Three-dimensional sculpturing of columnar morphology is most easily achieved by rotating the substrate about an axis normal to the substrate plane, during physical vapor deposition. The vapor flux density and the substrate rotation velocity have to be maintained at fixed values. Under suitable conditions, helicoidal columns with fixed pitch (i.e., structural period) grow. These are the solid-state analogs of chiral liquid crystals [343, 344], and therefore display optical rotation. Not only can all of the foregoing facts about STFs be traced back to a pioneering paper published by Young and Kowal in 1959 , but their morphological foundation is transparent in an 1898 paper of Bose .
Despite publication in a prestigious journal, the Young-Kowal achievement remained obscure for over three decades. Happily, the technique of rotating the substrate, the helicoidal morphology realized thereby, and the transmission optical activity of the fabricated thin films were rediscovered in the last decade [65â67]. Subsequent progress on their fabrication and optical characteristics has been very rapid.
The morphology of these STFs is chiral or handed, and their relative permittivity dyadic , where the rotation dyadic is given in Eq. (6.49) and the reference relative permittivity dyadic follows the prescription (6.55). Two examples of chiral STFs are shown in Fig. 9.1. Structural handedness does not require the nonhomogeneity to be periodic and the morphology to be helicoidal. But when STFs possess both of those attributes, analysis is simplified considerably and useful devices result.
This chapter is focused on chiral STFs, whose columns are nominally perfect helixes of elliptical cross-sections. The morphological parameters are illustrated in Fig. 9.2. Macroscopically, a chiral STF is a periodically nonhomogeneous dielectric continuum, with its axis of nonhomogeneity parallel to the z axis. Light propagation in a chiral STF is described in terms of a 4 Ã 4 matrix ordinary differential equation, whose exact analytical solution exists . Periodicity results in the display of the Bragg phenomenonâ which is of the circular type, as it permits discrimination between incident left and right circularly polarized (LCP and RCP) plane waves. Many uses of this phenomenon are presented in Chapter 10.