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Applied Physics Letters

Low noise NbN lattice-cooled superconducting hot-electron bolometric mixers at submillimeter wavelengths

Jonathan H. Kawamura, R. Blundell, Edward Tong, Gregory N Gol'tsman, Eugene M. Gershenzon, Boris M. Voronov, Serguei Cherednichenko

Published Mar 24, 1997• 40 citations
Publisher

Abstract

Lattice-cooled superconducting hot-electron bolometric mixers are used in a submillimeter-wave waveguide heterodyne receiver. The mixer elements are niobium nitride film with 3.5 nm thickness and ∼10 μm2 area. The local oscillator power for optimal performance is estimated to be 0.5 μW, and the instantaneous bandwidth is 2.2 GHz. At an intermediate frequency centered at 1.4 GHz with 200 MHz bandwidth, the double sideband receiver noise temperature is 410 K at 430 GHz. The receiver has been used to detect molecular line emission in a laboratory gas cell.

DOI
10.1063/1.118634

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We report on heterodyne mixing experiments between 630-820 GHz using a diffusion-cooled superconducting hot electron bolometer at intermediate frequencies from 1-2 GHz. The niobium bolometer which is cooled by diffusion of the hot electrons into a normal conductor has dimensions of 0.3 /spl mu/m/spl times/0.15 /spl mu/m and was fabricated at KOSMA with a two-step electron beam lithography process. The film thickness of the device is 30 nm with T/sub c/ of 6.1 K and /spl Delta/T/sub c/ of 0.7 K. The mixing experiments are performed with a tunerless waveguide mixer block previously used for very low-noise SIS receivers. At a physical temperature of 2.2 K, receiver noise temperatures at 666 GHz are 1500 K at 1.0 GHz intermediate frequency, increasing rapidly above 1.5 GHz. The dissipated local oscillator power is evaluated to be 65-90 nW.

IEEE Transactions on Applied Superconductivity•Jun 1, 1997•13 citations
Lattice-cooled superconducting hot-electron bolometric mixers are used in a submillimeter-wave waveguide heterodyne receiver. The mixer elements are niobium nitride film with 3.5 nm thickness and ∼10 μm2 area. The local oscillator power for optimal performance is estimated to be 0.5 μW, and the instantaneous bandwidth is 2.2 GHz. At an intermediate frequency centered at 1.4 GHz with 200 MHz bandwidth, the double sideband receiver noise temperature is 410 K at 430 GHz. The receiver has been used to detect molecular line emission in a laboratory gas cell.
The heterodyne performance of lattice-cooled hot-electron bolometric mixers is measured at 200 GHz. Superconducting thin-film niobium nitride strips with ∼5 nm thickness are used as waveguide mixer elements. A double-sideband receiver noise temperature of 750 K at 244 GHz is measured at an intermediate frequency centered at 1.5 GHz with 500 MHz bandwidth and with 4.2 K device temperature. The instantaneous bandwidth for this mixer is 1.6 GHz. The local oscillator power required by the mixer is about 0.5 μW. The mixer is linear to within 1 dB up to an input power level 6 dB below the local oscillator power. A receiver incorporating a hot-electron bolometric mixer was used to detect molecular line emission in a laboratory gascell. This experiment unambiguously confirms that the receiver noise temperature determined from Y-factor measurements reflects the true heterodyne sensitivity.
We have developed a low-noise waveguide heterodyne receiver for operation near 1 THz using phonon-cooled NbN hot-electron bolometers. The mixer elements are submicron-sized microbridges of 4 nm-thick NbN film fabricated on a quartz substrate. Operating at a bath temperature of 4.2 K, the double-sideband receiver noise temperature is 760 K at 1.02 THz and 1100 K at 1.26 THz. The local oscillator is provided by solid-state sources, and power measured at the source is less than 1 /spl mu/W. The intermediate frequency bandwidth exceeds 2 GHz. The receiver was used to make the first ground-based heterodyne detection of a celestial spectroscopic line above 1 THz.
NbN phonon-cooled superconducting hot-electron bolometric mixers with quasi-optical coupling were fabricated in a self-aligned process by means of e-beam-lithography. The mixer element consists of a 5 nm thick NbN film sputtered on a fused quartz substrate. Noise performance was investigated in the frequency range of 798 to 813 GHz for intermediate frequencies from 1.0 to 1.75 GHz. Uncorrected double sideband receiver noise temperatures of about 900 K were measured at an intermediate frequency of 1000 MHz with a 50 MHz bandpass filter. The gain bandwidth was determined by superposing two local oscillator signals and measuring the height of the discrete line at the intermediate frequency output. A 3dB-roll-off-frequency of some 1.3 GHz was obtained.
The performance of a hot-electron bolometric (HEB) mixer based on NbN from 0.9 to 2.5 THz was investigated using a quasi-optical receiver configuration. An HEB mixer is an ultra-thin superconducting NbN strip located at the feed point of a thick normal conducting Au spiral antenna on a high-resistivity Si substrate. The active area of the mixer was 3 nm thick, 0.4 μm long and 4 μm wide. The quasi-optics consisted of an MgO hyperhemisphere with anti-reflection caps made of Kapton-JP polyimide film and an offset parabola to reduce input losses. The frequency dependence of the double-sideband receiver noise temperature was investigated at several frequencies by using a backward wave oscillator or an optically pumped far-infrared laser as the local oscillator. Results demonstrated low-noise and wide-band characteristics, below 1 K GHz −1 over the measured frequency range. At 917 GHz, the measured receiver noise temperature was 550 K across a 500 MHz intermediate-frequency bandwidth centred at 1.5 GHz, which is slightly better than that of other HEB mixers at around this frequency. This paper was presented at the 8th International Superconductive Electronics Conference, Osaka, Japan, 19–22 June 2001.
Experimental work on the development of a quasi-optical heterodyne receiver for the THz frequency range is reported. The mixing device is a superconducting hot electron bolometer (HEB) made from ultrathin, d = 3 - 5 nm, niobium nitride (NbN) films. Work described in this thesis comprises: The design of a prototype receiver using quasi-optical coupling. Device fabrication involving reactive magnetron sputtering of NbN films and lithographic definition of the antenna integrated mixer. An investigation of the receiver performance at frequencies between 0.6 THz and 2.5 THz. The best performance is obtained with HEB mixers made from NbN films with a thickness d = 3-3.5 nm and a critical temperature T c = 9 - 9.5 K. Double-side band receiver noise temperatures are 530 K at 0.6 THz, 650 K at 1.6 THz and 1100 K at 2.5 THz. Corrected for losses in the receiver input optics, these values amount to only 10 times the theoretical quantum limit. MgO as the substrate material has been shown to provide improved acoustic coupling between NbN film and the substrate compared to other materials such as Si or sapphire. A maximum IF gain bandwidth of 4.5 GHz has been measured under the same operating conditions which yield lowest noise temperature. The local oscilltor power requirement is ~250 nW absorbed in the bolometer. Quasi-optically coupled HEB mixers have become the device-of-choice for radio astronomical applications in the frequency regime at and above 1 THz.
We report on heterodyne mixing experiments between 630-820 GHz using a diffusion-cooled superconducting hot electron bolometer at intermediate frequencies from 1-2 GHz. The niobium bolometer which is cooled by diffusion of the hot electrons into a normal conductor has dimensions of 0.3 /spl mu/m/spl times/0.15 /spl mu/m and was fabricated at KOSMA with a two-step electron beam lithography process. The film thickness of the device is 30 nm with T/sub c/ of 6.1 K and /spl Delta/T/sub c/ of 0.7 K. The mixing experiments are performed with a tunerless waveguide mixer block previously used for very low-noise SIS receivers. At a physical temperature of 2.2 K, receiver noise temperatures at 666 GHz are 1500 K at 1.0 GHz intermediate frequency, increasing rapidly above 1.5 GHz. The dissipated local oscillator power is evaluated to be 65-90 nW.