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The chiral Bragg stack consisting of alternating layers of refractive indices and , and chiral strengths and . The widths of the layers are and , so the lattice constant . Circularly polarized light is incident at an angle from the surrounding medium with refractive index .
Band structures for a chiral Bragg stack with layers of thickness and , refractive indices and , and . The blue (red) lines indicate the real (imaginary) part of the wavevectors. The full lines are for the chiral Bragg stack with , while the dotted lines are for the nonchiral case, . Results are shown for on-axis and oblique incidence of light, respectively. Note the shift of the band edges for oblique incidence only.
(a) Chiral Bragg stack with negligible index contrast between layers. The incident light is circularly polarized and at an angle to the axis of the Bragg stack. This situation can be directly mapped onto (b) a nonchiral Bragg stack with index contrast, where linearly polarized light is incident on axis.
Band gaps for the chiral Bragg stack. The gray areas indicate the regions in which the imaginary part of the wavevector is nonzero, i.e., the band gap regions. Results are for a structure with , , , , and oblique incidence, . (a) No index contrast, i.e., . The dashed lines show the analytical expressions Eq. (6) for the first order shift in the central band gap wavelength with the chiral parameter. Blue (green) lines indicate band gaps for right (left) circularly polarized light. (b) Index contrast, . Here, the dashed lines show the approximate analytical results, but with replaced by . The longest-wavelength XP band gaps are indicated.
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