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.

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.

AN EFFECTIVE IMPEDANCE MATCHING METHOD FOR RFID SLOT TAG ANTENNASKey words: radio frequency identification; tag antenna; slot antenna INTRODUCTIONRadio frequency identification (RFID) technology is the use of radio communication to identify an object automatically. Recently, an increasing number of research studies have been done in the field of the 860-960 MHz ultra high frequency (UHF) band RFID system owing to great demands for long reading range, high data rate, and small antenna size. An UHF RFID system is comprised of tags, readers, and a host computer. The tag, which consists of an antenna and a chip, plays a critical role in the performances of the RFID system. During the UHF tag design, conjugate impedance matching between the tag antenna and the chip must be realized for maximum power transfer. Typical UHF tag antennas were designed as modified dipoles [1], which can be printed on very thin Piezoelectric transducer (PET) materials at low cost. This type of antenna is suitably adhered to the object with low dielectric constant. However, it does not work efficiently when mounted on the object with high conductivity. Slot antennas are widely used in modern 50-X microwave communication systems, especially where low-profile mounting is required. Because slot antenna is easily integrated into metallic objects, it can be considered a potential candidate on the RFID tag operation for metallic object identification. So far, only few attempts have been made on the designs of the slot-type tag antennas for RFID applications [2][3][4][5]. It should be noted that the complex impedance matching for most of the reported slot antennas are accomplished by varying the shape of the radiating slot and/or using an offset-center feed. However, there has been relatively little research conducted on the design rule for the complex impedance matching of the slot tag antenna. In this article, we present a simple matching method used for two slot tag antennas. By using a matching slot, the proposed slot tag antennas have an impedance tuning capability, and good impedance matching between the slot antenna and the chip can be easily achieved. Detailed design methods and experimental results are discussed and presented. ANTENNA DESIGNThe configuration of the compact slot tag antenna with a matching slot is shown in Figure 1 . The radiating slot is bent into an M-shaped structure for achieving compact antenna size, and it is placed symmetrically with respect to the centerline (y axis) of the metal plane. The matching slot with a length of t and a width of w is connected to its two arms of the M-shaped slot and used to match the antenna with the tag chip. To verify the validation of the impedance matching method for various slot-type tag antennas, reference antenna that has a simple slot structure with a matching slot is also studied, and its design parameters are shown in Figure 1(b).The proposed antenna is designed to operate in the UHF frequency band of 915 MHz. The tag chip used in this design is Alien Higgs in strap package with ...

We present the use of magnetically controlled birefringence in a nematic liquid crystal cell for phase shifting of electromagnetic waves in the range of terahertz frequencies. This device can be operated at room temperature. A maximum phase shift of 141° has been demonstrated at 1.025 THz and the results are in good agreement with theoretical predictions.

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.