Cho-Fan Hsieh

Liquid-crystal-based terahertz tunable Lyot filter

A tunable Lyot filter operating in the terahertz frequency range is demonstrated by using fixed and variable nematic liquid crystals.

Voltage-controlled liquid-crystal terahertz phase shifter and quarter-wave plate

Phase shift exceeding tau/2 at 1 THz is demonstrated by using electrically controlled birefringence in a homeotropically aligned nematic liquid crystal (E7) cell, 570 microm in thickness. The driving voltage required for a phase shift of 90 degrees is 125 V (rms). We demonstrate that the phase shifter works as an electrically switchable quarter-wave plate at 1 THz. The device can also be used as an electrically tuned phase compensator around the quarter-wave point near 1 THz.

Magnetically tunable room-temperature 2 pi liquid crystal terahertz phase shifter.

Tunable phase shift up to 360 degrees at 1 THz is achieved with a liquid crystal (LC) device. The key to this design is (1) the use of a nematic LC, E7, which exhibits a birefringence of ~ 0.17 (0.2 - 1.2 THz); (2) a LC cell (3-mm in thickness) with sandwiched structure to increase the interaction length while minimizing interface Fresnel losses; and (3) the use of magnetic field to align the thick LC cell and achieve continuous tuning of phase from 0 to 360 degrees . This device can be operated over a broad range near room temperature.

Control of enhanced THz transmission through metallic hole arrays using nematic liquid crystal

We demonstrate frequency tuning of enhanced THz radiation transmitted through a two-dimensional metallic hole array (2D-MHA) by controlling the index of refraction of the medium filling the holes and adjacent to the 2D-MHA on one side. The medium is a nematic liquid crystal (NLC) and its index of refraction is varied using magnetically controlled birefringence of the NLC. With the NLC, the peak transmission frequency of the 2D-MHA shift to the red by 0.112 THz and can be tuned from 0.193 to 0.188 THz. The peak transmittance is as high as 70% or an enhancement of 2.42 times, considering the porosity of the 2D-MHA. As a tunable THz filter, this device exhibits a continuous tuning range of 4.7 GHz , a low insertion loss of 2.35 to 1.55 dB and a quality factor of ~ 4-5.

Polarizing terahertz waves with nematic liquid crystals.

A Feussner-type terahertz polarizer with a nematic liquid crystal (NLC) layer between two fused-silica prisms is demonstrated. The polarization factor and extinction ratio of the NLC-based terahertz polarizer can exceed 0.99 and 10(-5), respectively.

Temperature-dependent optical constants and birefringence of nematic liquid crystal 5CB in the terahertz frequency range

We have measured the frequency dependence and temperature dependence of the optical constants of a liquid crystal 4′-n-pentyl-4-cyanobiphenyl (5CB) in both nematic and isotropic phases by using terahertz time-domain spectroscopy. The extinction coefficient of 5CB is less than 0.02 and without sharp absorption features in the frequency range of 0.2–1.0 THz. The extraordinary and ordinary indices of refraction at 25 °C are around 1.77 and 1.58, respectively, giving rise to a birefringence of 0.20±0.02 in this frequency range. The temperature-dependent order parameter extracted from birefringence agrees with the results from the visible region quite well.We have measured the frequency dependence and temperature dependence of the optical constants of a liquid crystal 4′-n-pentyl-4-cyanobiphenyl (5CB) in both nematic and isotropic phases by using terahertz time-domain spectroscopy. The extinction coefficient of 5CB is less than 0.02 and without sharp absorption features in the frequency range of 0.2–1.0 THz. The extraordinary and ordinary indices of refraction at 25 °C are around 1.77 and 1.58, respectively, giving rise to a birefringence of 0.20±0.02 in this frequency range. The temperature-dependent order parameter extracted from birefringence agrees with the results from the visible region quite well.

Electrically tunable room-temperature 2/spl pi/ liquid crystal terahertz phase shifter

Tunable phase shift up to 360/spl deg/ at 1 THz is demonstrated using electrically controlled birefringence in a vertically aligned nematic liquid crystal (E7) cell, 1.83 mm in thickness. The driving voltage and corresponding field required for a phase shift of 360/spl deg/ at 1 THz are 100 V and 90.5 V/cm, respectively.

Fabrication of Terahertz Planar Metamaterials Using a Super-Fine Ink-Jet Printer

Super-fine ink-jet (SIJ) printing technology is applied to the fabrication of terahertz metamaterials. A silver film is fabricated using an SIJ printer with silver paste ink, and it is confirmed that the film behaves as a good conductor in the terahertz frequency region. Then, basic terahertz metamaterials such as metal wire-grid structures and split-ring resonators are printed on high-resistivity silicon substrates. The terahertz responses of the printed samples agree with those expected from their structures. SIJ printing is one of the ideal methods for fabricating terahertz metamaterials owing to its rapidity, simplicity, flexibility, and sufficient accuracy.

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.

Monophasic TiO2 films deposited on SrTiO3(100) by pulsed laser ablation

Single phase TiO2 thin films, of either rutile or anatase structure, have been prepared on SrTiO3(STO)(100) substrates by in situ pulsed laser deposition (PLD). Thermodynamically unfavorable, for films deposited on STO(100) substrate directly, pure anatase TiO2(00l) films were formed even when a rutile TiO2(110) substrate was used as a target. On the other hand, pure rutile TiO2(110) films were obtained by oxidizing PLD TiN films in-situ at temperatures higher than 700 °C. The optimized deposition conditions for preparing TiN and TiO2 films were reported. The crystalline structure, surface morphology, and electronic structure of these films were examined. A mechanism of the process of film formation is also proposed.