350 GHz NbN Hot Electron Bolometer Mixer

K. H. Gundlach, Thomas Lehnert, C. Rösch +4 more

K. Ilin, Serguei Cherednichenko, Gregory N Gol'tsman +2 more

We report the results of the bandwidth measurements of NbN hot-electron bolometers, perfomied in the terahertz frequency domain at 140 GHz and 660 GHz and in time domain in the optical range at the wavelength of 395 nm.. Our studies were done on 3.5-nm-thick NbN films evaporated on sapphire substrates and patterned into ilin-size microbridges. In order to measure the gain bandwidth, we used two identical BWOs (140 or 660 GHz), one functioning as a local oscillator and the other as a signal source. The bandwidth we achieved was 3.5-4 GHz at 4.2 K with the optimal LO and DC biases. Time-domain measurements with a resolution below 300 fs were performed using an electro-optic sampling system, in the temperature range between 4.2 K to 9 K at various values of the bias current and optical power. The obtained response time of the NbN hot-electron bolometer to —100fs-wide Ti:sapphire laser pulses was about 27 ps, what corresponds to the 5.9 GHz gain bandwidth. Introduction The bandwidth of hot-electron bolometer (HEB) mixers has reached values which are record for bolometers. The gain bandwidth for phonon-cooled NbN HEB mixers of 4 GHz for 140 GHz and 650 GHz frequencies [1, 2] and the noise bandwidth of 8 GHz for 650 GHz [2] have been demonstrated. For diffusion-cooled Nb HEB mixers, the bandwidth has reached 6 GHz, when measured at 20--40 GHz [3]. However, there is a definite practical interest to even further extend the bandwidth of these devices. The possibilities of the HEB mixer technology in this direction are far from being exhausted. For example, the future of the NbN HEB technology requires that critical temperature Tc > 10 K and critical current density j c > 10 6 Akm at 4.2 K are simultaneously obtained for d < 3 nm-thick films M. The reduction of the optimal power of the local oscillator PLID below 100 nW leads to the decrease of the device length down to L < 0.2 pm, which, in turn, should bring an additional cooling channel for hot electrons—diffusion into metal contacts and a further bandwidth extension.

Shigehito Miki, Akira Kawakami, Yoshinori Uzawa +1 more

We propose a superconducting hot electron bolometer (HEB) mixer with an upper heat sink to improve the phonon diffusion efficiency in phonon cooled HEB mixers. We designed and fabricated NbN HEB mixers, and investigated the cooling effect of the upper heat sink by measuring intermediate frequency (IF) bandwidths of the HEB mixers. The SiO films for the upper dielectric heat sink, ranging in thickness from 9 to 35 µm, were deposited on the top of the NbN strip by high vacuum evaporation. We measured the IF bandwidths of HEB mixers, with and without the SiO heat sink, and observed that the 3-dB IF roll-off frequency increased from 0.4 to 0.7 GHz by using a 35-µm-thick SiO heat sink.

W. Zhang, P. Khosropanah, Tanish Aggarwal +4 more

H. Kataoka, Akira Kawakami, Yoshinori Uzawa +2 more

Hot electron bolometers (HEBs) are expected to be used as low-noise heterodyne mixers for applications in radio astronomy and remote sensing in the terahertz frequency region. We fabricated and tested HEB mixers for these applications; however, those we made with the conventional process easily broke due to thermal stress and electrical surges. To improve their durability, a new process involving fluoride radical etching was developed. In this process, HEB mixers were fabricated using a relatively thick NbN strip (several tens of nm). Finally, the NbN strip was etched that was less than 4-nm thick enough to operate as HEB mixers by using the fluoride radical etching. We confirmed that fluoride radical etching caused no damage to the etched NbN strip. The dc properties of the HEB mixers fabricated with the new process did not change for fourteen 4.2-300 K thermal cycles for 50 days. The receiver noise temperature of the HEB mixer was evaluated and it was about 615 K at 780 GHz.

Jonathan H. Kawamura, R. Blundell, Edward Tong +3 more