Tailoring the light distribution of micro-LED displays with a compact compound parabolic concentrator and an engineered diffusor.

We propose a novel optical design to tailor the angular distribution of a micro-LED (µLED) display system and use vehicle display as an example to illustrate the design principles. The display system consists of a µLED array with a tailored LED structure, a small formfactor compound parabolic concentrator (CPC) system, and a functional engineered diffusor. It provides high efficiency, high peak brightness, and small formfactor. In the design process, a mix-level optical simulation model, including the angular distribution of polarized emission dipole (dipole emission characteristics), Fabry-Perot cavity effect (wave optics), and light propagation process (ray optics), is established to analyze the angular distribution of µLEDs. Such an optical design process from dipole emission to display radiation pattern can be extended to other µLED display systems for different applications.

Angular color shift of micro-LED displays.

Sidewall emission of a micro-scale light emitting diode (micro-LED) improves the light extraction efficiency, but it causes mismatched angular distributions between AlGaInP-based red micro-LED and InGaN-based blue/green counterparts due to material difference. As a result, color shift of RGB micro-LED displays may become visually noticeable. To address this issue, we first analyze the angular distributions of RGB micro-LEDs and obtain good agreement between simulation and experiment. Next, we propose a device structure with top black matrix and taper angle in micro-LEDs, which greatly suppresses the color shift while keeping a reasonably high light extraction efficiency.

Compensation of blue phase I by blue phase II in optoeletronic device.

Compensation effect of blue phase I (BP I) with blue phase II (BP II) liquid crystal was demonstrated. BP I and BP II were co-exist in the optoeletronic device by polymer stabilization. Consequently, disadvantages of BP I and BP II were greatly improved by compensation effect and resulted in high contrast ratio, low hysteresis and fast falling time. Mechanism of compensation effect was explained by relaxation ability of lattice structure under electrical field and compensation structure was well confirmed by Bragg's reflectance spectrum and Commission International de l'Éclairage chromaticity diagram.

Turning film for widening the viewing angle of a blue phase liquid crystal display.

We report a new turning film structure for widening the viewing angle of a vertical-field-switching blue-phase liquid crystal display, which employs a directional backlight. The turning film consists of periodic prismatic structures: each period has three prisms with different base angles so that the output light can be spread to a larger angle. Simulation results show that a full width at half-maximum of ±80° in the horizontal direction and ±45° in the vertical direction can be achieved.

Concentration effect of carbon nanotube based saturable absorber on stabilizing and shortening mode-locked pulse.

We comprehensively investigated the concentration effect of dispersed single-walled carbon nanotubes (SWCNTs) in polymer films for being a saturable absorber (SA) to stabilize the mode locking performance of the erbium-doped fiber laser (EDFL) pulse through the diagnosis of its nonlinear properties of SA. The measured modulation depth was from 1 to 4.5% as the thickness increased 18 to 265 microm. The full-width half-maximum (FWHM) of the stable mode-locked EDFL (MLEDFL) pulse decreased from 3.43 to 2.02 ps as the concentrations of SWCNTs SA increased 0.125 to 0.5 wt%. At constant concentration of 0.125 wt%, the similar pulse shortening effect of the MLEDFL was also observed when the FWHM decreased from 3.43 to 1.85 ps as the thickness of SWCNTs SA increased 8 to 100 microm. With an erbium-doped fiber length of 80 cm, the shortest pulse width of 1.85 ps were achieved at 1.56 microm with a repetition rate of 11.1 MHz and 0.2 mW of the output power under an output coupling ratio of 5%. An in-depth study on the stable mode-locked pulse formation employing SWCNTs SA, it is possible to fabricate the SWCNT films for use in high performance MLEDFL and utilization of many other low-cost nanodevices.

Poly(N-isopropylacrylamide)-tethered silicate platelets for colloidal dispersion of conjugated polymers with thermoresponsive and photoluminescence properties.

Poly(N-isopropylacrylamide)-tethered nanosilicate platelets (NSP-PNiPAAm) have been synthesized by covalently bonding the polymer onto the surfaces of silicate platelets of nanometer dimension, and this class of nanohybrids has proved to be effective for dispersing water-insoluble conjugated polymers (CPs). Simple pulverization of poly[2-methoxy-5-(2'-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV) with NSP-PNiPAAm rendered the powder material dispersible in water, whereupon it displayed thermoresponsive properties at 37.5 degrees C and CP particle size variation between ca. 50 and 100 nm by SEM observation. The same dispersion had a maximum UV-vis absorption at 524 nm and PL emission at 605 nm. The PL emission was significantly higher at 4 degrees C than at 45 degrees C. Being coated as a film, it showed an orange emission under an ultraviolet lamp, consistent with the PL measurement. The water-borne process of dispersing the CP in aqueous media by the presence of NSP-PNiPAAm and followed by film formation to demonstrate a unique method of manipulating hydrophobic conjugated polymers in a facile manner.

Enhancing the Efficiency of Color Conversion Micro-LED Display with a Patterned Cholesteric Liquid Crystal Polymer Film.

Color-converted micro-light-emitting diode (micro-LED) displays with wide color gamut, high ambient contrast ratio, and fast response time are emerging as a potentially disruptive technology. However, due to limited optical density and thickness of the color-conversion film, the blue light leakage and low color-conversion efficiency still hinder their widespread applications. In this paper, we demonstrate a patterned cholesteric liquid crystal (CLC) polymer film with two special optical functionalities. On the green and red sub-pixels, the corresponding planar CLC texture acts as a distributed Bragg reflector for the blue light, which in turn improves the color conversion efficiency and expands the color gamut. On the blue sub-pixels, the corresponding focal-conic CLC texture acts as light scattering medium, which helps to reduce the angular color shift. Further analysis reveals that the patterned CLC film can alleviate the crosstalk between green and blue color filters. Therefore, compared to the display system without such a CLC film, our proposed device structure increases the color conversion efficiency by 143% (at ~90% Rec. 2020) and reduces average angular color shift Δu'v' from 0.03 to 0.018 at the viewing angle with the most severe color shift. Such a patterned CLC film is applicable to all kinds of color-conversion display systems, including organic and inorganic phosphors.

Observation of carbon nanotube and clay micellelike microstructures with dual dispersion property.

Micellar microstructures of carbon nanotubes (CNTs) with clay platelets were formed by physically pulverizing both materials in powder form. The resultant CNT-clay mixtures were enabled to decrease the level of aggregation of the CNTs from their original state in water as well as in organic mediums including toluene, dimethylformamide, and ethanol. The presence of clay significantly enhanced the CNT dispersion in the following trend: anionic synthetic fluorinated mica (300 x 300 x 1 nm(3)) > anionic sodium montmorillonite (80 x 80 x 1 nm(3)) > cationic layered double hydroxide (200 x 200 x 1 nm(3)). Both geometric dimensions and ionic charge could be the predominant factors for decreasing the CNT entanglement. The CNT-Mica demonstrated an amphiphilic property for dispersing in water and toluene, but in an irreversible manner. It is explained that the original CNT and clay's noncovalent bonding forces are randomized during the contact with solvent. The formation of micellelike microstructures, resembling oil-in-water and water-in-oil surfactants, was proposed. Ultraviolet-visible absorbance and transmission electronic microscopy have verified the existence of two different microstructures, which also exhibited differences in thermal stability (600 vs 650 degrees C onset temperature) by thermal gravimetric analysis as well as electrical conductivity (10(-4) vs 10(-6) S/cm).

Aqueous dispersion of conjugated polymers by colloidal clays and their film photoluminescence.

The plate-shaped clays enabled us to disperse water-insoluble conjugated polymers (CPs) into a colloidal form and an aqueous process for making CP films. Simple pulverization of poly[2-methoxy-5-(2'-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV) with silicate clays rendered the powder mixture an unusual dispersing behavior in water. The most effective clay was selected from screening several natural and synthetic clays including synthetic fluorinated mica (Mica), sodium montmorillonite (MMT), synthetic smectite (SWN), and cationic layered double hydroxide (LDH). A high aspect ratio and intensive charge density (2.1 e nm(-1)) of Mica were highly effective for promoting the CP dispersion. The enhancement for dispersing MEH-PPV is rated in the following trend: Mica > MMT > SWN > LDH. The result is rationalized by the influences of their geometric shape and ionic charge of clay on MEH-PPV in water. Two other representative CPs, sulfonated polyaniline (SPA) and triphenyl phosphine oxide cored polyaniline (TPOPA), were used to generalize the CP/clay colloidal behavior. All of the three CPs as in an aqueous form can be coated into hybrid films. Under an ultraviolet lamp, these films showed color emissions, orange for MEH-PPV/Mica, olive for SPA/Mica, and green for TPOPA/Mica, which are consistent with the photoluminescence measurements.

Hierarchical synthesis of silver nanoparticles and wires by copolymer templates and visible light.

Self-assembled silver wires in micro-meter scale were obtained from aqueous silver nitrate solution in the presence of a comb-like copolymer as the sole organic component. The requisite copolymer was easily prepared by the grafting poly(oxyethylene)-monoamine (POE-amine) onto poly(styrene-co-maleic anhydride) (SMA). Upon storage at ambient temperature with exposure to daylight, the aqueous AgNO(3)/SMA-POE solution gradually underwent a color changed from transparent pale-yellow to dark-violet over a period of hours, and after several months a solid precipitate was deposited. The formation process was monitored by ultraviolet-visible spectrometer, particle size analysis, scanning electron microscope, and transmission electron microscope. Silver wires were hierarchically formed by progressive transformation from the initial appearance of silver nanoparticles (ca. 10nm in diameter), followed by the intermediate rectangles (0.6-1.0µm in width and 0.4µm in length) in solution and ultimately the precipitates in micro-scale of silver wires at 1.6-6.4µm in diameter and 100-370µm in length. The progressive formation of the precipitated silver wires was accelerated by the exposure of visible light as a photo-reducing energy source. The micron-scale wires have a silver content over 97.4wt.% and a sheet resistance of 5.5×10(1)Ω/square.