We prove a new theorem on polarized light: when un-polarized (natural) light is incident to
a lossless optical element, the polarized reflected intensity equals the polarized transmitted
intensity. We have confirmed the theory both through algebraic simulation and through
experiments carried out on a Titanium Oxide chiral thin film.
We show that a chiral film that is not matched to the refractive indices of the surrounding media behaves as a Bragg resonator for elliptically polarized light with a given handedness, ellipticity, and axial alignment. Equations are derived for the polarization parameters, auxiliary angle (chi) and azimuthal angle (alpha), of the polarization ellipse. Thickness modulated chiral films and chiral-birefringent composite media are considered as elliptical Bragg mediums. Experimental results are presented for coatings fabricated with titanium oxide.
Currently there is a need for retarders that do not degrade under elevated temperatures and intense illumination for use in displays and devices such as rear projection televisions. A stack of inorganic planar layers with alternating high and low refractive indices behaves as a form birefringent uniaxial material, but the sign of the birefringence is always negative. In the presentation we outline the use of serial bideposition with 90 deg incremental substrate rotations to generate positive uniaxial materials with a typical difference of 0.12 between the ordinary and extraordinary refractive indices. Optical methods for displaying the axial symmetry of the coatings and for measuring the birefringence are described and contrasted with previous and current work on inorganic biaxial materials.
The problem of coupling a surface dielectric grating consisting of a sinusoidal corrugation to a structurally chiral volume grating has been formulated and solved with results showing the robust co-existence of phenomena characteristic of both types of grating: namely Rayleigh-Wood anomalies and Bragg zones. A mechanism to strengthen the coupling between both grating types has now been included in the model, namely a quarter wave plate whose birefringence converts normally incident linearly polarised light of the chiral Bragg wavelength into transmitted circularly polarised light.
It is possible to tailor the period of the sinusoidal surface grating such that the scattering of obliquely incident light from above, by the surface grating, will lead to a specific non-zero order being scattered onto a path which is normally incident on the rest of the structure. Significantly, such orders exhibit the circular Bragg phenomenon in reflection and transmission only if both grating types are present, making this a truly coupled grating effect.
Optical and electron microscopies reveal complexity in the multilayered chiral coatings that produce green metallic-like reflections from manuka (scarab) beetles. In particular the reflectors are shown to have the form of small concave pits and troughs that are filled with contouring chiral material. Each chiral micro-reflector presents a range of pitch and tilt to an incident beam of light. The presentation attempts to relate these physical properties to optical properties such as spectral reflectance, angle of spread and perceived color of the beetles.
Effective medium theory is applied to nanoengineered inorganic
coatings in which a primary chiral medium (A) threads through a
second medium (B) that may be isotropic, birefringent or chiral.
In general A and B are represented by thin locally-birefringent
layers with axes that rotate through the thickness of the
coating. Local properties of the complex medium are represented
by a Herpin birefringent period (A/2)B(A/2), with effective
principal refractive indices and with thickness much less than
the chiral period.
The effect of changing the fraction fB of material B is
considered. With simplicity of fabrication in mind, the special
case in which A and B are identical apart from thickness and
azimuthal orientation is developed. Then, by maintaining a
constant optical thickness for the Herpin period, the circular
Bragg resonant wavelength of the primary medium can be retained.
As an example of the potential use of threaded-chiral materials,
handed resonant reflectors for elliptically-polarized light are
considered.
The optical properties of narrow-band Fabry-Perot filters for circulalry polarized light are surveyed theoretically. Here the general filter is considered to be formed by two chiral reflectors each of N turns, one with an abrupt twist of ξ and separated by an isotropic spacer of refractive index n and thickness d. The spacerless filter with d = 0, ξ = π/2 which may be regarded as a special case, is known to suffer from severe limitation of finesse and purity of polarization as N becomes large. However from previous work, there is reason to anticipate that general chiral supercavities with virtually unlimited finesse may be possible. In the presentation performance maps of the degree of polarization and the finesse confirm that chiral supercavities are possible, and conditions on n, d, and ξ are explored.
In order to simplify the structure of the birefringence dual frequency laser in which a quart crystal or an artificial stress plate is settled, improve the convenience of adjustment, and improve stability of FD (frequency difference), a novel structure of dual frequency laser is constructed, in which the FD is produced by the cavity mirror coated with complex multilayer birefringent film. Different layers of the multilayer film perform different functions for the whole film system of sandwich structure. From the outermost to the innermost near the substrate, there are anti-reflectance film, birefringent film and reflectance film sequentially. Utilizing both the transverse Zeeman effect and intra-cavity birefringence effect in a He-Ne laser, frequency-difference (FD) about 7 MHz is achieved in experiments.
A new global approach, called 'Generalized Ellipsometry', is now capable to characterize the optical and structural properties of general anisotropic layered systems, including absorption, and can be applied, in general, to determine the linear response tensor elements for wavelengths from the far IR to the deep UV. This technique enables new insights into physical phenomena of layered anisotropic mediums, and can provide precise structural and optical data of novel compound materials. Experimental results are presented for stibnite single crystals as example for an arbitrary biaxial absorbing material, a wurtzite GaN thin film with uniaxial anisotropy grown on sapphire, a spontaneously atomically ordered III-V semiconductor alloy thin film, and a sculptured titanium dioxide film with symmetrically dielectric tensor properties.
Thin film optical materials deposited with a helical nanostructure can be used to fabricate devices that respond differently to right-handed and left-handed circularly polarized light. Together these inorganic materials and devices promise the framework of chiral optics, in which the basic polarization states of propagating light are circular. In the presentation we show that one period of a continuous chiral material can be replaced with three or more sub- layers of oriented biaxial material, and compare the performance that is expected of elements and devices such mirrors, spacerless spectral-hole filters and circularly polarized lasers that use continuous and layered chiral materials respectively. The investigation shows that, for the same number of turns and local linear birefringence, the performance of the layered material is slightly poorer than that of the continuos material. However, in practice the small loss in performance of the layered chiral material may be offset by inherently larger local linear birefringence and improved structural fidelity due to superiority of optical monitoring over quartz-crystal monitoring.
The serial bideposition (SBD) technique is used to produce biaxial thin films with large linear birefringence and chiral (handed) media with large circular birefringence. Thin film wave plates and chiral reflectors of SBD silicon that have been fabricated show promise for applications in the wavelength range 800-2200nm. In particular we have deposited quarter-wave plates of metric thickness less than 1micrometers for the 800nm wavelength used in CD players, and half-wave plates of metric thickness about 3micrometers for the 1550nm optical communications wavelength. An observation of the Bragg resonance in a silicon chiral film in the 600-800nm wavelength range where the absorption is high but the linear birefringence is approximately equals 0.35, suggests possible applications even at the 633 HeNe wavelength. Polarizing elements that we have designed and fabricated for use with circularly polarized light in the visible and near infrared spectral regions are described. These include a two-layer Fabry-Perot filter that uses a structural phase discontinuity instead of a physical spacer layer to define the wavelength of a narrow spectral hole at the center of the Bragg dip. Further strategies, also based on phase discontinuities, are explored for adapting other filter designs from isotropic thin film filter theory to the circular case.
We consider anisotropic stress in substrates coated with vacuum- deposited optically-anisotrpic thin films. Birefringent films with tilted-columnar and normal-columnar nanostructures and thin film helocoidal bianisotropic mediums are considered. In the case of tilted- columnar nanostructures, the sign and relative magnitude of the stresses parallel to the deposition plane and perpendicular to the deposition plane depend on deposition angle. We show that an ion-assisted overcoat, applied with the primary purpose of protecting optical properties, can be designed to provide effective compensation of stress.
The properties of anisotropic coatings fabricated in vacuum by serial bideposition are described. With appropriate choice of deposition parameters the technique can be used to engineer large linear form birefringence in films that have nanostructural columns aligned perpendicular to the surface of the substrate. Conversely, the biaxial films can be engineered so that all principal axes are inclined to the substrate, or the target of engineering can be large circular form birefringence. Several general and specific applications are discussed, including the fabrication of waveplates, linear polarizers, and a double-layer anisotropic antireflection coating used to select the polarization direction of an open cavity HeNe laser.
Thin film form birefringence depends on the shape or bunching of nanostructural columns and on the difference in the refractive indices of the columns and the surrounding voids. When moisture from the atmosphere enters the nanostructure the linear form birefringence may decrease by a large amount, of the order of 50%. However, current data refers to tilted-columnar films. We report here on moisture penetration effects in films fabricated by serial bideposition and engineered for large linear form birefringence or large circular form birefringence. As well, we consider post-deposition processes that may retard the uptake of moisture.
Vacuum deposition results in oxygen deficient surfaces which rapidly absorb oxygen from the atmosphere either in the form of O2 or H2O. In order to retard water uptake this deficiency must be reduced, possibly by annealing samples in an oxygen rich atmosphere subsequent to deposition. Alternatively, surface oxide and hydroxyl groups can be rendered hydrophobic via reaction with silane derivatives. Both methods have been tested for their effectiveness in retarding water uptake.
Microstructural and optical properties of obliquely deposited thin films are simulated in a computer 3D method. Identical hard sphere particles arrive serially as the simulation proceeds and relax into the nearest site where they touch the particle of impact and two others. Structural and optical properties are calculated from the resulting 3D array of particles. Peaks in the radial distribution function of simulated films are in agreement with particle to particle distances in small clusters and indicate that the films are amorphous. Columnar directions are identified by a maximum in the variance of the film density and results are given for several deposition geometries. Form birefringence is indicated by anisotropy in the 2D angular distribution function viewed down the columns, and values of the three principal refractive indices are computed by an iterative method.
Vacuum deposited anisotropic films offer unique optical properties but currently suffer from several problems, including instability and excessive haze. We discuss the use of transmission- mode perpendicular incidence ellipsometry and waveguiding for measuring the birefringent properties of tilted columnar films, and describe an in situ method for measuring anisotropic optical scatter distributions. Our results show that the properties of the films depend on deposition conditions, including substrate temperature.
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