Birefringence-dependent linearly-polarized emission in a liquid crystalline organic light emitting polymer.

We investigated the linearly polarized emission of uniformly aligned poly(9,9-di-n-octylfluorenyl-2,7-diyl)-alt-(benzo[2,1,3]thia-diazol-4,8-diyl) (F8BT) with a liquid crystalline phase on a rubbed alignment layer. The polarization ratio, defined by the ratio of luminous intensities polarized parallel and perpendicular to the rubbed direction, gradually decreased with increasing thickness of the F8BT film. In the photoluminescence (PL) process, the polarized light is emitted throughout the whole F8BT film, while in the electroluminescence (EL) process, the polarized light is emitted at a certain region within the F8BT film. The thickness-dependent polarization ratios in both PL and EL processes were successfully described based on a simple model wherein the mean optical birefringence was expressed as a function of the thickness of the F8BT film.

Reduction of the operating voltage of a nanoencapsulated liquid crystal display by using a half-wall structure.

We proposed a half-wall structure in the in-plane switching (IPS) configuration of the nanoencapsulated liquid crystal (LC) display for reducing a driving voltage. The IPS electrodes were fabricated on top of the half-walls to enhance electric field strength through the whole LC layer. In addition, we demonstrated a self-masking process for the half-wall structure and the IPS electrodes without any additional mask-aligning process.

Polarizer-free liquid crystal display with double microlens array layers and polarization-controlling liquid crystal layer.

We propose a polarizer-free liquid crystal display (LCD) consisting of two microlens array (MLA) layers, a twisted nematic (TN) LC layer, and two light-blocking masks. By changing the polarization state, focal length of the LCD can be controlled. Since two light-blocking masks have a circular stop pattern and a complementary open pattern, entire gray-scale spectrum may be realized by controlling the intensity of light passing through masks. Ultimately, fast response time characteristics could be achieved due to the alignment of LC molecules on the flat MLA surface.

Switchable reflective lens based on cholesteric liquid crystal.

We demonstrated a switchable reflective lens based on choleteric liquid crystal (CLC) with a plano-convex shape. The plano-convex CLC lens was fabricated by assembling a planar substrate and a concave substrate. The reflective CLC lens exhibits wavelength selectivity and handedness sensitivity like the conventional CLC cell. The plano-convex CLC lens acts as a biconvex lens with the same curvature due to the Bragg reflection of the CLC layer. In addition, the reflective CLC lens was defocused by applying external voltage.

Temperature-independent pitch invariance in cholesteric liquid crystal.

We report a pitch invariance in cholesteric liquid crystals (CLCs) independent of temperature by mixing two chiral dopants. One dopant tends to shorten the helical pitch of the CLC, but the other makes the pitch longer, with increasing temperatures. From an analysis of temperature dependencies of the pitch for each dopant, we determined the mixing ratio of two chiral dopants for the pitch invariance. Finally, we obtained the pitch-invariant CLCs to temperature and the helical twisting power of the mixed dopant was estimated.

A hemispherical electronic eye camera based on compressible silicon optoelectronics.

The human eye is a remarkable imaging device, with many attractive design features. Prominent among these is a hemispherical detector geometry, similar to that found in many other biological systems, that enables a wide field of view and low aberrations with simple, few-component imaging optics. This type of configuration is extremely difficult to achieve using established optoelectronics technologies, owing to the intrinsically planar nature of the patterning, deposition, etching, materials growth and doping methods that exist for fabricating such systems. Here we report strategies that avoid these limitations, and implement them to yield high-performance, hemispherical electronic eye cameras based on single-crystalline silicon. The approach uses wafer-scale optoelectronics formed in unusual, two-dimensionally compressible configurations and elastomeric transfer elements capable of transforming the planar layouts in which the systems are initially fabricated into hemispherical geometries for their final implementation. In a general sense, these methods, taken together with our theoretical analyses of their associated mechanics, provide practical routes for integrating well-developed planar device technologies onto the surfaces of complex curvilinear objects, suitable for diverse applications that cannot be addressed by conventional means.

Optically isotropic switchable microlens arrays based on liquid crystal.

We present an optically isotropic switchable microlens array (MLA) based on liquid crystals (LCs) using the Joule heating electrode structure. The LC molecules were initially aligned vertically on the lens and electrode surfaces. By applying voltage to the transparent electrodes, the temperature of the LC layer could be changed. Above the clearing point temperature of LCs, the LC layer shows an averaged refractive index that differs from the nematic state refractive index. The MLA could have switching characteristics by index matching between the LC layer and polymer lens structure. The proposed switchable MLA shows high light efficiency with truly optically isotropic properties.

Polarizer-free liquid crystal display with electrically switchable microlens array.

We propose a polarizer-free liquid crystal display (LCD) with an electrically switchable microlens array. The incident lights are controlled to focused or defocused states by index matching of the lens polymer and LC layer. By adopting two light-blocking masks that have a circular stop pattern and the complementary open pattern, the LCD was able to realize the entire gray scale. Additionally, to achieve fast response time characteristics, we introduce polymerized RMs within the alignment layers.

Reflective three-dimensional displays using the cholesteric liquid crystal with an inner patterned retarder.

We propose a reflective three-dimensional (3D) display using a cholesteric liquid crystal (ChLC) with an inner patterned retarder producing half-wave retardation. The inner patterned retarder, fabricated by selective ultra-violet exposure to the aligned reactive mesogen, divides the circularly polarized light reflected from the ChLC layer into two orthogonal circular polarizations. These reflected orthogonal polarizations construct stereoscopic 3D images without any optical components such as a polarizer and backlight unit.

Single pixel transmissive and reflective liquid crystal display using broadband cholesteric liquid crystal film.

We propose a single mode transflective liquid crystal display (LCD) which is operated as the transmissive and reflective modes in a single pixel without dividing into sub-pixels. The single pixel transflective LCD was composed of the cross-polarized nematic LCD as a light modulator and the broadband cholesteric liquid crystal film (BCLCF) as a half mirror. The BCLCF, simply prepared by the exposure of ultraviolet light to the mixture of the nematic LC and the reactive mesogen with chirality, selectively reflects a certain circular polarization but transmits the orthogonal circular polarization in entire visible light. The electro-optical properties in both transmissive and reflective modes coincide with each other.

Microlens array fabricated using electrohydrodynamic instability and surface properties.

We fabricated polarization-dependent and polarization-independent microlens arrays (MLA) through the electrohydrodynamic instability of the optically anisotropic organic layer. The anisotropic flow induced by the instability of the organic layer leads to making the lens profile on the patterned electrode. We can easily control the polarization dependence of the MLA by controlling the surface alignment properties, even with the optically anisotropic organic layer. This method is a straightforward, fast, and reliable process for MLA fabrication since it does not require cumbersome developing and molding processes.

Continuous viewing angle-tunable liquid crystal display using temperature-dependent birefringence layer.

We demonstrated a continuous viewing angle controllable liquid crystal display (LCD) adopting a thermally variable retardation layer (TVRL) using nematic liquid crystal with transparent electric-heating lines to control temperature. The simulated and experimental results of the proposed LCD show continuous and symmetric viewing angle characteristics by tuning the retardation of TVRL using Joule heating.

Surface-controlled patterned vertical alignment mode with reactive mesogen.

We proposed a patterned vertical alignment (PVA) mode controlled by a modified surface with ultraviolet (UV) curable reactive mesogen (RM) mixed with vertical alignment material for a liquid crystal display (LCD) with fast response time. In the surface-controlled PVA (SC-PVA) mode, the RM monomers in the alignment layer are polymerized along the LC directors by UV exposure under an applied voltage. The polymerized RMs produce a pretilt against the substrate normal depending on the applied field direction in the patterned electrode configuration. In such SC-PVA mode, fast response time was achieved at whole grey levels with the predetermined rotational preference of the LC directors governed by the pretilt direction.

Fast vertical alignment mode with continuous multi-domains for a liquid crystal display.

We proposed a fast liquid crystal display (LCD) in a vertical alignment (VA) mode with continuous multi-domains for wide-viewing characteristics. The fast VA LCD was fabricated by the mixed vertical alignment layer with UV curable reactive mesogen (RM) polymer memorizing the switching directions of the LC molecules. The wide-viewing and fast response characteristics were obtained by axially symmetric switching directions and the memorization of them without any specific electrode patterns or surface structures.

Fast switching characteristics of a microlens array using the electroclinic effect of SmA* liquid crystals.

We present a microlens array characterized by the electroclinic effect of chiral smectic A (SmA(*)) liquid crystals, which show the very fast dynamic switching characteristics required in high-speed optical devices. In order to easily control the intensity at the focal length of the proposed dynamic microlens structure, we adopt a solid-type liquid crystal polymer with optical anisotropy, which can split the beam intensity into two directions, depending on the vectorial portion of the polarization state of the light. The proposed microlens shows a focal intensity tunable by controlling the polarization of light at the SmA(*) liquid crystal. The lens has a very fast switching time of about 24 micros, which is several times faster than conventional microlens arrays with surface-stabilized ferroelectric liquid crystals.

Simultaneous emission of orthogonal handedness in circular polarization from a single luminophore.

The direct emission of circularly polarized (CP) light improves the efficiency of an organic light-emitting diode and characterizes the secondary structure of proteins. In most cases, CP light is generated from a luminescent layer containing chiral characteristics, thereby generating only one kind of CP light in an entire device. Here, we propose direct CP light emissions using a twisted achiral conjugate polymer without any chiral dopant as an emitting layer (EML). The twisted structure is induced in the mesogenic conjugate polymer due to its elasticity by applying different alignment directions to its upper and lower interfaces. Furthermore, we demonstrate the simultaneous emission of orthogonal CP light in a single luminescent device by patterning different alignment directions on the surfaces of the EML. The light source with multipolarization including the orthogonal CP states is applicable to many applications in biosensors and optical devices.

Control of Circularly Polarized Electroluminescence in Induced Twist Structure of Conjugate Polymer.

An extremely high degree of circularly polarized photoluminescence (CPPL) and electroluminescence (CPEL) (dissymmetry factor values: |gPL | = 0.72 and |gEL | = 1.13) are generated from twisted stacking of achiral conjugated polymer induced by nonemitting chiral dopant of high helical twisting power for the first time. Using a theoretical analysis incorporating the Stokes parameter, the twisting angle and birefringence of the aligned conjugated polymer, and the degree of linear polarization in the emitted light are found to make a roughly equal contribution to the degree of CPEL as to the degree of CPPL. Moreover, it is also found that the location of the recombination zone within the emitting layer is a crucial parameter for determining the difference in the dissymmetry factor between CPEL and CPPL. This result is applied to an organic light-emitting display to improve the luminous efficiency by 60%.

High-speed infrared phase modulators using short helical pitch ferroelectric liquid crystals.

A fast phase modulator based on ferroelectric liquid crystal (FLC) is demonstrated and its performances characterized. For uniform alignment and pure phase modulation, we propose a new FLC device configuration using short helical pitch material and homeotropic alignment structure. This device is driven by periodic in-plane electrode stripes implemented on the surface of both cell substrates. As a result, we have obtained large phase modulation (> 2pi at lambda=1.55 microm) and fast response (< 200 microsec).

Printed assemblies of inorganic light-emitting diodes for deformable and semitransparent displays.

We have developed methods for creating microscale inorganic light-emitting diodes (LEDs) and for assembling and interconnecting them into unusual display and lighting systems. The LEDs use specialized epitaxial semiconductor layers that allow delineation and release of large collections of ultrathin devices. Diverse shapes are possible, with dimensions from micrometers to millimeters, in either flat or "wavy" configurations. Printing-based assembly methods can deposit these devices on substrates of glass, plastic, or rubber, in arbitrary spatial layouts and over areas that can be much larger than those of the growth wafer. The thin geometries of these LEDs enable them to be interconnected by conventional planar processing techniques. Displays, lighting elements, and related systems formed in this manner can offer interesting mechanical and optical properties.