Oscillations have been obtained at frequencies from 100 to 712 GHz in InAs/AlSb double-barrier resonant-tunneling diodes at room temperature. The measured power density at 360 GHz was 90 W cm-2, which is 50 times that generated by GaAs/AlAs diodes at essentially the same frequency. The oscillation at 712 GHz represents the highest frequency reported to date from a solid-state electronic oscillator at room temperature.

Low-temperature-grown (LTG) GaAs is used as an optical-heterodyne converter or photomixer, to generate coherent continuous-wave output radiation from microwave frequencies up to 3.8 THz. The photomixer consists of an epitaxial layer of LTG GaAs with interdigitated electrodes fabricated on the top surface. Terahertz photocurrents are generated in the gaps between the electrodes, and power is radiated into free space through a three-turn self-complementary spiral antenna. In a photomixer having a 0.27-ps electron-hole lifetime and small electrode capacitance, the output power is practically flat up to about 300 GHz and then rolls off at a rate of approximately 12 dB/oct.

open, and the other cells are closed. In configuration 2, the MEMS switches in PBG cells 2, 4, 6, and 8 are open, and the other cells are closed. In fact, configurations 1 and 2 are symmetrical configurations, except for the feed point. NUMERICAL AND MEASURED RESULTSAnsoft HFSS 8.0 is used to analyze the antenna. The antenna operates at around 10 GHz. The size of the substrate is 50 ϫ 30 mm, and the metal ground 60 ϫ 40 mm. In this work, the open or closed states of a switch are simulated by the absence or presence of a metal pad of size 0.4 ϫ 0.4 mm, which is approximately the size of a MEMS switch [5]. According to the simulated antenna, three prototypes are fabricated and measured.Figures 2(a), 3(a), and 4(a) show the simulated and measured return losses of configurations 0, 1, and 2, respectively. As can be seen, the simulated results are in good agreement with the measured results, except for the operating frequency shifting downward slightly. According to the measured results, 9.65 GHz is selected as the radiation-pattern measurement frequency. At this frequency, antenna configurations 0, 1, and 2 have the return losses of Ϫ11.2, Ϫ11.2, and Ϫ11.5 dB, respectively.The measured far-field radiation patterns of configurations 0, 1, and 2 are shown in Figures 2(b), 3(b), and 4(b), respectively. In Figure 2(b), the main beam in the E-plane directs to Ϫ24°, and the 3-dB beamwidth covers a range from Ϫ52°to 25°. In Figure 3(b), direction of the main beam in the E-plane is 34.5°and the range of 3-dB beamwidth is from 16°to 63°. The side-lobe level is less than Ϫ7 dB. In Figure 4(b), direction of the main lobe in the E-plane points at Ϫ35°and the 3-dB beamwidth ranges from Ϫ66°to Ϫ18°. The side-lobe level is less than Ϫ7 dB. The measured results are in good agreement with the simulation. The simulated directivities of configurations 0, 1, and 2 are 8.87, 10.08, and 9.71 dB, respectively. All three configurations of the antenna have low cross-polarization of less than Ϫ21 dB. CONCLUSIONA pattern reconfigurable quasi-Yagi microstrip antenna has been presented and three prototypes have been measured. By using switch-controlled PBG structure, the pattern-reconfigurable antenna scans from Ϫ66°to ϩ63°in the upper-half space while maintaining the operating frequency around 9.65 GHz. The PBG structure also increased the antenna gain. NONUNIFORM MEANDERED AND FORK-TYPE GROUNDED ANTENNA DESIGN

The application of THz to medical imaging is experiencing a surge in both interest and federal funding. A brief overview of the field is provided along with promising and emerging applications and ongoing research. THz imaging phenomenology is discussed and tradeoffs are identified. A THz medical imaging system, operating at ~525 GHz center frequency with ~125 GHz of response normalized bandwidth is introduced and details regarding principles of operation are provided. Two promising medical applications of THz imaging are presented: skin burns and cornea. For burns, images of second degree, partial thickness burns were obtained in rat models in vivo over an 8 hour period. These images clearly show the formation and progression of edema in and around the burn wound area. For cornea, experimental data measuring the hydration of ex vivo porcine cornea under drying is presented demonstrating utility in ophthalmologic applications.

Recent optical heterodyne measurements with distributed-Bragg-reflector diode-laser pumps demonstrate that low-temperature-grown ͑LTG͒ GaAs photomixers will be useful in a compact all-solid-state terahertz source. Electrical 3 dB bandwidths as large as 650 GHz are measured in mixers with low electrode capacitance. These bandwidths appear to be independent of pump-laser wavelength over the range 780-850 nm. Shorter wavelength pumping results in a significant reduction of the bandwidth. The best LTG-GaAs photomixers are used to generate coherent continuous-wave output radiation at frequencies up to 5 THz.

The photonic crystal is investigated as a substrate material for planar antennas in the microwave and millimeter-wave bands. Experimental results are presented for a bow-tie antenna on a (111)-oriented facecentered-cubic photonic-crystal substrate with a band gap between approximately 13 and 16 GHz. When driven at 13.2 GHz, the antenna radiates predominantly into the air rather than into the substrate. This suggests that highly efficient planar antennas can be made on photonic-crystal regions fabricated in semiconductor substrates such as GaAs.

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An analysis has been carried out of optical heterodyne conversion with an interdigitated-electrode photomixer made from low-temperature-grown (LTG) GaAs and pumped by two continuous-wave, frequency-offset pump lasers. The analytic prediction is in excellent agreement with the experimental results obtained recently on a photomixer having 1.0~pm-wide electrodes and gaps. The analysis predicts that a superior photomixer having 0.2~pm-wide electrodes and gaps would have a temperature-limited conversion efficiency of 2.0% at a low difference frequency, 1.6% at 94 GHz, and 0.5% at 300 GHz when connected to a broadband 100 fi load resistance and pumped at hv=2.0 eV by a total optical power of 50 mW. The predicted 3-dB bandwidth (193 GHz) of this photomixer is limited by both the electron-hole recombination time (0.6 ps) of the LTG-GaAs material and the RC time constant (0.5 ps) of the photomixer circuit.