Transparent electrically-conductive nanoporous thin films can be used as electrodes to attract dye ions from solution to
modulate the reflectance of a surface. Here we demonstrate that this technique can be used to create diffraction gratings
that can be modified by the application of a small electrical potential with a selected spatial distribution. Using
nanoporous ITO films fabricated by Glancing Angle Deposition, we have produced variable diffraction gratings,
fabricated by a variety of methods, including lithography combined with etching techniques or direct patterning using
focused ion beam etching. We demonstrate modulation of the diffraction pattern by employing electric force to attract
the dye ions into the nanoporous electrode, thereby introducing a substantial local change in the effective refractive
index value and thus altering the resultant diffraction pattern and, in some cases, yielding diffractive orders that lie
between those associated with the underlying grating. These new orders are easily distinguished and their intensity can
be substantially modified by controlling the applied voltage. Because this technique can work with very small pitch
gratings, this approach has the potential to enable new applications that may not be readily achieved using conventional
liquid crystal technology.
We present a novel method of modulating total internal reflection (TIR) from an optical surface using a solution of dye ions in combination with a nanostructured electrode. Previous work using the electrophoretic movement of pigment particles to modulate TIR was limited by agglomeration of the pigment over time. Dye ions do not suffer from this limitation, but because of their small size they have significantly smaller absorption cross-section per unit charge than pigment particles which are generally two orders of magnitude larger. This significantly limits the maximum absorption caused by electrostatic attraction of the ions to a transparent conductive electrode. This can be overcome by using a transparent conductive nanoporous thin film as the electrode in which the porosity increases the effective surface area, allowing more dye ions to move into the evanescent wave region near the nanoporous transparent electrode and thus substantially increases the amount of absorption. In this paper, we demonstrate the modulation of TIR by observing the time-dependent variation of the reflectance as the dye ions are moved into and out of the evanescent wave region. This approach may have applications in reflective displays and active diffractive devices.
Photoluminescent nanostructured thin films have been fabricated using physical vapour deposition and the glancing angle deposition (GLAD) technique. Precision controlled substrate motion and oblique incidence (>75o) enable the fabrication of a variety of 3-D morphologies including vertical posts, helical (chiral) columns and chevrons. Scanning electron microscopy and X-ray diffraction were used to characterize the film nanostructure. These experiments focussed on the chiral morphology which exhibits intriguing polarization behaviour such as selective transmission of circularly
polarized light and circularly polarized photoluminescence. Helical films of Y2O3:Eu and Alq3 were fabricated with thicknesses in excess of 2 μm and densities nominally 60% of bulk. Transmission spectroscopic ellipsometry measurements were used to determine the degree of selective transmission of polarized light through the samples. The degree of circular polarization for the photoluminescent light emitted from helical films was measured with the use of a quarter waveplate and linear polarizer. Polarized photoluminescence efficiencies were consistent with the observed selective transmission of circularly polarized light through the films. The use of GLAD to control the nanoscale morphology of the films allows for the spectral location and strength of these polarization effects to be easily and accurately selected.
Optical studies of porous nano-engineered thin film materials fabricated using Glancing Angle Deposition (GLAD) have been a focal point of research since the inception of the GLAD technique over ten years ago. As the sophistication of porous nano-engineered thin film designs has increased over the years, photonic device applications of these materials have been explored. We will review some of our recent advancements in the study and fabrication of porous nano-engineered thin films for optical applications including our group's work with helical films and devices, square spiral photonic crystal films, and graded-index (GRIN) films and devices. Initial optical studies of helical films focused upon the circular Bragg effects and optical rotatory dispersion exhibited by such structures. In recent years, the exploration of different materials and the fabrication of liquid crystal (LC) cells using these films have brought the prospect of using such film-LC hybrids in display applications much closer. Helical films made from luminescent materials have also been investigated and were found to emit partially circularly-polarized light. Our work with square spiral structures focuses upon the fabrication of periodic arrays of such structures in order to yield a three-dimensional photonic bandgap. Our techniques also enable the formation of designed defects in the array with relative ease, opening the door to a myriad of potential applications. Finally, we will discuss graded-index structures which are made by varying the porosity of the film structure during film growth. Films of this nature have been designed and fabricated for use as wide-band antireflection coatings, rugate filters, spectral-hole filters, and optical humidity sensors.
Nanostructured europium-doped yttrium oxide (Y2O3:Eu) films were fabricated using electron beam evaporation, in combination with the Glancing Angle Deposition (GLAD) technique. GLAD makes use of controlled substrate motion during physical vapour deposition (PVD) of a thin film resulting in a high degree of control over the nanostructure of the film. Films were deposited using pre-doped Y2O3:Eu source material. Scanning electron microscopy was used to characterize film nanostructure, while the light emission properties of these films were characterized by photoluminescence measurements. Films of four different nanostructures were used in this study: chevrons, pillars, helices, and normally-deposited solid thin films. For each film nanostructure, measurements of the angular dependence of the intensity of the emitted light, as well as absolute brightness, were obtained and compared. The polarization of the light emitted from the chevron film was also examined using a linear polarizer to analyze the polarization state. Measurements of the selective transmission of circularly polarized light through the helical samples were obtained using variable angle spectroscopic ellipsometry.
Thin films of europium-doped yttrium oxide (Y2O3:Eu), a well-known luminescent material, were grown using electron beam evaporation, in combination with the Glancing Angle Deposition (GLAD) technique. GLAD makes use of controlled substrate motion during physical vapour deposition (PVD), resulting in a high degree of control over the nanostructure of the film. Until recently GLAD had not been used with luminescent materials. Films were deposited using pre-doped Y2O3:Eu source material, with either 4.0% (wt) Eu doping or 5.6% (wt) Eu doping. The nanostructure of these films was characterized through scanning electron microscopy, while the light emission properties of these films was characterized by photoluminescence measurements. In order to optimize the light emission properties of the films the partial pressure of oxygen during the deposition of the films was varied. Films were deposited on both silicon and sapphire substrates, in order to compare how different substrates affect the growth and light emission of the films.