FTS measurements of 2.5 and 4.7 THz double-slot antennas on SiO2/Si3N4 membrane

Receivers for astronomical observation above 1 THz have been developed the last years, such as the ground-based radio telescope in the south pole TREND, the airborne-based observatory SOFIA or the space-based Herschel observatory. All these observatories use NbN hot electron bolometer on bulk silicon chips, since it has been demonstrated to be the best mixing element at above 1.4 THz, showing better noise performances than SIS, an intermediate frequency about 4 GHz and LO power requirement around a few hundreds of mW. Detection of submillimeter lines of OH and OI at 2.5 and 4.7 THz presents a high interest for radioastronomers. At those very high frequencies, the fabrication of waveguides becomes very difficult. In recent years, the micromachining technology has been proposed for the fabrication of millimeter wave circuits on very thin dielectric membranes. Membrane-based NbN HEB mixers are being developed for new type of receivers. Taking advantage of the quasi-optic design allowed by the freestanding SiO₂/Si₃N₄ membrane, those detectors are expected to show numerous advantages (it allows us to use a mirror instead of a silicon lens, decreasing the overall noise temperature, the structure of the antenna is enlarged and the processing at very high frequencies is made easier, a back-short can be added at λ/4 to double the antenna gain) and permit to easily design and process double-slot antennas up to several THz. Planar antennas are used to couple the RF power to the bolometer and it is mandatory to experimentally determine the RF bandwidth of those antennas. This work presents the Fourier transform spectrometer (FTS) measurements made with membrane-based double-slot antennas, designed at 2.5 and 4.7 THz, and using an NbN HEB mixer.