Abstract
The transmittance spectrum of a slab of an electro--optic structurally chiral material (SCM) that is helicoidally nonhomogeneous in the thickness direction and is endowed with a central 90$^\circ$--twist defect, shows evidence of an ultranarrowband spectral hole when a sufficiently high dc electric field is applied between the entry and the exit planes and the incident light is circularly polarized in opposition to the structural handedness of the SCM. This spectral hole migrates on the wavelength axis as the applied dc electric field is altered in magnitude, thereby suggesting the possible use of a centrally defective, electro-optic SCM slab as an electrically tunable, circular-polarization rejection filter with ultranarrow bandwidth.
Highlights
Volume gratings are generally made of isotropic dielectric materials
Haus and Shank [3] proposed the insertion of a phase defect half–way along a volume grating, in order to create a spectral hole in the reflection spectrum — a transmission feature in the center of the Bragg regime which is useful for narrow–bandpass filtering [4]
The Bragg phenomenon is insensitive to the polarization state of the incident light, because the volume gratings are made of isotropic materials
Summary
Volume gratings are generally made of isotropic dielectric materials. The relative permittivity of a volume grating varies periodically in the thickness direction from the entry plane to the exit plane [1, 2], and it exhibits the Bragg phenomenon: the existence of a high–reflectance wavelength– regime, provided the grating thickness spans a large number of relative–permittivity periods. As the relative permittivity matrix of a SCM varies helicoidally in the thickness direction, the high–reflectance characteristic of the Bragg regime is observed only when the incident light is circularly polarized (CP) in such a way that its handedness matches the structural handedness of the material; but incident CP light of the other handedness is mostly transmitted This distinction suggests that the term circular Bragg phenomenon (CBP) be used for the most important optical characteristic of SCMs. The introduction of a phase defect half–way inside a SCM slab gives rise to a spectral hole — but only when the CP state of the incident light matches the structural handedness [8, 9]. Space (i.e., vacuum) μ0/ 0, respectively, with μ0 and 0 being the permeability and permittivity of free space
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