Electro-optically active materials

PP chromophores into electro-optically active materials suitable for fabrication of prototype devices. Our initial focus is a review of problems associated with intermolecular chromophore electrostatic interactions and how these can be niimmuzed by simple structural modification of chromophores. We summarize our experiences with defining maximum achievable optic nonlinearity, minimum optical loss (including processing associated loss), and maximum material stability. This... 

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. 

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. 

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...

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... 

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-... 

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. 

In contrast to the SHG coefficient, the electro-optic coefficient r,y/t is symmetric in its first two indices for a lossless, not optically active, material. This follows from Eq. (11), which relates the electro-optical coefficient to the impermeability tensor Tiy (the matrix inverse of the dielectric permeability tensor y), a real and symmetric tensor under the stated conditions [3],... 

In the fabrication of practical E-O devices, all of the three critical materials issues (large E-O coefficients, high stability, and low optical loss) need to be simultaneously optimized. One of the major problems encountered in optimizing polymeric E-O materials is to efficiently translate the large P values of organic chromophores into large macroscopic electro-optic activity (r33). According to an ideal-gas model, macroscopic optical nonlinearity should scale as (M is the chromo-... 

Electro-optic activity in guest-host materials can be quantified in terms of the electro-optic coefficient, which can be expressed as... 

Robinson, B.H., Dalton, L.R. Monte carlo statistical mechanical simulations of the competition of intermolecular electrostatic and poling-field interactions in defining macroscopic electro-optic activity for organic chromophore/polymer materials, J. Phys. Chem. A 104(20), 4785 795 (2000)... 

The photorefractive effect in polymeric systems is different from that in inorganic crystals because the charge photogeneration quantum yield, the carrier mobility and (in case of low-Tg materials) the electro-optic activity are clearly field-dependent parameters. Polymeric materials optimized for photorefractive response have to fulfill a number of important requirements in addition to those known for conventional applications. These include spectrally optimized charge... 

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