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International Conference on Infrared, Millimeter, and Terahertz Waves (IRMMW-THz)

High speed response of a Hot Electron Bolometer to a picosecond optical THz source and to a THz QCL comb up to 31GHz

G. Torrioli, Andres Forrer, Urban Senica, Mattias Beck, P. Carelli, F. Chiarello, Jérôme Faist, Alessandro Gaggero, E. Giovine, F. Martini, Roberto Leoni, Giacomo Scalari, Sara Cibella

Published Aug 28, 2022• 1 citation
Publisher

Abstract

We have investigated the frequency response of a phonon-cooled Hot Electron Bolometer (HEB) made from an ultrathin superconducting niobium nitride film. We have used short optical pulses and a broadband spectrum analyzer to record the impulse response of the HEB. To interpret the experimental results, we have used numerical calculations based on the two temperature (2T) theoretical model.

DOI
10.1109/irmmw-thz50927.2022.9895724

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Stability of a Quasi-Optical Superconducting NbTiN Hot-Electron Bolometer Mixer at 1.5 THz Frequency Band

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We are developing quasi-optical superconducting hot-electron bolometer (HEB) mixer receivers for astronomical and atmospheric remote sensing applications. The microbridge of the HEB mixer was fabricated at room temperature from a 6.8-nm-thick niobium titanium nitride (NbTiN) film deposited on a 20-nm-thick aluminum nitride (AlN) buffer layer, using a helicon sputtering technique at a slow deposition rate. The mixer was cooled to 4.2 K using a closed-cycled mechanical 4 K pulse-tube cryocooler with a temperature fluctuation of$\pm$1.6 mK. The stability of a large-volume NbTiN HEB mixer was studied at 1.47 THz by changing local oscillator (LO) power with the mixer bias voltage fixed. The intermediate frequency (IF) signal measured at 1.5 GHz had a maximum peak at a certain mixer bias current. The receiver noise temperature was lowest at around the IF maximum peak. It was also found that the IF signal was most stable at around the IF maximum peak under conditions in which the instability of LO pumping level, induced by small mechanical vibrations of the cryostat, remained in the optical system.

IEEE Transactions on Applied Superconductivity•Jan 24, 2011•6 citations
We have investigated the frequency response of a phonon-cooled Hot Electron Bolometer (HEB) made from an ultrathin superconducting niobium nitride film. We have used short optical pulses and a broadband spectrum analyzer to record the impulse response of the HEB. To interpret the experimental results, we have used numerical calculations based on the two temperature (2T) theoretical model.
We study the performance of a hot-electron bolometer (HEB) operating at THz frequencies based on superconducting niobium nitride films. We report on the voltage response of the detector over a large electrical detection bandwidth carried out with different THz sources. We show that the impulse response of the fully packaged HEB at 7.5 K has a 3 dB cutoff around 2 GHz. Remarkably, detection capability is still observed above 30 GHz in an heterodyne beating experiment using a THz quantum cascade laser frequency comb. Additionally, the HEB sensitivity has been evaluated and an optical noise equivalent power NEP of 0.8 pW/√ H z has been measured at 1 MHz.
We present a characterization of a 1.5 THz waveguide niobium titanium nitride (NbTiN) superconducting hot electron bolometer (HEB) mixer which can be pumped by a commercial solid state tunable local oscillator (LO) source. The NbTiN HEB mixer is made from a 12 nm thick NbTiN thin film deposited on a quartz substrate at room temperature. A gold electrode is formed in situ on the NbTiN thin film without breaking vacuum to ensure good contact. The uncorrected DSB receiver noise temperature is measured to be 1700 K at 1.5 THz, whereas the mixer noise temperature is derived to be 1000 K after corrections for losses of the input optics and the intermediate frequency (IF) amplifier chain. The required LO power absorbed in the HEB mixer is evaluated to be 340 nW by using an isothermal technique. The IF gain bandwidth is supposed to be about 1.3 GHz or higher. The present results show that good performance can be obtained at 1.5 THz even with a relatively thick NbTiN film (12 nm), as in the case of 0.8 THz. In order to investigate the cooling mechanism of our HEB mixers, we have conducted performance measurements for a few HEB mixers with different microbridge sizes both at 1.5 and 0.8 THz. The noise performance of the NbTiN HEB mixers is found to depend on the length of the NbTiN microbridge. The shorter the microbridge is, the lower the receiver noise temperature is. This may imply a contribution of the diffusion cooling in addition to the phonon cooling.
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.
Superconducting Hot Electron Bolometers (HEBs) are good candidates for detecting weak signals in the submillimeter or terahertz range. In this thesis work, a novel fabrication method was developed to make two types of niobium HEBs for different applications. HEBs were designed, fabricated, and then characterized at dc, microwave, and THz frequencies. The first type is a diffusion-cooled HEB, made with a short bridge that determines its cooling time. In this thesis, bridges were typically 400 nm long with bandwidths of about 1 GHz. These diffusion-cooled HEBs were developed as part of a collaboration with the University of Arizona (UA), to develop a proof-of-concept heterodyne array submillimeter camera. Devices were fabricated on thin fused quartz and silica substrates for waveguide coupling in the UA system for the astrophysically interesting 345 and 810 GHz atmospheric windows. The goal of this collaboration is to provide a basis of comparison between Nb diffusion-cooled HEB mixers and superconductorinsulator-superconductor mixers at these frequencies. The second type is a phonon-cooled HEB, made with a ∼3 µm long bridge. Its thermal response is dictated by the electron-phonon relaxation time. These devices were developed in collaboration with Prof. C. Schmuttenmaer’s lab in the Yale Chemistry department, Prof. G. Blake at Caltech,
Heterodyne detection of weak THz signals can be obtained with ultrathin superconducting microbridge bolometers where frequency mixing is achieved by rapid relaxation of hot electrons in resistive hot spots. These devices do not suffer from the superconducting band gap limit of conventional SIS mixers. Phonon cooled HEBs from NbN allow mixing with IF bandwidths up to several GHz. We discuss NbN thin film optimization and present characterization methods which allow to investigate the phonon cooling path.
We are developing quasi-optical superconducting hot-electron bolometer (HEB) mixer receivers for astronomical and atmospheric remote sensing applications. The microbridge of the HEB mixer was fabricated at room temperature from a 6.8-nm-thick niobium titanium nitride (NbTiN) film deposited on a 20-nm-thick aluminum nitride (AlN) buffer layer, using a helicon sputtering technique at a slow deposition rate. The mixer was cooled to 4.2 K using a closed-cycled mechanical 4 K pulse-tube cryocooler with a temperature fluctuation of$\pm$1.6 mK. The stability of a large-volume NbTiN HEB mixer was studied at 1.47 THz by changing local oscillator (LO) power with the mixer bias voltage fixed. The intermediate frequency (IF) signal measured at 1.5 GHz had a maximum peak at a certain mixer bias current. The receiver noise temperature was lowest at around the IF maximum peak. It was also found that the IF signal was most stable at around the IF maximum peak under conditions in which the instability of LO pumping level, induced by small mechanical vibrations of the cryostat, remained in the optical system.