Electrooptic Ceramics for Optical Devices

By applying an E field the optical properties, especially n, of electrooptic (EO) materials can be changed. Many transparent solids and liquids show EO effects, but only a few of these are useful for practical applications. 

If an electrooptic ceramic is noncentrosymmetric then 5n is proportional to E. Examples of such materials include 

The most important EO ceramics are solid-solution phases in the PbZrOs-PbTiOs-LaiOs system known collectively as PLZT materials. 

The linked network of oxygen octahedra has B ions (Ti , Zr ) occupying the sites within the oxygen octahedra (B sites) and A ions (Pb, La ) situated in the interstices (A sites) created by the linked octahedra. When off-valent ionic substitutions are made into this structure (e.g., La for Pb ) electrical neutrality is automatically maintained by the creation of A-site or B-site vacancies. 

FIGURE 32.21 The PLZT phase diagram. A, memory B, linear  

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A special class of materials that is also ferroelectric are electrooptic ceramics. Materials such as lanthanum-modified lead zirconate titanate (PLZT) produce excellent electrooptic devices. These polycrystalline ceramics exhibit voltage-variable behavior—that is, they can be switched from optically transparent to opaque by the application of voltage. Most of these devices, which are used for shutters, modulators, and displays, are processed by hot pressing to full density. Experiments in many laboratories are being carried out to tape-cast these materials into thin sheets. The main problem encountered to date has been the ability to sinter to full density. The use of nanosized powders has helped in this regard. The ability to tape-cast large sheets could open a wide variety of applications for these materials. 

The PLZT ceramics show large changes in birefringence by switching the polarization and these materials are of potential use in optical memories and displays. " However, the electrooptic properties utilized for such devices depend on the grain size. ... 

Traditional ceramics are quite common, from sanitary ware to fine chinas and porcelains to glass products. Currently ceramics are being considered for uses that a few decades ago were inconceivable applications ranging from ceramic engines to optical communications, electrooptic applications to laser materials, and substrates in electronic circuits to electrodes in photoelectrochemical devices. Some of the recent applications for which ceramics are used and/or are prime candidates are listed in Table 1.1. 

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