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

High-TC THz HEB mixers: Progress and prospects

Boris S. Karasik, Daniel Cunnane, Andrei V. Sergeev

Published Aug 1, 2015• 0 citations
Publisher

Abstract

We analyze the pathways for achieving the THz hot-electron bolometer (HEB) mixers using high-TCsuperconductors. Requirements to the material in order to obtain large (up to 10 GHz) intermediate frequency bandwidth as well as recent results on MgB2HEB mixer devices are discussed. Based on a thermal model of the energy relaxation in a thin film, we explain the difference between the mixing behaviors in previously studied HEB materials. We also introduce the concept of the HEB mixer based on the low electron density MBE-grown quasi-2DEG LaCuO/LaSrCuO superconductors with tunable critical temperature.

DOI
10.1109/irmmw-thz.2015.7327411

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Photonics•Jan 25, 2019•6 citations
We analyze the pathways for achieving the THz hot-electron bolometer (HEB) mixers using high-TCsuperconductors. Requirements to the material in order to obtain large (up to 10 GHz) intermediate frequency bandwidth as well as recent results on MgB2HEB mixer devices are discussed. Based on a thermal model of the energy relaxation in a thin film, we explain the difference between the mixing behaviors in previously studied HEB materials. We also introduce the concept of the HEB mixer based on the low electron density MBE-grown quasi-2DEG LaCuO/LaSrCuO superconductors with tunable critical temperature.
High-TCsuperconductor (YBaCuO) hot electron bolometers (HEB) are promising THz mixers due to the large predicted bandwidth and the low local oscillator (LO) power requirement. Recently, we introduced the hot-spot model, initially dedicated to low-TCNbN devices, to predict YBaCuO mixer performance. Major improvements to the model have been introduced: local description of the power absorption along the HEB constriction, THz dependence of the resistive transition, impedance matching between the THz antenna and the constriction. Mixer noise temperature TNand conversion gain G were predicted up to 2.5 THz. For instance, TN= 2700 K (DSB) and G = -13 dB are obtained with 9 μW of LO power at 1 THz.
Superconducting hot-electron bolometers (HEB) represent a promising candidate for heterodyne mixing at frequencies exceeding 1 THz. Nb HEB mixers offer performance competitive with tunnel junctions without the frequency limit imposed by the superconducting energy gap. Although the performance of YBa/sub 2/Cu/sub 3/O/sub 7-/spl delta// HEB mixers is not projected to be superior to that of Nb devices, which operate at low temperatures, they introduce the possibility of sensitive, low power heterodyne detectors operating at temperatures approaching 90 K for applications requiring portability and closed-cycle refrigeration. We report on the fabrication and characterization, both DC and RF, of high-T/sub c/ mixers based on ultra-thin (/spl les/20 nm) YBa/sub 2/Cu/sub 3/O/sub 7-/spl delta// films patterned to micrometer dimensions and incorporated into 2.5 THz planar mixer circuits.
This work is devoted to the investigation of the applicability of NbZr as a new material for wideband hot- electron bolometer (HEB) mixer, which is a device of choice for most of low noise heterodyne receivers operating above 1 THz in astronomy applications. Although currently 10-nm NbZr films offer a maximum gain bandwidth of only 450 MHz at Tc on sapphire substrates, there is every reason to believe that the above result can be improved by decreasing the film thickness and using other substrate material which will provide better acoustic match with NbZr. operation can be described by relatively simple equations for the heat balance, and the mixer's roll-off frequency is completely determined by several time constants of the film material ((10),(11)). The values of these characteristic times can be obtained from the dependence of the gain bandwidth on temperature and film thickness. For the electron temperature to be uniform throughout the bridge it is necessary that either the LO frequency be greater than the superconductor gap frequency, or that the ambient temperature be about Tc (in the latter case the LO frequency need not exceed the gap frequency of the unheated superconductor). Also the bolometer bias should be small and the power levels of the oscillators should not influence the operating point of the bolometer. In this paper we study deposition process of ultrathin NbZr films on sapphire substrates, gain bandwidth of NbZr bolometers and the possibly of using this material as a new material for HEB mixers.
Diffusion-cooled aluminum hot-electron bolometer (HEB) mixers are of interest for low-noise high resolution THz-frequency spectroscopy within astrophysics. Al HEB mixers offer operation with an order of magnitude less local oscillator power, higher intermediate frequency bandwidth and potentially lower noise than competing devices made from other materials. We report on mixer experiments at 618 GHz with devices fabricated from films with sheet resistances in the range from about 55 /spl Omega/ down to about 9 /spl Omega/ per square. Intermediate frequency bandwidths of up to 3 GHz were measured (1 /spl mu/m long device), with absorbed local oscillator power levels of 0.5 to 6 nW and mixer conversion up to -21.5 dB. High input coupling efficiency implies that the electrons in the device are able to thermalize before escaping from the device. It was found that the long coherence length complicates mixer operations due to the proximity of the contact pads. Also, saturation at the IF frequency may be a concern for this type of device, and warrants further studies.
Hot electron bolometers (HEB) made from high-TC superconducting YBa2Cu3O7–x (YBCO) oxide nano-constrictions are promising THz mixers, due to their expected wide bandwidth, large mixing gain, and low intrinsic noise. The challenge for YBCO resides, however, in the chemical reactivity of the material and the related aging effects. In this paper, we model and simulate the frequency dependent performance of YBCO HEBs operating as THz mixers. We recall first the main hypotheses of our hot spot model taking into account both the RF frequency effects in the YBCO superconducting transition and the nano-constriction impedance at THz frequencies. The predicted performance up to 4 THz is given in terms of double sideband noise temperature TDSB and conversion gain G. At 2.5 THz for instance, TDSB ≅ 1000 K and G ≅ − 6 dB could be achieved at 12.5 μW local oscillator power. We then consider a standoff target detection scheme and examine the feasibility with YBCO devices. For instance, detection at 3 m through cotton cloth in passive imaging mode could be readily achieved in moderate humidity conditions with 10 K resolution.