Polarization-universal bandgaps realized with chiral sculptured thin films

Abstract

Two thin-film structures comprising chiral sculptured thin films (CSTFs) were fabricated to exploit the circular Bragg phenomenon (CBP) exhibited by CSTFs for realizing polarization-universal bandgaps. In a matched ambidextrous bilayer (MAB), a CSTF overlays its enantiomer, both identical except in structural handedness. A tightly interlaced MAB (TIMAB) comprises multiple bilayers, each bilayer containing a one-period-thick CSTF and its enantiomer of the same thickness. Optical transmission measurements for obliquely incident light in the visible spectrum showed that both MABs and TIMABs can exhibit polarization-universal bandgaps with maximum transmittance of less than 20%. The center wavelengths of these bandgaps are in the proximity of the center wavelength of the CBP exhibited by the constituent CSTFs, but the bandgaps are considerably narrower than the CBP spectral regime. The bandwidth variation of the polarization-universal bandgap of the TIMAB is considerably less than that of the MAB. These bandgaps can be tuned by adjusting the angle of incidence.