Hot electron bolometers (HEB) exploiting the properties of the high-TC superconductor (HTS) Y-Ba-Cu-O, are offering a competitive alternative to THz Schottky mixers, which require moderate cooling (e.g., 60 to 80 K). This arises from the HTS HEBs expected wide bandwidth (tens of GHz), and low local oscillator (LO) power requirements: tens of microwatts, whereas several milliwatts are necessary to pump Schottky diode LOs efficiently. In fact, the large instantaneous bandwidth is related to the extremely short electron to phonon relaxation time (1 to 2 ps, typically) in YBa- Cu-O. It is much longer in low-TCsuperconductors (LTS), e.g., about 20 ns in NbN. Besides, as required for LTS materials, it is mandatory to fabricate high quality and ultra thin HTS epitaxial films, so to process nano-bolometers exhibiting good mixing performances (i.e., double sideband noise temperature Tn and conversion gain G). Most of all, the challenge for Y-Ba-Cu-O remains the chemical reactivity and the aging effects, as reported previously. The main objectives of this paper are: i) to predict Y-Ba-Cu-O HEB heterodyne mixer performances, and ii) to exploit those predictions to simulate a stand-off passive detection system for, e.g., screening or security applications.
In the THz range, high-TC superconductor (HTS) hot electron bolometers (HEB) are offering a competitive alternative to moderately cooled (e.g., 60 to 80 K) Schottky mixers. This is due to HTS HEBs large expected bandwidth (tens of GHz), and low local oscillator power requirements (tens of microwatts, as compared to several milliwatts required for Schottky diode pumping). Indeed, the large instantaneous bandwidth is driven by the very short electron to phonon relaxation time in Y-Ba-Cu-O HTS oxide − 1 to 2 ps, typically, whereas it is about 20 ns in NbN, a low- TC superconductor (LTS). Besides, as for the LTS counterparts, it is mandatory to grow ultra thin high quality HTS epitaxial films, in order to process micro or nano-bolometers (nano-constrictions) exhibiting good mixing performances. Early HEB models were based on the point bolometer approach, which describes the device in terms of thermal reservoirs only. We have extended the hot spot model (initially introduced for LTS HEBs) to Y-Ba-Cu-O HEBs, taking into account the spatial dependence of the electron and phonon temperatures along the nano-constriction. We have also introduced the THz frequency effects in the Y-Ba-Cu-O superconducting transition as well as the impedance matching between the nanoconstriction and the antenna. We have checked the feasibility of stand-off target detection operating in the passive mode with an Y-Ba-Cu-O HEB THz heterodyne mixer. For instance, detection at 5 m through cotton cloth in passive imaging mode could be readily achieved in standard humidity conditions with 10 K resolution at 2.5 THz.
High-TC superconducting (HTS) hot electron bolometers (HEB) are promising THz mixers due to their large expected
bandwidth and low local oscillator (LO) power requirements at 60-80 K operating temperature. To obtain HEB efficient
mixing, it is mandatory to grow very thin high quality HTS films leading to good micro or nano-bolometer
superconducting properties. The challenge for Y-Ba-Cu-O resides, however, in the chemical reactivity of the material
and the related aging effects. Early HEB models described the device in terms of thermal reservoirs only, namely the
electrons and the phonons of the superconductor. The electron-phonon interaction time, which drives the HEB mixer
ultimate response, is 1-2 ps for Y-Ba-Cu-O, with an expected bandwidth close to 100 GHz. Recently, we introduced the
hot spot model for Y-Ba-Cu-O HEBs, taking - more realistically - the spatial dependence of the electron temperature
along the nano-bolometer (or constriction) length into account. From DC analysis, the I-V characteristics could be
deduced. In this paper, we further consider a full description of the constriction impedance at THz frequencies, which
allows to work out the mixer performance in terms of double sideband noise temperature TDSB and conversion gain G.
For a constriction of technologically achievable dimensions, i.e., 400 nm long x 400 nm wide x 35 nm thick, minimum
TDSB = 1900 K at 9 μW LO power, with G = -9.5 dB, is obtained at 400 GHz, assuming impedance matching with a selfcomplementary
planar antenna.
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