suggestion that here is an upcoming technology with new and alternative methods to
replace the traditional electronic telecommunications technology. If we analyse this con-
cept, especially if we ask : "How 'optical' is optical telecommunications today?" we find
an answer which is rather disappointing : only one unique function is optical, what is
depicted in Figure 1 (for further reading see textbooks [1], [2], [3], [4], [5]).
4 W. Frank
In Figure 1 a general block diagram of an information transmission system is shown.
In order to elucidate the basic concept of telecommunications we will restrict our consi-
derations to a telephone system. Only the transmission from the emitting diode to the
receiving photodiode has something to do with optics; normally the transmission channel
is a single mode fiber made of silica. For silica optical fibers see [7]. All other steps such
as encoding of the source signal, switching or lumping of several phone calls for transmis-
sion by a single fiber (time multiplexing, Figure 3 ) are processed in traditional electronic
technology.
optical
Transmitter Receiver
Regenerator
code decode
Figure 1. Block diagram of an optical transmission system. The only "op-
tical" element is the fiber between the laser diode and the photo-
diode.
Basic optical concepts such as phase control or definite polarization are not important.
Interference of waves, just the most powerful tool in understanding propagation and in-
teraction of electromagnetic waves, is considered as an undesired perturbation. Concepts
from crystal optics such as anisotropy of the refractive index, also known as birefringence,
may it be naturally or "man made", is undesired. There is also no frequency manipulation
that is: switching the signal from one carrier frequency to another.
Considered this way the present state of the art in "optical" telecommunications does
not deserve the proud name "optical". This statement, however, does not mean that we
want to underestimate that concept. On the contrary : we will discuss the basic principles
of different functions of the transmission process and learn the kind of possibilities hidden
in the concept of optics for the use in future telecommunications systems. The objective
is to expand the optical domain beyond the pure point-to-point transmission to the region
of signal processing, as far as possible.
Optronics 5
3. ASPECTS OF A PURELY OPTICAL TRANSMISSION SYSTEM
In order to reach high transmission capacity (e.g. thousands of phone calls via a single
channel) the process of transmission must be digital.
125 \isec
Voice signal
(microphone)
Sampled signal
digital signal
II t
« 15 [jsec
Figure 2. Principle of digitalization of an electrical microphone signal.
Above : An analogue voltage signal from a microphone. Mid :
"Sampling" of the voltage signal. Below : Quantization of sam-
ples resulting in pulse sequences.
The concept of digitizing is depicted in Figure 2 , e.g. of a time dependent acousti-
cal signal converted into an electrical one. Using the "sampling theorem" of information
theory [1], found by Shannon, one of the elementary principles of all digital telecom-
munications, it is sufficient to take "samples" from the continuous signal at certain time
Intervalls, which are a little shorter than the inverse of the double value of the transmission
bandwidth. In our example, this means that if we want to transmit a bandwidth for the
human voice of 3.6 kHz sufficient for good understanding during phone calls (identifying
the speaking partner by voice!) every 125 //sec a "sample" is taken from the signal. The
rest is abandoned. It is the essential statement of Shannon's theorem, that the sequence
of samples taken in that manner contains the complete information. The remaining time
difference between two pulses can now be used to represent the actual value of a sample
by binary coded pulse sequences, the so called quantization. So the density of pulses is
64 kbit/sec with a 4 bit coding and the duration of the single pulse is 15 //sec.
If we want to increase the number of phone calls transmitted through one channel (e.g.
an optical fiber) we have to stagger the pulse sequences of the phone calls transmitted
and shorten the single pulses. For instance : When staggering 2000 phone calls results in
a transmission bit rate of 140 Mbit/sec the duration of a single pulse shrinks to 7 nsec.
This process is called "time multiplexing", see Figure 3 .
6 W. Frank
C3 subscriber
DMUX^\
Figure 3. Staggering different channels of phone calls, time multiplexing,
MUX and demultiplexing, DMUX
Leaving this process for the moment within the electrical domain we can control a
laser diode which converts them into light pulses in the infrared region of the spectrum,
e.g. at 1300 nm. ("Light pulse" means that the laser diode is switched on and off for the
time of the pulse duration emitting its quasi monochromatic frequency). The light pulses
generated by the laser diode are coupled into an optical fiber.
The next step is to increase the transmission capacity of that fiber. The simplest
process is to use further optical carrier frequencies. With proposition of linear behaviour
of the system different carrier frequencies have no mutual interaction. This procedure is
called "wavelengths division multiplexing" (WDM) and is depicted in Figure 4 .
Figure 4. Different optical carrier frequencies are fed into one optical fiber,
wavelength division multiplex, WDM.
So as to get closer to the demands of optical communications today and in the future,
we have to look at the laser. In order to use the transmission capacity of modern single
mode fibers, i.e. in the Gbit/sec region, the laser needs to be indirectly modulated.
Present modulation frequencies are used directly up to 10 Gbit/sec, see Figure 5 a.