Architecting One-dimensional and Three-dimensional Columnar Morphologies of Chiral Sculptured Thin Films

Open Access
- Author:
- Fiallo, Ricardo
- Graduate Program:
- Engineering Science and Mechanics
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- July 17, 2024
- Committee Members:
- Laura Cabrera, Program Head/Chair
Michael Lanagan, Major Field Member
Mark Horn, Dissertation Co-Advisor
Akhlesh Lakhtakia, Chair & Co-Dissertation Advisr
Julio Urbina, Outside Unit & Field Member - Keywords:
- chiral sculptured thin film
circular bragg phenomenon
architected morphology
3D periodicity
physical vapor deposition
photonic crystal
metamaterials - Abstract:
- Photonic crystals are periodic materials that can control the propagation of plane waves. They are found in nature and have been fabricated industrially as well as in laboratories. The wavelength range of the plane waves they control is directly related to the feature size of the photonic crystals. For this reason, a contemporary challenge in the field of metamaterials is the fabrication of photonic crystals with periodicity in three dimensions for the optical wavelength regime. A well-studied example of lab-grown one-dimensional photonic crystals is the chiral sculptured thin film (CSTF). A CSTF is fabricated by directing a collimated vapor flux at a substrate at an angle χv while rotating the substrate, an example of temporally periodic physical vapor deposition (TP-PVD). This causes the growth of parallel nanohelixes that collectively exhibit polarization-discriminatory bandgaps. A CSTF is an example of an architected morphology. Other architected morphologies have been proposed with the capability of exhibiting polarization-universal bandgaps. These are the matched ambidextous bilayer (MAB), the tightly interlaced MAB (TIMAB), and the equichiral STF (ECSTF). These were all realized experimentally in my research. Optical characterization of these architected morphologies revealed that they exhibit the theoretically predicted polarization-universal bandgaps with slight characteristic distinctions for each morphology. The bandgaps are tuneable by changing the angle of incidence. Fabrication of the architected morphologies by TP-PVD implicitly requires a planar (smooth) substrate. This was proven in my research by implementing a surface roughening procedure on glass and silicon substrates before attempting TP-PVD with the intention of depositing a CSTF. The subsequent morphological and optical characterization revealed that a CSTF cannot be fabricated if the surface is too rough. I fabricated three-dimensional photonic crystals with nanoscale features by etching two-dimensional periodic patterns (either square or triangular lattices) in fused silica and silicon substrates and then depositing a CSTF thereon such that the nanohelixes grow on top of a two-dimensionally periodic array (with the CSTF being periodic along the remaining dimension). These three dimensional photonic crystals were optically characterized (before and after CSTF deposition) in transmission and reflection, for both linear (s- and p-polarized) and circular (right-handed and left-handed) polarization states, in relation to the direction of the incident plane wave and the wavelength in the 500-900 nm range. Before CSTF deposition, the patterned substrates exhibited opal-like iridescence, and the optical characterization detected nonspecular transmission modes and Rayleigh-Wood anomalies in accordance with the mathematical predictions of the two-dimensional photonic crystals. After CSTF deposition, the substrates maintained their iridescence and the morphological characterization showed discrete clumps of nanohelixes growing on the unetched regions. Optical characterization revealed an overlap of the CSTF bandgap, Fabry-P´erot resonance, Dyakonov-Tamm surface waves, and a high-loss regime; the last named phenomenon being exclusive to the substrates and dependent on the lattice orientation and features.