Chia-Jen Lin

The complex refractive indices of the liquid crystal mixture E7 in the terahertz frequency range

We have used terahertz time-domain spectroscopy to investigate the complex optical constants and birefringence of a widely used liquid crystal mixture E7 in both nematic and isotropic phases (26°C–70°C). The extinction coefficient of E7 at room temperature is less than 0.035 and without sharp absorption features in the frequency range of 0.2–2.0 THz. The extraordinary (ne) and ordinary (no) indices of refraction at 26°C are 1.690–1.704 and 1.557–1.581, respectively, giving rise to a birefringence of 0.130–0.148 in this frequency range. The temperature-dependent (26°C–70°C) order parameter extracted from the birefringence data agrees with that in the visible region quite well. Further, the temperature gradients of the terahertz optical constants of E7 are also determined. The optical constants of E7 in the terahertz or sub-millimeter wave range are found to deviate significantly from values predicated by the usual extended Cauchy equations used in the visible and near-infrared.

Manipulating terahertz wave by a magnetically tunable liquid crystal phase grating

This investigation demonstrates the feasibility of a magnetically tunable liquid crystal phase grating for the terahertz wave. The phase grating can be used as a beam splitter. The ratio of the zeroth and first-order diffracted THz-beams (0.3 THz) polarized in a direction perpendicular to that of the grooves of the grating can be tuned from 4:1 to 1:2. When the THz wave is polarized in any other direction, this device can be operated as a polarizing beam splitter.

Electrically Controlled Liquid Crystal Phase Grating for Terahertz Waves

This work demonstrates the feasibility of electrically controlled liquid-crystal-based phase grating for manipulating the terahertz (THz) waves. This device can be utilized as a THz beam splitter and the beam splitting ratio of the zeroth- to the first-order diffractions can be tuned from 10:1 to 1:1.

Mechanism in determining pretilt angle of liquid crystals aligned on fluorinated copolymer films

This work explores the surface treatment of copolymer materials with fluorinated carbonyl groups in various mole fractions by ultraviolet irradiation and ion-beam (IB) bombardment and its effect on liquid crystal (LC) surface alignments. X-ray photoemission spectroscopic analysis confirms that the content of the grafted CF2 side chains dominates the pretilt angle. A significant increase in oxygen content is responsible for the increase in the polar surface energy during IB treatment. Finally, the polar component of the surface energy dominates the pretilt angle of the LCs. (Some figures in this article are in colour only in the electronic version)

THz Optical Constants of the Liquid Crystal MDA-00-3461

We have measured the far infrared optical constants of MDA-00-3461 at 25°C by using terahertz time-domain spectroscopy. The extraordinary and ordinary refractive indices of MDA-00-3461 are ne ≈ 1.74 and no ≈ 1.54, or a birefringence of 0.20 from 0.3 to 1.4 THz. The extinction coefficient of MDA-00-3461 is relatively small. There are no absorption peaks in the frequency range investigated.

Achromatic Liquid Crystal Phase Plate for Short Laser Pulses

This work demonstrates the feasibility of reducing pulse broadening by using liquid crystal achromatic half and quarter wave plates for ultrafast pulsed lasers. Each wave plate is made of two and three liquid crystal cells stacked together. Total phase retardation and its broadening reducing of the achromatic half wave plate have been measured. The experimental results are in good agreement with theoretical calculation.

Alignment Control of Rubbed Polyimide Layers by UV-Irradiation

This investigation demonstrates the feasibility of re-aligning the assembled liquid crystal cells based on rubbed substrates with photoalignment treatment. A pre-exposure process is proposed to achieve the photo-reorientation effectively. This study shows that the fine-tuning on the LC alignment after a cell is assembled by adjusting UV irradiation conditions can be obtained easily.

A Simple Method of Determining the Pitch of a Chiral Nematic Liquid Crystal

The pitch value P of a chiral nematic liquid crystal is a critical parameter for azimuthal anchoring strength measurement. The Grandjean-Cano wedge method is a conventional technique for pitch determination, but the perfect wedge cell with known angle and uniform disclination lines are required. In this work, we have developed an improved Grandjean-Cano method, which is simple and gives accurate pitch value. The sensitivity of this new method is ±λ/2Δn.

Magnetically tunable metallic photonic crystals immersed in liquid crystal for terahertz wave

We investigated the phenomena of a metallic photonic crystal (MPC) immersed in liquid crystal. According to our design, the photonic crystal has specific photonic band gap (PBG) and can be utilized as a filter. The device is filled with nematic liquid crystal (NLC), MDA-00-3461. The refractive indices of NLC can be magnetically controlled by reorienting the NLC molecules. Consequently, the corresponding PBG and the filtering performance of the device are tunable. According to our experimental results, the low frequency boundary of PBG at 0.121 THz can be blue shifted by 6.17 GHz, and the high frequency boundary of PBG at 0.175 THz can be shifted to the blue by 11.04 GHz. As a tunable THz filter, the peak transmittance at 0.187 THz can be blue shifted by 3.66 GHz.

Surface Alignment with High Pretilt Angle Using the Photoreactive Fluorinated Polymer Films

A newly developed fluorinated polymer film for liquid crystal alignment by applying ultra-violet (UV) irradiation or ion beam treatment is studied in this work. The surface alignment with high pretilt angle is achieved by the UV treatment. The induced pretilt angle by ion beam treatment on this photo-reactive polymer film, however, is relatively small. Surface analyses by x-ray photoemission spectroscopy and contact angle measurement are carried out to deduce the underlying mechanism. The result shows that the higher content of the fluorinated side-chain in polymer contribute to the lower surface energy and the higher pretilt angle. The atomic ratio of fluorine is largely decreased by ion beam treatments. The alignment uniformity and stability of the surfaces treated by linearly-polarized UV light and ion beam bombardment are also demonstrated.