Spectral hole filters using tilt-modulated chiral sculptured thin films

A defect-free chiral sculptured thin film (STF) reflects light of one circular polarization state and transmits that of the other in a spectral regime, called the Bragg regime. A tilt-modulated chiral STF reflects light of both circular polarization states in the Bragg regime if the amplitude of modulation is sufficiently large. A twist defect in an unmodulated chiral STF results in either a narrow passband or an ultranarrow stopband filter depending upon the thickness of that STF. An ultranarrow passband filter can also be realized using a twist defect in the tilt-modulated chiral STF, if that STF is sufficiently thick. Furthermore, it was seen that the polarization-insensitive mirrors fabricated using tilt-modulated chiral STFs are very tolerant of twist defects if the amplitude of modulation is sufficiently large.