Polymers for Second-Order Nonlinear Optics - American Chemical

Chapter 12

Downloaded by UNIV OF MASSACHUSETTS AMHERST on May 30, 2018 | https://pubs.acs.org Publication Date: August 11, 1995 | doi: 10.1021/bk-1995-0601.ch012

Techniques of Ultrastructure Synthesis Relevant to the Fabrication of Electrooptic Modulators 1

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L. R. Dalton , B. Wu , A. W. Harper , R. Ghosn , Y. Ra , Z. Liang , R. Montgomery , S. Kalluri , Y. Shi , W. H. Steier , and Alex K - Y . Jen 1

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Loker Hydrocarbon Research Institute, University of Southern California, Los Angeles, CA 90089-1661 Department of Electrical Engineering, University of Southern California, Los Angeles, CA 90089-0483 Tacan Corporation, 2330 Faraday Avenue, Carlsbad, CA 93111 EniChem America, Inc., Research and Development Center, 2000 Cornwall Road, Monmouth Junction, N J 08852

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The production of thermally stable second order nonlinear optical lattices and the processing of these lattices into buried channel waveguides which can be integrated with fiber optic transmission lines and electronic drive circuitry is discussed. Three techniques for the production of hardened NLO lattices are reviewed; namely, (1) the use of DEC chromophores, with reactive functionalities at both the donor and acceptor ends, to created a hardened lattice by two-step processing with the final step an intermolecular crosslinking reaction, (2) the generation of polyimides containing covalently incorporated NLO chromophores, and (3) the exploitation of thermosetting schemes including sol-gel processing where the NLO chromophore is covalently attached to the reacting species. The adaptation of these three schemes to incorporate high µ β chromophores is discussed. Also reviewed is the production of buried channel waveguides in hardened NLO materials by (1) photochemical processing, (2) reactive ion etching and electron cyclotron resonance etching, and (3) spatially selective poling.

Rapid developments in telecommunications, including the increased use of high bandwidth fiber optic transmission cable, are creating a need for improved electro optic modulators and directional couplers. However, before commercial utilization of polymeric modulators can be contemplated a variety of criteria must be satisfied including reasonable optical nonlinearity; low optical loss; adequate thermal stability to withstand the temperature variations encountered in device fabrication, integration, and operation; processibility to permit coupling to existing fiber optic lines and electronic drive components; and low manufacturing cost. Indeed, commercial viability will likely require that the performance/cost ratio of polymeric modulators significantly exceed that of the competition (modulated lasers and inorganic modulators). 0097-6156/95/0601-0158$12.00yO © 1995 American Chemical Society Lindsay and Singer; Polymers for Second-Order Nonlinear Optics ACS Symposium Series; American Chemical Society: Washington, DC, 1995.


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DALTON ETAL.

Ultrastructure Synthesis & Electrooptic Modulators

159

Although octopolar chromophores have been proposed (7), modulator development to date has focused upon the use of dipolar chromophores. Unfortunately, dipole-dipole repulsion opposes the straight forward assembly of such chromophores into noncentrosymmetric lattices; thus, production of stable lattices by self-assembly techniques has yet to prove useful for the general fabrication of electro-optic modulators (2,3). Sequential synthesis techniques, exploiting Langmuir-Blodgett techniques, Merrifield-type covalent coupling reactions, ionic coupling, and molecular beam epitaxy methods, have produced interesting structures but have not yet produced materials useful in fabricating electro-optic modulators. The method of choice in the fabrication of early prototype modulators has been electric field poling near the glass transition temperature of polymers containing NLO chromophores either as dopants or a covalently bonded components of the polymer structure (4J5,6,7). Of course, this approach suffers from the dilemma that electricfieldpoling does not produce a thermodynamically stable lattice and relaxation of poling-induced order following removal of the polingfieldis a problem which must be addressed. A simple solution would seem to be tofinda polymer with a glass transition temperature sufficiently greater than operational and processing temperatures that no significant relaxation of poling-induced order would occur. However, although such polymers are known to exist, unrealistic thermal stability requirements are placed upon NLO chromophores if they are to withstand the temperatures used in poling such materials. A more practical approach appears to be to find polymer materials, which incorporate NLO chromophores, that can be spin cast into optical quality films (that is, exhibit reasonable solubility in processing solvents), can be poled at modest temperatures, and then chemically hardened into a high glass transition lattice capable of withstanding subsequent processing and in-field use. Exploration of this latter approach has been a major objective of our laboratory and is the primary focus of this article. In pursuing hardened NLO lattices, two quite philosophically different approaches have become popular, the first involves doping of chromophores into polymeric or oligomeric materials to form composite materials while the second focuses upon covalent coupling of the chromophore so that the chromophore is attached by one or more points of attachment to afinalhardened polymer lattice. The obvious advantage of the former approach is that of ease and cost of producing NLO materials; frequently, commercially available chromophores can simply be blended with commercially available polymers, e.g., the isotropic poly(ether imide) Ultem (General Electric). Disadvantages of such composite materials include (1) poor chromophore loading (number density) due to finite solubility of the chromophore guest in the polymer host, (2) phase separation and aggregation of chromophores, (3) sublimation of chromophores at high processing temperatures, (4) a plasticizing effect on the host lattice, and (5) dissolution of chromophores with application of cladding layers. In practice, the above limitations of guest/host composites translate into inadequate optical nonlinearity (because of poor loading and problems encountered in poling) and unacceptable limitations placed upon processing for the fabrication of devices. Consequently, we have chosen to focus upon schemes which involve covalent incorporation of chromophores even though such schemes require initially greater synthetic effort. Lattice Hardening Reactions Utilizing Covalently Incorporated Chromophores. We proceed to describe the production of hardened NLO materials employing the three schemes alluded to above. A fourth approach, namely the production of

Lindsay and Singer; Polymers for Second-Order Nonlinear Optics ACS Symposium Series; American Chemical Society: Washington, DC, 1995.

160

POLYMERS FOR SECOND-ORDER NONLINEAR OPTICS


interpenetrating networks (S), will not be discussed here but is discussed elsewhere in this volume. Two-Step Processing of Hardened Lattices Using Double-End Crosslinkable (DEC) Chromophores. The general concept of employing DEC chromophores has been introduced elsewhere (9). A representative example, employing an azobenzene chromophore, together with representative data on the thermal and temporal stability of the second order optical nonlinearity (measured by second harmonic generation) is given in figure 1. The advantage of the DEC scheme is that spin casting and initial poling of the NLO prepolymer can be carried out without interference from the lattice hardening (crosslinking) reaction. Indeed, AdTech has taken advantage of this feature to make the prepolymer shown in figure 1 commercially available. Recent work has focused upon adaptation of high |x(3 chromophores, e.g., those incorporating heteroaromatic groups and acceptor groups such as thiobarbituric acid and 3-phenyl-5-isoxazolone, to the DEC format. A representative example is shown in synthesis scheme 1. Scheme MEMO

JDMEM

1.

MEMO^. -*03MEM N

1. DMF/POCI3 2.^-CH P(0)(OEt)2 2

CH 0 1. cast into film w i t h O C N - Q ~ Q - N C O 2. electric field poling OCH 3. heat to effect crosslinking 3

3

heavily

crossllnked

3 D network

Although a number of such materials have been successfully synthesized and chemically characterized, NLO characterization has not been completed at this time. Thus, it is not possible at thistimeto quantitatively specify the improvement in optical nonlinearity that is to be realized with high \i$ chromophores incorporated into hardened DEC lattices or to specify the thermal stability of such lattices.



DALTON ET AL.

^CH CH OH 2

H C=C-CO(CH ) 2

2

CH

2

CH CH OH

6

2

2

O

3

DEC Monomer Polymerized with M M A and H E M A


s/\/>y\/\y\/>y\/v\y\y^

I A

Ho'

I

A

D

Ho'

OH

I

A

D

OH

Ho'

D.

OH

Polymer 2

.OCH3 OCN

1) Casting film with X L 2) Electric poling 3) Thermal cross-linking

NCO

H CO" 3

Cross! inker (XL)

I III H A

A

A

A

A

A

D

D

D

D

D

D

v/\/\/\A/\/\y\Xr\y\A^ Crossllnked

NLO stability of crosslinked polymer at 125°C

NLO stability of crosslinked polymer at 90°C

1000 Time (hours)

polymer

0.4

100 Time (hours)

Figure 1. The production of a hardened NLO lattice from a DEC chromophore is shown. Such a chromophore has reactive functionalities at both donor and acceptor ends of the NLO chromophore. Also shown is the thermal stability of the optical nonlinearity for the chromophore shown. The temporal stability of second harmonic generation (relative second harmonic generation efficiency, d(t)/d(0)) is shown at 90 and 125°C.


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Two Step Processing of Hardened Polyimide Lattices Covalently Incorporating NLO Chromophores. In schemes 2 and 3, we illustrate two general schemes for producing hardened polyimide lattices containing covalently incorporated NLO chromophores. Again, a two step processing protocol is pursued. Spin casting and initial poling is accomplished using a polyamic acid precursor polymer which is then hardened by thermally or chemically effecting an intramolecular condensation. As with intermolecular crosslinking of the DEC approach discussed above, imidization results in a dramatic improvement in glass transition temperature and in the thermal stability of NLO activity (10). Representative thermal stability data is given in figure 2. Again, more recent research efforts have focused upon incorporating chromophores with larger p.p values than the azobenzenes of schemes 2 and 3. An example of our efforts in this regard are discussed in detail elsewhere in this volume (11). Scheme 2 .

N

. ' N

NO2

Production of Hardened NLO Lattices by Thermosetting Reactions. A large number of thermosetting reactions have been explored as has been noted in recent reviews by Burland and coworkers (72) and by Dalton, et al. (13). Here we will focus upon schemes 4 and 5. In thermosetting reactions, the monomers involved in the thermally-induced condensation reactions are reacted to the point of producing materials with sufficient viscosity to permit spin casting of films and poling offilmswithout problemsfromionic conductivity. Care must be exercised that the hardening reactions are not permitted to proceed to the point of interfering with reorientation of the chromophores under the influence of the poling field.



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The importance of poling protocol is illustrated in figure 3. When following scheme 5, a poling protocol which involves stepped increases in temperature results in a factor of 1.5 increase in optical nonlinearity compared to simply ramping the temperature to the final temperature (180°C). An optical nonlinearity (second harmonic coefficient, d33) of 27 pm/V (at 1064 nm) was obtained by the stepped protocol. The thermal stability of the optical nonlinearity obtained for the sol-gel material of scheme 5 is shown in figure 4. For the material of scheme 4, optical nonlinearities (d33 values) of 59 pm/V and 95 pm/V (at 1064 nm) were obtained for the tri and tetralink thermosetting chromophores, respectively. Representative thermal stability data is shown in figure 5. Scheme 4 . HO^ -sÔH N

0 N

JL*NCO

1- Spin cast onto substrate

QT " NCO

2. EI.crtcfeldp.llno * 3. Heat to effect crosslinking

»

+

THIInk 3

° , NLO matrix

O S:0 (CH2) OH 2

HO

^„,^

6

( C H Z , 6 V

N^

0 H

b "*N + N

N

N

oso

CH3 NCO O T S^* NCO

1. Spin cast onto substrate Tetralink r - 5 — • . ,„ ,. »*-3D stabilized . Electric field poling _ . . 3. Heat to effect crosslinking NLO matrix

2

M

I

oso

(CH2) OH

2

(CH2) OH

2


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1.0-

ÔOn

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0.80.6-

oo o o

• 0

o o o

0.4-

o Downloaded by UNIV OF MASSACHUSETTS AMHERST on May 30, 2018 | https://pubs.acs.org Publication Date: August 11, 1995 | doi: 10.1021/bk-1995-0601.ch012

0.2-

o Imidized • Non-imidized

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n

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o o oo

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0.0100

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1

1

150

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250

Temperature (°C)

Figure 2. A dynamic assay of the thermal stability of the material synthesized in scheme 2 is shown. The second harmonic generation efficiency is monitored while increasing the temperature of the NLO material at a rate of 10°C per minute. This assay of the thermal stability of poling-induced order is analogous to the assay of the thermal stability of chemical structure effected by thermal gravimetric analysis. To 180 °C

(b)

I

100

T"

T"

200 300 400 time (in minutes)

500

a x 0.0-, 0

1

100

r T 200 300 time (minutes)

1

400

r

500

Figure 3. (a) A normal poling protocol is shown. Here the temperature is increased to 180°C while the poling field is applied (b) A step poling protocol is shown where the temperature is increased in 20 to 30°C steps in approximately 3 hour intervals, (c) The second harmonic generation signal is shown for the step poling protocol. The initial drop in SHG immediately following a step can likely be attributed to ionic conductivity which occurs as the sample temperature exceeds the glass transition temperature. Toward the end of the 3 hour period a leveling off of SHG is observed which reflects the fact that the lattice has hardened to the point of inhibiting further poling-induced alignment of chromophores.



DALTON ET AL.

Infrastructure Synthesis & Electrooptic Modulators

50

100 150 Temperature (°C)

200

250

500 Time (hours)

Figure 4. Upper. Dynamic assay of the thermal stability of second harmonic generation for the sol-gel materials of scheme 5 as a function of curing conditions. Lower. The long term thermal stability of the 180°C cured material is shown at 100°C.



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•B

o z

• • —l 500

J

i

I

i

i

Trilink Tetralink

1 „ 1000

Time (hours)

•

_L_ - Tetralink - Trilink

3

N

O

0

8

"

1

0.4 H

Z

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20

1

1 40

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1 60

«

1 80

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1 ' 100

1

1 12ft ^

%

1 140

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1 160

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1 180

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Temperature (°C)

Figure 5. NLO thermal stability data is given for the tri and tetralink thermosetting materials.


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DALTON ETAL.

infrastructure Synthesis & Electrooptic Modulators Scheme 5 . W

HCKCHafe-S-^-N'

CH CH20H 2

I OCNtCHâSHOErta

°

(EtO) Si(CH2)3HNCOO(CH )2-S-^^-N 3

%

Polymers for Second-Order Nonlinear Optics - American Chemical

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