BROADBAND OPTICAL RESPONSES OF CHIRAL SCULPTURED THIN FILMS DEPOSITED ON PLANAR AND NON-PLANAR SUBSTRATES

Open Access
Author:
Erten, Sema
Graduate Program:
Engineering Science and Mechanics
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
October 27, 2017
Committee Members:
  • Dr. Akhlesh Lakhtakia, Dissertation Advisor
  • Dr. Dinesh Agrawal , Committee Chair
  • Dr. Jian Xu , Committee Member
  • Dr. Akhlesh Lakhtakia, Committee Member
  • Dr. Thomas Mallouk, Outside Member
Keywords:
  • chiral sculptured thin films
Abstract:
Sculptured thin films (STFs) are classified as nanoengineered metamaterials whose morphology can be altered during deposition. STFs are generally fabricated via physical vapor deposition (PVD) techniques [1]. A chiral STF is an assembly of identical and parallel nanohelixes. In order to deposit a chiral STF, the substrate holder in the vacuum chamber of a thermal evaporation system should be tilted at a fixed angle and rotated at a constant rate during deposition. Chiral STFs exhibit the circular Bragg phenomenon (CBP) when interacting with circularly polarized incident light. Also, they can be used to excite multiple surface-plasmon-polariton (SPP) waves when deposited on top of a metallic grating. The CBP is described as circular-polarization-selective reflection. It occurs when circularly polarized light is incident on a film of a structurally chiral material exemplified by cholesteric liquid crystals (CLCs) and chiral STFs. When the handedness of the circularly polarized incident light matches the structural handedness of the structurally chiral material with a sufficient thickness, the incident light is highly reflected in a specific wavelength regime. If the handedness of the circularly polarized light and structural handedness of the film are mismatched, the reflection is very little [2]. The CBP has been exploited for various optical applications such as circular-polarization filters and reflectors. SPP waves propagate along the interface of a metal and a dielectric material. Similar to any other surface wave, SPP waves are sensitive to irregularities near the interface. Only one SPP wave can be excited when the dielectric material is homogeneous. When the dielectric material is replaced by a non-homogeneous one, the direction of non-homogeneity being normal to the interface, more than one SPP wave can be excited. The multiplicity of SPP waves at a specific frequency when the non-metallic partnering material is periodically non-homogeneous offers advantages for optical sensing of analytes. For this dissertation, the broadband responses of chiral STFs deposited on planar and non-planar substrates were investigated. The objective was to experimentally investigate and qualitatively, if not quantitatively, prove theoretical preiii dictions about the optical responses of chiral STFs. The motivation was to design and engineer chiral STFs for optical applications and devices. The dissertation has four main chapters based on: (i) The CBP exhibited by chiral STFs for oblique angles of incidence, (ii) The CBP exhibited by mirror-backed chiral STFs for oblique angles of incidence, (iii) Dual-band circular polarization filters, and (iv) Excitation of multiple SPP waves guided by the interface of a chiral STF and a one-dimensional (1D) metallic grating. The first main chapter is focused on the investigation of the CBP for oblique angles of incidence to experimentally prove theoretical predictions. Chiral STFs of zinc selenide were deposited on BK7 glass substrates by thermal evaporation. Specular and non-specular reflectance and transmittance data were collected using a custom-made experimental set-up. The results clearly indicated the circular Bragg regime for all angles of incidence investigated. A blue-shift in the wavelength regime of the CBP with increasing angle of incidence was observed. The results are in good agreement with previous theoretical predictions, qualitatively. The second one is a follow-up study to understand the exhibition of the CBP when the transmittance is blocked. A chiral STF was backed by a highly reflective mirror and the reflectance spectrums were measured. The results indicated that when the transmittance is blocked, all remittances show evidence of the CBP instead of just one remittance. The third one is focused on a novel strategy for fabricating dual-band circular polarization filters. For this study, chiral STFs of zinc selenide were deposited on both sides of a substrate by the serial bideposition (SBD) technique [3]. The transmittance data were collected for normally incident light. Two different circular Bragg regimes were observed centered at different wavelengths. The last main chapter is focused on exciting multiple SPP waves guided by the interface of a chiral STF and a metallic grating. The chiral STFs were deposited by the thermal evaporation technique on 1D metallic gratings. The specular and non-specular transmittance and reflectance measurements were completed using a custom-made variable-angle spectroscopic system. At least three different SPP waves were excited experimentally. The results can be used to increase the sensing capabilities of SPP wave based sensors by increasing the number of analytes that can be detected at a time. The practicality of such sensors can be increased due to the fact that there is no need for a prism to excite SPP waves when employing the grating-coupled configuration.