Rolf A. Wyss

A traveling-wave THz photomixer based on angle-tuned phase matching

A traveling-wave THz photomixer based on a free-space optical-THz phase-matching scheme is proposed. A dc-biased coplanar strip line fabricated on low-temperature-grown GaAs serves as the active area of the device, and is illuminated by two noncollinear laser beams which generate interference fringes that are accompanied by THz waves. The device with the laser-power-handling capability over 300 mW and a 3-dB bandwidth of 1.8 THz was experimentally demonstrated. The results show that traveling-wave photomixers have the potential to surpass small-area designs.

Optimal choice of material for HEB superconducting mixers

We demonstrate that a potential distinction in ultimate performance of phonon-cooled and diffusion-cooled HEB mixers is not due to the cooling mechanisms but rather due to the different properties of available superconductors. The only available material for a phonon-cooled mixer with sufficiently large IF bandwidth (/spl sim/4 GHz) is NbN, whereas a variety of clean materials (e.g., Nb, NbC, Al) are suitable for a diffusion-cooled mixer. For a readily achievable device length of 0.1 /spl mu/m for example, the diffusion-cooled IF bandwidth can be /spl ges/10 GHz. The requirement of low local oscillator (LO) power can also be more easily met in diffusion-cooled devices by selection of a material with lower critical temperature and low density of electron states. In contrast, the parameters in the NbN-based mixer cannot be widely varied because of the high resistivity and high transition temperature of the material and the necessity of using ultrathin films. Given the limited availability of LO power from compact solid-state sources at frequencies above 1 THz a diffusion-cooled mixer based on aluminum is a very attractive choice for low-background radioastronomy applications.

Hot-electron superconductive mixers for THz frequencies

Superconductive hot-electron bolometer (HEB) mixers have been built and tested in the frequency range from 1.1 THz to 2.5 THz. The mixer device employs diffusion as a cooling mechanism for hot electrons. The double sideband receiver noise temperature was measured to be approximately equals 2750 K at 2.5 K at 2.5 THz; and mixer IF bandwidths as high as 9 GHz are achieved for 0.1 micrometers long devices. The local oscillator power dissipated in the HEB microbridge was in the range 20- 100 nW. Further reductions in LO power and mixer noise can be potentially achieved by using Al microbridges. The advantages and parameters of such devices are evaluated. A distributed-temperature model has been developed to properly describe the operation of the diffusion-cooled HEB mixer. The HEB mixer is a primary candidate for ground based, airborne and spaceborne heterodyne instruments at THz frequencies.

Low-noise and wideband hot-electron superconductive mixers for THz frequencies

Superconductive hot-electron bolometer (HEB) mixers have been built and tested in the frequency range 1.1-2.5 THz. The mixer device is a 0.15-03 /spl mu/m microbridge made from a 10 nm thick Nb film. This device employs diffusion as a cooling mechanism for hot electrons. The double sideband noise temperature was measured to be /spl les/3000 K at 2.5 THz and the mixer IF bandwidth is /spl ap/9-10 GHz for a 0.1 /spl mu/m long device. The local oscillator (LO) power dissipated in the HEB microbridge was 20-100 nW. Further improvement of the mixer characteristics can be potentially achieved by using Al microbridges. The advantages and parameters of such devices are evaluated The HEB mixer Is a primary candidate for ground based, airborne and spaceborne heterodyne instruments at THz frequencies. HEB receivers are planned for use on the NASA Stratospheric Observatory for Infrared Astronomy (SOFIA) and the ESA Far Infrared and Submillimeter Space Telescope (FIRST). The prospects of a submicron-size YBa/sub 2/Cu/sub 3/O/sub 7-/spl delta// (YBCO) HEB are also discussed. The expected LO power of 1-10 /spl mu/W and SSB noise temperature of /spl ap/2000 K may make this mixer attractive for various remote sensing applications.

Design and characterization of optical-THz phase-matched traveling-wave photomixers

Design and characterization of optical-THz phase-matched traveling-wave photomixers for difference-frequency generation of THz waves are presented. A dc-biased coplanar stripline fabricated on low-temperature-grown GaAs is illuminated by two non-collinear laser beams which generate moving interference fringes that are accompanied by THz waves. By tuning the offset angle between the two laser beams, the velocity of the interference fringe can be matched to the phase velocity of the THz wave in the coplanar stripline. The generated THz waves are radiated into free space by the antenna at the termination of the stripline. Enhancement of the output power was clearly observed when the phase-matching condition was satisfied. The output power spectrum has a 3-dB bandwidth of 2 THz and rolls off as approximately 9 dB/Oct which is determined by the frequency dependent attenuation in the stripline, while the bandwidth of conventional photomixer design has the limitation by the RC time constant due to the electrode capacitance. The device can handle the laser power of over 380 mW, which is 5 times higher than the maximum power handring capability of conventional small area devices. The results show that the traveling-wave photomixers have the potential to surpass small area designs, especially at higher frequencies over 1 THz, owing to their great thermal dissipation capability and capacitance-free wide bandwidth.