Birefringence Of Films Research Articles

Simple system for evaluating retardation of liquid crystal cells using grating type liquid crystal polarization splitters

We propose a unique optical system for measuring the retardation of birefringent films using a pair of liquid crystal (LC) gratings; that is, the examined birefringent films are inserted between two LC gratings. Because the LC grating functions as a polarization beam splitter for circularly polarized light, the proposed system is optically equivalent to the measurement system using a pair of two circular polarizers. First, the polarization splitting performance of the LC grating is discussed. It is found that a sufficiently high voltage (such that the retardation is less than a half wavelength) has to be applied for the almost pure circularly polarized diffracted light. Next, the measurement of the retardation of a homogeneous LC cell as an examined birefringent film was demonstrated using the proposed method. The proposed method is revealed to have the same measurement performance as that of the conventional method using a pair of linear polarizers and has an advantage that there is no need for the optic axis of the test birefringent specimen to be set at a specific angle.

Compact linearly polarized ceramic laser made with anisotropic nanostructured thin films.

A novel method is proposed for the fabrication of polarizing laser mirrors for compact solid-state lasers using glancing angle deposition. Changing the inclination angle and the azimuthal orientation of the substrate during deposition allows one to create and control in-plane birefringence of a deposited thin film by changing its nanostructure. Principal refractive indices of tungsten trioxide films were determined for various deposition angles using transmission and reflection ellipsometry. High-reflectance contrast between orthogonal linear polarization directions was obtained using a single material without any additional processing steps. These birefringent films were the building blocks of a Bragg mirror that was tested as an output coupler of a (Yb0.1Y0.9)3Al5O12 ceramic laser in a laser-diode end-pumped configuration. Continuous-wave, linearly polarized, transverse single-mode laser emission was obtained at a wavelength of 1030 nm with a polarization extinction ratio higher than 973 (30 dB).

Paper No S6.2: An Advanced Optical Thin Film Structure for Fabricating Multidomain Display

AbstractIn this paper, an advanced optical thin film structure, which can be used for fabricating multidomain display is introduced. It is composed of multilayer patterned birefringent thin films, of which each thin film is formed by a liquid crystal (LC) polymer layer with spatially varying optical axes controlled by a patterned photoalignment layer so that it can spatially modulate light propagation directions and polarizations. The overall thickness of the multilayer films, excluding the substrate, can be less than 1 µm, which offers the flexibility to generate high resolution light wavefront structure and the light diffraction within the film is also suppressed. As a result, the optical thin film can be ideally working as a photomask for exposure on a LC photoalignment layer to generate multidomain alignment structures in a single exposure, which can be applied for the manufacturing of LC multidomain displays. To verify the effectiveness of the proposed photomask, a periodic two‐domain thin film structure has been designed and fabricated and a liquid crystal cell with two‐domain alignment structure has been successfully fabricated in a single exposure with the photomask.

Highly birefringent polymer films from the photo-crosslinking polymerisation of bistolane-based methacrylate monomers

The photo-polymerisation of mixtures of mono- and di-methacrylates, containing a bistolane moiety with a central fluorine-substituted benzene ring, resulted in the formation of highly birefringent polymer films (Δn = 0.40), which were obtained in a nematic liquid crystal (NLC) phase. While the dimethacrylate forms enantiotropic NLCs at T = 110–138°C, smectic phases at T = 50–138°C and crystallises at T = 50°C, whereas the monomethacrylate forms NLCs at a wider temperature range (T = 98–185°C) and crystallises at a lower temperature (T = 98°C). These methacrylates were infinitely miscible and a 20/80 (w/w) mixture of the dimethacrylate/monoacrylate was able to form an NLC phase over a broad temperature range (T = 73–179°C). The mixed NLC phase exhibited a Δn value of 0.36, even though the Δn values of the mono- and di-methacrylates were determined as 0.35 and 0.25, respectively, suggesting that the Δn of the mixture follows an additivity rule. Furthermore, the Δn of the NLC phase could be increased to 0.40 by photo-polymerisation.

Negative dispersion retarder using two negative birefringence films.

The achromatic response and wide viewing angle for varying wavelength of incident light are of long waiting research to be utilized it for the display devices. Such response can be obtained by employing the retarder that exhibits negative birefringence and negative dispersion. In this paper, negative dispersion half-wave retarder and negative dispersion quarter-wave retarder have been demonstrated by optimizing the retardation and the angle between the extraordinary axes of polystyrene and poly-methylmethacrylate films. The optimum angles for half and quarter-wave retarders were found to be 40° and 70°, respectively for different retardation values of polystyrene and poly-methylmethacrylate films.

Dual-layer ultrathin film optics: I. Theory and analysis

This paper revisits dual-layer ultrathin film optics, which can be used for functional graded refractive index thin film stack. We present the detailed derivation including s-polarized and p-polarized light under arbitrary incidence angle showing the equivalence between the dual-layer ultrathin films and a negative birefringent thin film and also the approximations made during the derivation. Analysis of the approximations shows the influence of thickness of dual-layer thin films, the incidence angle and desired refractive index of the birefringent film. Numerical comparison between the titanium dioxide/aluminum oxide based dual-layer ultrathin film stack and the equivalent birefringent film verifies the theoretical analysis. The detailed theoretical study and numerical comparison provide a physical insight and design guidelines for dual-layer ultrathin film based optical devices.

Dual-layer ultrathin film optics: II. Experimental studies and designs

This paper presents an experimental study of using dual-layer ultrathin films with a high refractive index and low refractive index to approximate a single-layer thin film with an equivalent intermediate refractive index, which is the basis of the functional graded refractive index thin film stack using dual-layer ultrathin films. Using ion beam deposition, five sets of ultrathin films of titanium dioxide (TiO2) and aluminium oxide (Al2O3) are deposited on silicon with different volume fractions and characterized by spectroscopic ellipsometer for the ordinary/extraordinary refractive index and by spectrophotometer for the reflectance. By designing the sequence of TiO2 and Al2O3 ultrathin films, the measured results show a good agreement between the experiments and simulation of the equivalent birefringent thin film. An application example of using the TiO2 and Al2O3 dual-layer ultrathin films to approximate a single-layer antireflection coating for silicon is presented.

Effect of molecular architectures in photoinduced birefringence in films of azo-modified diblock copolymers

Langmuir films of polystyrene-block-poly(4-vinylpyridine) (PS-b-P4VP) with an azo dye, 4-[4-(phenylazo)-1-naphthylazo]phenol (Disperse Orange 13, DO13), hydrogen-bonded to the pyridine group of P4VP blocks were investigated. The surface pressure isotherms were considerably different for the copolymer dissolved in a nonselective (chloroform) and selective (toluene) solvents. The Langmuir–Blodgett (LB) and Langmuir–Schaefer (LS) films produced with PS-b-P4VP(DO13) copolymers displayed higher optically induced birefringence than the cast films owing to their nanostructured nature. The photoinduced birefringence properties were particularly enhanced when toluene was used owing to the confinement of azo chromophores in micelles consisting of a P4VP core and a PS corona.

Determination of the Dispersion of the Principal Refractive Indices for Birefringent Polypropylene Films

We present a novel method for determining the dispersion of the refractive indices of birefringent films, based on treatment of transmission spectra, in which we observe interference of light. The dispersion curves n x (λ) and n y (λ) were determined by treatment of transmission spectra obtained for normal incidence of radiation on a P2-25 birefringent fi lm, and n z (λ) was determined for oblique incidence of radiation. From the results of determination of the dispersions of the principal refractive indices of a birefringent P2-25 polypropylene film (Mogilevkhimvolokno OAO, Belarus), we established that the sample is a negative biaxial retarder with N z = 2.9.

Novel theory analysis and new perspective on angular selectivity

Wave propagation of symmetry incidence on the biaxial birefringent film is discussed in this paper based on the characteristic matrix method. Furthermore, based on the refractive index surface, the refractive indices and effective optical admittances of the forward and backward propagating extraordinary waves (p-polarized wave) are analyzed in detail. With regard to the symmetry incidence, optical properties, such as the input optical admittance, transmittance and reflectance, are not identical for an extraordinary wave under common condition, i.e., angular selectivity. Based on an analysis of refractive index surface, the effective optical admittances of the forward and backward propagating extraordinary waves are equal in this particular case; transmittance and reflectance under the symmetry incidence are completely overlapped, similar to findings in the isotropic thin film. We validate our analysis using the developed software package. Based on the novel theory analysis, we can gain a new perspective of symmetry incidence about angular selectivity for the biaxial birefringent film.

Observation of a large optical birefringence effect in a (110) oriented porous silicon layer

We account the observation of the largest reported birefringence effect in a semi-artificial material. An in-plane birefringence of more than 0.7 was observed at the blue-green optical spectral range in a porous silicon film fabricated by electrochemical etching of a (110) Si wafer. Birefringent films are of interest in a variety of applications including sensing and polarization control.

Enhanced Solar Cell Conversion Efficiency Using Birefringent Liquid Crystal Polymer Homeotropic Films from Reactive Mesogens

Novel birefringent liquid crystal polymer homeotropic films have been coated on semiconductor solar cells to improve the effective incident sunlight angles. The liquid crystal polymer precursor, based on reactive mesogens, is fluidic and flows like liquid. It would distribute uniformly on the solar cell sample surface by any traditional coating technique. The birefringence for light, due to the liquid crystal retardation properties, manipulated the optical length and the deflection of incident light, thus allowed an increase in the energy conversion efficiency. The expensive sunlight tracking systems could be avoided. The processing parameters can be tuned such as different mesogen concentrations and plate speeds of spin-coating. The results showed that the solar cell conversion efficiency was improved from 14.56% to 14.85% at an incident sunlight angle of 15°. It was further improved from 13.40% to 13.81% when the angle was 30°. The interesting angular dependency on solar cell efficiency enhancement has been evaluated.

Gel setting of hybrid carrageenan solutions under steady shear

We study the gel formation under steady shear and the final gel properties of hybrid-carrageenan solutions in NaCl and KCl, with a systematic comparison with kappa- and iota-carrageenan solutions. For high enough shear rates corresponding to homogeneous flow, the structural development of the gels is overall the same for the different salts and carrageenans, and shows the following features: (a) helices are formed during cooling and the onset of formation is not affected by the flow; (b) the flow-induced orientation of structures results in the shear thinning behaviour of the cooling fluids; (c) structural orientation is locked upon the gel setting which takes place immediately after flow cessation and results in a significantly weaker gel elasticity, longer gel structural build-up, a shift of the spectrum of gel relaxation times to longer times, and in the eventual birefringence of films obtained from dried gels. A new phenomenological model that contemplates all experimental results is proposed.

Analysis of angular-selective performances of obliquely deposited birefringent thin film

A slanted columnar TiO2 sculptured anisotropic thin film is prepared using the glancing angle deposition (GLAD) technique. We extend the inference regarding the optical properties of a uniaxial birefringent film based on theoretical analysis to include the more general case of a multilayer biaxial birefringent thin film, the optic axis of which is in the incident plane. We also investigate in detail the symmetrical angularselective transmittance performances of TiO2 biaxial anisotropic thin films for light incident at the same angle but coming from opposite sides of the surface normal. The tilted nanocolumn microstructures of the birefringent thin films induce optical anisotropy. The transmisson spectra for the p-polarized wave of TiO2 biaxial anisotropic thin film measured in the experiment almost overlap at the symmetrical oblique incidence at the ±0 angles, which validates our theoretical derivation.

Phase Shift of Polarized Light after Transmission through a Biaxial Anisotropic Thin Film

Based on the theoretical analysis of biaxial birefringent thin films with characteristic matrix method, we investigate the phase shift on transmission of a tilted columnar biaxial film at normal and oblique incidence over 300–1200 nm for s- and p-polarized waves. Compared with the simplified calculation method, the interference effects of the birefringent thin film are considered to yield more accurate results. The quarter wavelength phase shift calculated with the characteristic matrix method is consistent with that monitored with in situ measurement by two-angle ellipsometry, which validates our complied program for the calculation of the phase shift of the biaxial anisotropic thin film. Furthermore, the characteristic matrix method can be easily used to obtain continuous adjustable phase retardation at oblique incidence, whereas the simplified calculation method is valid for the case of normal incidence. A greater generality and superiority of the characteristic matrix method is presented.

Large Area Self‐Assembly of Nematic Liquid‐Crystal‐Functionalized Gold Nanorods

AbstractFascinating nematic‐ and smectic‐like self‐assembled arrays are observed for gold nanorods partially capped with either laterally or terminally attached nematic liquid crystals upon slow evaporation of an organic solvent on TEM grids. These arrays can be manipulated and reoriented by applying an external magnetic field from quasi‐planar to vertical similar to a Fréedericksz transition of common organic nematic liquid crystals. Birefringence and thin film textures of these self‐assembled gold nanorod arrays observed by polarized optical microscopy are strongly reminiscent of common organic nematic liquid crystal textures between crossed polarizers and, additionally, support the formation of ordered liquid crystal‐like anisotropic superstructures. The ordering within these arrays is also confirmed in bulk samples using small angle X‐ray scattering (SAXS).

Photoinduced Cooperative Orientation and Pattern Formation in Copolymer Films with 4-Methoyazobenzene and Benzoic Acid Side Groups

Photoinduced cooperative molecular reorientation in liquid crystalline (LC) methacrylate copolymers with hexamethylene spacer groups terminated with 4-methoxyazobenzene and benzoic acid in the side chain, and the influence of the copolymerization ratio on the thermally enhanced photoinduced molecular reorientation behavior were investigated. All copolymers exhibited a LC phase, and sufficient thermally enhanced photoinduced in-plane molecular reorientation was achieved by irradiating with linearly polarized (LP) UV light and subsequent annealing when the azobenzene composition was greater than 5 mol%. Adjusting the exposure energy and the polarization of the LPUV light attained a patterned oriented structure, which can be useful as alignment layer for functional materials. Finally, a patterned birefringent film was fabricated using photocurable liquid crystals on the patterned copolymer film.

Colors from polarizers and birefringent films

In this paper we analyze the interesting artwork of Austine W. Comarow that creates colored images from colorless birefringent films. This artwork consists of birefringent polymer films sandwiched between two polarizers, similar to a polariscope. We explain quantitatively the main features observed in Comarow's work. We derive the equations that predict the colors generated in the system as a function of the thickness and the angles between the films. We illustrate the phenomena with simple homemade devices.

Design of an achromatic optical coating waveplate

We propose a design method of an achromatic waveplate that comprises anisotropic thin films as a multi-cell layered system. Each cell is a symmetrical three-layered ABA structure with equivalent refractive indices and phase thicknesses. A merit function defined as the rate of change of the phase retardation with respect to wavelength is introduced to control the variation of phase retardation by selecting proper birefringence and thickness for each film. The allowed thickness of each birefringent film is investigated for different requirement about the uniformity of phase retardation. An achromatic waveplate with a smaller number of cells is demonstrated by arranging the two composed films made of different materials with high contrast of refractive index between them. The arrangement of a matching layer, which is associated with the performance of the achromatic waveplate, is also discussed.

Effects of block copolymer self-assembly on optical anisotropy in azobenzene-containing PS-b-PMMA films

Polystyrene-b-polymethylmethacrylate (PS-b-PMMA) was selected as the host for 4-(4-nitrophenylazo)aniline (Disperse Orange 3, DO3) based on a previous study of DO3/PMMA and DO3/PS binary blends. Selective location of DO3 into the PMMA block of the copolymer was expected during self-assembly of the block copolymer since a preferential interaction of DO3 with PMMA has been demonstrated. However, surface segregation of DO3 was found during the thermal annealing used to nanostructure the copolymer. To avoid this, a thermoplastic polymer (Azo-TP) was synthesized from the bulk reaction of DO3 and diglycidyl ether of bisphenol A (DGEBA). The choice of DGEBA as a co-reactant was an attempt to encourage the selective location of azo groups in the PMMA phase of PS-b-PMMA. An inspection of solutions of Azo-TP in PS and PMMA, corroborates the preferential affinity of Azo-TP for PMMA. The Azo-TP could be satisfactorily dissolved in PS-b-PMMA. We have investigated the growth and decay processes of the optically induced birefringence in films of PS-b-PMMA containing 12 wt% Azo-TP. The resulting materials showed a good photoinduced time response, high maximum birefringence and an elevated fraction of remnant anisotropy.