Poled polymers

Mitata S, Sasabe H (1997) Poled polymers and their applications to SHG and EO devices. Gordon and Breach Science Publishers, Amsterdam... 

This chapter provides an overview of the basic principles and designs of such sensors. A chemical sensor to detect trace explosives and a broadband fiber optic electric-field sensor are presented as practical examples. The polymers used for the trace explosive sensor are unpoled and have chromophores randomly orientated in the polymer hosts. The electric field sensor uses a poled polymer with chromophores preferentially aligned through electrical poling, and the microring resonator is directly coupled to the core of optical fiber. 

Electro-optic modulation (Pockels effect) using a poled polymer is shown schematically in Figure 5.33, where an optical signal confined in a wave-guide is modulated by the application of an external voltage, known as a Mach-Zender Interferometer. 

Figure 5.33 Electro-optic modulation in a wave-guide using a poled polymer.

In the EFISH method, the molecule of interest is dissolved in an appropriate solvent and put into a cell of the type shown in Figure 9. Electrodes above and below the cell provide the means for a D.C. electric field, which orients the solute (and solvent) molecules through its interaction with the molecular dipoles. Similar to the poled polymer approach, the average molecular orientation is increased along the field direction and an oriented gas model used to extract p. 

The application of electro-optic polymer materials to practical devices will require some significant advances in the development of the materials. The primary advance required is the achievement of suitable thermal stability of the poled state over temperature ranges determined by fabrication, assembly, and end use requirements. Table in illustrates some of the temperatures and conditions under which a poled polymer must remain poled to within a few percent of its initial value over the life of a device. 

Table ID. Temperature Ranges and Conditions for Stability of Poled Polymers, °C...

The general constraints for the design of any dyes for ADPM SHG in poled polymer systems rapidly narrow the choice of chromophores. The dyes should be overall charge neutral (to facilitate poling) and highly soluble in polymer matrices. The first excited electronic state should be well separated from higher energy states for two reasons (1) since the vibronic envelope associated with an... 

Figure 2 shows the tradeoffs between crystal and polymer organic NLO materials for device applications. Although cither 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. 

The electro-optic coefficient in a poled polymer film can be related to the second-order susceptibility as shown in Equation 5. 

The results are to some extent inconclusive and suggest that a two-photon state may have to be included. Also reported here are some further major improvements in molecular second order nonlinearities of particular importance to poled-polymer electrooptic applications (EO). Thus, it is found that appropriate replacement of benzene moieties with that of thiazole in certain azo dyes results in a factor of three increase in i-p, the molecular dipole ( io) projected molecular second order nonlinear optical susceptibility, p. 

In the view of the practical use, polymeric materials are expected to overcome the disadvantages of organic molecular crystals in mechanical properties and processability. Most polymers are usually in a symmetric structure as a whole, so they should be considered as third-order rather than second-order NLO materials, except for poled polymer systems. 

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