Electro-Optic Activity

Efforts aimed at understanding the structure of this fascinating type of conglomerate are in progress. Unfortunately, the most fundamental aspect of the structure of B4—whether it is crystalline or a liquid crystalline—is not firmly established. That is, at this point it is not clear whether the B4 phase is a fluid conglomerate or a very novel version of a crystal conglomerate. This ambiguity, however, is emphatically absent in the B2 electro-optically active family of phases, as discussed below. 

For an organic material to show electro-optical activity it must exhibit high second order non-linear behaviour. This is one of the terms in the equation that defines the polarisation (u) of an organic molecule as shown below, where E is the perturbing electric field. 

The dielectric tensor describes the linear response of a material to an electric field. In many experiments, and particularly in optical rheometry, anisotropy in is the object of measurement. This anisotropy is manifested as birefringence and dichroism, two quantities that will be discussed in detail in Chapter 2. The nonlinear terms are responsible for such effects as second harmonic generation, electro-optic activity, and frequency tripling. These phenomena occur when certain criteria are met in the material properties, and at high values of field strength. 

The order parameter, , will thus play a critical role in defining the maximum achievable electro-optic activity that can be realized, particularly if this quantity is dependent on chromophore concentration. 

From the above introduction to the basic phenomenon of electro-optic activity, several points are clear. First, a conceptually attractive class of organic elec-... 

Third, achieving the largest possible order parameter will be important for optimizing electro-optic activity. Moreover, this order parameter must be stable over the operational lifetime of a device utilizing the electro-optic material. 

TTL chips) and on the order of 1 V or less for the fastest semiconductor chips. If EO modulators are to be used in fiber optic communication links, it can be noted that the link gain is inversely proportional to (VTC)2 and the noise figure is directly proportional to (V )2. The higher the electro-optic activity of materials (and thus the lower the of the EO device) the better Drive voltages in the order of 1 V or less are required for lossless (transparent) communication links. 

If the host polymer does not have a high glass transition temperature then the structures of polymer and chromophore must be such that post-poling lattice hardening can be carried out to assure thermally stable electro-optic activity [2, 5,50,63,64,110,132-150]. 

As with the measurement of p, it is important to compare electro-optic coefficients determined by different methods. It is particularly important to establish consistency between electro-optic coefficients obtained from initially poled thin films and those existing in functional prototype devices. Such consistency not only adds credibility to reported material electro-optic activity, but also as-... 

Fig. 3. Device for measuring electro-optic activity of chromophore/polymer materials near the glass transition temperature of the material matrix. This configuration avoids rotational relaxation of chromophores in the absence of an applied poling field...

In addition to characterization of molecular and macroscopic electro-optic activity, it is important to define optical loss. Optical loss can be influenced both by absorption and by scattering effects. In order to minimize overall loss, it is important to understand the independent contributions made by scattering and absorption. To separate these effects, we need to determine the contributions made by both chromophore and polymer host to the optical absorption at device operating wavelengths. Chromophore interband electronic absorption can be measured on resonance by traditional UV-Visible spectrometry however, we will typically be concerned with optical absorption at telecommunication wavelengths of 1.3 and 1.55 microns where such techniques do not provide accurate information. Total optical absorption at 1.3 microns is occasionally determined by both the interband electronic absorption of the chromophore and by C-H vi-... 

As is evident from a consideration of Figs. 7-9, each of these chromophores has exhibited electro-optic activity exceeding that of lithium niobate while at the same time exhibiting auxiliary properties of chemical stability (Td >300 °C) and solubility that permits preparation of device quality materials [183,210-212]. These materials also illustrate another major direction in the preparation of electro-optic materials namely, the development of bridging segments that lead to improved chemical stability, improved solubility in spin-casting solvents, improved compatibility with polymer host materials, and which inhibit unwanted intermolecular electrostatic interactions (we shall discuss such interactions... 

Fig-9. EO coefficient data, as a function of chromophore number density, for CLD-type chromophores with (solid circles) and without (open triangles) isophorone protection of the polyene bridge. The maximum achievable electro-optic activity is smaller for the naked polyene bridge structure and the maximum of the curve is shifted to lower number density. The dipole moment and molecular first hyperpolarizability values are comparable (The unprotected polyene bridge variation may exhibit slightly higher values of p(3)... 

From 1990 to 1997, a number of chromophores with ever improving pp values were synthesized by research groups led by Alex Jen, Seth Marder, Tobin Marks, Tony Garito and many others [2, 3, 5, 63]. Unfortunately, these chromophores did not translate to materials with ever-increasing electro-optic activity. Thus, as late as 1997, it was not clear that any organic material had surpassed the elec-... 

In the following sections, we shah demonstrate that the observed behavior of electro-optic activity with chromophore number density can be quantitatively explained in terms of intermolecular electrostatic interactions treated within a self-consistent framework. We shall consider such interactions at various levels to provide detailed insight into the role of both electronic and nuclear (molecular shape) interactions. Treatments at several levels of mathematical sophistication will be discussed and both analytical and numerical results will be presented. The theoretical approaches presented here also provide a bridge to the fast-developing area of ferro- and antiferroelectric liquid crystals [219-222]. Let us start with the simplest description of our system possible, namely, that of the Ising model [223,224]. This model is a simple two-state representation of the to-... 

Clearly, theory provides very good guidance for optimizing macroscopic electro-optic activity by control of chromophore shape and the magnitude of electrostatic interactions. One wants to keep chromophores as short and as spherical as possible. An extreme prolate ellipsoidal shape is the worst possible shape for optimizing electro-optic activity. Making the shape even longer by the addition... 

A detailed consideration of more advanced theoretical treatments clarifies the role played by the polymer dielectric constant. In the absence of intermo-lecular electrostatic interactions, one would desire the lowest possible dielectric constant, e.g., PMMA would be a better host matrix than polycarbonate. This is because the dielectric constant of the polymer host would act to attenuate the poling field felt by the chromophores. On the other hand, in the presence of intermolecular electrostatic interactions, optimum electro-optic activity will be achieved for polymer hosts of intermediate dielectric constant. The dielectric constant of the host acts not only to attenuate the externally applied poling field, but also fields associated with intermolecular electrostatic interactions. 

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. 19. Dynamical thermal stability of electric field poling-induced electro-optic activity for two samples. The data were obtained as described in [121] by slowly increasing temperature while monitoring second harmonic generation. The chromophore is a DEC chromophore described in [138]. Uncrosslinked refers to the precursor polymer where only one end of the DEC chromophore is attached to the polymer lattice. Crosslinked refers to the situation where both ends of the DEC chromophore have been reacted to achieve covalent coupling to the polymer lattice. The ends of this DEC chromophore are asymmetrically functionalized so that attachment reactions can be carried out independently...

While propagation loss in electro-optically active waveguides remains a great concern, typically the greatest contribution to device insertion loss comes from mode mismatch between silica fibers and electro-optic waveguides. At... 

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