A titanium dioxide (TiO2) chiral sculptured thin film (STF) fabricated using a serial bi-deposition (SBD) method based on electron beam evaporation has been studied using spectroscopic Mueller matrix ellipsometry (MME). Complete Mueller matrices for the SBD TiO2 chiral STF have been measured using a dual-rotating compensator spectroscopic ellipsometer over the spectral range from 250 to 825 nm in transmission mode, both at normal incidence (i = 0°) and over a range of oblique angles (5° i 60°). A multilayer structurally-graded optical model has been applied to deduce spectra in the three principal indices of refraction that characterize the locally biaxial structure, using as input the complex amplitude transmission ratios deduced from the Mueller matrix measured at normal incidence. A Bragg resonance feature has been observed, and this feature blue-shifts with increasing angle of incidence. Predictions of the transmittance for circularly polarized light normally incident upon the SBD TiO2 chiral STF can be obtained simply by multiplying the unnormalized Mueller matrix by the appropriate Stokes vector, and the results are in excellent agreement with direct measurements.
Magnesium is an essential mineral in the human body and has recently been studied as a bioabsorbable material for use
in cardiac stents. New areas of application can be found in bone plates, bone screws, and orthopedic implants.
Magnesium alone has a corrosion rate much too high for use in such applications and has been alloyed with various
elements to improve corrosion resistance. The use of vapor deposition to create Mg alloys for the above applications has
not been attempted although certain properties of non-equilibrium alloys, namely corrosion resistance, can be improved.
Using vapor deposition the characterization of the growth of magnesium alloy thin films has been done utilizing various
alloying elements, substrate temperatures, post-deposition treatments, and substrate positions. The results point towards
a growth mode controlled by crystallization of the Mg. Mg Sculptured thin films (STFs) are used to demonstrate these
effects and potential solutions while also providing a route to control nanoscale surface morphology to enhance cell
growth, cell attachment, and absorption properties. The results of the study are presented in terms of x-ray diffraction
data, microscopy analysis of growth evolution, and corrosion testing. This magnesium alloy research utilizes a dual
source deposition method that has also provided insight about some of the growth modes of other alloy STFs.
Engineering of surface morphology using dip coatings and etching has been used in biomedical materials to enhance
certain application specific surface properties. STF technology potentially provides a path to merge the advantages of
non-equilibrium alloy formation and engineering nanoscale surface morphology.
We construct an external cavity diode laser (ECDL) comprising structurally left-handed chiral sculptured-thin-film
(STF) mirrors for pure circular-polarized (CP) emission, and observed single mode, left-handed CP lasing performance.
The extinction ratio of CP output was found to increase rapidly near the threshold of the injection-current for the laser
Chiral sculptured thin films (STFs) of TiO2 deposited by the serial bi-deposition method can be modified post-deposition to tailor the circular Bragg phenomenon which occurs due to their periodic helical morphology. This post-deposition tailoring is accomplished through annealing and/or wet chemical etching. Annealing of the chiral STF in spectral-hole-filters (SHFs) transforms the material structure from non-crystalline to an anatase structure which is more stable and provides better optical performance. The transformation causes a blue-shift of the circular Bragg phenomenon (CBP) which is quantified and correlated with material properties. Further annealing causes a significant change in morphology, which resembles a "string of pearls", and may be the cause of a loss of transmission. Wet chemical etching is also capable of blue-shifting the Bragg regime of a non-crystalline SHF, while having very little effect on a crystalline SHF. The spectral shift of the Bragg regime is limited by the ability of the nanowires, of which the STFs are composed, to remain attached to the substrate. Wet chemical etching and annealing can be used in combination to allow more control of post-deposition processing.
Postdeposition chemical etching of spectral-hole filters, which were fabricated as chiral sculptured thin films with central 90-deg-twist defects, decreases the cross-sectional dimensions of the helical columns that such films comprise and blueshifts the spectral holes, thereby establishing the efficacy of postdeposition chemical etching as a means to tune the optical response characteristics of sculptured thin films.