Molecular Optics: Nonlinear Optical Properties of Organic and

1 Molecular Optics: Nonlinear Optical Properties of Organic and Polymeric Crystals 1

Downloaded by GEORGETOWN UNIV on July 8, 2013 | http://pubs.acs.org Publication Date: September 29, 1983 | doi: 10.1021/bk-1983-0233.ch001

A. F. G A R I T O , K. D. SINGER , and C. C. T E N G University of Pennsylvania, Department of Physics and Laboratory for Research on the Structure of Matter, Philadelphia, PA 19104

Studies of the basic physics of nonlinear second order o p t i c a l processes in solids remain critically important to the f i e l d of n o n l i n e a r o p t i c s and to further advances in telecommunications technologies. This fact receives continued emphasis today in problems of current interest such as phase conjugation and o p t i c a l bistability. Research activities have centered on second order o p t i c a l processes occurring in inorganic d i e l e c t r i c insulators and semiconductors primarily because of the i m p o r t a n t scientific achievements and r e s u l t a n t large body of a v a i l a b l e information from e a r l i e r studies of p i e z o e l e c t r i c and f e r r o e l e c t r i c properties and semiconductor transport. Consequently, n e a r l y all of the n o n l i n e a r o p t i c a l m a t e r i a l s widely under study are i n o r g a n i c s o l i d s . Recently, however, much i n t e r e s t has focused on organic and polymeric solids b a s i c a l l y due to their e x c e p t i o n a l l y large n o n l i n e a r second order o p t i c a l p r o p e r t i e s and the promise of virtually unlimited numbers of c r y s t a l l i n e structures.(1-9) This a r t i c l e by i t s nature is l i m i t e d to a b r i e f summary of recent developments i n the authors' research on fundamental studies of nonlinear second order o p t i c a l p r o p e r t i e s of organic and p o l y m e r i c crystals and f i l m s . In p a r t i c u l a r , we d e s c r i b e several important results from these studies that have systematically led to microscopic understanding of the nature of the e l e c t r o n i c s t a t e s and o r i g i n of the large nonlinear optical responses in organic solids and c r y s t a l l i n e polymers while r e p o r t i n g recent progress in on-going studies of polymers. 1

Current address: Western Electric Laboratories, Princeton,ΝJ08540. 0097-6156/83/02330001$07.50/0 © 1983 American Chemical Society

In Nonlinear Optical Properties of Organic and Polymeric Materials; Williams, D.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

2

NONLINEAR OPTICAL PROPERTIES

Second Harmonic Effect in Solids

Generation

and

Linear

Electrooptic

N o n l i n e a r s e c o n d o r d e r o p t i c a l p r o p e r t i e s s u c h as s e c o n d h a r m o n i c g e n e r a t i o n and t h e l i n e a r electrooptic effect arise from the f i r s t n o n - l i n e a r term i n the constitutive r e l a t i o n f o r the p o l a r i z a t i o n P ( t ) of a medium i n an a p p l i e d e l e c t r i c f i e l d E ( t ) = Ε c o s (.Jt. 1

P(t)

=ε χ( )

E(t)

0

+e ) (2) 2 0

C

E

( t )

+ ε

^ ( 3) 3 ( ) χ

E

t

+


The f i r s t and t h i r d o r d e r t e r m s i n odd p o w e r s o f the applied electric f i e l d are p r e s e n t for a l l materials. In the s e c o n d o r d e r t e r m , a p o l a r i z a t i o n i s i n d u c e d p r o p o r t i o n a l to the s q u a r e o f the a p p l i e d electric field, and the . n o n l i n e a r second order optical susceptibility X'^ ) ( _ ω . ω ^ ω ) must, therefore, v a n i s h i n c r y s t a l s that p o s s e s s a c e n t e r of symmetry. In a d d i t i o n to the no ne e n t r ο s y m m e t r i c structure, efficient second harmonic g e n e r a t i o n r e q u i r e s crystals to p o s s e s s p r o p a g a t i o n d i r e c t i o n s where the crystal birefringence cancels the natural dispersion leading to p h a s e m a t c h i n g . For crystalline solids, comparative quantities for second harmonic generation and the linear electrooptic coefficient a r e g i v e n by M i l l e r ' s delta and the polarization optic coefficient f. The q u a n t i t y 6 i s d e f i n e d by the r e l a t i o n f

*ijic

-

^

ψ

χ

where t e r m s s u c h as c o m p o n e n t s , and d ^ ^ , is defined through X

(

ijk

-

2 ω

5

ω

,

ω

)

e

1

i

j

the

optic

i j k

(

i fk

ε

ω

(2)

k

the second

linear susceptibility harmonic coefficient,

(-20); ω , ω)

coefficient

f

ijk= o( k- ) ijk

where linear X

£

*

^ i ^ a r e

= 2 d

The p o l a r i z a t i o n expression r

^

2

f

( ) 3

is

defined

ω,

0)

= 1/2

the

(4)

^ i s the d i e l e c t r i c c o n s t a n t , and electrooptic coefficient, is defined

(- *

by

i ^ f n ^ r ^

(_

ω ;

ω

,

0

)

r

ijk> as

the

(5)

where

n ^ i s the i n d e x o f r e f r a c t i o n . Figure 1 shows the standard quantities for s e l e c t e d o r g a n i c and p o l y m e r i c e x a m p l e s whose δ and f


1.

Molecular Optics

G A R I T O ET A L .


+

3

1000

1000-

4

MNA

MNA

100

ORGANIC SOLIDS 100-

m-NA

m-Ν A", M A P

POM LiNb0 10

f

LiNbO^ LiI0 '

ι

3

3

10-

ADP-j DIELECTRIC KDP, CdS *

INSULATORS

a

7

LiI0

3

i> K D P

4

CdS

SEMICONDUCTORS

a-QUARTZ GaAs

ADPiGoAs

8

2

(10" m /C) 2

f (lôWc)

Figure 1. Comparative quantities for selected tensor components of second harmonic generation (left) and the linear electro-optic effect (right) (measured at 1.06 μm wave length). (The strain free quantity, f, was measured at 0.633 μm wavelength except in the case of GaAs which was measured at - 0.9 μm). (Reproduced with permission from Ref. 8. Copyright 1982, Laser Focus.)


4



2

1

2

s u s c e p t i b i l i t y x{ ] ( - ω ; ω, θ ) o f 5 4 0 +_ 100 χ 1 0 " m/V. The f i n d i n g that the electronic contribution dominates t h e f r e q u e n c y r a n g e f r o m z e r o to o p t i c a l f r e q u e n c i e s has a l s o been r e p o r t e d i n o t h e r o r g a n i c molecular solids and a p p e a r s to be a p r o p e r t y common to o p t i c a l l y n o n l i n e a r o r g a n i c s o l i d s . This is an outstanding property in other respects in that such electronic excitations provide extremely fast i n t r i n s i c s w i t c h i n g t i m e s of o r d e r 1 0 " ^ seconds. The 7T-electron excitations are viewed as occuring on m o l e c u l a r s i t e s w e a k l y c o u p l e d to their neigbors and p r o v i d i n g sources of nonlinear optical response t h r o u g h the o n - s i t e m i c r o s c o p i c second o r d e r nonlinear electronic susceptibility ^j. j k (- ^? ι > 2 ) ' In second order p e r t u r b â t iô^n ^ h e o r y with the perturbing H a m i l t o n i a n H' = e Ε · r cos wt, and b o t h t h e f u n d a m e n t a l and c r e a t e d c o m b i n e d f r e q u e n c i e s b e l o w electronic resonances but w e l l above v i b r a t i o n a l and r o t a t i o n a l modes, c a n be e x p r e s s e d as ω

~ 4 Ï [ £ ' < « »'» "* r

7

/ V

V

+ r

" ' * ' " ' " ' ^ ( ( u v , - ω)(ω„ +ω)

+

ω

Ω

( α ν , +ω)(ω„ - ω))

n*g

+ {τ„.'τ . 'τ„"

+

Λ Λ

^..ν..Λ„')(

2

Κΐί

)

+2ω

+ω)

{ωηι

l

+ 4 Σ [ ^ „ ν „ * Δ ^ ; ( ω „ / - 4ω ) +r„ (r„'u>r >

+^ J _) _2

(6)

ω)

2

+r j l r ^ j

n

)(We

+2ω )] 2

2

Κ/-ω )Κ, -4ω*)

η where s u m m a t i o n s a r e o v e r c o m p l e t e

s e t s o f 1e i g e n s t a t e s ' is reduced to experimental and theoretical studies of the corresponding microscopic 3 of single molecular units, or p o l y m e r chains, comprising the optically nonlinear organic solid. Such s t u d i e s a r e i l l u s t r a t e d i n the n e x t s e c t i o n s . 01


DC I n d u c e d

Second

Harmonic

Generation

The value of 3 and its dispersion for a molecule, or p o l y m e r c h a i n , c a n be experimentally determined by DC i n d u c e d second harmonic generation (DCSHG) m e a s u r e m e n t s of liquid solutions(9-12). The e x p e r i m e n t a l a r r a n g e m e n t r e q u i r i n g an a p p l i e d DC f i e l d E ° to r e m o v e t h e n a t u r a l c e n t e r o f i n v e r s i o n symmetry of the s o l u t i o n is described in Figure 4. The s e c o n d h a r m o n i c p o l a r i z a t i o n of the s o l u t i o n i s e x p r e s s e d as (β)

w h e r e the p r o d u c t Γ ^ ^ ι ^ ι o f t h e t h i r d o r d e r n o n l i n e a r ogtical susceptiblity o f t h e s o l u t i o n and t h e DC f i e l d acts as ,aji e f f e c t i v e second harmonic susceptibility ( - 2 ω ; ω , ω ). contains îik and t h e s m a l l e r m i c r o s c o p i c t h i r d o r d e r susceptibility Yjikl for electronic excitations and t h e h y p e r Raman effect. After statistical averaging over the liquid and with a l l e l e c t r i c fields aligned parallel, îjki i s g i v e n by Γ

1

ι

.

η

H

f

o

(

f

»

)

2

2 «

f

(

Y

. | u

j

(

9

)

D and 1/3 ^ ( 2 B

Bus Ύ

£

u

1

/

1

5

v

u,v(

2 Y

u

u

v

3

y + v

uuvV

+

Y

u

v

v

y ) u

uvvJ

where 3 i s the molecular second order nonlinear o p t i c a l s u s c e p t i b i l i t y , y the m o l e c u l a r d i p o l e moment; and Ύ t h e t h i r d o r d e r s u s c e p t i b i l i t y w i t h γ

Ο

>

3 m

r

§

Os



Molecular Optics

0.01 m e a l / s e c

10

II

12

13

ISOTHERMAL

14 ANNEAL

15 TIME

16

17

18

(hr)

Figure 11. Spontaneous heat evolved vs. time of isothermal polymerization of DNP. (Reproduced with permission from Ref. 15. Copyright 1981, Makromol. Chem.J



18


molecular units placed in a strong polymer s t r u c t u r e . A l t h o u g h c e n t r ο s y m m e t r i c at room t e m p e r a t u r e , DNP crystals u n d e r g o an a p p a r e n t f e r r o e l e c t r i c transition at 3 OK (Figure 12) to a low temperature η ο n c e n t r ο s y m m e t r i c s t r u c t u r e ( J_6) which provided important structural information for ultimately stabilizing other no ne e n t r o s y m m e t r i c diacetylene p o l y m e r s t r u c t u r e s a t room t e m p e r a t u r e ( 7 _ ) . I n a d d i t i o n to the ι τ - e l e c t r o n s y s t e m c o n t a i n e d i n t h e R and R' s i d e g r o u p s , the p o l y m e r c o n j u g a t e d chain possesses a ^ - e l e c t r o n band s t r u c t u r e that in general exhibits optical transparency from a p p r o x i m a t e l y the m i d - v i s i b l e r e g i o n i n t o the n e a r i n f r a r e d , leading to i n d i c e s o f r e f r a c t i o n i n the range 1 . 6 - 2 . The n a t u r a l structural anisotropy of the ordered polymer chains l e a d s to l a r g e optical birefringence important to phase m a t c h i n g . Under p o l a r i z e d v i s i b l e l i g h t , single crystals exhibit a lustrous metal-like reflectance for p o l a r i z a t i o n a l o n g t h e c h a i n a x i s d i r e c t i o n and a r i c h highly colored reflectance for polarization p e r p e n d i c u l a r to t h e c h a i n a x i s . Diacetylene polymer crystals also possess exceptional radiation and m e c h a n i c a l damage r e s i s t a n c e ; for instance, radiation damage thresholds have been o b s e r v e d as h i g h as 1 GW/cm^ f o r 25 n a n o s e c o n d p u l s e s at 1 . 8 9 y wavelength. V a l u e s o f the Y o u n g ' s m o d u l u s and t e n s i l e strength a l o n g the c h a i n a r e c o m p a r a b l e to t h o s e o f d i a m o n d . Phase matched second harmonic generation in single c r y s t a l noncentrosymmetric p o l y m e r s was first observed i n MNA s u b s t i t u t e d diacetylene polymers(8). Two asymmetric monomer e x a m p l e s f o r m i n g t h i s new polymer c l a s s , NTDA and MNADA, are d i s u b s t i u t e d with MNA a n d an e t h y l a m i d e g r o u p ( F i g u r e 1 3 ) , a n d s i n g l e c r y s t a l s u n d e r g o s o l i d s t a t e p o l y m e r i z a t i o n by t h e r m a l a n n e a l i n g , u v , and x - r a y i r r a d i a t i o n . The 3 v a l u e s for t h e MNADA and NTDA monomers as m e a s u r e d by DCSHG a r e e s s e n t i a l l y t h e s a m e a s t h a t o f MNA a l o n e . The r e s u l t h a s b e e n e x p l a i n e d by l o c a l i z a t i o n o f t h e ττelectron contributions t o 3 o f t h e i n d i v i d u a l MNA, diacetylene, and e t h y l a m i d e g r o u p s f o r m i n g t h e monomer structure. The l i n k i n g ("^2~^n m e t h y l e n e c a r b o n s a c t as τ τ - e l e c t r o n b l o c k i n g c e n t e r s b e t w e e n t h e g r o u p s , and the observed 3 e s s e n t i a l l y r e s u l t s from the dominant MNA g r o u p . In the solid state, the phase matched second harmonic signal increases with increased polymerization(7_). F i g u r e 14 shows p r e l i m i n a r y d a t a for the second harmonic i n t e n s i t y of NTDA microcrystals p o l y m e r i z e d under x - r a y r a d i a t i o n . As the polymer forms, the second harmonic intensity



Molecular Optics

0.5


0.4

0.3

0.2

0.1

0.0

20

40

60

80

100

T E M P E R A T U R E (Κ)

Figure 12. Temperature dependence of the inverse dielectric susceptibility ) of DNP along the principal axis for polymerization. (Reproduced with permission from Ref. 16. Copyright 1980, Ferroelectric^

β

\

CH

CH-C=C-C=C-(CH )-C-N-CH CH, 2 η 2 3 2

3

9

P

μ

Figure 13. Diacetylene monomer structures of ΜΝ ADA (η- 2) and NTDA (n = 8).


20


steadily increases to 10-15 t i m e s the L i l O ^ reference signal. The increase in signal intensity with i n c r e a s e d p o l y m e r c o n c e n t r a t i o n can r e s u l t from d i r e c t c o n t r i b u t i o n s to X ^ ' f r o m t h e p o l y m e r τ τ - e l e c t r o n band structure, or from more e f f i c i e n t phase matching b e t w e e n t h e f u n d a m e n t a l and s e c o n d h a r m o n i c w a v e s as the a n i s o t r o p i c i n d e x of r e f r a c t i o n c h a n g e s . These aspects together with newly developed diacetylene structures exhibiting even h i g h e r second harmonic i n t e n s i t i e s r e m a i n at the f o c u s o f o n - g o i n g studies.


Diacetylene

Films

and

Optical

Guided

Wave S t r u e t u r e s

In addition to studies of diacetylene single crystals, current research, a c t i v i t i e s are focused on studies of the second x' ' and third χ'^ ' order nonlinear optical responses of disubstituted diacetylene p o l y m e r f i l m s as a c t i v e o p t i c a l guided wave s t r u c t u r e s . Diacetylene polymers possess X ^ ' values c o m p a r a b l e t o g e r m a η i u m (j_7 ) . In the first s t a g e , three major q u e s t i o n s are being addressed: (i) use o f t h e n a t u r a l l y l a r g e i n d e x o f r e f r a c t i o n o f diacetylene polymers to satisfy the principle waveguiding requirement for the refractive index of the g u i d e to be g r e a t e r t h a n t h a t o f t h e substrate n-j and t h e s u p e r s t r a t e (n^ < > n^); (ii) m i n i m i z e o p t i c a l l o s s by c o n t r o l o f t h e guide t h i c k n e s s to m o n o m o l e c u l a r l a y e r d i m e n s i o n s ; and ( i i i ) r e n d e r p a t t e r n e d g u i d e d wave s t r u c t u r e s o f m i c r o n d i m e n s i o n by s t a n d a r d o p t i c a l , x-ray, and electron beam l i t h o g r a p h i c p r o c e d u r e s . In one a p p r o a c h , considerable progress is being made u t i l i z i n g d e p o s i t i o n o f d i s u b s t i t u t e d diacetylene monomolecular films onto solid substrates by the Langmuir-Blodgett technique!18,19) · In this technique, a monomolecular layer of amphiphilic diacetylene monomer m o l e c u l e s is s p r e a d onto a airwater interface and c o m p r e s s e d . As shown i n Figure 15, typical ρres sure-area isotherms exhibit a characteristic rapid pressure rise as the spread d i a c e t y l e n e m o l e c u l e s a r e o r i e n t e d u p r i g h t to t h e a i r water interface into a condensed ph a s e (j_9) · The condensed monolayer is t r a n s f e r r e d onto a v e r t i c a l l y dipped solid substrate and multilayer films are systematically built up by sequential monolayer deposition thereby controlling film thickness at the monomolecular l e v e l . S t u d i e s of the s o l i d s t a t e p h o t o p o l y m e r i z a t i o n of deposited multilayer films reveal that uv i n d u c e d p h ο t ο ρο 1 y m e r i ζ a t i ο η of disubstituted diacetylenes 2


1.


21

Molecular Optics

15 POLYMER


1r

X

20

30

X-RAY

DOSE

= 1.89/1

0

CuK

:x

X-RAY

a

40 N-3

110 m

io

120

J

2ω

Figure 14. The second harmonic \ of NTDA microcrystals relative to the second harmonic intensity of lit hum iodate (LilO^) powder Ρ with increasing x-ray-induced polymerization. (Reproduced with permission from Ref. 7. Copyright 1980, J. Opt. SocJ ω

\^9.6°C 5 0 h

I7.5°C ^ ^ 22.7 °C

40 7Γ ( dynes/cm)

\

30

20

\

10

0

0

0.10

0 20

0.30

040

Γ ( n m / molecule ) 2

Figure 15. Pressure-area isotherms for spread diacetylene monolayer of cadmium pentacosa-10,12-diynide.


22


p r o c e e d s w i t h quantum e f f i c i e n c i e s i n the range 10-100 depending on the p a r t i c u l a r monomer molecular structure (j_8). Separate s t u d i e s of x - r a y i n d u c e d polymerization of film forming disubstituted d i a c e t y l e n e s have d e m o n s t r a t e d quantum e f f i c i e n c i e s as h i g h as 1 0 . ( j _ 9 ) I n c o n t r a s t t o uv p h o t o n s , absorbed x-ray photons p r o v i d e a n a t u r a l g a i n m e c h a n i s m by c r e a t i n g l a r g e numbers of secondary electrons and o t h e r photons that a c t u a l l y cause p o l y m e r i z a t i o n . An example o f p e n t a c o s a - 1 1 ,1 2 - d i y n o i c acid is shown in Figure 16. High resolution patterns c a n be rendered in disubstituted diacetylene films by selective polymerization using s t a n d a r d uv, x-ray, and e-beam l i t h o g r a p h i c m e t h ο d s ( J_9). An e x a m p l e o f a 1 m i c r o n size t e s t p a t t e r n of a developed d i a c e t y l e n e polymer film deposited on an oxidized silicon wafer by L a n g m u i r - B l o d g e t m e t h o d s i s shown i n F i g u r e 1 7 · Such p a t t e r n s a r e d e v e l o p e d , and i f d e s i r e d , t r a n s f e r r e d to t h e u n d e r l y i n g s u b s t r a t e by l i q u i d r e a g e n t s . However, i t was r e c e n t l y f o u n d t h a t s u c h p a t t e r n s can a l s o be p r o c e s s e d by d r y p l a s m a e t c h i n g , w h i c h i s i m p o r t a n t to avoiding polymer swelling, peeling and other u n d e s i r a b l e p a t t e r n i r r e g u l a r i t i e s o f t e n i n t r o d u c e d by using l i q u i d reagents., . . . I n a d d i t i o n t o X* ^ ' a n d X ' ' p r o c e s s e s i n t h e s e diacetylene polymer guided wave structures, current s t u d i e s a r e f o c u s e d on r e c e n t l y d e v e l o p e d f i l m f o r m i n g disubstituted diacetylenes exhibiting phase matched second harmonic generation s u c h a s DCNQDA ( F ι gu r e 18)(20^). In the q u i n o i d r i n g , t h e ττ - e l e c t r o n system is substituted w i t h the c y a n o (CN) e l e c t r o n acceptor and a m i n e ( - N H ) d o n o r g r o u p s . Development of these new structures is based on earlier theoretical calculations for the parent molecular unit DCN0DA(20). An e x c e p t i o n a l l y large value of -170 χ 1 0 ~ * ° c n P / e s u at 1.06y had been p r e d i c t e d resulting from charge c o r r e l a t i o n s i n the τ τ - e l e c t r o n excited s t a t e s and s u b s e q u e n t l y c o n f i r m e d by DCSHG e x p e r i m e n t s on a r e l a t e d d e r i v a t i ve(2jD).


1 2

Conclusion In summary, we h a v e b r i e f l y r e v i e w e d current research highlights from studies of second order n o n l i n e a r o p t i c a l r e s p o n s e s i n o r g a n i c and p o l y m e r i c media. We h a v e s t r e s s e d how f u n d a m e n t a l s t u d i e s h a v e led to microscopic understanding of important electronic states that c o m p r i s e the o r i g i n of the large second order nonlinear responses in these


1.

23

Molecular Optics



1.0

0.8 Ζ

t

ο Ι Ο

7

0.6

< oc 0.4

•

I

° α.

!I

i

0.2Î-

I

i 1

r

0.0 i

1x10'r3

2x10"·

X - R A Y DOSE

3x10"^

(mJ/cm')

Figure 16. x-Ray inducedpolymerization ofpentacosa-10,12-diynoic acid with quantum efficiency of 10~ (CuKJ. 12




Figure 17. Photomicrograph of a UV lithographic Τ feature showing I'μηι-wide spaces and 2μm-wide bars in the center of a 300μm-wide square.

N

C

\

/=\

jC-( NC

W

/

)-Q

X

N

" *

C

H

2

\\

^C-O-C-KHpJp-C-C-C-C-iCHJnCHa N-CH Η ' 2

Figure 18. Quinoid structure of DC ΝDA. (20).


1.

GARITO ET AL.

Molecular Optics

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systems. M o r e o v e r , the f e a s i b i l i t y of phase matched second h a r m o n i c ger r a t i o n i n s i n g l e c r y s t a l polymers and high resoluti η lithography for guided wave structures has been d e m o n s t r a t e d with disubstituted diacetylene polymers. This multidisciplinary field has w i t n e s s e d i m p o r t a n t a d v a n c e s both i n theory and experiment. However, t h i s p r o m i s i n g f i e l d w i l l remain in its i n i t i a l phases without progress on t h e major p r o b l e m s o f c r y s t a l g r o w t h and f i l m f a b r i c a t i o n o f these materials. Finally this short review has centered on s e c o n d o r d e r p r o c e s s e s , b u t we anticipate s i m i l a r progress i n the u n d e r s t a n d i n g of important t h i r d order processes. Acknowledgments T h i s work was s u p p o r t e d by the D e f e n c e Advanced Research Projects A g e n c y u n d e r No. DAAK-70-77-C-0045 (5-26502) and the N a t i o n a l S c i e n c e F o u n d a t i o n MRL p r o g r a m u n d e r g r a n t No. D M R - 7 9 2 3 6 4 7 .

Literature

Cited

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13. S . J . Lalama and A . F . G a r i t o , Phys. Rev. A20, 1179 (1979). 14. G. Wegner i n M o l e c u l a r M e t a l s , W. Halfielded, Plenum (1979) and r e f e r e n c e s t h e r e i n . 15· A . R . M c G h i e , G . F . L i p s c o m b , A . F . Garito, K . N . Desai and P.S. Kalyanaraman, Makromol. Chem. 182, 965 (1981). 16. G.F. Lipscomb, A.F. Garito and T . S . W e i , Ferroelectric 23, 161 (1980). 17. C. S a u t e r e t , J . - P . Hermann, R. F r e y , F. P r a d e r e , J . D u c u i n g , R . H . Baughman and R.R. C h a n c e , P h y s . Rev. L e t t . 36, 956 (1976). 18. B. T i e k e and G. Wegner, M a k r o m o l . Chem. C 1 7 9 , 1639 (1978). 19. J . E . S o h n , A . F . Garito, K . N . Desai, R . S . N a r a n g , and M. K u z y k , M a k r o m o l . Chem. 180, 2975 (1979); and also S.E. Rickert, A . F . Garito, K. H a y e s , K . N . D e s a i and M.E. Filipkowski, (to be p u b l i s h e d ) . 20. S . J . L a l a m a , K . D . S i n g e r , A . F . Garito and K . N . D e s a i , A p p l . P h y s . L e t t . 39, 940 (1981) and to be published. RECEIVED August 5, 1983


Molecular Optics: Nonlinear Optical Properties of Organic and

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