Organic Polymers as Guided Wave Materials - American Chemical

Chapter 20

Organic Polymers as Guided Wave Materials

Downloaded by UNIV OF CALIFORNIA SANTA BARBARA on September 1, 2015 | http://pubs.acs.org Publication Date: March 11, 1991 | doi: 10.1021/bk-1991-0455.ch020

Karl W. Beeson, Keith A. Horn, Michael McFarland, Ajay Nahata, Chengjiu Wu, and James T. Yardley Research and Technology, Allied-Signal, Inc., Morristown, NJ 07962

Organic polymers with chemically-engineered linear and nonlinear optical properties offer great promise for the integration of optical structures on silicon and gallium arsenide semiconductors. Waveguide structures can be delineated in organic polymers with a variety of processing techniques that are compatible with the fabrication of microelectronic features. We report that ultraviolet and visible radiation can be used to photochemically delineate index of refraction profiles in solution spin-coated organic films thereby generating a wide variety of waveguide structures. The technique can use classical photoresist type masks or laser writing techniques to produce the desired features. The formation of single mode passive waveguide structures is demonstrated using PMMA films containing photochemically active nitrones. The methodology is readily extended to poled polymer films with electro-optic response, and is demonstrated in the preparation of a channel waveguide electro-optic amplitude modulator. The recent rapid development of powerful compact diode lasers operating at wavelengths from the visible to the near infrared, the need for rapid information processing and communications systems, and the microminiaturization of electronic components have been key factors driving the field of integrated optics. In order to implement many of the desired integrated optical devices, systems engineers require new materials with large optical nonlinearities capable of being processed into waveguide structures. To realize the promise of single-chip integrated structures containing laser sources, passive and active spatially delineated waveguides, detectors, and direct output for other applications, the processing

0097-6156/91/0455-0303$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.


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MATERIALS FOR NONLINEAR OPTICS: CHEMICAL PERSPECTIVES

technologies for these new materials must be compatible with existing silicon and gallium arsenide electronic and optical devices. Organic polymer films offer interesting advantages for these purposes when compared to the existing waveguide technologies based on inorganic crystals such as lithium niobate and potassium titanyl phosphate. For example, organics have been shown to have optical nonlinearities (1-3) that can exceed those of the inorganic crystals. Since the large nonlinearities have their origin in electronic (4) rather than lattice motions, they have the potential for frequency response greater than that possible in the materials routinely used today. In addition, organics have low dielectric constants (5) and dielectric losses even at GHz frequencies (6). Another potentially key advantage of the organic polymers is that they can be chemically modified to vary their linear optical properties such as optical transmission (which is far greater than many of the semiconductors in the .7 to 2 micron wavelength region), refractive index, adhesion, mechanical properties, and thermal stability. In addition to these advantages, organic polymers can be processed at temperatures compatible with sensitive silicon and gallium arsenide electronic features using methodologies analogous to existing photoresist technologies. Waveguide Fabrication The properties of organic waveguides (7) which can be used to advantage in integrated optical applications include the confinement of light to micron dimensions, the diffractionless propagation of light in spatially delineated regions, and the generation of high intensities in small volume elements with long interaction lengths that can lead to novel nonlinear optical effects. Because of the potential offered by organic polymers in waveguide configurations, a variety of fabrication methods have been previously investigated for the creation of the necessary spatial refractive index profiles. Included in these methodologies are photolithography (8) followed by reactive ion or wet etching processes, gas or solution indiffusion (9Λ0\ plasma polymerization (11), single crystal film formation (12J3), Langmuir Blodgett film preparation (14), electric field poling (15), and "photolocking" (16,17). We have developed a new method for the spatial delineation of refractive index profiles which allows for the efficient and rapid generation of single-mode organic waveguides structures at room temperature using both coherent and noncoherent light sources. This "photodelineation" (18-21) method is based on the change in refractive index that results from the photochemical transformation of reactive chromophores mixed in polymeric matrices. The dispersion in refractive index of such organic materials is well described by the single oscillator Sellmeier equation (22) (Equation 1) where λ^, is the wavelength of the primary oscillator, λ is the measurement wavelength, A is proportional to the oscillator strength and Β accounts for nondispersive contributions from all other oscillators.

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

20. BEESON ET A L


305

If the material has a dominant long wavelength absorption with high oscillator strength, as is the case for many materials used for χ processes, then the refractive index of the material is dominated by this single oscillator. If this absorption feature, or indeed any electronic absorption feature, is associated with a photoreactive state, irradiation into this band will result, after photochemical reaction, in a new absorption band either at longer or shorter wavelength than the original absorption maximum. This effects a corresponding increase or decrease in the refractive index for wavelengths longer than the resulting absorption wavelength (λ > λ^,). Two photochemical technologies, lamp/mask patterning and laser direct writing, can be used to delineate waveguide structures in solution spin-coated organic polymer films. The first of these uses incoherent light sources and standard photoresist type masks with the desired positive or negative patterns. Both contact and projection methodologies can be used. The laser direct writing process uses a focussed coherent light source to generate the refractive index profiles. Analogous to photoresist systems, there are two methods of pattern formation for each of these methods. The photochemical generation of increased refractive index in regions to be used for waveguides is designated as a positive system, while systems in which the refractive index is reduced in regions adjacent to the waveguide are negative systems.


2

Nitrone/PMMA Waveguides: A Negative Pattern System It is well established experimentally and theoretically that organic materials exhibiting large second-order nonlinear optical effects have a polarizable electronic structure (often π conjugated) with asymmetric charge distribution (aromatic charge-transfer states) and a noncentrosymmetric macroscopic orientation (1). In order for these same materials to be useful for the photochemical delineation of waveguides the materials must also have a photochemically reactive state characterized by a U V absorption band at a readily accessible wavelength, and a moderate to high quantum efficiency for reaction. It is also desirable that the reaction not generate secondary photoproducts. As a class of materials the nitrones (shown in Equation 2) exhibit many of the desired features. They are readily synthesized by the reaction of aromatic aldehydes with substituted phenylhydroxylamines, are thermally stable and undergo a photocyclization reaction to the corresponding oxaziridine with high quantum efficiency (23,24). x

x


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MATERIALS FOR NONLINEAR OPTICS: CHEMICAL PERSPECTIVES

Since the cyclization results in the destruction of the conjugation between the two rings, the absorption maximum of the oxaziridine is blue shifted compared to that of the starting nitrone. The same photochemical conversion occurs efficiently in spin coated films consisting of (4-N,N-dimethylaminophenyl)-N-phenyl nitrone (DMAPN) in low molecular weight P M M A . Figure 1 shows the photochemicallyinduced spectral transformation of a 0.76 micron film consisting of DMAPN(23 wt%) in P M M A as a function of fluence (mJ/cm ) at 361 nm. Table I shows the dispersion of the refractive index of the D M A P N / P M M A film before and after irradiation (1000 W xenon arc lamp, 361 nm interference filter, 1 h).


2

Table I. The dispersion of the index of refraction of DMAPN(23 wt %)/PMMA films before and after photolysis through a 361 nm interference filter

A t ) S

m

a

(nm)

Refractive Index

x

5 4 3 n m

6 3 3 n m

6 7 0 n m

8 1 5 n m

Sellmeier Α(μπι)

2

Β

λ (ητη) 0

^N(23wt%)/PMMA (before irradiation) DMAPN(23wt%)/PMMA (after irradiation)

^

1.5394 1.5309 1.5310 1.5337 0.467

1.28

340

The intrinsic refractive index of D M A P N / P M M A films can be varied from that of P M M A (1.48) to greater than 1.57 by changing the weight percent of D M A P N . Figure 2 shows the measured refractive indices of D M A P N / P M M A films as a function of the weight percent of D M A P N both before and after irradiation through a 360 nm broad band filter. At 633 nm, a wavelength far from resonance, the observed changes in refractive index can be as large as 0.02. Micron scale multi-mode waveguide structures were demonstrated in these films using both standard mask and laser writing techniques. Figure 3 shows optical micrographs (taken with crossed polarizers) of a multi-mode " Y splitter" and a "crossover" written using an argon ion laser writing apparatus. A l l U V lines of the argon laser were used and were focussed with a 10X microscope objective onto the sample which was translated under computer control. The writing process can be readily followed using a T V camera and monitor. The laser written lines are regions of decreased refractive index adjacent to the waveguide. Measured losses in these multimode structures were typically between 1 and 2 dB/cm at 815 nm. The solution of Maxwell's equations for the propagation of optical radiation with the appropriate boundary conditions for an asymmetric step index channel waveguide with the structure shown in Figure 4a provides for a set of guided waveguide modes characterized by indexes j and k with corresponding propagation constants p . A n analysis of the mode structure for this geometry has been carried out according to the procedure of Marcatili (25). From this analysis the effective jk


20.

BEESONETAL.


CH, CH,


X|r

200

300

400

500

600

Wavelength (nm) Figure 1. Spectral changes of a DMAPN(23wt%)/PMMA film as a function of fluence (mJ/cm ). 2

1.5 8

1.56

χ

Organic Polymers as Guided Wave Materials - American Chemical

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