PERIODIC DOMAIN INVERSION IN CRYSTALS: "DESIGNER" OPTICAL MATERIALS. P.A.Thomas, Department of Physics, University of Warwick, Coventry CV4 7AL, UK.

Periodic domain inversion (PDI) refers to the deliberate introduction of an array of domains of alternating structural polarity into a polar, usually ferroelectric, crystal. The crystals which have been the subject of PDI thus far are nonlinear optical materials such as LiNbO₃, LiTaO₃ and KTiOPO₄, all of which show ferroelectric behaviour at room temperature. The purpose of PDI is to confer tailor-made nonlinear optical properties to the resulting materials.

The most stringent requirement of crystals to be used effectively for nonlinear optical applications such as second-harmonic generation (frequency-doubling), is that the crystal should be phase-matchable, i.e. that the input wave at the fundamental frequency, [[omega]], and the output wave at the second-harmonic, 2[[omega]], should propagate in phase-synchronization. In terms of the refractive indices, n_[[omega]] and n_2[[omega]], of the crystal, this requirement is most simply expressed by n_[[omega]] = n_2[[omega]]. This condition is met fortuitously in a very few crystals through a combination of birefringence and dispersion: however, for meeting the vast range of new applications of optical materials, it would be preferable to find a method of "designing-in" phase-matching properties to existing materials. Periodic domain-inversion is one such technique.

In this talk, the principles and practice of PDI will be described. The technique will be viewed in the context of the crystal structures and ferroelectric properties of crystals which have been successfully domain-inverted so far. The potential for the extension of the technique to new materials will then be evaluated. Finally, methods for examining the arrays of inversion domains established in the crystals will be considered with particular emphasis on our recent work using high-resolution x-ray topography and diffraction^[1]-[2].

References

_{[1] Z. W. Hu, P. A. Thomas, M. C. Gupta & W. P. Risk. Appl. Phys. Lett. (1995) 66(1), 13-15.}

[2] Z. W. Hu, P. A. Thomas & J. Webjorn.J. Phys. D: Appl. Phys. (1995) 28, A189-194.