Electro-optic polymeric materials

Over the past five years, a new class of electro-optic polymeric materials has evolved which provides for the first time the capability to fabricate simple and inexpensive electro-optic devices on a variety of substrates. More importantly, these materials possess optical dielectric constants (or refractive indexes) comparable to radio-frequency dielectric constants allowing for fabrication of devices in which the electric field and the optical field propagate at the same velocity. Finally, the low dielectric constant of these materials relative to inorganic ionic crys s provides for operation of devices at much higher efficiency. Although the above facts have been clear for some time, the practical applications of these materials cannot be realized until materials can be created which satisfy a host of practical requirements and until device architectures and fabrication techniques appropriate for these materials can be developed. We will describe here research directed toward both of these ends. 

Li, J., P.J. Neyman, M. Vercellino, J. R. Heflin, R. Duncan, and S. Evoy. 2004. Active photonic crystal devices in self-assembled electro-optic polymeric materials. Mater Res Soc Symp Proc 817 133-138. 

Dalton, L.R., A.W. Harper, B. Wu, R. Ghosn, J. Laquindanum, Z. Liang, A. Hubbel, and C. Xu. 1995. Polymeric electro-optic modulators Materials synthesis and processing. Adv Mater 7 519-540. 

Electro-optic materials can be made using liquid crystal polymer combinations. In these applications, termed polymer-stabilized liquid crystals [83,86], the hquid crystal is not removed after polymerization of the monomer and the resulting polymer network stabilizes the liquid crystal orientation. 

This review will highlight the interrelationships between basic photopolymer science and practical applications of this technology. Each application of photopolymer technology can be described in terms of three primary descriptors the mode of exposure, the mechanism of the photopolymer reaction employed and the visualization method used. Using this foundation, the widely diverse applications of photopolymer technology to electronic materials, printing materials, optical and electro-optical materials, the fabrication of devices and polymeric materials, adhesives and coating materials will be discussed. 

More complex geometries have been developed [40] and the influence of the geometrical structure has been examined. Although straight-through microchannel emulsification has been developed [39,41], the production rates are still low compared to those obtained with standard emulsification methods. However, the very high monodispersity makes this emulsification process very suitable for some specific fechnological applicafions such as polymeric microsphere synfhesis [42,43], microencapsulation [44], sol-gel chemistry, and electro-optical materials. 

Fig. 6. Measurement of photostability of two polymeric electro-optic materials as carried out by researchers at IPITEK (TACAN) Corporation. The data represented by the solid line correspond to the LRD-3 DEC material of Dalton and co-workers [138] while the data represented by open circles correspond to a diaminonitrostilbene chromophore/poly(methyl methacrylate) guest/host material produced by IBM Almaden Laboratories. The dramatic improvement observed for the DEC material can be associated with increased lattice hardness from the chromophore coupling to adjacent polymer chains...

Our detailed theoretical treatment tells us that the extremely optimistic early (pre-1985) predictions for electro-optic activity for polymeric materials will not be realized. Certainly, electro-optic activity will not increase in a linear manner with N and with p. The quantity p 3 divided chromophore molecular weight is not a good chromophore figure of merit as was assumed until recently. However, theoretical guidance provided by theories that explicitly take into account intermolecular interactions has permitted macroscopic electro-optic coefficients to be routinely achieved that significantly exceed those of lithium niobate. 

Fig. 21. Dynamic thermal stability [121] of a variety of polymeric electro-optic materials. All materials involve the Disperse Red chromophore. Trace 1 PMMA composite material trace 2 chromophore covalently attached by one end to a soft (PMMA-like) matrix [138] trace 3 DEC chromophore with both ends attached to a soft polymer matrix [138] trace 4 Disperse Red chromophore covalently attached at one end to a polyimide polymer matrix [121] trace 5 DEC-type chromophore with both ends attached to a sol-gel type matrix [139]...

As already noted, resistive loss in metal electrode structures and in transition from millimeter wave waveguides to the electrode structure is the greatest problem in achieving 100 GHz and higher electro-optic modulation. Fetterman and co-workers [301] have shown by pulse techniques that the 3-dB bandwidth of polymeric electro-optic materials is typically in the order of 360 GHz for 1 cm of material. Stripline electrode structures have been used to achieve operation to somewhat above 100 GHz. Fetterman and co-workers [282] have recently described a novel finline transition between a millimeter waveguide and such elec-... 

Optical propagation loss for polymeric electro-optic materials is typically in the order of 1 dB/cm when care is taken to avoid scattering losses associated with processing and poling-induced damage [2, 3, 5, 63, 64, 257]. Lower loss values can be obtained by isotopic replacement of protons with deuterium and with halogens [211, 304, 305]. With effort, electro-optic material losses can be reduced to approximately 0.2 dB/cm for the telecommunication wavelengths of 1.3 and 1.55 microns. 

TACAN has prepared a summary table of the performance of polymeric electro-optic materials and comparison with inorganic materials (Table 5). Note that, although this compilation by TACAN is only a year old, the future performance of polymeric electro-optic materials has already been realized. 

At the present time, no significant commercialization of polymeric electro-optic modulators exists. However, that situation appears be changing rapidly. Pacific Wave Industries now offer a variety of broad bandwidth modulators for purchase and firms such as Radiant Research, IPITEK and Lumera Corporations are dramatically expanding their activities. Figure 35 shows the PWI40 GHz modulator fabricated from CLD-l/APC polymer material. 

---