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Vanadium dioxide (VO2) thin films were deposited at room temperature on Corning 2947 glass substrates by direct current (DC) magnetron sputtering with a high purity VO2 target. Crystal structure, surface topography, surface morphology and optical properties of the deposited VO2 thin films were investigated. The deposited films exhibited a single orientation of (110) with a crystallite size of 41.3 nm as confirmed by the X-ray diffraction analysis and Scherrer formula, respectively. From the surface topography analysis, the film surface had root mean square surface roughness of ~6.8 nm and consisted of round-shaped grains. Similarly, from the surface morphology analysis, spherical-like grains were observed on the surface of the deposited VO2 thin films with estimated average grain size of 34.2 nm. The deposited thin films showed high transmittance and low reflectance in the visible and near-infrared wavelength regions at room temperature. In addition, from the optical transmittance against temperature measurements, only a few transmittance variation and a slight change in hysteresis loop were detected during heating and cooling between room temperature and 100 °C. Hence, the deposited VO2 thin films were found to exhibit lack of phase transition.

We propose a simple structure of photonic crystal fibers (PCFs) with high birefringence and low confinement loss based on one rectangular centric ring of smaller circular air holes (CAHs) in the fiber core, and three rings of larger CAHs in the fiber cladding. This simple geometry (using all CAHs with two different air hole sizes) is capable of achieving a flexible control of the birefringence, B = 5.501 × 10−3, and ultra-low confinement loss, 7.30 × 10−5 dB/km, at an excitation wavelength of λ = 1550 nm. The birefringence value is ∼5.0 times greater than that obtained for conventional CAH PCF. This simple structure has the added advantage from the view point of easy fabrication, robustness, and cost. A full-vector finite element method combined with anisotropic perfectly matched layers was used to analyze the various fiber structures. We have analyzed four cases of CAH PCFs, focusing on the core asymmetry design as opposed to the conventional approach of CAHs or elliptical air holes on the cladding and core. The robustness against manufacturing inaccuracies of the proposed structure has also been further investigated in this work.

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