Hwan-young Choi

Ultralarge-area block copolymer lithography enabled by disposable photoresist prepatterning.

We accomplished truly scalable, low cost, arbitrarily large-area block copolymer lithography, synergistically integrating the two principles of graphoepitaxy and epitaxial self-assembly. Graphoepitaxy morphology composed of highly aligned lamellar block copolymer film that self-assembled within a disposable photoresist trench pattern was prepared by conventional I-line lithography and utilized as a chemical nanopatterning mask for the underlying substrate. After the block copolymer film and disposable photoresist layer were removed, the same lamellar block copolymer film was epitaxially assembled on the exposed chemically patterned substrate. Highly oriented lamellar morphology was attained without any trace of structure directing the photoresist pattern over an arbitrarily large area.

Two-dimensional photonic crystal color filter development

Reflective color filters using two-dimensional photonic crystals based on sub-wavelength gratings were proposed and constructed. Using low-cost nanoimprint lithography, an amorphous silicon layer was deposited through the low-temperature PECVD process and patterned into two-dimensional structures. The isolated amorphous silicon patterns were readily crystallized using a multi-shot excimer laser annealing at low energy. A study of the close relationship between color filter reflectance and silicon pattern crystallinity is introduced. Theoretical and experimental results show that the proposed color filters have high reflectance and, moreover, decrease the dependence on incident angle compared to one-dimensional photonic crystal color filters.

High angular tolerant color filter using subwavelength grating

A reflective color filter based on a two-dimensional subwavelength grating is proposed. The filter has been designed by performing numerical simulations and has been fabricated on 5×5 mm2 quartz glass using electron beam lithography. The grating color filter shows reflectance of 74% and spectral width of about 80 nm. Furthermore, it has good angular tolerance, up to ±45°, for unpolarized incident light. By analyzing the reflectance spectral distributions in the band diagram, we found that high angular tolerance is achieved due to the high refractive index contrast of grating structures.

Simple liquid crystal display backlight unit comprising only a single-sheet micropatterned polydimethylsiloxane (PDMS) light-guide plate

A simple liquid crystal display (LCD) backlight unit (BLU) comprising only a single-sheet polydimethylsiloxane (PDMS) light-guide plate (LGP) has been developed. The PDMS LGP, having micropatterns with an inverse-trapezoidal cross section, was fabricated by backside 3-D diffuser lithography followed by PDMS-to-PDMS replication. The fabricated BLU showed an average luminance of 2878 cd/m(2) with 73.3% uniformity when mounted in a 5.08 cm backlight module with four side view 0.85cd LEDs. The developed BLU can greatly reduce the cost and thickness of LCDs, and it can be applied to flexible displays as a flexible light source due to the flexible characteristic of the PDMS itself.

Switching of liquid-crystal devices between reflective and transmissive modes using long-pitch cholesteric liquid crystals

We propose liquid-crystal (LC) devices capable of switching between reflective and transmissive modes using the scattering and transparent states of long-pitch cholesteric LCs (CLCs). Two different device configurations can be realized by changing the location of a CLC layer. Low-power operation without the parallax problem can be achieved using the bistable switching of CLCs. We believe that the proposed devices are potential candidates for highly efficient transflective displays.

Pretilt angle control and multidomain alignment of liquid crystals by using polyimide mixed with liquid crystalline prepolymer

We propose a method to control the pretilt angle of liquid crystals by employing a mixture of the vertical alignment polyimide and the liquid crystalline prepolymer (LCP). The pretilt angle between 10° and 90° can be controlled continuously by changing the mixing ratio of the LCP. Multidomains with different pretilt angles can be formed because the pretilt angle between 50° and 90° can be obtained by varying the UV exposure time region by region under a fixed mixing ratio. We confirmed experimentally that proposed alignment layers are thermally stable.

Design and fabrication of a micropatterned polydimethylsiloxane (PDMS) light‐guide plate for sheet‐less LCD backlight unit

Abstract— A polydimethylsiloxane (PDMS) light-guide plate (LGP) having micropatterns with an inverse-trapezoidal cross section was developed for a sheet-less LCD backlight unit (BLU). The micropatterned PDMS LGP was fabricated by backside 3-D diffuser lithography followed by two consecutive PDMS replication processes: photoresist-to-PDMS and PDMS-to-PDMS replications. The fabricated LGP showed an average luminance of 2878 nits and a uniformity of 73.3% in a 2-in. backlight module with four side-view 0.85-cd LEDs. It also could feasibly be applied to a light source for flexible displays owing to the flexible characteristic of the PDMS itself.

Single cellgap transflective liquid crystal cell with high contrast and high cellgap tolerance.

In this paper an optical configuration of a transflective liquid crystal (LC) cell driven by a horizontal electric field is proposed, which shows high contrast, high cellgap tolerance, and single gamma, simultaneously. The dark state of the reflective part is realized by a polarizer (0 degrees), a half-wave plate (15 degrees), LC layer (120 degrees), and a quarter-wave layer (-15 degrees), while a wide-band quarter-wave plate (45 degrees) and a polarizer (90 degrees) are added for the dark state of the transmissive part. Since the optic axis of the homogeneously aligned LC layer is set to be parallel to the polarization direction of the light passed through the half-wave plate, the dark state is rarely affected by the cellgap of the LC layer. Due to the different directions of the electric fields, LCs are rotated to 97.5 degrees for the bright state of the reflective part, but to 75 degrees for that of the transmissive part. With the proposed configuration, a high contrast single-gamma transflective display with high cellgap tolerance can be realized in a single-cellgap structure.

Nanoparticle Doped In-Cell Retarder for Low Operating Voltage in Transflective Liquid Crystal Displays

In this paper, we propose a method of suppressing the increase in operating voltage caused by the in-cell retarder in transflective liquid crystal displays (LCDs), by doping nanoparticles with a high dielectric constant in the in-cell material. Doping with nano-particles increases the dielectric constant of the in-cell material by more than 5 times that of a pure in-cell material. The in-cell material with a high dielectric constant reduces the magnitude of the electric field, within the volume of the in-cell retarder, to decrease the operating voltages of a transflective LCD with an in-cell retarder. It is confirmed that the increase in operating voltage, in a transflective vertical alignment (VA) cell with an in-cell retarder, can be suppressed, from 3.8 to 0.7 V, by doping an in-cell material with 5 wt % TiO2.

Optical configurations for nematic liquid crystal device switchable between reflective and transmissive modes

We propose an optical configuration for a nematic liquid crystal (LC) device that is switchable between the reflective and the transmissive modes. By placing a reflective polarizer between the two LC layers, we obtain higher reflectance and reduce the parallax effect in the reflective mode. We can eliminate the parallax effect by using a wire-grid polarizer or other in-cell reflective polarizers. We expect that the proposed device can be used in various outdoor applications.