We report on the development of waveguide-based mixers for operation beyond 2 THz. The mixer element is a
superconducting hot-electron bolometer (HEB) fabricated on a silicon-on-insulator (SOI) substrate. Because it is beyond
the capability of conventional machining techniques to produce the fine structures required for the waveguide embedding
circuit for use at such high frequencies, we employ two lithography-based approaches to produce the waveguide circuit:
a metallic micro-plating process akin to 3-D printing and deep reactive ion etching (DRIE) silicon micromachining.
Various mixer configurations have been successfully produced using these approaches. A single-ended mixer produced
by the metal plating technique has been demonstrated with a receiver noise temperature of 970 K (DSB) at a localoscillator
frequency of 2.74 THz. A similar mixer, produced using a silicon-based micro-machining technique, has a
noise temperature of 2000 K (DSB) at 2.56 THz. In another example, we have successfully produced a waveguide RF
hybrid for operation at 2.74 THz. This is a key component in a balanced mixer, a configuration that efficiently utilizes
local oscillator power, which is scarce at these frequencies. In addition to allowing us to extend the frequency of
operation of waveguide-based receivers beyond 2 THz, these technologies we employ here are amenable to the
production of large array receivers, where numerous copies of the same circuit, precisely the same and aligned to each
other, are required.
The Stratospheric TeraHertz Observatory (STO) is a NASA funded, Long Duration Balloon (LDB) experiment designed to
address a key problem in modern astrophysics: understanding the Life Cycle of the Interstellar Medium (ISM). STO will
survey a section of the Galactic plane in the dominant interstellar cooling line [C II] (1.9 THz) and the important star
formation tracer [N II] (1.46 THz) at ~1 arc minute angular resolution, sufficient to spatially resolve atomic, ionic and
molecular clouds at 10 kpc. STO itself has three main components; 1) an 80 cm optical telescope, 2) a THz instrument
package, and 3) a gondola . Both the telescope and gondola have flown on previous experiments [2,3]. They have been reoptimized
for the current mission. The science flight receiver package will contain four [CII] and four [NII] HEB mixers,
coupled to a digital spectrometer. The first engineering test flight of STO was from Ft. Sumner, NM on October 15, 2009.
The ~30 day science flight is scheduled for December 2011.
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.
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 is a 0.15 - 0.3 micrometer 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 less than or equal to 3000 K at 2.5 THz and the mixer IF bandwidth is expected to be at least 10 GHz for a 0.1 micrometer 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</SUB>Cu<SUB>3</SUB>O<SUB>7-(delta</SUB> ) (YBCO) HEB are discussed. The expected LO power of 1 - 10 (mu) W and SSB noise temperature of approximately equals 2000 K may make this mixer attractive for various remote sensing applications.
We report on the development of quasioptical Nb hot-electron bolometer mixers for heterodyne receivers operating at 1 THz 3 THz. The devices have submicron in-plane sizes, thus exploiting diffusion as the electron cooling mechanism. Quasioptical mixer circuits have been developed with planar double-dipole or twin-slot antennas. The measured (DSB) receiver noise temperatures are 1670 K at 1.1 THz, with an estimated mixer noise temperature of approximately equals 1060 K, and 2750 K at 2.5 THz, with an estimated mixer noise temperature of approximately equals 900 K. The IF bandwidth is found to scale as the length-squared, and bandwidths as high as 8 GHz have been measured. These results demonstrate the low-noise, broadband operation of the diffusion-cooled bolometer mixer over a wide range of far-infrared wavelengths.
We report on the first heterodyne measurements with a diffusion-cooled hot-electron bolometer mixer in the submillimeter wave band, using a waveguide mixer cooled to 2.2 K. The best receiver noise temperature at a local oscillator frequency of 533 GHz and an intermediate frequency of 1.4 GHz was 650 K (double sideband). The 3 dB IF roll-off frequency was around 1.7 to 1.9 GHz, with a weak dependence on the device bias conditions.