Materials for Nonlinear Optics - ACS Publications - American

Chapter 13

Downloaded by UNIV OF CALIFORNIA SAN DIEGO on September 1, 2015 | http://pubs.acs.org Publication Date: March 11, 1991 | doi: 10.1021/bk-1991-0455.ch013

Applications of Organic Second-Order Nonlinear Optical Materials G. C. Bjorklund, S. Ducharme, W. Fleming, D. Jungbauer, W. E. Moerner, J. D. Swalen, Robert J. Twieg, C. G. Willson, and Do Y. Yoon Almaden Research Center, IBM Research Division, San Jose, CA 95120-6099

The history of research on second order organic NLO materials is reviewed, with particular emphasis on crystals and poled polymers. Crystals are best for second harmonic generation applications, while polymers are best for electro-optic waveguide devices such as modulators and switches. Recent results on cw intracavity second harmonic generation using the organic nonlinear crystal DAN (4-(N,N-dimethyl-amino)-3-acetamidonitrobenzene) in an optically pumped Nd:YAG laser cavity are presented, demonstrating for the first time that laser grade optical quality can be achieved with organic NLO crystals. Progress toward high frequency electro-optic phase modulators using both organic crystals and poled polymers is discussed. A family of thermoset poled NLO polymers based on epoxy chemistry is reported. One of these materials has a second harmonic coefficient d = 42 pm/V that is stable for at least 14 days at 80C. 33

It is by now well recognized that organic nonlinear optical materials have the potential to supplant inorganic crystals as the materials of choice for frequency doubling, modulation, and switching (I). Key advantages of all types of organic NLO materials include the high intrinsic nonlinearities of individual organic molecules, the ability to use molecular engineering to tailor properties to specific applications, low dc dielectric constant, and low temperature processing.

0097-6156/91/0455-0216$06.00/0 © 1991 American Chemical Society In Materials for Nonlinear Optics; Marder, S., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1991.


13. BJORKLUND ET AL.

Second-Order Nonlinear Optical Materials

111

Figure 1 schematically illustrates the history of research on 2nd order organic NLO materials. Soon after the invention of the laser and the birth of the field on nonlinear optics, second harmonic generation and two photon absorption were observed in a variety of organic molecules. Systematic studies of the relationship of molecular structure to molecular nonlinearities done during the 1970's brought out the importance of electron derealization and charge transfer for high nonlinearity. In the 1980's research began on ways to incorporate these highly nonlinear organic molecules into oricntationally ordered bulk materials that would exhibit useful bulk NLO properties. Two main approaches were initiated that are still being followed with great energy today: crystal growth and poled polymers. Crystal growth is typically performed using a variety of techniques such as solution, melt, vapor phase or Bridgman to produce noncentrosymmetric crystals of pure N L O molecular chromophores. Unfortunately only a small fraction of the available N L O chromophores form suitable crystals with the necessary noncentrosymmetric orientational order. However, in those cases where crystals with the proper symmetry can be grown, the high concentration of NLO chromophores can result in very large bulk nonlinearities. For instance organic NLO crystals have figures of merit for second harmonic generation (SHG) that exceed the best inorganic materials by an order on magnitude. In addition, these crystals also often have sufficient birefringence to allow angle tuned phase matching enhancement of the SHG conversion efficiency. The poled polymer approach involves incorporating N L O chromophore molecules into a host polymer matrix and establishing orientational order by heating the polymer above its glass transition temperature, aligning the N L O chromophores using a strong dc electric field, and then cooling in the presence of the field. The host NLO molecules can be simply doped into the host polymer, or in more advanced materials, chemically bonded to the polymer mainchain. The great advantages of the poled polymer approach are the ability to use almost any N L O chromophore and the ability to cheaply and easily fabricate thin film optical waveguides on a variety of substrate materials, including electronics components. One disadvantage of poled polymers is a gradual room temperature relaxation of the poling induced orientational ordering that occurs in many cases. (See Section IV) Figure 2 shows the tradeoffs between crystal and polymer organic NLO materials for device applications. Although either type of materials could in principle be used for both applications, crystals are best for second harmonic generation, and poled polymers are best for electro-optic waveguide devices such as modulators and switches.

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

218

MATERIALS FOR NONLINEAR OPTICS: CHEMICAL PERSPECTIVES

1960 T Birth of Nonlinear Opiics Y


1965 Random Organic SHG Expts. V

1970

1975

Systematic Studies of Molecular 2nd Order Nonlinearities

Systematic Studies of Molecular 3rd Order Nonlinearities

1980

1985

Development of Useful Organic SHG Crystals

A

1990

Research on Organic Photorefractive Materials 2nd Order

Figure 1.

Research on Conjugated Polymers

Research on Poled Polymers

Photorefractive

3rd Order

Schematic representation of the major themes in the history

of research on organic nonlinear optical materials.




POLYMER

219

CRYSTAL


General 0

transparency

0

0

dielectric properties

0

Fabrication + + + x

waveguide

—

poling required

x

xtal g r o w t h r e q u i r e d

+

p a t t e r n i n g by p o l i n g

+

d y e m o l e c u l e flexibility

x

Stability + +

mechanical robustness

0

optical (damage)

0

n o n l i n e a r (orientational)

+ +

EO + +

l e v e l of i n t e g r a t i o n

++

V

+ +

o n l y 033 n e e d e d

V2

a

L/d —

SHG phase matching enhancement + + —

+ +

waveguide enhancement cavity enhancement

+ +

l a r g e off d i a g o n a l cfy

+ +

Figure 2. Tradeoffs between polymer and crystal organic nonlinear optical materials. EO refers to applications for electro-optic waveguide devices such as modulators and switches. SHG refers to applications for frequency doubling of moderate and low power laser sources. A + indicates favored, - indicates disfavored, 0 indicates neither favored nor disfavored, and x indicates not relevant.


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II. Intracavity Second Harmonic Generation Using an Organic Crystal Intracavity second harmonic generation and frequency mixing using inorganic crystals has recently been demonstrated to be a practical means of obtaining milliwatts of cw blue or green laser light from infrared diode laser sources. A further enhancement in the achievable conversion efficiency could in principle be obtained using organic crystals with intrinsically higher figures of merit for SHG, provided that the optical quality is sufficient to allow cw intracavity laser operation. We have recently conducted a set of intracavity second harmonic generation experiments using the organic nonlinear material D A N (4-(N,N-dimethylamino)-3-acetamidonitrobenzene) and an optically pumped cw Nd:YAG laser (2). Figure 3 shows the experimental setup. Quasi-cw operation was achieved with crystal samples immersed in index matching fluid in an antircflection coated cuvette that was placed internal to the Nd:YAG laser cavity. This technique permits rapid surveying of crystal samples obtained directly from solution growth without polishing or antircflection coating them. Up to 0.56mW peak power of 532nm light was generated for 2.3W of circulating intracavity 1064nm peak power using 0.5W of 810nm pump. Figure 4 shows the dependence of the SHG power on the fundamental power. In addition, we have achieved true cw operation using antircflection coated D A N crystals. These results represent the first cw intracavity application of an organic N L O material of any type and demonstrate that laser grade optical quality can be achieved with organic N L O crystals. III. Electro-Optic Phase Modulators Using Organic Materials In an attempt to demonstrate high frequency electro-optic phase modulation with organic N L O materials, we have tested several candidate crystals in a specially designed test fixture that incorporates a stripline electrode structure to produce a transverse traveling wave electrical field. The electrical response of the stripline structure was tested and found to be flat up to 3 G H Z . A single frequency laser beam was directed through the crystal and a high finesse scanning ctalon was used to directly detect the optical power in the resulting F M sidebands. A schematic of the experiment is shown in Figure 5. Using a crystal of MNMA (2-methyl-4-nitro-N-mcthylanilinc), a modulation index of M = .014 was achieved at a drive frequency of 400 MHz. This represents the first demonstration of high speed electro-optic phase modulation in an organic crystal. Experiments are underway to fabricate a waveguide phase modulatorusing the thcrmoset poled N L O polymers described in Section IV. So far, metal bottom electrode / polymer buffer layer / N L O polymer layer /




Lens

Nd:YAG Rod Cuvette

Pump Laser

r High Shutter Reflector


Detectors

Output Coupler

Mirror

Figure 3. Experimental setup for intracavity SHG using an organic NLO crystal placed in the cuvette. 0.05

rO 0.01

200

400

600

800

Intracavity YAG power (mW) Figure 4.

Peak SHG output vs peak intracavity power.

EOM

FPetalon

laser to scope

rf gen

Figure 5.

Experimental setup for electro-optic phase modulation.


222


polymer buffer layer / top metal electrode structures have been fabricated and waveguiding has been achieved.


IV. Thermoset Poled N L O Polymers One of the problems that has plagued both guest-host and covalently functionalized linear mainchain types of poled N L O polymers has been relaxation of the poling induced orientational order and hence of the nonlinearity. The time scale of this relaxation in guest-host materials can be on the order of tens of hours at room temperature (3). Covalent attachment of the N L O chromophores to linear polymer mainchains improves the time scale to a few thousand hours at room temperature (4), but this is still far short of the ten year room temperature lifetime required for practical devices. In addition, compatibility with electronics requires long term stability at elevated temperatures in the 50 to 80C range as well as short term tolerance of processing temperatures that could exceed 200C. Motivated by these concerns, we have pursued the development of thermoset poled N L O polymers where the N L O chromophores are covalently attached with multiple chemical bonds to a cross linked polymermatrix, as shown schematically in Figure 6. In our first experiments a tetrafunctional nonlinear chromophore, 4-nitro 1,2 phcnylcncdiamine was reacted with an optically passive bifunctional cpoxy monomer, diglycidylcther of bisphenol-A to form a soluble prepolymcr composed 21% by weight of NLO moieties that could be spin coated onto flat substrates (5). A precuring step at 100C was then necessary to increase the viscosity of the polymer to withstand the high poling electric Fields without breakdown. The sample was then heated to HOC and subjected to > 1 MV/cm dc electric field from a corona discharge. After 16 hours the fully cured sample was cooled in the presence of the poling field and the SHG coefficient d measured using the Maker fringe technique with a. 1.064 um fundamental wavelength. Figure 7 schematically illustrates these processing steps. A value of d =14 pm/V was measured immediately after poling and found to be stable for at least 500 hours at room temperature and to exhibit no detectable decay after 30 minutes at 80C. In subsequent experiments aimed at extending this approach to produce thermoset poled NLO polymers with higher nonlinearities (6), the epoxy monomer was also functionalized to contain an N L O moiety, as shown in Figure 8. The polymer thus formed by reacting bifunctional N,N-(diglycidyl)-4-nitroaniline and trifunctional N-(2-aminophenyl)4-nitroaniline was composed 63% by weight of NLO moieties. However, the N L O moieties in this polymer arc only singly attached to the crosslinked matrix as opposed to the double attachment of the previous polymer. After a processing sequence similar to that of Figure 6, except that the temperature was ramped up step by step for the final cure, a d = 50 pm/V was measured immediately after poling (for comparison, d = 30 3 3

3 3

3 3

3 3





Figure 7. Schematic of thermal processing and poling procedure for a thermoset N L O polymer.


223


224


+

p N0

Epoxy Monomer (a)

9

Amine Monomer (b)

T = 80°C CO C

CO J

g

33

1

42 pm/V

CO

10

15 20 Time/min.

25

30

Figure 8. Schematics of the epoxy monomer N,N-(diglycidyl)4-nitroaniline (a.) and of the amine monomer N-(2-aminophenyl)4-nitroaniline (b.) Also shown is the SHG signal vs time at SOC for the already annealed sample.




225

pm/V for LiNb0 ). Upon heating to 80C, a small decay to 42 pm/V occurred in the first 15 minutes, but thereafter, as shown in Figure 7, no decay was observed for 30 minutes at 80C. Waveguide birefringence studies indicate that no further decay occurred after 14 days at 80C. 3

Literature Cited


1.

Zyss, J.; Chemla, D. S., "Nonlinear Optical Properties of Organic Molecules and Crystals", Academic Press. 1987. 2. Ducharme, S.; Risk, W. P.; Moerner, W. E.; Lee, V. Y.; Tweig, R. J.; Bjorklund, G. C., accepted for publication in Applied Physics Letters. 3. Hampsch, H. L.; Yang, J.; Wong, G. K.; Torkelson, J. M. Macromolecules. 1988, 21, 526. 4. Ye, C.; Minemi, N.; Marks, T. J.; Yang,J.;Wong, G. K. Macromolecules. 1988, 21, 2899. 5. Eich, M.; Reck, B.; Yoon, D. Y.; Willson, C. G.; Bjorklund, G. C. J.Appl. Phys. 1989, 66, 3241. 6. Jungbauer, D.; Reck, B.; Twieg, R. J.; Yoon, D. Y.; Willson, C. G.; Swalen, J. D., accepted for publication in Applied Physics Letters. RECEIVED July 18, 1990


Materials for Nonlinear Optics - ACS Publications - American

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