Sketch of the HEB structure. It shows the NbN bridge in between the contact pads which consist of NbTiN and Au. Also shown is (part of) the thick Au antenna layer on the NbN film and partly on the contact pads.
SEM picture showing a top view of the twin-slot antenna coupled HEB mixer and part of the filter structure. The dark structure in the center of the twin slot is the remaining e-beam resist used as an etching mask for the NbN bridge. The antenna is designed for a center frequency of . Relevant parameters are marked by white bars.
Resistance vs temperature of the small area HEB. is the normal state resistance at , the normal state resistance of the antenna, the resistance just above with the critical temperatures of the stack layer consisting of the NbN film with contact pad on top, the critical temperatures of the antenna, and the critical temperatures of the NbN bridge, defined at . Inset: zoomed SEM micrograph of the HEB bridge between the contact pads covered by an e-beam resist layer.
(a) Normalized direct response of the twin-slot antenna coupled HEB mixer measured (corrected for optics) with a Fourier-transform spectrometer. The calculated response (normalized) is also included for comparison. (b) Measured (normalized) and calculated beam pattern (-plane) and fundamental mode Gaussian fit of the uncoated antenna-lens combination at .
Schematical picture of the experimental setup.
(a) Uncorrected double sideband receiver noise temperature at an LO frequency of in a two-dimensional plot of voltage vs current. The latter is a function of the applied LO power. The minimum value is . Contour lines indicate absolute values of . (b) The direct detection current in a two-dimensional plot of voltage vs current. Contour lines indicate absolute values of .
The double sideband receiver noise temperature at in the small signal limit, . The minimum value of is . The contour lines indicate absolute values of .
Uncorrected double sideband receiver noise temperature at in a two-dimensional plot of bias voltage vs the estimated LO power at the lens front of the mixer unit.
DSB receiver noise temperature as a function of IF frequency at at the optimal operating point of the mixer. Dark gray line: using the MMIC LNA (by Weinreb) and isolator. Black line: Without the isolator but with the Kuo-Liang LNA. The dotted line represents a single pole roll-off at , with at zero frequency of . Inset: Original noise bandwidth measurement (gray line) of the actual device but using the old design bias board and a single pole roll-off at (black dotted line).
Mixer gain bandwidth obtained at using the Weinreb MMIC LNA and a hot/cold load technique. Asterisk symbols: Mixer gain bandwidth at measured using two coherent sources. The dotted line represents a single pole fit with a roll-off at .
Continuum Allan variance of the small area NbN HEB receiver operated at obtained in a bandwidth around 2.6, 2.7, and . The theoretical white noise behavior and stability behavior of the HEB driven into normal state by high dc bias are included for comparison.
Optical losses in the LO path measured at .
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