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
fabrication process.
Luminescent thin films are used for many applications, including light-emitting diodes, lasers and flat panel
displays. Glancing angle deposition (GLAD) is a physical vapor deposition technique which relies on highly
oblique flux angles to create porous thin films. When combined with real-time substrate motion control and
measurement of deposition rates, it is possible to produce high quality nanostructered thin films. A rugate filter
uses a sinusoidally varying index profile to produce a stop band. Using the GLAD technique, it is possible to
produce a rugate filter from a single material. The central wavelength, depth and width of the stop band can be
designed by adjusting the film nanostructure. In this paper, rugates composed of Y2O3:Eu are used to control
the angular emission profiles of the photoluminescent thin film. Confined, annular and isotropic emission profiles
film is nearly uniform for emission angles
between ~ -60° and ~60°.
We have used the glancing angle deposition technique to fabricate highly porous nanostructured optical thin films that
act as humidity sensors. The responsiveness and repeatability of these sensors has been investigated for samples stored
under different environmental conditions. It has been found that samples stored in air have a more stable performance
than those stored in a dry nitrogen environment. It has also been found that annealing impacts the responsiveness of the
optical thin film sensors.
Oblique evaporation of inorganic materials has long been used to induce alignment in liquid crystals, often for
the purpose of controlling the pretilt angle in a liquid crystal cell. These alignment layers are relatively dense,
keeping the liquid crystals above the surface of the inorganic layer. By evaporating at increasingly oblique angles
(> 80°), the alignment layer can be made porous, allowing liquid crystals to infiltrate the film and to align to
individual nanostructures. By coupling simultaneous computer controlled substrate motion during evaporation,
a process known as glancing angle deposition (GLAD), the nanostructures can be grown in a variety of useful
shapes, including helices, polygonal spirals, zigzags and periodically bent S-shaped columns. Alone, these films
exhibit properties such as linear and circular polarization selective Bragg reflection, and full three-dimensional
photonic bandgaps. By infiltrating liquid crystals into the voids of the film, one can align liquid crystals in three
dimensions, as well as tune and switch the film's optical properties. Additionally, the GLAD film can be used
to template polymerizable liquid crystals for subsequent monomer infiltration. In this work, using spectroscopic
ellipsometry, we examine the effects of liquid crystal infiltration on various film structures made from a variety
of metal oxides, for both varying film thickness and deposition angle. Techniques for filling a porous film with a
known volume of liquid crystals are also presented. Additionally, we examine the switching behaviour for these
films under applied electric fields. Finally, we compare experimental and simulated results used to predict and
optimize the optical properties of these hybrid films.
Titanium dioxide thin films were formed by electron-beam evaporation onto fused silica substrates using serial bideposition (SBD). The SBD technique combines rapid substrate rotation and oblique-angle physical vapor deposition (PVD) to create optical coatings that are composed of nanostructured columns which exhibit large birefringence values in the plane of the substrate. In this study, post-deposition annealing was used to crystallize amorphous TiO2 thin films formed by SBD to improve birefringence without significantly increasing optical absorption or scattering. Birefringent thin films were fabricated at deposition angles ranging from 60° to 75° and annealed in air at temperatures ranging from 200°C to 900°C to form anatase and rutile TiO2. Changes in the optical properties, crystallinity, and nanostructure were characterized by ellipsometry, x-ray diffraction, atomic force microscopy, and scanning electron microscopy. It was found that optical anisotropy increases strongly upon formation of anatase, yielding in-plane birefringence values that doubled from 0.11 to 0.22 in the case of TiO2 thin films deposited at 60° and annealed at 400°C. Raising the annealing temperature to 900°C to form rutile thin films increased the thin film birefringence further but also led to low optical transparency due to increased absorption and diffuse scattering.
Porous thin films of dielectric materials have been deposited using e-beam evaporation onto substrates held at highly oblique angles (> 80o), coupled with simultaneous computer controlled substrate motion about two independent axes. This technique, known as glancing angle deposition (GLAD), enables the formation of shaped, isolated nanostructures, including vertical posts, zig-zags, and both helical and polygonal spirals, which exhibit chiral optical properties. GLAD films form the backbone of liquid crystal (LC) hybrid optical materials and devices, and afford key advantages. The
porous nature of the GLAD structures allows LCs to uniformly penetrate the film and modify its optical properties. Addition of LCs to GLAD films improves the properties of the films by reducing optical scattering, enhancing transmission, and accentuating existing chiral and linear optical anisotropies. Further, by mixing a dichroic dye with the LCs, polarization selective optical properties can be introduced into the film which can be used to augment the functionality of GLAD films. It has been found that addressing hybrid GLAD films with an electric field reorients the LCs, allowing one to switch the optical properties of the composite film. This behaviour extends to LCs mixed with dichroic dye, allowing one to switch the selective polarization properties with an applied voltage. Using results based on spectroscopic ellipsometry, we will examine the optical properties and switching behaviour of LC/dichroic dye hybrid
GLAD films and discuss how the results allow one to infer the alignment of LCs in GLAD films, as well how the addition of dichroism to the film affects the selective transmission of both linearly and circularly polarized light.
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.
Thin films with chiral or helical microstructures exhibit circular birefringence effects. Glancing angle deposition (GLAD) is a fabrication method capable of producing chiral thin films with controllable porosity and microstructure. In this paper, the effects of porosity on the circular birefringence exhibited by helical TiO2 films are presented. Transmittance measurements reveal two optimal film growth angles: one corresponding to a maximum in form birefringence and another corresponding to strong anisotropic scattering. Reflectance data support the transmittance measurements in the regime where scattering is minimized.
Titanium dioxide was evaporated onto rotating substrates at highly oblique deposition angles to create thin films exhibiting a nanostructure which resembles a polygonal helix. Abrupt, periodic rotations of the substrate were used to create triangle, square, pentagon, and star-shaped film morphologies. Experimental optical measurements show that polygonal-helix thin-films exhibit double-handed circular Bragg phenomena. Unlike a standard chiral filter, a polygonal-helix thin-film reflects left-handed circularly polarized light at one frequency band and right-handed circularly polarized light at a second frequency band. The relative wavelength-dependence of the reflection bands is controlled by the angular rotation between arms of a polygonal helix. Spectral-hole polarization filters, produced by adding twist and layer defects to a polygonal helix, are also reported. Twist-defects tend to produce a narrow passband within both circular Bragg reflection bands of a polygonal helix, while a spacing layer defect can be used to produce a passband within only one of the reflection bands.
In this paper, we present the growth and optical characterization of the preliminary stages of amorphous silicon square spiral growth on pre-patterned and unpatterned sections of silicon substrates. The periodicity of the seeding was set to 1 μm using electron beam lithography, and a seed enhancement layer was deposited on top of the seeds, followed by a quarter-turn square spiral on top of that. It was found that the optical constants in the wavelength region of 1000 nm to 1700 nm for the film materials were higher for the patterned sections of the film as compared with the unpatterned sections of the film.
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.
Porous thin films have been fabricated by physical vapor deposition at an extremely oblique angle of incidence (85°). This deposition technique, called glancing angle deposition (GLAD), was used to create thin films composed of isolated helical columns. By investigating a variety of dielectrics, we found that helical GLAD films fabricated from titanium dioxide produce the strongest chiral optical response because of its large refractive index. Further improvements were made by using post-deposition annealing to form anatase and rutile polycrystalline phases of TiO2. By tailoring the pitch of the helical structures, the circular Bragg reflection band was tuned to preferentially reflect red, green, and blue light. The high porosity of a GLAD film (>50%) permits liquid crystals (LC) to be incorporated into the pores of the helical nanostructure, which creates chiral alignment in otherwise non-chiral LCs. This technique improves circular Bragg reflection and can create addressable hybrid materials with potential applications to high-efficiency reflective displays.
Porous thin film structures have been fabricated by physical vapor deposition at an incident flux angle that was typically greater than 80°. This deposition technique, often called glancing angle deposition (GLAD), was used to create thin films composed of isolated helical columns. Modification of the deposition parameters was used to control the porosity, the handedness, and the pitch of the helical structure. The high porosity of the GLAD film (>50%) permits fluids, and in particular liquid crystals (LC), to be incorporated into the voids of the nanostructure. We present the results of a study assessing the effect of film material, chiral morphology, and liquid crystalline material on the optical performance of helical GLAD films. Films fabricated from TiO2, a high refractive index material, exhibited strong optical rotation of linearly polarized light and selective reflection of circularly polarized light. By increasing the number of turns of the helix the chiral optical response was enhanced, and by tailoring the pitch of the helical columns, the wavelength-dependence of the reflection band was tuned to preferentially reflect red, green, or blue light.
The classic challenge faced by researchers dealing with liquid crystals is to control the LC molecular orientation and hence optimise the optical properties. Well known techniques for influencing LC texture include the use of surfactants or thin film alignment layers. The underlying limitation common to such techniques is that while excellent control of LC anchoring at the substrate surface is achieved, molecular alignment in the bulk of the LC is reliant entirely upon the cooperative effects and resulting elastic properties of the LC material. Generally, this has worked sufficiently well in practice, but unfortunately, the complete dependence on the intermolecular forces of the LC means that unencumbered, reversible switching is not always possible. Our group has taken a unique approach to influence LC orientation. Using glancing angle deposition (GLAD), highly porous thin films can be grown possessing isolated columnar microstructure whose shape can be tailored via substrate motion during film deposition. In particular, we can grow films of helical columns with controlled pitch and handedness. These films exhibit circular dichroism and optical activity similar to that seen in chiral LCs. The high porosity of GLAD films permits fluids such as LCs to be introduced into the pores, leading to a new type of hybrid optical material. Most significantly, initial work showed that when achiral LCs were embedded in chiral GLAD media, there was an enhancement of the circular dichroism and optical activity as the chiral GLAD film served to induce a chiral orientation in the LC. In this report, we start with a brief overview of the GLAD process and some relevant optical studies, leading to a review of GLAD/LC hybrid materials, switchable devices, and finally, a discussion of recent research optical characterisation and some ideas for future avenues of investigation.
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.
The glancing angle deposition (GLAD) technique may be used to fabricate thin films with a very porous chiral or helical microstructure, with geometry of the helices controlled and engineered to have specific porosity, helical pitch, and handedness. Such films with chiral morphology exhibit optical rotation and circular dichroism similar to certain classes of liquid crystals, with the pitch and handedness of the chiral film determining the resultant optical properties. When nematic liquid crystals (LCs) are introduced into the pores of chiral GLAD films, a composite material is formed that shows a significant enhancement of the chiral optic response, suggesting an alignment of LC molecules by the underlying inorganic chiral film 'backbone'. Results are presented from a study of the optical behaviour of chiral films fabricated from materials such as silicon dioxide, alumina, and magnesium fluoride, and from study of the effects of parameters such as helical pitch, film thickness, and film porosity on the optical properties. The enhancement of optical response created by impregnating these films with nematic LC is reported. The construction of electro-optic cells containing the composite GLAD/LC films is described, demonstrating reversible electro-optic switching of the LC component within the film.
Porous, chiral thin films with controlled microstructure fabricated by glancing angle deposition (GLAD) exhibit unique optical properties. The observed optical activity and circular birefringence in these films have been compared to those of cholesteric (or chiral) liquid crystals. Porous GLAD films have been previously demonstrated as alignment 'backbone' structures for liquid crystals (LC) embedded in the pores of the films, leading to a new class of composite optical materials. GLAD films with chiral, or 'helical,' microstructure have been found to impose a chiral nematic-like molecular ordering in non-chiral nematic liquid crystals. The addition of nematic LCs to the films was found to enhance significantly the chiral optic response compared to that of the film alone. Recently, we demonstrated electro-optic switching of the LC component in optical devices based on GLAD-LC composites. In an unaddressed state, the GLAD film induces chiral nematic-like alignment in the embedded LC, with the GLAD film controlling pitch and handedness of the aligned LC. In an addressed state, the LC molecules align parallel to the field and by index matching, the chiral optic response of the device vanishes. In this work, we present extensive optical characterization of the GLAD-LC composite materials including measurements of optical rotation and circular dichroism, switching behavior, and analysis of structure- property relationships.
The orientational ordering of liquid crystal (LC) materials directly determines their optical properties. Controlling the orientational order allows the optical properties to be engineered for display and switching applications. Recent advances in LC ordering with LCs embedded in porous networks have resulted in materials with exciting new properties, enabling new display and switching technologies. A new technique called Glancing Angle Deposition (GLAD), based on conventional thin film fabrication, allows engineering of porous structures of inorganic materials in three dimensions on a nanometer scale. By impregnating the void spaces in these porous inorganics with various polymers and LCs, we have created a new type of hybrid material where the orientational order of the impregnate is controlled by the inorganic backbone structure. Optical measurements of GLAD materials with various impregnates demonstrate that simple rodlike liquid crystalline materials (nematics) are oriented by a helical inorganic backbone to form a phase similar to the chiral nematic phase seen in other (cholesteric) liquid crystals. This new hybrid material appears promising for optical switching and display applications.
Unique thin film microstructures have been fabricated with the Glancing Angle Deposition (GLAD) technique. These porous, thin films can be engineered with a variety of different morphologies to sub-micron dimensions, including helical, post, and chevron or zigzag microstructures. This paper reports some recent results in study and application of films deposited using GLAD, namely: the use of low pressure, long throw sputtering to produce porous titanium films; deposition of porous, structured ZrO2 films for use as thermal barriers; and measurement of the mechanical response of chiral or `microspring' thin films.
Glancing angle deposition (GLAD), developed recently by Robbie and Brett, is an advanced technique of thin film deposition that can produce porous thin films with columnar microstructural features controllable on a ten nanometer scale. GLAD combines highly oblique angle deposition with computer controlled substrate motion to allow engineering of thin film microstructure for a diverse range of applications. Of particular promise for optical applications are chiral thin film morphologies, including films fabricated by GLAD possessing helical microstructure. Previous optical characterization has demonstrated rotation of the plane of polarization in these films. In this work, circularly polarized spectroscopic transmission measurements on helical GLAD films have shown selective reflection/scattering of the circular polarization which matches the handedness of the film, with the helical pitch controlling the peak wavelength. The geometric properties of films fabricated with GLAD, such as film density, helical rise angle, and helical radius can be controlled independently and easily allowing optical properties to be tailored as desired.
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