The chief optical characteristic of chiral STFs is their display of the circular Bragg phenomenon. As discussed in Sec. 9.3.4, this phenomenon is the distinction between the responses to normally incident plane waves of orthogonal circular polarization states in a certain wavelength regime; i.e., the creation of a circular-polarization-sensitive stopband. Depending on the application being considered, the Bragg regime can be classified as either narrowband or broadband. The Bragg regime can be broadened further, if necessary, and ultranarrowband features can be excited in it too. Furthermore, it is tunable and can be shifted in a variety of ways. No wonder, all known optical applications of chiral STFs are based on the circular Bragg phenomenon.
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