Polymers for Integrated Optics - American Chemical Society

Chapter 33 Polymers for Integrated Optics

Downloaded by NANYANG TECHNOLOGICAL UNIV on June 1, 2016 | http://pubs.acs.org Publication Date: August 26, 1987 | doi: 10.1021/bk-1987-0346.ch033

John E. Sohn, Kenneth D. Singer, and Mark G. Kuzyk Engineering Research Center, AT&T, Princeton, NJ 08540

The potential applicability of organic and polymeric materials to integrated optics is large owing to both their microscopic and bulk properties. Two of the advantages of using such materials areflexibilityin the fabrication of optical structures and the tailoring of optical properties through material engineering. For application in guided-wave nonlinear optical devices high opt ical quality and low dielectric constant are but two of the requisite properties. Polymer glasses have been shown to possess these properties, and recently have been rendered optically nonlinear using electricfieldpoling of nonlinear optical molecular-doped polymer glasses. Organic and polymeric materials are dis cussed in the context of the requirements of integrated optics, including development of doped poled polymer glasses. Presently, in order for optical information to be processed, the information mustfirstbe converted into electrical information before it can be operated upon by control electronics. Once the processing is complete, conversion to optical information is done before transmission. The transmission medium of today is optical (opticalfiber)and the switching medium is elec tronic (integrated circuit). Electro-optic devices, e.g. titanium indiffused lithium niobate (Ti:LiNb0 ), will shortly see commercial application. With these devices, an electronic control is used directly on an optical signal, obviating the need for optical to electrical conversion and reconversion. Further down the road, all-optical processing (optical control of an optical data stream) may be real ized. These developments require materials with large optical nonlinearities and the ability to be processed and integrated with optical sources, detectors and drive electronics, and where necessary, the ability to be formed into structures capable of supporting guided waves. Numerous investigations into the nonlinear optical properties of certain organic and polymeric materials have shown that these materials possess the largest observed optical nonlinearities. These nonlinearities, whose physical mechanisms were elucidated by basic studies over the last two decades, arise from charge correlated features present in conjugated π-electron moieties constituting these organic materials. A variety of approaches to the fabrication of bulk organic nonlinear optical materials have been pursued including molecu lar crystals, crystalline polymers, Langmuir-Blodgettfilms,liquid crystals and liquid crystal polymers. Significant efforts in crystal growth and single crystal thinfilmfabrication ' as well as our recent demonstration of optically nonlinear polymer glasses show promise for using organic and polymeric materials in applications using both second and third order optical nonlinearities. 3

111 1 2 1 1 3 1 1 4 1

151 1 6 1 [ 7 1 1 8 1

111 1 3 1

131

101

191

1111

0097-6156/87/0346-0401$06.00/0 © 1987 American Chemical Society

Bowden and Turner; Polymers for High Technology ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

POLYMERS FOR HIGH T E C H N O L O G Y

402 Nonlinear Optics

The polarization response of a material to external electromagnetic fields is given by (in the electric dipole approximation)

P (Etotal) " X

+ X ' Εtotal + Χ

:

Εtotal Εtotal + X · E E E i total

total

+ *

tota

where the %'s are the susceptibilities and E is the sum of electric fields of various frequen cies and polarizations. χ is the permanent polarization, χ the linear susceptibility, and the higher order x's are the nonlinear optical susceptibilities. For simplicity, tensor notation is ignored. Linear processes, such as refraction and absorption, arise from the linear susceptibil ity χ , and nonlinear processes arise from the higher order terms. When electromagnetic fields, ω ,ω , traverse a material whose higher order susceptibili ties are zero or negligible, no interaction of the two fields occurs. However, with an optically nonlinear material ( χ and/or higher order terms nonzero), these fields do interact and fields of different frequency can be generated. For example, for a material possessing nonzero χ , ωχ — o> results in second harmonic generation, that is, a field of twice the incident frequency ( 2 0 ^ ) is produced. Frequency conversion or parametric mixing occurs when ω ^ o> and a wave at 0 ) 3 — 0 ? ! ± o> is gen erated. The linear electro-optic (Pockels) effect, ω — 0, yields a change in the index of refrac tion of the material, thus changing the optical path length. Parametric amplification occurs when energy is exchanged between incident beams. These effects lead to potential applications such as frequency doublers, optical mixers, optical amplifiers, switches, and modulators. Processes arising from the third order susceptibility χ include optical bistability, the intensity dependent index of refraction, and optical phase conjugation. The odd order susceptibilities are nonzero in all materials. However, owing to the fact that χ is a third rank tensor, the second order susceptibility is nonzero only in noncentrosymmetric materials, that is, materials possessing no center of symmetry. The focus of this paper is on second order processes, and the relationships between the bulk susceptibility, second har monic generation, and the linear electro-optic effect. For second harmonic generation, χ $ is symmetric in ijy leading to the relationship between the second harmonic coefficient dijk and the bulk second order susceptibility tota!


( 0 )

( 1 )

( 1 )

1

2

( 2 )

( 2 )

2

2

χ

2

2

( 3 )

( 2 )

x

( 2 ) l 1 2 1

χ $ (-2ω;ω,ο>) - 2d

ijk

(-2ω;ω,ω).

(2)

Since the electro-optic coefficient r is defined by the electric field dependence of the optical indicatrix, r is related to the second order bulk susceptibility through ijk

1121

ijk

X i j l 0 )

- -γ€ (ω)ejj(ω)r (-o>;o>,0). ι7

(3)

ijk

The intrinsic nonlinearity of certain organic materials is substantially higher than the nonlinearity of inorganic and semiconducting materials. The origin of the linear electro-optic effect in organic compounds arises from their electronic structure. A n understanding of the molecular origins of the nonlinearity is essential in the development of optically nonlinear organic and polymeric materials. The electronic contribution to the bulk second order suscepti bility is related to the microscopic susceptibility β by the van der Waals sum 141

1131

υκ

Χ?^(-ω ;ω ,ω ) 3

Nft'fj'fl

1

1

2

Σ Σ ™*0./W)

cos(j,J(s))

cos(k,K(s))

β„ ω κ


(4)

(l)

33.

SOHN ET A L .

Polymers for Integrated

403

Optics

where s is summed over molecules in the unit cell, Ν is the number of unit cells per unit volume, fs are local field factors, the cosines transform the molecular to bulk axes, and β is the electronic molecular nonlinear optical susceptibility. The major contributions to the molec ular susceptibility, for 2ω < E\ (below the first excited state), are the transition moment μ ^ , the change in dipole moment Δμ between the ground state and first excited state of the molecule, and the excited state energy Ε χ, and are related by υκ

g


P

"

3

4Λ

6μ ,£ Δμ 2

2

2

(£ -ω )(£ -4ω )' 2

2

2

2

Thus, the larger the transition moment and change in dipole moment, the larger the microscopic susceptibility. Approaches to increasing this susceptibility that have proved suc cessful include the placement of strong electron-donor and electron-acceptor groups at opposite ends of a conjugated ττ-electron system and increasing the conjugation length (See Table l ) . [ni [is] 1 1 4 1

TABLE 1. Values of βμ measured at λ-1.9 μm

βμ

Molecule 2-methyl-4-nitroaniline

(MNA)

(\0- cm D/esu) 30

1 2 0

5

[i3]

Disperse Red 1 *

525

M P C merocyanine**

-2600

l n l

1141

* 4-[7V-ethyl-7V-(2-hydroxyethyl) ]amino-4'-nitroazobenzene ** 7V-methyl-[4(lH)-pyridinylidene ethylidene]-2,5-cyclohexadien-l-one

Construction of a bulk material possessing a large bulk susceptibility χ requires not only molecular constituents with large microscopic susceptibilities but a noncentrosymmetric system where the orientation of the molecular species results in additivity of the molecular susceptibili ties. ( 2 )

Doped Poled Polymer Glasses A variety of approaches to macroscopic structures have been r e p o r t e d . The majority of these approaches involve the engineering and use of crystalline materials. W e have chosen to use amorphous polymer glasses, and recently demonstrated that molecularly-doped poled polymer glasses possessing reasonably large second order nonlinear optical susceptibilities can be produced. This approach has the advantages of material processability, the capability of integration with sources, detectors, and drive electronics, and material properties required for application to integrated optics, namely high optical quality, and low dielectric constant and dielectric loss. The concept involves the preparation of a solid solution of an optically nonlinear organic molecule dissolved in a polymer glass, processing of the solid solution into a thin film, and elec tric filed poling to align the dipolar dye molecules, thus removing the inversion center inherent to amorphous glasses, and allowing the nonlinear optical properties to be additive. Orientationally ordered films are prepared by heating the film above the glass-rubber transition 1 1 1 1 3 1 1 9 1 1 1 0 1

1111


POLYMERS FOR HIGH T E C H N O L O G Y

404

temperature where molecular motion is enhanced. A strong electric field is applied which aligns the nonlinear optical species in a Boltzmann distribution. Cooling the sample and removing the electric field below the glass-rubber transition temperature results in locking in the induced polarization, yielding a noncentrosymmetric material. The nonlinear optical susceptibility can be calculated by assuming a one-dimensional molecule and a non-interacting molecular ensemble at the poling conditions, and is given by 11

[4]


5

(6)

105

where tin

2

n

2

+ 2)

βΕ

+

kT '

(7)

ρ

2e

Ν is the number density, 0 3 3 3 is a component of the molecular nonlinear optical susceptibility, μ is the static dipole moment, E is the poling field, « is the static dielectric constant, η is the index of refraction, and where the f's are local field factors at the appropriate frequencies (Λ " ( « « +2)/3). The model embodied in Equations (6) and (7) can be used to evaluate the potential of doped poled polymer films in nonlinear optics. Using reasonable values for the parameters of the model and molecules with large values of βμ ~~ 0 μ , susceptibilities comparable to those measured in crystals can be realized (See Table 2). p

333

3

T A B L E 2. Potential second harmonic coefficient for various molecular dopants in poled poly (methyl methacrylate) (PMMA) using Eqs. (6) and (7) with Ν - 3 x l O / c m , E - 0.5 MV/cm, η - 1.52, and € - 3.6. Values of βμ are measured separately at λ-1.9 μm 2 0

3

p

βμ

Molecule

(\(T™cm Dlesu) s

d

33

GO

- 9

2-methyl-4-nitroaniline ( M N A )

120

1.6

Disperse Red 1

525

6.6

M P C merocyanine

-2600

-32

esu)

To demonstrate the concept, we used the organic dye Disperse R e d 1 (4-[7V-ethyl-7V-(2hydroxyethyl)]amino-4'-nitroazobenzene) and the polymer glass poly (methyl methacrylate) ( P M M A ) . Solid solutions of 0-12wt% of the dye in P M M A were prepared and, using standard coating techniques, rendered into thin films on indium tin oxide coated glass. The thin indium tin oxide layer is transparent and acts as one electrode for the poling process. The other elec trode is transparent gold deposited on top of the film. The film is heated above its glass-rubber transition temperature (T — 1 0 0 C ) . A n intense electric field of 0.2-0.6MVcm~ is applied to align the nonlinear dopant. The field is maintained until the sample is well below T . The second-order nonlinear optical properties of the poled film are investigated using second harmonic generation in transmission. The sample preparation technique results in a poled glassy film possessing a unique axis in the direction parallel to the field direction within g

e

l

g


33.

Polymers for Integrated

SOHN ET A L .

405

Optics

the point group of mm. Thus the symmetry operations are an infinite-fold rotation axis and an infinity of mirror planes. There are then five non-zero tensor components, three of which are independent. The second harmonic polarization has the form 1161

Ρΐ " 2d E E P* -2d E E P?-d E +d E +d El ω

l5

u

n

2

x

3l

l5

2

y

y

x

g

(8)

z

i3

where the standard contracted notation is used. Kleinman symmetry gives i/i5-

Polymers for Integrated Optics - American Chemical Society

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