Optical anisotropy is an inherent property of columnar dielectric films, such as those fabricated by the glancing
angle deposition (GLAD) technique. This process utilizes physical vapor deposition combined with computer-controlled
substrate motion to finely tune the direction of column growth and vital morphological parameters
such as column cross-section and inter-columnar spacing. Control over the anisotropic properties of the porous
film provides an opportunity to design polarization-selective photonic devices and films with improved band
gap properties. Anisotropic defects in multilayer films also result in a polarization-sensitive position of resonant
transmission modes. We employed the finite-difference time-domain and frequency-domain methods to
theoretically analyze and design columnar films with unique band-gap properties. The following morphologies
were considered: (i) S-shaped columnar films with polarization-dependent band-gap position and width. Using
numerical simulations we have shown that the competitive effect of different sources of anisotropy can be used
to engineer photonic band gaps with strong selectivity to linearly-polarized light; (ii) Rugate thin films with an
anisotropic defect, which exhibit resonant mode splitting. Optical devices were fabricated using titanium dioxide
because it has good transparency in the visible range of the optical spectrum and a large bulk refractive index.
Experimental results were compared to simulations to verify the designs and understand the limitations of the
We present the design, fabrication, and characterization of a GLAD-fabricated photonic crystal sensor with a
bandgap located in the visible optical spectrum. The photonic crystal is fabricated from TiO2 using electron-beam
evaporation in a GLAD capable vacuum deposition system. Changes in humidity over a wide range (from 3% to
90% relative humidity) are detected by a colour change in the film due to movement of the photonic bandgap.
The colour changes are quantified by measuring the transmittance of white light. Coupling the sensitivity of the
film with a simple visual feedback system eliminates the need for complicated measurement techniques. This is
desirable to minimize the cost and power consumption of the sensor device, making it amenable to large-scale
production and deployment.
Glancing angle deposition (GLAD) is a thin film fabrication method providing dynamic control over the
internal columnar microstructure of the deposited film. Using the GLAD technique it is possible to control the porosity
of the coating allowing precise tailoring of the optical properties. Therefore, in a single material system, the refractive
index profile of the film can be engineered to create a variety of multilayer structures. The focus of this research is on the
optical properties of these structured thin films. When the structure is periodic, incident radiation is subject to
constructive and destructive scattering which lead to photonic bandgap effects. Also of interest are the optical properties
of aperiodic systems, such as the Thue-Morse multilayer, which are deterministic but non-periodic. The complex
structural correlations in aperiodic systems lead to interesting bandgap-like properties. Applying the GLAD technique,
periodic and aperiodic optical lattices are fabricated with titanium dioxide, a dielectric material commonly used in
optical coating devices. The bandgap properties of these systems are investigated using transmittance spectroscopy and
transfer matrix calculations.
The fabrication of one dimensional photonic bandgap nanostructures is described and the optical properties of
these structures are examined. Using a deposition technique known as a glancing angle deposition (GLAD),
porous films with a predefined nanoscale geometry are created. Specifically, in the present work we consider
GLAD fabricated thin films characterized by periodically varying refractive index in one-dimension. We
introduce a variety of planar defect layers into the structures and investigate the resulting changes observed in
the photonic bandgap of the system. Theoretical simulation of transmittance spectra of GLAD fabricated films
is performed with the finite-difference time-domain (FDTD) method and the results are compared with
experimental measurements. Modifications of the transmittance spectra are investigated by changing the
geometry of the defect layer and varying the void region effective index. It is shown that the spectral width
and location of states within the bandgap is controlled by the geometry of defect and film microstructure.
Active tunability of the defect states is obtained by considering infilling of the void regions of the structure
with nematic liquid crystals and then analyzing the optical spectrum for various orientations of the liquid
crystal director axis.
Porous thin films of TiO2 exhibit interesting and useful optical properties when the glancing angle deposition (GLAD) technique is used to impart controlled structural variations on the nanometer scale. Specifically, helically structured thin films possess optical properties sensitive to the polarization state of incoming light, including selective reflection of circular polarizations and optical rotation of the vibration ellipse of light as it passes through the film. By adjusting the deposition parameters, the helical structures can be transformed into vertically aligned columns with nanometer diameter variations. These films possess a continuously varying refractive index along the substrate normal. This index profile can be tailored so that it varies sinusoidally along the substrate normal to form a rugate interference filter. With the addition of a constant index layer of thickness equal to the sine period located in the center of the film, a narrow bandpass appears within the filter’s larger reflectance band.