Jacob Y L Ho

Variable liquid crystal pretilt angles by nanostructured surfaces

Variable liquid crystal pretilt angles of any value from 0 to 90° can be obtained by using a nanostructured alignment layer. This layer is robust and reliable. The pretilt angles obtained are stable against high storage and operating temperatures, and have strong anchoring energies.

Variable liquid crystal pretilt angles generated by photoalignment of a mixed polyimide alignment layer

A method to generate any liquid crystal pretilt angle is proposed and demonstrated. This method is based on deep ultraviolet photoalignment of a layer consisting of nanostructured domains of horizontal and vertical alignment polyimides. Competition between the two types of domains produces the controlled pretilt angle. The obtained anchoring energy is reasonably large.

High-speed and high-sensitivity silicon-on-insulator metal-semiconductor-metal photodetector with trench structure

The frequency response and quantum efficiency (QE) of silicon‐on‐insulator (SOI) metal‐semiconductor‐metal photodetectors in the near‐infrared (∼800 nm) are greatly enhanced with a simple reactive ion etching to form electrodes inside the interdigitated trenches. Detectors with 1.25 μm trench spacing were fabricated on a SOI substrate with a 6‐μm‐thick silicon top layer. The unique device structure isolates carriers generated deep inside the semiconductor substrate and at the same time provides a highly uniform electric field throughout the active region of the detector, resulting in an instrumentation limited response time of 23 ps at 5 V bias and a −3 dB bandwidth of 2.3 GHz as measured at 790 nm. The dc responsivity is 0.12 A/W, corresponding to an external QE of 18.7% and an internal QE of 88.5%. The large bandwidth and good responsivity at the wavelength of interest, combined with their low operating voltages, make these detectors attractive for use in short‐distance optical communication systems.

Photoalignment of Ferroelectric Liquid Crystals by Azodye Layers

The photoinduced alignment of ferroelectric liquid crystals (FLCs) onto photochemically stable azo-dye films was studied. The alignment quality of FLC display cells depends mainly on the difference between the FLC surface energy and the aligning substrates surface energy; however, the structure and thickness of FLC layers are also important. The effect of the thickness of photoaligning azodye layer on the alignment quality and multiplex operation of passively addressed FLC display cells has been investigated. An optimal (about 3–5 nm) azo-dye layer thickness that provides both the highest multiplex operation steadiness and the best contrast ratio of the FLC display cells was found. The photoaligned FLC display cells showed the contrast ratio CR>500:1 at the wavelength λ=0.63 µm both in surface stabilized and deformed helix FLC electrooptical modes.

Nanopyramid structure for ultrathin c-Si tandem solar cells.

Recently, ultrathin crystalline silicon solar cells have gained tremendous interest because they are deemed to dramatically reduce material usage. However, the resulting conversion efficiency is still limited by the incomplete light absorption in such ultrathin devices. In this letter, we propose ultrathin a-Si/c-Si tandem solar cells with an efficient light trapping design, where a nanopyramid structure is introduced between the top and bottom cells. The superior light harvesting results in a 48% and 35% remarkable improvement of the short-circuit current density for the top and bottom cells, respectively. Meanwhile, the use of SiOx mixed-phase nanomaterial helps to provide the maximum light trapping without paying the price of reduced electrical performance, and conversion efficiencies of up to 13.3% have been achieved for the ultrathin tandem cell employing only 8 μm of silicon, which is 29% higher than the result obtained for the planar cell.

Bandwidth enhancement in silicon metal-semiconductor-metal photodetector by trench formation

We report on fabrication and characterization of a high-speed silicon metal-semiconductor-metal photodetector with novel structure. Metal contacts on the sidewalls of the interdigitated trenches are used to generate a highly uniform electric field well below the detector surface so that carriers deep within the semiconductor bulk can easily attain their saturation velocities. The detector, with 9-/spl mu/m-deep interdigitated trenches and a trench spacing of 1 /spl mu/m, has a pulse width of 28.2 ps and a -3-dB bandwidth of 2.2 GHz at 5 V. The responsivity measured at 790 nm is 0.14 A/W, corresponding to an external quantum efficiency of 22.1%. Our results also show that a bias as low as 2 V is sufficient for this diode to operate at high-speed without reducing responsivity.

Bridged-Grain Polycrystalline Silicon Thin-Film Transistors

We introduce a new structure for low-temperature polycrystalline silicon thin-film transistors (TFTs). This bridged-grain structure can reduce the threshold voltage and the subthreshold swing, increase the on-off ratio, and suppress leakage current and kink effect of TFTs significantly. This technique can be applied to all polycrystalline silicon TFTs, including those made by solid-phase crystallization, metal-induced crystallization, metal-induced lateral crystallization, and excimer laser annealing.

Azodyes as Photoalignment Materials for Polymerizable Liquid Crystals

We demonstrate the excellent photoalignment capability of sulfuric azodyes for polymerizable liquid crystals (PLC) whose anisotropic films are extensively studied as basic elements of many passive optical devices (e.g., retardation films, polarizers, and color filters). These dyes exhibit a high affinity to various substrates and insolubility in PLC and organic solvents commonly used for their dilution. An extremely low exposure dose (less than 50 mJ) is needed to induce an excellent planar alignment of PLC. This alignment can be easily patterned using minimal numbers of masks and exposure steps. No sign of alignment deterioration is observed after photopolymerization. These imply that new photoalignment materials can be effectively used for the industrial production of optical films based on PLC. Particularly, they suit the continuous manufacturing process for such films.

Extended Jones matrix method for oblique incidence study of polarization gratings

A fast and accurate extended Jones matrix method for characterizing the transmitted field of polarization gratings at oblique incidence is reported. The far field diffraction properties of the mth order of the grating are determined by vectorial Fourier coefficients of the transmitted field. With this method, polar plots of the various transmission orders can be obtained. The dependence of viewing angle performance on the grating pitch and grating thickness is studied. Liquid crystal polarization gratings with different pitches were fabricated with azo-dye SD-1 as photoalignment material, by polarization holography. The experimental results show very good agreement with the calculated data.

Simultaneous Determination of Large Pretilt Angles and Cell Gap in Liquid Crystal Displays

A method for the measurement of arbitrary pretilt angles in liquid crystal displays is proposed and demonstrated. The cell gap can also be determined accurately in the same experiment as well. This method makes use of a photo-elastic modulator and is capable of measuring pretilt angles from 0 to 90deg precisely.