Superconducting NbN Hot Electron Bolometer Research Articles

Performance improvements of a terahertz direct detector for imaging arrays

We studied and improved the performance of a terahertz (THz) direct detector based on the superconducting NbN hot electron bolometer (HEB) integrated with microwave readout for imaging arrays and other applications. A 6 nm thick NbN microbridge at the feed point of a twin-slot antenna is embedded into a high-Q quarter-wavelength coplanar waveguide (CPW) resonator made from a 350 nm thick NbN film which is weakly coupled to a CPW feedline. THz direct detection was performed at a signal frequency of 0.66 THz at a bath temperature of 4.2 K. When operating at a low input microwave power level, the device acting as a microwave kinetic inductance detector with a kinetic inductance fraction of 0.45 changes the surface impedance of the resonator in response to incident THz radiation. The estimated optical noise equivalent power (NEP) is ∼0.23 pW · Hz−0.5 which shows about one order of magnitude improvement compared to that of the traditional voltage-biased NbN HEBs. The improved performance of the THz direct detectors with lower NEP and easy readout makes them promising in imaging with a large number of the array pixels.

10.6 μm heterodyne receiver based on a superconducting hot-electron bolometer mixer and a quantum cascade laser

We report on the development of a heterodyne receiver at mid-infrared wavelength for high-resolution spectroscopy applications. The receiver employs a superconducting NbN hot electron bolometer as a mixer and a room temperature distributed feedback quantum cascade laser operating at 10.6 μm (28.2 THz) as a local oscillator. The stabilization of the heterodyne receiver has been achieved using a feedback loop controlling the output power of the laser. Improved Allan variance times as well as a double sideband receiver noise temperature of 5000 K and a noise bandwidth of 2.8 GHz of the receiver system are demonstrated.

Temperature dependence of the receiver noise temperature and IF bandwidth of superconducting hot electron bolometer mixers

In this paper we study the temperature dependence of the receiver noise temperature and IF noise bandwidth of superconducting hot electron bolometer (HEB) mixers. Three superconducting NbN HEB devices of different transition temperatures (Tc) are measured at 0.85 THz and 1.4 THz at different bath temperatures (Tbath) between 4 K and 9 K. Measurement results demonstrate that the receiver noise temperature of superconducting NbN HEB devices is nearly constant for Tbath/Tc, less than 0.8, which is consistent with the simulation based on a distributed hot-spot model. In addition, the IF noise bandwidth appears independent of Tbath/Tc, indicating the dominance of phonon cooling in the investigated HEB devices.

Heterodyne detection at near-infrared wavelengths with a superconducting NbN hot-electron bolometer mixer

We report on the development of a highly sensitive optical receiver for heterodyne IR spectroscopy at the communication wavelength of 1.5μm (200THz) by use of a superconducting hot-electron bolometer. The results are important for the resolution of narrow spectral molecular lines in the near-IR range for the study of astronomical objects, as well as for quantum optical tomography and fiber-optic sensing. Receiver configuration as well as fiber-to-detector light coupling designs are discussed. Light absorption of the superconducting detectors was enhanced by nano-optical antennas, which were coupled to optical fibers. An intermediate frequency (IF) bandwidth of about 3GHz was found in agreement with measurements at 300GHz, and a noise figure of about 25dB was obtained that was only 10dB above the quantum limit.

Direct Measurement of the Input RF Noise of Superconducting Hot Electron Bolometer Receivers

A good understanding of input radio-frequency (RF) noise contribution is of particular importance to the development of high-sensitivity superconducting hot electron bolometer (HEB) receivers. In terms of simulation results based on the hot spot model, we found that the output noise of HEB mixers is nearly linearly decreased with the mixer's bath temperature when the HEB mixer is operated at its optimum DC-bias regime under appropriate local-oscillator signal pumping, and the effect of this decrease on the receiver output power can be well corrected. Thus, we used an intersecting lines technique to measure the input RF noise temperature of a quasi-optical superconducting NbN HEB receiver, and the measured input RF noise temperature of the HEB receiver is around 300 K at 850 GHz. For comparison, we also evaluated the losses and equivalent noise temperatures of the quasi-optical components in the RF signal path. The evaluated input RF noise temperature is found to be smaller than the measured one.

Hot electron bolometer heterodyne receiver with a 4.7-THz quantum cascade laser as a local oscillator

We report on a heterodyne receiver designed to observe the astrophysically important neutral atomic oxygen [OI] line at 4.7448 THz. The local oscillator is a third-order distributed feedback quantum cascade laser operating in continuous wave mode at 4.741 THz. A quasi-optical, superconducting NbN hot electron bolometer is used as the mixer. We recorded a double sideband receiver noise temperature (TrecDSB) of 815 K, which is ∼7 times the quantum noise limit (hν2kB) and an Allan variance time of 15 s at an effective noise fluctuation bandwidth of 18 MHz. Heterodyne performance was confirmed by measuring a methanol line spectrum.

Heterodyne Mixing and Direct Detection Performance of a Superconducting NbN Hot-Electron Bolometer

The heterodyne mixing and direct detection performance of a superconducting NbN hot-electron bolometer (HEB) integrated with a log-spiral antenna have been thoroughly characterized. The corrected receiver noise temperature and IF gain bandwidth are approximately 800 K at 0.5 THz, 750 K at 0.85 THz and 3 GHz at its optimum bias point. In addition, both the receiver noise temperature and IF gain bandwidth were found insensitive to the bath temperature, while the bias point was fixed, in good agreement with those simulated with the hot-spot model taking account of the HEB's current-dependent resistive transition. Furthermore, the HEB's frequency response was measured by a Fourier transform spectrometer at different bias points and bath temperatures. The estimated noise equivalent power (NEP) was close to 10-13 W/Hz0.5 around the HEB's transition temperature.

Noise temperature and beam pattern of an NbN hot electron bolometer mixer at 5.25 THz

We report the measured sensitivities of a superconducting NbN hot electron bolometer (HEB) heterodyne receiver at 5.25 THz. Terahertz (THz) radiation is quasioptically coupled to a HEB mixer with a lens and a spiral antenna. Using a measurement setup with black body calibration sources and a beam splitter in vacuo, and an antireflection coated Si lens, we obtained a double sideband (DSB) receiver noise temperature (TrecDSB) of 1150 K, which is nine times hν/2k, where h is the Planck constant, ν the frequency, and k the Boltzmann constant. In addition, the measured far field beam patterns of the integrated lens antenna show nearly collimated beams from 2.5 to 5.3 THz that allow reliable measurement of TrecDSB using the vacuum setup. Our experimental results in combination with an antenna-to-bolometer coupling simulation suggest that the HEB mixer can work well at least up to 6 THz, making it suitable for next generation of high-resolution spectroscopic space telescopes and, in particular, for the detection of the neutral atomic oxygen line at 4.7 THz.

Quantum noise in a terahertz hot electron bolometer mixer

We have measured the noise temperature of a single, sensitive superconducting NbN hot electron bolometer (HEB) mixer in a frequency range from 1.6 to 5.3 THz, using a setup with all the key components in vacuum. By analyzing the measured receiver noise temperature using a quantum noise (QN) model for HEB mixers, we confirm the effect of QN. The QN is found to be responsible for about half of the receiver noise at the highest frequency in our measurements. The β-factor (the quantum efficiency of the HEB) obtained experimentally agrees reasonably well with the calculated value.

3.4 THz heterodyne receiver using a hot electron bolometer and a distributed feedback quantum cascade laser

We report a heterodyne receiver using a superconducting NbN hot electron bolometer (HEB) integrated with a tight winding spiral antenna as mixer and a distributed feedback (DFB) terahertz quantum cascade laser (QCL) operating at 3.42 THz as local oscillator. The aim is to demonstrate the readiness of both devices for the detection of OH lines at 3.5 THz in a real instrument. We show that the improved single-spot beam of the terahertz QCL can easily pump the HEB mixer. We measured a double sideband receiver noise temperature of 2100 K at the optimum local oscillator power of 290 nW. This noise temperature can be further reduced to 1100 K if we correct the loss due to the use of an uncoated lens, and the losses of the window and the air. Therefore, the combination of a HEB and such a DFB QCL can in principle be used to detect an OH line at 3.5 THz. However, a high input power of several watts, which is needed to operate the QCL in a liquid-helium cryostat, poses a big challenge to the receiver stability.

Low noise NbN hot electron bolometer mixer at 4.3THz

We have studied the sensitivity of a superconducting NbN hot electron bolometer mixer integrated with a spiral antenna at 4.3THz. Using hot/cold blackbody loads and a beam splitter all in vacuum, we measured a double sideband receiver noise temperature of 1300K at the optimum local oscillator (LO) power of 330nW, which is about 12 times the quantum noise (hν∕2kB). Our result indicates that there is no sign of degradation of the mixing process at the superterahertz frequencies. Moreover, a measurement method is introduced which allows us for an accurate determination of the sensitivity despite LO power fluctuations.

Noise Behaviour of a THz Superconducting Hot-ElectronBolometer Mixer

A quasi-optical superconducting NbN hot-electron bolometer (HEB) mixeris measured in the frequency range of 0.5–2.5 THz for understanding ofthe frequency dependence of noise temperature of THz coherentdetectors. It has been found that noise temperature increasing withfrequency is mainly due to the coupling loss between the quasi-opticalplanar antenna and the superconducting HEB bridge when taking accountof non-uniform distribution of high-frequency current. With thecoupling loss corrected, the superconducting HEB mixer demonstrates anoise temperature nearly independent of frequency.

Superconducting hot-electron bolometer mixer for terahertz heterodyne receivers

We present recent results showing the development of superconducting NbN hot-electron bolometer mixer for German receiver for astronomy at terahertz frequencies and terahertz limb sounder. The mixer is incorporated into a planar feed antenna, which has either logarithmic spiral or double-slot configuration, and backed on a silicon lens. The hybrid antenna had almost frequency independent and symmetric radiation pattern slightly broader than expected for a diffraction limited antenna. At 2.5 THz the best 2200 K double side-band receiver noise temperature was achieved across a 1 GHz intermediate frequency bandwidth centred at 1.5 GHz. For this operation regime, a receiver conversion efficiency of -17 dB was directly measured and the loss budget was evaluated. The mixer response was linear at load temperatures smaller than 400 K. Implementation of the MgO buffer layer on Si resulted in an increased 5.2 GHz gain bandwidth. The receiver was tested in the laboratory environment by measuring a methanol emission line at 2.5 THz.