Suppression of circular Bragg phenomenon in chiral sculptured thin films produced with simultaneous rocking and rotation of substrate during serial bideposition
Open Access
- Author:
- Swiontek, Stephen Edward
- Graduate Program:
- Engineering Science
- Degree:
- Master of Science
- Document Type:
- Master Thesis
- Date of Defense:
- April 19, 2012
- Committee Members:
- Akhlesh Lakhtakia, Thesis Advisor/Co-Advisor
Jian Xu, Thesis Advisor/Co-Advisor - Keywords:
- Sculptured thin films
ZnSe
circular Bragg phenomenon
PVD - Abstract:
- A sculptured thin film (STF) is produced by physical vapor deposition, whereby a collimated vapor flux is directed obliquely onto a moving substrate on which the STF is deposited. The STF comprises parallel and identical nanocolumns of a certain shape sculptured by substrate motion. Substrate rotation produces chiral STFs whose nanocolumns are helical. Chiral STFs produced by serial bideposition, which either actually or effectively involves two vapor fluxes, possess large local linear birefringence and evince highly intense optical activity, both of which properties are attractive for optical sensor, filter, and display technologies. Specifically, chiral STFs exhibit the circular Bragg phenomenon for normally incident light: in a certain spectral regime, either left- or right-circularly polarized light is preferentially reflected. Theoretical modeling has shown that if the substrate is rocked, in addition to rotation, the circular Bragg phenomenon is suppressed, even though the deposited thin film should still have a chiral morphology. Instead, an ordinary Bragg phenomenon is exhibited which is insensitive to the polarization state of the incident light. Therefore, an ordinary Bragg filter can be made with just one material, instead of as a quarter-wave stack that requires the deposition of two materials. For this work, ZnSe chiral STFs were fabricated with/without substrate rocking, and their transmittance spectrums for linear- and circular-polarized incident light were measured. During fabrication, a collimated vapor flux was directed at a vapor flux angle of either 20° or 25° with respect to the substrate plane, which was rotated at 0.5 rpm in a low-pressure vacuum chamber at a base pressure of 1.5 10-5 Torr. At the same time, the substrate was rocked with an amplitude of 5°, 10°, 15°, or 20° depending on the initial starting value of vapor flux angle. The rocking period and the rotation period were identical. All chiral STFs produced were 9-periods thick, with the spatial period being 330 nm. With sufficient rocking amplitude, the discrimination between left- and right-circularly polarized (RCP and LCP) light nearly vanished, whereas similar Bragg phenomenons for s- and p-polarized light were observed. Thus, chiral STF technology can be used to produce both ordinary and CP Bragg filters.