SNTFs

SNTFs have received somewhat less attention than chiral STFs. The local constitutive relations for a chiral STF may not be known exactly. However, because % is constant, the local constitutive relations will remain constant. Thus the chiral STF may be modeled, at least in form, relatively easily by selecting reasonable constants. In a SNTF, however, % is a function of z and, as a result, the local constitutive relations are expected to vary as a function of z. As mentioned earlier, this can be attributed to two causes: change in material density, and nanowire cross-sectional morphology. The dependences of the local constitutive relations on % are not functions that one would reasonably expect to guess. However, limited experimental data obtained by Hodgkinson et al. (1998) on CTFs for some materials have become available from which the dependence of constitutive relations on % have been inferred (Hodgkinson and Wu 1998; Hodgkinson et al. 2001; Chiadini and Lakhtakia 2004). Recently, remittances have been calculated for SNTFs resulting from deposition during which the vapor deposition angle, %v, is varied sinusoidally as a function of film thickness (Polo and Lakhtakia in press). When linearly polarised light is incident on the film in the x-z plane (plane parallel to planes in which the SNTF nanowires lie) the spectrums show multiple Bragg regimes. Unlike the case for chiral STFs, Bragg reflections of many orders have strong peaks. Even films with a small number of structural cycles are highly reflective, particularly for large modulation amplitudes about an average value of 90o. The spectral positions of the Bragg regimes differ for s polarisation (electric field perpendicular to the x-z plane) and p polarisation (electric field in the x-z plane). The height and width of the Bragg regimes grows with the amplitude of the oscillation of %v that is used to produce the SNTF. Like chiral STFs, the Bragg regimes shift to shorter wavelengths with incident angle.

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