High-Tc superconducting microbolometer for terahertz applications

Superconducting hot electron bolometers (HEBs) are widely used as terahertz (THz) heterodyne detectors (mixers) and becoming the most advantageous candidate for low noise receiver at frequencies above 1.2 THz. However, there is still lack of accurate theoretical modeling which explains the complex mechanism of different aspects of superconducting HEBs. These difficulties do not prevent researchers around the world from making experimental progress. In the last few years, superconducting HEBs are also used as THz direct detectors to develop the array for the applications in biology, medicine and security systems. Recently, a noise equivalent power (NEP) of 0.2 pW/√Hz at 2.5 THz is reported for the superconducting HEBs with thermal biasing. Instead of thermal one, microwave (MW) which was first used to stabilize the HEB mixer is also applied to bias the device as a direct detector. Furthermore, THz can be used to bias the HEBs, if THz sources can be provided easy. Although MW biasing can improve sensitivity of the superconducting HEB direct detectors, little work has been done to compare the responsivity and NEP among the THz direct detectors with thermal, THz and MW biasing. To find a best biasing method, we have studied the similarities and differences of the direct detectors based on the superconducting HEBs with three different biasing methods. The fabricated NbN HEB detectors consist of a complementary logarithmic-spiral antenna made of gold and an NbN film (bridge) connecting across the antenna's inner terminals. We have fabricated the niobium nitride (NbN) superconducting HEB mixers with the system double sideband (DSB) noise temperature better than 8 times of the quantum limit: hf/kB (where h is the Planck constant, kB is the Boltzmann constant and f is the operating frequency) at the frequencies higher than 1 THz. Here, the mixer chips with the system DSB noise temperature of about 400 K and the intermediate frequency (IF) gain bandwidth (GBW) of larger than 5 GHz at 4.2 K and 0.65 THz have been used. As the direct detectors, the current responsivity of 290 A/W and 105 A/W are obtained for MW and THz biasing, which about one order higher than that of thermal biasing. The NEP of 2.7 pW/√Hz is obtained with MW biasing, which can be expected to be improved in the future.

Superconducting Hot Electron Bolometer (HEB) mixers are a competitive alternative to Schottky diode mixers or other conventional superconducting receiver technologies in the terahertz frequency range because of their ultrawide bandwidth (from millimeter waves to the visible), high conversion gain, and low intrinsic noise level, even at 77 K. A new technological process has been developed to realize HEB mixers based on high temperature superconducting materials, using 15 to 40 nm thick layers of YBa 2 Cu 3 O 7-δ (YBCO), sputtered on MgO (100) substrates by hollow cathode magnetron sputtering. Critical temperature values of YBCO films were found in the 85 to 91 K range. Sub-micron HEB bridges (0.8 μm x 0.8 μm) were obtained by combining electronic and UV lithography followed by selective etching techniques. Realization of YBCO HEB coupling to planar integrated gold antennas was also considered.

Hot electron bolometric mixers: new terahertz technology

In this paper we focus mainly on the investigation of the performance of a quasi-optical (planar log-spiral antenna) phonon-cooled NbN superconducting hot electron bolometer (HEB) mixer, which is cryogenically cooled by a close-cycled 4-K cryocooler, at 500 and 850 GHz frequency bands. The mixer's noise performance, stability of IF output power, and local oscillator (LO) power requirement are characterized for three NbN superconducting HEB devices of different sizes. The transmission characteristics of Mylar and Zitex films with incidence waves of an elliptical polarization are also examined by measuring the mixer's noise temperature. The lowest receiver noise temperatures (with no corrections) of 750 and 1100 K are measured at 500 and 850 GHz, respectively. Experimental results also demonstrate that the bigger the HEB device is, the higher the stability of IF output power becomes.

We present microwave (30 GHz) measurements on aluminum superconducting hot-electron bolometer (HEB) mixers. Aluminum HEB mixers have a lower superconducting transition temperature than niobium and niobium nitride devices, and are predicted to have improved sensitivity and require less local oscillator power. The devices studied consist of a narrow superconducting aluminum microbridge with contacts comprised of thick aluminum, titanium, and gold. A perpendicular magnetic field is used to suppress superconductivity in the contacts. The device length is much shorter than the electron–phonon inelastic length [L≪(Dτe–ph)1/2] and thus the intermediate frequency (IF) bandwidth is set by the diffusion time of hot electrons from the microbridge into the contacts. We discuss limitations on the IF bandwidth in very short HEB mixers with fully normal contacts. Overall, the microwave heterodyne mixing results show good performance with a mixer noise temperature inferred from the conversion efficiency and output noise of TM⩾4 K, double side band (DSB). Measurements at 618 GHz yield a conversion efficiency approximately 10 dB lower than observed at 30 GHz. Output saturation by background radiation is postulated to contribute significantly to this difference. A similar decrease in conversion efficiency in the microwave mixing measurements is observed when broadband noise is added to the rf input signal. We discuss the tradeoff between increased sensitivity and the occurrence of saturation effects in low transition temperature HEBs.

NbN phonon-cooled hot electron bolometer mixers (HEBs) have been realized withnegligible contact resistance between the bolometer itself and the contact structure. Usinga combination of in situ cleaning of the NbN film and the use of an additionalsuperconducting interlayer of a 10 nm NbTiN layer between the Au of the contactstructure and the NbN film superior noise temperatures have been obtained aslow as 950 K at 2.5 THz and 750 K at 1.9 THz. Here we address in detail the DCcharacterization of these devices, the interface transparencies between the bolometers andthe contacts and the consequences of these factors on the mixer performance.

We have demonstrated the phase-locking of a THz quantum cascade laser (THz-QCL) toward the realization of an accurate and stable local oscillator for a high-resolution receiver. The beat note between the THz-QCL and a THz reference was obtained by heterodyne mixing in a superconducting hot electron bolometer mixer (HEBM) and used for stabilizing the phase of 3.1 THz radiation from the THz-QCL. The phase-locked 3.1 THz radiation was fully evaluated with a superlattice harmonic mixer operating in the THz band, and this revealed that the THz-QCL synchronized with the microwave reference with a fractional frequency instability of 3×10 -15 at an averaging time of 100 s, corresponding to a center frequency deviation within 1 mHz, and the imposed phase noises during the heterodyne mixing were negligibly small.

In this paper, we present our studies on a technology demonstrator for a balanced waveguide hot-electron bolometer (HEB) mixer operating in the 1.6–2.0 THz band. The design employs a novel layout for the HEB mixer combining several key technologies: all-metal THz waveguide micromachining, ultra-thin NbN film deposition and a micromachining of a silicon-on-insulator (SOI) substrate to manufacture the HEB mixer. In this paper, we present a novel mixer layout that greatly facilitates handling and mounting of the mixer chip via self-aligning as well as provides easy electrical interfacing. In our opinion, this opens up a real prospective for building multi-pixel waveguide THz receivers. Such receivers could be of interest for SOFIA, possible follow up of the Herschel HIFI, and even for ground based telescopes yet over limited periods of time with extremely dry weather (PWV less than 0.1 mm).

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.

In this paper we study the effect of NbN-Au contact pads on the performance of a superconducting NbN hot electron bolometer (HEB) mixer. The receiver noise temperature (Trec) and IF noise bandwidth (BN) at a LO frequency of 1.3 THz are measured at different bath temperatures between 3.3 K and 8.5 K. The measured Trec and BN of the superconducting HEB mixer are nearly constant for Tbath/Tc less than 0.7, and increase quickly with the further increase of Tbath. Measurement results demonstrate that the NbN-Au contact pads in superconducting state nearly have no influence on Trec and Bn, but the contact pads in resistive state make Trec and BN increase quickly with Tbath, indicated by the slope change of current-voltage curves and IF output power peak.

There is a strong need for wideband and sensitive THz receivers for radio astronomy and remote sensing applications, for which superconducting Hot Electron Bolometer (HEB) mixers are very competitive. Besides, new THz applications have arisen because of interesting interaction with various media, for which room temperature detectors are highly attractive. We have used YBa2Cu3O7-d (YBCO) oxides to fabricate bolometers, either of high-Tc superconducting HEB type (high oxygen content, d 0.5). Firstly, we fabricated HEBs made from superconducting YBCO ultrathin films (15 to 40 nm thick) etched to form submicrometer constrictions. In order to investigate the feasibility of highly sensitive HEB linear arrays for passive THz imaging applications, extensive technological runs were performed to prevent ageing effects on both the pixel electrical and optical characteristics. Secondly, we designed YBCO semiconducting bolometric pixels for room temperature operation. Due to the reduced sensitivity and bandwidth with respect to superconducting HEBs, we considered the feasibility of 2D arrays for active THz imaging. As a first experimental step, pixel responsivity and thermal crosstalk between pixels were studied in the 1 Hz to 100 kHz modulation frequency range, so to evaluate the adequate frame refreshing rate.

The stability of a Hot Electron Bolometer (HEB) mixer can be improved by the use of microwave injection. In this article we report a refinement of this approach. We introduce a microchip controller to facilitate the implementation of the stabilization scheme, and demonstrate that the feedback loop effectively suppresses drifts in the HEB bias current, leading to an improvement in the receiver stability. The measured Allan time of the mixer's IF output power is increased to > 10 s.

The intermediate frequency (IF) bandwidth of a hot electron bolometer (HEB) mixer is an important parameter of the mixer, in that it helps to determine its suitability for a given application. With the availability of wideband low noise amplifiers, it is simple to measure the performance of an HEB mixer over a wide range of IF at a fixed LO frequency using the standard Y-factor method. This in-situ method allows us to measure both the gain and noise bandwidths simultaneously. We have also measured mixer output impedance with a vector network analyser. Intrinsic time constant has been extracted from the impedance data and compared to the mixer's bandwidths determined from receiver Y-factor measurement.

We report on a scheme whereby the local-oscillator (LO) of a THz heterodyne receiver can be phase-locked by the mixer of the heterodyne receiver. This scheme is demonstrated for the phase-locking of an 847.6 GHz Gunn oscillator and multiplier chain combined source with a superconducting hot-electron bolometer (HEB) mixer. We show that with this technique the phase-locked beat signal can reach a signal-to-noise ratio higher than 70 dB in a resolution bandwidth (RBW) of 1 Hz. This phase-locking scheme should find good use in THz heterodyne spectrometers.

GaAs devices and circuits for terahertz applications