Third-Order Nonlinear Optical Effects in Molecular and Polymeric

Chapter 3

Third-Order Nonlinear Optical Effects in Molecular and Polymeric Materials Paras N. Prasad

Downloaded by UNIV OF MINNESOTA on May 18, 2013 | http://pubs.acs.org Publication Date: March 11, 1991 | doi: 10.1021/bk-1991-0455.ch003

Photonics Research Laboratory, Department of Chemistry, State University of New York at Buffalo, Buffalo, NY 14214

This review is aimed at seeking the participation of the chemical community in the exciting new field of nonlinear optics. Chemists and chemical engineers with backgrounds ranging from synthesis to theory can make valuable contributions in this field as it offers challenges both for fundamental and applied research. The article focuses specifically on third order nonlinear optical processes in molecular and polymeric materials. Basic concepts are briefly reviewed along with a discussion of some structural requirements for third order effects. Some widely used measurement techniques are presented. The current status of third-order nonlinear optical material is reviewed along with a discussion of the relevant fundamental and technological issues. The article concludes with a discussion of the important areas in which chemists and chemical engineers can make important contributions (1). Nonlinear optics is currently at the forefront of research because of i t s potential applications in the future technology of optical processing of information. For a l l o p t i c a l processing which involves l i g h t control by l i g h t , third-order nonlinear o p t i c a l processes provide the key operations of o p t i c a l l o g i c , o p t i c a l switching and o p t i c a l memory storage. Molecular and polymeric materials have emerged as an important class of nonlinear o p t i c a l materials because of the tremendous f l e x i b i l i t y they offer both at the molecular and bulk levels for structural modifications necessary to optimize the various f u n c t i o n a l i t i e s needed for a specific device application (1 3). Since the nonlinear response of these molecular materials i s primarily determined by their molecular structure, one can use molecular modeling and synthesis to design and custom t a i l o r molecular structures with enhanced nonlinear responses simultaneously introducing other desirable f u n c t i o n a l i t i e s . Polymeric structures have the additional advantage that one can incorporate structural

0097-6156/91/0455-OO5O$O6.00/0 © 1991 American Chemical Society In Materials for Nonlinear Optics; Marder, S., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1991.

3. PRASAD

Third-Order Nonlinear Optical Effects

51

modifications not only in the main chain but also in the side chain


(3). At the bulk l e v e l , molecular and polymeric materials also offer f l e x i b i l i t y of fabrication in various forms such as c r y s t a l s , films, f i b e r s , as well as monolayer and multilayer Langmuir-Blodgett films. In addition, one can make composite structures to introduce multifunctionality at the bulk l e v e l . The interest in this area i s not only technological. This f i e l d also offers a tremendous challenge for basic research. The focal point of the nonlinear optics of molecular materials i s a basic understanding of the relationship between the molecular structure and microscopic o p t i c a l nonlinearity. Especially for third-order nonlinear o p t i c a l processes this understanding i s in i t s infancy. We have to s i g n i f i c a n t l y improve this understanding so that theoretical/computational c a p a b i l i t i e s can be developed for predicting structural requirements necessary for large nonlinearities. The dynamics of various quantum states i s another important area since one, two, and multiphoton resonances s i g n i f i c a n t l y influence the nonlinear optical response. The relationship between microscopic nonlinearities and the corresponding bulk effect i s yet another area which also warrants detailed investigation. Chemists can play a v i t a l role in making s i g n i f i c a n t contributions to the issues of both fundamental and technological importance. Theoretical and synthetic chemists working together to simultaneously develop r e l i a b l e computational methods and synthetic routes to systematically derivatized structures on which experimental measurements are made can provide valuable input for developing a microscopic understanding of o p t i c a l n o n l i n e a r i t i e s . An experimental study of dynamics of various resonances coupled with theoretical analysis i s another important contribution. Use of chemical processing to make various forms of molecular assemblies for a given class of compounds ( c r y s t a l l i n e , spin coated films, Langmuir-Blodgett films) can y i e l d useful insight into relationship between the microscopic nonlinearities and the corresponding bulk n o n l i n e a r i t i e s . Materials chemists can also make an important contribution by developing processes using chemical synthesis routes by which high o p t i c a l quality films or fibers of a highly nonlinear material can be fabricated. This i s a t u t o r i a l a r t i c l e written to stimulate the interest of various chemists and chemical engineers in t h i s exciting new f i e l d . F i r s t , the basic concepts are reviewed. Then a survey of the current status of third-order nonlinear materials i s presented. This i s followed by a discussion of relevant issues and the valuable contributions chemists can make to this f i e l d . Basics of Nonlinear Optics Optical response of a material i s generally described in the approximation of e l e c t r i c - d i p o l e interaction with the radiation (**). In this model, the o s c i l l a t i n g e l e c t r i c f i e l d of radiation induces a polarization in the medium. When a material i s subject to a strong o p t i c a l pulse from a laser the e l e c t r i c f i e l d i s intense and the

In Materials for Nonlinear Optics; Marder, S., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1991.

52

MATERIALS FOR NONLINEAR OPTICS: CHEMICAL PERSPECTIVES

induced polarization shows a nonlinear behavior which can be expressed by the following power series expansion . P =

( 1 ) X

'E

+

X

( 2 )

(

:EE +

3

EEE + . . . = x

)

X

e f f

'E

(1)

In Equation 1, i s the l i n e a r s u s c e p t i b i l i t y which is generally adequate to describe the optical response in the case of a weak optical f i e l d . The terms x X second and t h i r d order nonlinear o p t i c a l s u s c e p t i b i l i t i e s which describe the nonlinear response of the medium. At o p t i c a l frequencies (4_) a

n U)

d

a

r

e

t

n

e

= 1 + 4itxU)

= (o>)

2


n

e

(2)

For a plane wave, we have the wavevector k = — and the phase velocity v = —. In a nonlinear medium, x(^) = X f f ^ ) Equation 1 is dependent on E; therefore, n, k and v are a l l dependent on E . The two important consequences of the third-order o p t i c a l nonlinearities represented by x third-harmonic generation and intensity dependence of the refractive index. Third-harmonic generation (THG) describes the process in which an incident photon f i e l d of frequency (GO) generates, through nonlinear polarization in the medium, a coherent optical f i e l d at 3a>. Through x interaction, the refractive index of the nonlinear medium is given as n = n +n I where describes intensity dependence of the refractive index ana I is the instantaneous intensity of the laser pulse. There is no symmetry r e s t r i c t i o n on the third-order processes which can occur in a l l media including a i r . o f >

a

r

e

Q

Microscopic Optical

Nonlinearities

At the microscopic l e v e l , the nonlinearity of a molecular structure is described by the e l e c t r i c dipole interaction of the radiation f i e l d with the molecules. The resulting induced dipole moment and the Stark energy are given as (1,3) y

e

Stark

=

a

-

E

~ * o ~

1

/

ind E

=

u

+

2

6

''

E

a

:

E

E

"

E

+

1

/

y

3

E

E

E

E

+

^

« 3 : E E -1/4 E-Y EEE -

(4)

In the above equation a is the l i n e a r p o l a r i z a b i l i t y . The terms 3 and Y, called f i r s t and second h y p e r p o l a r i z a b i l i t i e s , describe the * nonlinear optical interactions and are microscopic analogues of x and x In the weak coupling l i m i t , as is the case for most molecular systems, each molecule can be treated as an independent source of nonylinear o p t i c a l effects. Then the macroscopic s u s c e p t i b i l i t i e s X are derived from the microscopic nonlinearities 3 and Y by simple orientationally-averaged s i t e sums using appropriate l o c a l f i e l d correction factors which relate the applied f i e l d to the l o c a l f i e l d at the molecular s i t e . Therefore (1,3) X (-o) ;o) u> ,u> ) = F(OJ )F(u) )F(o) )F(w ) I.

)+2

(O) o

f

1

(0)

3

In Equation 6, n (w.) is the intensity independent refractive index at frequency OJ. . ° T^e sum in Equation 5 i s over a l l the s i t e s (n); the bracket, < >, represents an orientational averaging over angles 6 and | . Unlike for the second-order effect, this orientational average for the third-order coefficient i s nonzero even for an isotropic medium because i t is a fourth rank tensor. Therefore, the f i r s t step to enhance third order o p t i c a l nonlinearities in organic bulk systems i s to use molecular structures with large Y. For this reason, a sound theoretical understanding of microscopic nonlinearities i s of paramount importance. Structural Requirements for Third-Order Optical Nonlinearity Electronic structural requirements for third-order nonlinear organic systems are different from that for second order materials. Although the understanding of structure-property relationships for third-order effects is highly l i m i t e d , a l l microscopic theoretical models predict a large non-resonant third-order o p t i c a l nonlinearity associated with delocalized ir-electron systems (1 - 3 ) . These molecular structures do not have to be asymmetric because Y is a fourth rank tensor. Conjugated polymers with alternate single and multiple bonds in their backbone structures provide a molecular frame for extensive conjugation and have emerged as the most widely studied group of x organic materials. Examples of conjugated polymers are polydiacetylenes, poly-p-phenylenevinylene and polythiophenes. The o p t i c a l nonlinearity i s strongly dependent on the extent of ir-electron d e r e a l i z a t i o n from one repeat unit to another in the polymer (or oligomer) structure. This effective d e r e a l i z a t i o n i s not always equally manifested but depends on the details of repeat unit electronic structure and order. For example, in a sequentially b u i l t structure, the ir-delocalization effect on Y is found to be more effective for the thiophene oligomers than i t i s for the benzene oligomers (5). The largest component of the Y-tensor is in the conjugation d i r e c t i o n . Therefore,.even though no p a r t i c u l a r bulk symmetry i s required for nonzero x » medium in which a l l conjugated.polymeric chains align in the same d i r e c t i o n should have a larger x value along the chain direction r e l a t i v e to that in an amorphous or disordered form of the same polymer. Studies of x ordered or stretch-oriented polymers as discussed below confirm this prediction. F i n a l l y , the polymeric chains should pack as closely as possible in order to maximize the h y p e r p o l a r i z a b i l i t y density and hence x Extensive iT-conjugation i s also often associated with enhanced conductivity in organic systems (6). Polyacetylene and polythiophene which in the doped state exhibit very high e l e c t r i c a l conductivity also exhibit r e l a t i v e large third-order nonlinear o p t i c a l effects in a

i

n


54


the undoped (nonconducting) state. However, i t should be remembered that conductivity i s a bulk property which i s heavily influenced by intrachain as well as interchain c a r r i e r transports. In contrast, the o r i g i n of third-order nonlinearity in conjugated polymers i s primarily microscopic, determined by the structure of the polymer chain. Therefore, a conjugated polymer may be a very good x material but not necessarily a good conductor. Polydiacetylene i s a good example; i t exhibits a large non-resonant x value but is a wide band gap semi-conductor.


Measurement Techniques for Third-Order Nonlinear Optical Effects General Discussion. Experimental probes generally used to measure X are based on the following effects: (i) Third Harmonic Generation; ( i i ) E l e c t r i c F i e l d Induced Second Harmonic Generation; ( i i i ) Degenerate Four Wave Mixing, (iv) Optical Kerr Gate and (v) Self focusing. In addition, processes involving an intensitydependent phase-shift due,to intensity-dependent refractive index can also be used to measure x • The examples of these processes are found in nonlinear o p t i c a l waveguides, Fabry-Perot etalons and surface plasmon optics. The intensity-dependent phase s h i f t changes the resonance condition which defines the transmission characteristics of the wave through the waveguide, Fabry-Perot or the surface-plasmon coupling, as the intensity of the input (or pump beam) i s changed. . . Although one loosely uses a x value for a material, in r e a l i t y there are a number of relevant parameters which describe the third order o p t i c a l n o n l i n e a r i t i e s . These parameters are: (3)

(3)

(i) The x tensor, x fourth rank tensor which, even in isotropic media such as l i q u i d s , solutions, random solids or, amorphous polymers, has three independent components x\^i^ > i ? 1 2 1122* by the r e l a t i v e polarizations of the four waves. i

s

a

x

x

T

n

e

v

a

r

e

d

e

f

i

n

e

a

n

d

d

( i i ) Response time of the nonlinearity. The response time of the nonlinearity relates to i t s mechanism. Therefore, i t s determination i s of considerable value in establishing the mechanism of o p t i c a l nonlinearity. In addition, the response time i s a valuable parameter for device applications. The non-resonant electronic nonlinearity, which involves only v i r t u a l electronic states as intermediate levels for interaction, have the fastest response time, limited only by the laser pulse width. However, some resonant electronic nonlinearities can also have extremely fast response times when the excited state relaxation i s u l t r a - f a s t . Therefore, one needs to use the best time resolution a v a i l a b l e , preferably subpicosecond, to study the time-response of the nonlinearity when investigating i t s mechanism. (o)

(3)

( i i i ) Wavelength dispersion x • The x value i s dependent on the frequencies of the interacting waves. Therefore, s t r i c t l y speaking one should specify the x dispersion as x ("^^J i » ^2 u O while quoting a v a l u e . . This feature also cautions one to be careful in comparing the x values obtained by the various w


9

3. PRASAD

55


techniques. One measures x ("3oo; a>, oj, w) by t h i r d harmonic generation, and x (-a);oj,-oj,a)) by degenerate four wave mixing. The two values are not expected to be i d e n t i c a l because of the dispersion effect. S t i l l a qualitative c o r r e l a t i o n of the two values serves a useful purpose in identifying i f one i s measuring a non-resonant purely electronic nonlinearity. (o)

(3)

(iv) Sign of x • The nonlinear s u s c e p t i b i l i t y x also has a sign which is an important fundamental property r e l a t i n g to the microscopic nature of o p t i c a l nonlinearity.


(3)

(v) Real or complex. The x value may not just be a real number. It can also be a complex number. This s i t u a t i o n occurs when any frequency of the interacting waves approaches that of a onephoton, two-photon or three-photon electronic resonance (the l a t t e r only for third harmonic generation). It is often d i f f i c u l t to get complete information on a l l the relevant parameters of third-order nonlinearity using one single technique. However, one can,use a combination of techniques to probe the various aspects of the x behavior. Here only two s p e c i f i c techniques to measure x are discussed. Third Harmonic Generation. For third harmonic generation (THG) one generally u t i l i z e s a Q-switched pulse Nd:Yag laser which provides nanosecond pulses at low r e p e t i t i o n rates (10 to 30 Hz) (7). In making third harmonic generation measurements, the response time i s not important; the pulse width of the l a s e r , therefore, i s not as crucial. However, i f the longer pulses (higher photon flux) cause sample decomposition due to absorption, i t may be advisable to use a CW-Q-switched and mode-locked Nd-Yag laser where the strongest pulses are selected through an e l e c t r o - o p t i c pulse selector. Usually, the organic systems have limited transparency towards the u.v. spectral range. The selection of wavelength should f i r s t be made so that the third harmonic signal does not f a l l in a u.v. region of high absorption. For this reason, either the fundamental output of the Nd:Yag laser i s Raman-Stokes shifted in a gas c e l l to a longer wavelength in the near IR, or a dye i s pumped and mixed with the green (or fundamental) from the Yag to generate the difference frequency. After proper selection of wavelength and p o l a r i z a t i o n , the laser beam i s s p l i t into two parts, one being used to generate the t h i r d harmonic in the sample and the other to generate t h i r d harmonic in a reference. For the THG technique, glass i s generally taken as the reference. For the non phase-matched THG one uses the Maker fringe or wedge fringe method in which the path length of the sample (and the reference) i s varied and the third harmonic signal i s monitored as a function of the interaction length I to obtain the fringes (7). From the fringes one determines the coherence length, 1^, for both the sample and the reference as the separation between two maximum corresponds to 21 . The r a t i o of the t h i r d harmonic signals I(3oj) for the sample and the reference for the same input intensity and the same interaction length i s given by (7)*


56


K3u>) I(3u0

reference sample reference

sample

(7)

reference


(3) From this expression one can determine x of the sample by obtaining from the experiment, the third harmonic s i g n a l s .

translated to vary the pathlength which yields the wedge fringe. The third-harmonic generation method has the advantage that i t probes purely electronic n o n l i n e a r i t y . Therefore, orientational and thermal effects as well as other dynamic n onlinearit ies derived from excitations under resonance condition are eliminated (7). The THG method, however, does not provide any information on the timeresponse of o p t i c a l nonlinearity. Another disadvantage of the method is that one has to consider resonances at OJ, 2OJ and 3u> as opposed to degenerate four wave mixing discussed below which u t i l i z e s the intensity dependence of r e f r a c t i v e index and where only resonances at OJ and 2OJ manifest. Degenerate Four Wave Mixing. Degenerate fourxwave mixing (DFWM) provides a convenient method of measuring x > which includes both electronic and dynamic resonant n o n l i n e a r i t i e s , and obtaining i t s response time (7). In a backward wave phase conjugate geometry for DFWM two waves 1^ and 1^ are counterpropagating and a t h i r d beam, I , is incident at a small angle; the s i g n a l , 1^, is the phase conjugate of I as i t is produced counterpropagating to I . In this arrangement the phase-matching requirement i s automatically satisfied. Since a l l the input o p t i c a l frequencies and,the output o p t i c a l frequency are of the same value, one measures x ( - O J ; OJ, OJ, OJ) . This x value is an important parameter for the design of devices u t i l i z i n g o p t i c a l switching and b i s t a b i l i t y . Furthermore, as we have demonstrated from the measurements conducted in our laboratory, one.can conveniently measure the anisotropy and timeresponse of x • Since dynamic n o n l i n e a r it ies such as thermal effects and excited state gratings produced by absorption of photons also contribute to the degenerate four wave mixing s i g n a l , the c a p a b i l i t y to go to time resolution of femtoseconds i s helpful in separating the various contributions. An experimental arrangement which provides time-resolutions of ~350 femtoseconds and very high peak power consists of a CW mode-locked Nd-Yag laser, the pulses from which are compressed in a f i b e r - o p t i c pulse compressor, and subsequently frequency doubled. The frequency doubled output i s s t a b i l i z e d by a s t a b i l i z e r u n i t , and then used to sync-pump a dye laser. The dye pulses are subsequently amplified in a PDA amplifier (from Spectra-Physics) which i s pumped by a Quanta-Ray model DCR-2A pulse Nd-Yag l a s e r . The resulting pulses are ~350 femtoseconds wide with a pulse energy of 0.3 mJ and at a r e p e t i t i o n rate of 30 Hz. The effective x values of a sample can be obtained by using CS^ as the reference material. The x value then i s obtained by using the following equation (7):


3. PRASAD

57


(3) ^sample

R

_

*Cs!

(

n

o sample^

CS

%

^

2

1

2

f

>|

f

sample^1/2 CS

^ 2

S a m p l e

In Equation 8, n ° . and n ° are the linear r e f r a c t i v e indices of sample CS the sample and C S ; £ _ and % _ are the path lengths of the two 2* CS sample ° media. I and I are the respective DFWM signals from the s amp J. e uo sample and CS^. The term L i s the correction factor for absorption and scattering losses in the sample. To obtain the time response of the nonlinearity, the backward beam i s o p t i c a l l y delayed with respect to the two forward beams. 2

0

0

y

2


2

Measurement of Microscopic N o n l i n e a r i t i e s , Y The measurement of x of solutions can be used to determine the microscopic n o n l i n e a r i t i e s Y of a solute, provided Y of the solvent i s known. This measurement also provides information on the sign of Y and (hence x ) of the molecules i f one knows the sign of Y for the solvent (5,7). Under favorable conditions one can also use solution measurements to determine i f Y i s a complex quantity. The method u t i l i z e s two basic assumptions: (i) the n o n l i n e a r i t i e s of the solute and the solvent molecules are additive, and ( i i ) Lorentz approximation can be used for the l o c a l f f s l d correction. Under these two assumptions one can write the x of the solution to be ( X

*

3

)

4

= F [ N _ . . . + N _ . J solute solute solvent solvent

(9)

In Equation 9 the terms represent the o r i e n t a t i o n a l l y averaged second h y p e r p o l a r i z a b i l i t i e s defined as = 1/5(Y + Y + Y + 2Y + 2Y + 2Y ) xxxx yyyy zzzz xxyy xxzz yyzz

(10)

3 N i s the number density in the units of number of molecules per cm . F i s the Lorentz correction factor defined by Equation 6. If the solute i s in d i l u t e concentration, Equation 9 can be written as y *

( 3 )

4

( 3 )

= F [N ] + y solute solute solvent L

J

x

( 1 1J )

If the values for both the solute and the solvent.have the same sign, Equation 11 predicts a l i n e a r dependence of x with the concentration of the s o l u t i o n . By a least square f i t of t h i s concentration dependence, one can readily obtain of the solute molecule. If the signs of the n o n l i n e a r i t i e s are opposite but both are r e a l quantities, a concentration dependence study would y i e l d a behavior where the value of the resultant x solution decreases and goes to zero at some concentration. In the case when of the solute i s complex, Equation 11 y i e l d s the signal given by o

f

t

n

e



58 I

a

|

| A X

(

3

)

|

1

2

= | F % , , ] • x J J solute solute solvent' R

1

f

(3

2

A

1

, , | (12) solute solute' When the real part of for the solute has a sign opposite to that for the solvent, the resulting plot of the signal (or x ) as a function of concentration does not show the x value going through zero. To distinguish these s i t u a t i o n s , one must perform a concentration dependence study. Only then can one extract the value of for the solute. A one concentration measurement i s l i k e l y to give an erroneous r e s u l t . + \f\

Im

2

n

1


Some Representative Measurements Compared to a r e l a t i v e l y large database existing for the second-order materials, that for the third-order molecular materials i s rather limited. Since the third-order processes do not require any s p e c i f i c molecular or bulk symmetry, the scope of investigation therefore can be much wider to include many different types of structures and a l l the bulk phases. Measurements in solution or the l i q u i d phase have been used to extract the microscopic nonlinear coefficient Y using the procedure discussed above. Here are quoted the Y values (orientationally averaged) of some representative molecular and polymeric structures measured in the solution phase. Table I l i s t s these values, which are just randomly selected and do not necessarily represent the f i r s t measurement on these systems. Third harmonic generation (THG), degenerate four wave mixing (DFWM) and Kerr Gate methods have been used for the measurement of o p t i c a l nonlinearity. In going from benzene to $-carotene (the second structure), the Y value increases by more than three orders of magnitude showing the importance of increase in the effective conjugation length. The third structure exhibits N-phenyl substitution in the benzimidazole type structures to introduce a two dimensional conjugation. The l a s t structure, an organometallic polymer, has also been measured in the solution phase. Because of the resonance condition encountered, the Y value is,complex. The x values for some representative polymers in thin f i l m form are l i s t e d in Table I I . From the polydiacetylene group, two s p e c i f i c examples chosen are PTS and Poly-4-BCMU. It should be point that the values l i s t e d for polythiophene, phthalocyanine and polyacetylene are resonant values. In resonant cases, the dynamic n o n l i n e a r i t i e s derived from the excited state population make the dominant contribution in,the DFWM method (7). Consequently, the magnitude of effective x and i t s response time can be dependent on the pulse width and the peak i n t e n s i t i e s . One has to investigate these features carefully before a meaningful conclusion can be drawn. If the excited state lifetime i s longer than.or comparable to the pulse width, a larger value of effective x measured using longer pulses. Polysilanes and polygermanes are inorganic polymers which have no -rr-electrons but show interesting photophysics attributable to d e r e a l i z a t i o n of the o-electrons (20,21). They have the.advantage of having a wide o p t i c a l transparency range but their X values are smaller. i

s


3. PRASAD

59


Table I The microscopic nonlinearity y for some molecular structures measured In the liquid phase


Structure

CH 3

CH.

Wavelength of measurement

Method

1.907 fim (Fundamental)

THG

1.097xl0~ ref. 8

THG

4.8xl0" ref. 9

CH

3

CH

3

DFWM

l.lxlO* ref. 10

DFWM

- 10" ref. 11

36

CH 1.89

CH

y (esu)

tm

33

$6 33

602 nm

35

602 nm

X =1.064 fim Kerrgate puap A =0.532 nm probe

7 »8.56xl0" r««l »3.57xl0"

34

33

r

ref. 12


60

MATERIALS FOR NONLINEAR OPTICS: CHEMICAL PERSPECTIVES Table II

The * form

( 3 )

values of some molecular and polymeric materials measured In the thin film

Structure

Wavelength of measurement

-[-©-

Method

( 3 )

* (esu) Film study

10

- 4xl0(ref. 13)

602 nm

DFWM

undaaental "2.62 u

THG

1.6X10" Parallel to polymer chain (ref. 14)

585 nm

DFWM

4.0X10" red form (ref. 15)

585 nm

DFWM

- ID" (ref. 16)

Polythlophene

602 nm

DFWM

- ID" (ref. 17)

Polyacetylene

1.06 |i

THG

4X10" (ref. 18)

602 nm

DFWM

- lO" (ref. 19)

undaaental =1.064 ix

THG

1.5x10" (ref. 20)

THG

11.3xl0" (ref. 21)

—M


Poly-p-phenylenevlnylene (PPV)

-i

-

Polydiacetylene

R 0

10

R—CH -0-S—

CH

2

PTS

3

0

0

10

R«-( CH -) -O-C-NH-CH C00 (CH ) CH 2

4

2

2

3

3

Poly-4-BCMU

-

11

PBT —

M

9

10

9

Ri ~E?'"3""

p

°

i

v

s

i

i

a

n

e

R2 "

12

Ri •r-J— -Li \2 * F

Polygermane


12

3. PRASAD

61


Several measurements have been reported for oriented polymers. PTS polydiacetylene c r y s t a l was investigated by THG and the r e s u l t s confirmed the largest value of x along the polymer chain (14). Uniaxially and b i a x i a l l y oriented films of PPV as well as b i a x i a l l y oriented films of PBT have been investigated ( 1 3 , 1 6 , 2 2 ) . Again the largest component of x i n the uniaxial f i l m i s along the draw d i r e c t i o n , confirming the largest nonlinearity being along the polymer chain which p r e f e r e n t i a l l y a l i g n along the draw d i r e c t i o n . A simple model which involves transformation of the fourth rank tensor X from a f i l m based co-ordinate to the laboratory based coordinate can explain the polar plot of x obtained by rotating the f i l m with respect to the p o l a r i z a t i o n vectors of the beam ( 1 3 , 1 6 ) . For the measurements reported here using DFWM, a l l the e l e c t r i c f § } vectors were v e r t i c a l l y polarized giving r i s e to measurement of laboratory frame. , * The value of x ranges from 10 esu to smaller values. f

d


i

n

t

n

e

Relevant Issues and Opportunities for Chemists Compared to materials for second-order nonlinear devices, the t h i r d order materials are even further away from being ready for device applications. The relevant issues for third order materials are: (3)

( i ) Improvement in the currently achievable nonresonant x values. Inorganic multiple quantum well semiconductors exhibit large resonant n o n l i n e a r i t i e s (23). It is doubtful that molecular and polymeric materials w i l l offer any challenge to the inorganic semiconductors for resonant n o n l i n e a r i t i e s . The a t t r a c t i v e feature of molecular materials such as conjugated polymers is the high nonresonant x they exhibit which naturally has the fastest response time in femtoseconds. However, even the currently achievable highest nonresonant value (; u),u),o)) which w i l l be responsible for t h i r d harmonic generation i s given as (25) .4 i i n (~3o>;u>,a),oj) =- — ijkl >3 4ri e

Y

h

^

(

m

m

m

In t h i s c a l c u l a t i o n one computes the energies and various expectation values of the dipole operator for various excited states. These terms are then summed to compute Y. If one does an exact c a l c u l a t i o n , in p r i n c i p l e both the derivative and the sum-over-states methods should y i e l d the same r e s u l t . However, such exact calculations are not possible. The sum-over-states method requires that not only the ground states but a l l excited state properties be computed as w e l l . For this reason one resorts to semi-empirical calculations and often truncates the sum over a l l states to include only a few excited states. Electron c o r r e l a t i o n effects are expected to play an important role in determining o p t i c a l n o n l i n e a r i t i e s . Both the configuration interaction and Moeller-Plesset perturbation correction approaches have been used to incorporate e l e c t r o n - c o r r e l a t i o n effects (26,27). Although both the a b - i n i t i o derivative method and the semiempirical sum-over-states approach have been used with some success to predict q u a l i t a t i v e trends, they are not s u f f i c i e n t l y developed to have predictive c a p a b i l i t i e s for structure-property r e l a t i o n s h i p . C l e a r l y , there i s a need to develop semi-empirical theoretical methods which can r e l i a b l y be used to p r e d i c t , with costeffectiveness and with reasonable computational time, molecular and polymeric structures with enhanced o p t i c a l nonlinearity.


3. PRASAD

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Another chemical approach to improve our microscopic understanding of o p t i c a l nonlinearities i s a study of nonlinear o p t i c a l behavior of sequentially b u i l t and systematically derivatized structures. Most past work for third-order nonlinearities have focused on conjugated polymers. This ad hoc approach i s not helpful in identifying f u n c t i o n a l i t i e s necessary to enhance o p t i c a l nonlinearities. A systematic study and c o r r e l a t i o n of Y values of systematically varied structure i s an important approach for material development. Zhao, et a l have investigated the nonlinearities of the following series of oligomers (5,28,29):

Thiophene oligomers

Poly-p-phenyl oligomers

Pyridine oligomers

As a function on n, Y value increases much more rapidly for the thiophene oligomers than for the benzene and pyridine oligomers indicating that the ir-electron d e r e a l i z a t i o n from one ring to another i s much more effective for thiophene units. In addition, the d - o r b i t a l of sulfur may be contributing to o p t i c a l nonlineajjity. The Y value for the thiophene oligomers follows a power law - n which i s close to what is predicted by an a b - i n i t i o c a l c u l a t i o n on polyenes (28). Zhao et a l have also investigated systematically derivatized thiophenes and found that placing NCX, groups at the end 2,2 positions of thiophene enhances the nonlinearity (5). Recently, a j o i n t effort of material Laboratory at Wright Research and Development Center and Photonics Research Laboratory at SUNY at Buffalo has resulted in a comprehensive study of structurenonlinear o p t i c a l properties of a large number of systematically varied aromatic heterocyclic compounds involving fused ring benzimidazole and benzthiazole structures ( 1 0 , 3 0 ) . This study has provided many useful insights some of which are as follows: (a) a sulfur ring in a conjugated structure i s much more effective than a phenyl ring or other heteroaromatic ring such as furan or pyridine in increasing o p t i c a l nonlinearity (b) an o l e f i n i c double bond provides a highly effective n-delocalization and consequent increase of the third-order nonlinearity, and (c) grafting of pendent aromatic groups through attachment to a nitrogen atom in a fused benzimidazole ring provides a means for producing twodimensional ir-conjugation leading to an enhancement of Y and also improved s o l u b i l i t y . The search of third-order materials should not just be limited to conjugated structures. But only with an improved microscopic understanding of o p t i c a l n o n l i n e a r i t i e s , can the scope, in any useful way, be broadened to include other classes of molecular materials. Incorporation of polarizable heavy atoms may be a viable route to increase Y. A suitable example is iodoform (CHI ) which has no irelectron but has a x value (3J_) comparable to that of bithiophene f


64

( /M^


/ M ^ ).

Organometallic structures represent another vast


class of molecular materials which are largely unexplored. ( i i ) Improvement in Materials Processing through Chemistry. Another important issue concerning the conjugated polymeric structure as third-order nonlinear materials i s their p r o c e s s i b i l i t y . The conjugated linear polymeric structures tend to be insoluble and, therefore, cannot readily be processed into device structures. The lack of p r o c e s s i b i l i t y may render a material t o t a l l y useless for p r a c t i c a l application even i f i t may have a large x value. Synthetic chemists can play a v i t a l role by designing chemical approaches for processing of important nonlinear materials. Two specific examples presented here are: (a) Soluble precursor route and (b) Chemical d e r i v a t i z a t i o n for improving s o l u b i l i t y . In the soluble precursor route, a suitable precursor i s synthesized which can be cast into a device structure ( i . e . , film) by using solution processing. Then i t can be converted into the f i n a l nonlinear structure upon subsequent treatment (such as heat treatment). This approach has been used for poly-p-phenylenevinylene (PPV) as shown below ( 3 2 ) :

Precursor p o l y a e r

PPV

In the chemical d e r i v a t i z a t i o n approach one introduces a pendent long a l k y l or alkoxy group to increase s o l u b i l i t y . Polythiophene i t s e l f is insoluble but poly(3~dodecylthiophene), is soluble in common organic solvents. ( i i i ) Improving Optical Quality. Optical quality of the materials i s of prime concern for integrated optics applications which w i l l involve waveguide configurations. Most conjugated polymeric structures are o p t i c a l l y lossy. There i s a need for chemical approaches which w i l l provide a better control of s t r u c t u r a l homogeneities so that the o p t i c a l losses can be minimized. Another approach i s through the.use of composite structures where both the o p t i c a l quality and x can be optimized by a judicious choice of the two components. The best o p t i c a l quality medium i s provided by inorganic glasses such as s i l i c a . However, they by themselves have very low x • A composite structure such as that of s i l i c a and a conjugated polymer may be a suitable choice. Chemical processing of oxide glass using the s o l - g e l chemistry provides a suitable approach to make such composite structures. Using the s o l - g e l method, a composite of s i l i c a glass and poly(p-phenylene vinylene) has been prepared ( 3 3 ) in which the composition can be varied up to 50%. The


3. PRASAD


65

procedure involves molecular mixing of the silica sol-gel precursor and the polymer precursor in a solvent in which both are soluble. During the gelation, a film is cast. Subsequent heat treatment converts the precursor polymer to the conjugated poly(p-phenylene vinylene) polymeric structure. The optical quality of the film was found to be significantly improved and high enough to use them as optical waveguides at 1 .06jj.


Conclusions To conclude this article, it is hoped that the discussion of relevant issues and opportunities for chemists presented here will sufficiently stimulate the interest of the chemical community. Their active participation is vital for building our understanding of optical nonlinearities in molecular systems as well as for the development of useful nonlinear optical materials. It is the time now to search for new avenues other than conjugation effects to enhance third-order optical nonlinearities. Therefore, we should broaden the scope of molecular materials to incorporate inorganic and organometallic structures, especially those involving highly polarizable atoms. Acknowledgments The research conducted at SUNY at Buffalo was supported in part by the Air Force Office of Scientific Research, Directorate of Chemical and Atmospheric Sciences and Polymer Branch, Air Force WrightMaterials Laboratory, Wright Research and Development Center through contract number F49620-90-C-0021 and in part by the NSF Solid State Chemistry Program through Grant Number DMR-8715688. The author thanks his research group members as well as Drs. Donald R. Ulrich and Bruce Reinhardt for helpful discussions. Literature Cited 1. 2. 3. 4. 5. 6. 7. 8.

Nonlinear Optical Properties of Organic Molecules and Crystals; Chemla, D. S.; Zyss, J . , Eds.; Academic Press: Orlando, 1987; Vols. 1 and 2. Nonlinear Optical Properties of Polymers; Heeger, A. J.; Orenstein, J.; Ulrich, D. R., Eds.; Materials Research Society Symposium Proceedings: Pittsburgh, 1987; Vol. 109. Nonlinear Optical and Electroactive Polymers; Prasad, P. N.; Ulrich, D. R., Eds.; Plenum Press: New York, 1988. Shen, Y. R. The Principles of Nonlinear Optics; Wiley & Sons: New York, 1984. Zhao,M.T.; Samoc, M.; Singh, B. P.; Prasad, P. N. J. Phys. Chem. 1989, 93, 7916. Handbook of Conducting Polymers; Skotheim, T., Ed.; Marcel-Dekker: New York, 1987; Vols. 1 and 2. Prasad, P. N.; Williams, D. J. Introduction to Nonlinear Optical Effects in Molecules and Polymers; Wiley & Sons: New York, in press. Kajzar, F.; Messier, J. Phys. Rev. 1985, A32, 2352.


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9.

Hermann, J. P.; Richard, D.; Ducuing, J. Appl. Phys. Lett. 1973, 23, 178. 10. Zhao, M. T.; Samoc, M.; Prasad, P. N.; Reinhardt, B. A.; Unroe, M. R.; Prazak, M.; Evers, R. C.; Kane, J. J.; Jariwala, C.; Sinsky, M. submitted to Chem. Materials. 11. Ghosal, S.; Samoc, M.; Prasad, P. N.; Tufariello, J. J. J. Phys. Chem. 1990, 94, 2847. 12. Guha, S.; Frazier, C. C.; Porter, P. L.; Kang, K.; Finberg, S. E. Opt. Lett. 1989, 14, 952. 13. Singh, B. P.; Prasad, P. N.; Karasz, F. E. Polymer 1988, 29, 1940. 14. Sauteret, C.; Hermann, J. P.; Frey, R.; Pradene, F.; Ducuing, J.; Baughmann, R. H.; Chance, R. R. Phys. Rev. Lett. 1976, 36, 956. 15. Rao, D. N.; Chopra, P.; Ghoshal, S. K.; Swiatkiewicz, J.; Prasad, P. N. J. Chem. Phys. 1986, 84, 7049. 16. Rao, D. N.; Swiatkiewicz, J.; Chopra, P.; Ghosal, S. K.; Prasad, P. N. Appl. Phys. Lett. 1986, 48, 1187. 17. Logsdon, P.; Pfleger, J.; Prasad, P. N. Synthetic Metals 1988, 26, 369. 18. Sinclair, M.; Moses, D.; Akagi, K.; Heeger, A. J. Materials Research Society Proceedings, 1988; Vol. 109, p 205. 19. Prasad, P. N.; Swiatkiewicz, J.; Pfleger, J. Mol. Cryst. Liq. Cryst. 1988, 160, 53. 20. Kajzar, F.; Messier, J . ; Rosilio, C. J. Appl. Phys. 1986, 60, 3040. 21. Baumert, J. C.; Bjorklund, G. C.; Jundt, D. M.; Jurich, M. C.; Looser, H.; Miller, R. D.; Rabolt, J.; Sooriyakumaran, R.; Swalen, J. D.; Twieg, R. J. Appl. Phys. Lett. 1988, 53, 1147. 22. Swiatkiewicz, J . ; Prasad, P. N.; Karasz, F. E., unpublished results. 23. Optical Nonlinearities and Instabilities in Semi-Conductors; Huag, H., Ed.; Academic Press: London, 1988. 24. Chopra, P.; Carlacci, L.; King, H. F.; Prasad, P. N. J. Phys. Chem. 1989, 93, 7120. 25. Ward, J. F. Rev. Mod. Phys. 1965, 37, 1. 26. Grossman, C.; Heflin, J. R.; Wong, K. Y.; Zamani-Khamari, O.; Garito, A. F. In Nonlinear Optical Effects in Organic Polymers; Messier, J . ; Kajzar, F.; Prasad, P.; Ulrich, D. NATO ASI Series, Kluwer Academic Publishers: The Netherlands, 1989; Vol. 102, p 225. 27. Perrin, E.; Prasad, P. N.; Mougenot, P.; Dupuis, M. J. Chem. Phys. 1989, 91, 4728. 28. Zhao, M. T.; Singh, B. P.; Prasad, P. N. J. Chem. Phys. 1988, 89, 5535. 29. Zhao, M. T.; Perrin, E.; Prasad, P. N., unpublished results. 30. Prasad, P. N.; Reinhardt, B. A., submitted to Chem. Matls. 31. Samoc, A.; Samoc, M.; Prasad, P. N.; Willand, C.; Williams, D. J., submitted to J. Phys. Chem. 32. Gagnon, D. R.; Capistran, J. D.; Karasz, F. E.; Lenz, R. W.; Antoun, S. Polymer 1987, 28, 567. 33. Wung, C. J.; Pang, Y.; Prasad, P.N.; Karasz, F. E. Polymer, in press. RECEIVED July 18, 1990


Third-Order Nonlinear Optical Effects in Molecular and Polymeric

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