We have constructed and characterized THz phase shifters based on liquid crystals (LCs) with graphene grown by chemical vapor deposition (CVD) and indium-tin-oxide nanowhiskers (ITO NWhs) as transparent conducting electrodes. A graphene-based phase shifter can achieve a phase shift of π/2 at 1.0 THz with the operating voltage of ~2.2 V (rms) as opposed to ~ 5.6 V (rms) for ITO-NWhs-based phase shifter in previous work. On the other hand, 2π phase shift at 1.0 THz was achieved in an ITO-NWhs-based phase shifter with a multi-sandwiched structure by applying ~2.6 V (rms). The low operation voltage of both two kinds of phase shifters imply compatibility of both type of devices with thin-film transistor (TFT) and complementary metal-oxide-semiconductor (CMOS) technologies. The experimental results of phase shifters are in good agreement with the theoretical predictions.
The applications of liquid-crystal-based devices the sub-millimeter wave or THz (1 THz = 10<sup>12</sup> Hz) frequency range has
blossomed recently. In this paper, we review the methodology for determination of the THz optical constants of nematic
liquid crystals, using E7 as an example. To demonstrate potential applications, we report an electrically tuned THz Solc
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.
A method for liquid crystal surface alignment by using a one-step, ion beam sputtering on glass substrates is
demonstrated. Pre-coating by polyimide is not necessary. We use a diode-type sputter to treat the glass substrates with Ar
ion-beam. The homeotropic alignments for nematic liquid crystals are achieved. The alignments are characterized by
using the polarizing optical microscope and the conoscope. To find out the alignment mechanism, the studies by using
super conducting quantum interference device and scanning probe microscopy are carried out. The surveyed surface
morphology reveals that the films are amorphous and composed of nanoparticles with dimensions around 30 nm. The
magnetization anisotropy of the sputtered magnetic films is analyzed. The polar anchoring strengths of the coated films
with different thicknesses are measured and compared with their saturation magnetization. We deduce that the
homeotropic alignment is achieved due to the orientation of the diamagnetic nematogenic molecules in the magnetic
field caused by the γ-Fe<sub>2</sub>O<sub>3</sub> ferrimagnetic thin films. A simple model of alternatively distributed magnetic moments with
opposite direction is proposed. The profile of magnetic field strength near the surface is then calculated to compare with
the measured alignment strength.
In this work, we report recent progress in liquid-crystal-based electrically tunable THz optical devices. Tunable
phase shift up to 360° 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° at 1 THz are 100 V and 90.5 V/cm, respectively. A sandwiched NLC cell about 2 mm in total thickness
is used to increase the interaction length while minimizing Fresnel losses at the interfaces. A phase shift of 367° is
demonstrated at 1.05 THz, significantly improving the dynamic response of the device.
Recently, there have been increasing interests in the study of liquid-crystal-based devices for application in the submillimeter wave or THz frequency range. In this paper, we present recent progress in liquid crystal THz optics from our group. Using time-domain THz spectroscopy, we have determined the complex indices of refraction of nematic liquid crystals, 5CB, PCH5 and E7 from 0.2 to beyond 1 THz. Significantly, the birefringence of 5CB and E7 are found to be as large as 0.2 at THz frequencies, while the absorption is negligible. Electrical-field and magnetic-field-controlled birefringence in LC were also investigated. A tunable room-temperature THz phase shifter using magnetic-fieldcontrolled birefringence in nematic 5CB gives a phase shift as large as 108° at 1.0 THz. Phase shift exceeding 360° at 1 THz, an important milestone, was realized by using a sandwiched LC (E7, Merck) cell as thick as 3 mm. The magnetically tuned LC phase shifter served as key components in a two-element tunable LC Lyot filter. The tuning range of the filter is from 0.388 to 0.564 THz or a fractional tuning range of ~ 40%. Our work clearly demonstrates the potential of liquid crystal devices for THz applications. Finally, we will present initial works on control of enhanced THz transmission through a metallic hole array with nematic liquid crystals. Our work clearly demonstrates the potential of liquid crystal devices for THz applications.
Tunable semiconductor lasers are compact, versatile sources used extensively in dense-wavelength-division-multiplexing (DWDM) optical communication systems, precision metrology, environmental monitoring, and laser spectroscopy. We have developed a twisted nematic liquid crystal device, the liquid crystal pixel mirror (LCPM), successfully as electronically tunable spectral filters for wavelength selection in external cavity semiconductor lasers. In this talk, we report recent advances in this class of electronically tunable single- and multiple-wavelength semiconductor lasers at 650 and 830 nm. Preliminary results of operating the laser at 1.5 microns will also be shown. The laser output can be locked to the ITU grid at 100 GHz intervals. Output power of the laser is as high as several hundred milliwatts, with a tuning range of several tens of nanometers. The laser can be operated either in the continuous-wave (CW) or mode-locked configuration. The linewidth of the laser in the free-running CW mode is about 30 MHz. Fine-tuning of the cw output wavelength can be achieved by changing the driving voltage to the desired pixels of the LCPM. In the mode-locked configuration, the laser design allows intra-cavity dispersion compensation and pulse compression.
A reflection-type spatial light modulator using twisted nematic (TN) liquid crystal (the liquid crystal pixel mirror or LCPM) is employed to realize a new type of digitally tunable, narrow line-width (less than 0.1 nm, instrument-limited), multi-wavelength semiconductor laser. The laser is based on a novel folded telescopic grating-loaded external cavity with LCPM at the focal plane of the folded telescope. With a 50-pixel LCPM, the single wavelength digitally tunable range of a visible laser diode was from 650.8 to 661.24 nm in 0.21 nm steps by biasing the individual pixels. Further, the wavelength can be switched and reset with a response time of 13.6 ms. By biasing two pixels at the same time, obtain dual-wavelength output with the wavelength tunable from 0.21 to 10.4 nm. Generation of tunable triple wavelengths with equal or arbitrary wavelength separation are also demonstrated. Preliminary results on such a laser operating at 1550 nm is also shown.
We report a new configuration of tunable optical filter for DWDM applications. In this design, first-order diffracted signal light by a grating is directed to a lens and focused on to a transmission-type liquid crystal spatial light modulator (LC-SLM). Wavelength channels are selected by opening the appropriate pixels of the LC-SLM for transmission. The device is demonstrated by using a broad-band AR-coated laser diode as the signal light. For conceptual demonstration, we use optical fibers with a core diameter of 50 microns instead of the LC-SLM. A total of 37 channels, spanning 33.6 nm were successfully selected with channel spacing of 0.94 nm and bandwidth of 0.4 nm. With 100 micron-wide pixels separated by 5 microns in the LC-SLM and a 1800 lines/mm grating, we show selection of five channels with channel spacing of 0. 78 nm. The bandwidth is 0.43 nm. The channel isolation is better than 20 dB. At a wavelength of 1.5 micron, channel spacing as small as 12.4 GHz can be realized. This device is also expected to be useful for other DWDM applications, e.g., switching and routing.