166 research outputs found
Mechanism of transition to turbulence in a circular cylinder wake in a channel
© 2017 The Authors, published by EDP Sciences. Transition to turbulence in the circular cylinder wake has been studied experimentally and numerically at growing Reynolds number. Good agreement of calculation results with the flow visualization and measurements of instantaneous vector fields of velocity and vorticity has been demonstrated. The growing Reynolds number is shown to make large-scale vortex generation onset move upstream. It also triggers the transition to 3D flow pattern in the cylinder wake. This process is accompanied by non-monotonous behavior of the profiles of velocity and its turbulent fluctuations at equal distances from the cylinder. Non-monotonous behavior of the cylinder drag has been revealed for the Reynolds numbers ranging from 120 to 300
Superconducting parallel nanowire detector with photon number resolving functionality
We present a new photon number resolving detector (PNR), the Parallel
Nanowire Detector (PND), which uses spatial multiplexing on a subwavelength
scale to provide a single electrical output proportional to the photon number.
The basic structure of the PND is the parallel connection of several NbN
superconducting nanowires (100 nm-wide, few nm-thick), folded in a meander
pattern. Electrical and optical equivalents of the device were developed in
order to gain insight on its working principle. PNDs were fabricated on 3-4 nm
thick NbN films grown on sapphire (substrate temperature TS=900C) or MgO
(TS=400C) substrates by reactive magnetron sputtering in an Ar/N2 gas mixture.
The device performance was characterized in terms of speed and sensitivity. The
photoresponse shows a full width at half maximum (FWHM) as low as 660ps. PNDs
showed counting performance at 80 MHz repetition rate. Building the histograms
of the photoresponse peak, no multiplication noise buildup is observable and a
one photon quantum efficiency can be estimated to be QE=3% (at 700 nm
wavelength and 4.2 K temperature). The PND significantly outperforms existing
PNR detectors in terms of simplicity, sensitivity, speed, and multiplication
noise
Resonant Terahertz Detection Using Graphene Plasmons
Plasmons, collective oscillations of electron systems, can efficiently couple
light and electric current, and thus can be used to create sub-wavelength
photodetectors, radiation mixers, and on-chip spectrometers. Despite
considerable effort, it has proven challenging to implement plasmonic devices
operating at terahertz frequencies. The material capable to meet this challenge
is graphene as it supports long-lived electrically-tunable plasmons. Here we
demonstrate plasmon-assisted resonant detection of terahertz radiation by
antenna-coupled graphene transistors that act as both plasmonic Fabry-Perot
cavities and rectifying elements. By varying the plasmon velocity using gate
voltage, we tune our detectors between multiple resonant modes and exploit this
functionality to measure plasmon wavelength and lifetime in bilayer graphene as
well as to probe collective modes in its moir\'e minibands. Our devices offer a
convenient tool for further plasmonic research that is often exceedingly
difficult under non-ambient conditions (e.g. cryogenic temperatures and strong
magnetic fields) and promise a viable route for various photonic applications.Comment: 19 pages, 12 figure
Effect of the wire width on the intrinsic detection efficiency of superconducting-nanowire single-photon detectors
Thorough spectral study of the intrinsic single-photon detection efficiency
in superconducting TaN and NbN nanowires with different widths shows that the
experimental cut-off in the efficiency at near-infrared wavelengths is most
likely caused by the local deficiency of Cooper pairs available for current
transport. For both materials the reciprocal cut-off wavelength scales with the
wire width whereas the scaling factor quantitatively agrees with the hot-spot
detection models. Comparison of the experimental data with vortex-assisted
detection scenarios shows that these models predict a stronger dependence of
the cut-off wavelength on the wire width.Comment: 16 pages, 6 figure
Quantum interference and manipulation of entanglement in silicon wire waveguide quantum circuits
Integrated quantum photonic waveguide circuits are a promising approach to
realizing future photonic quantum technologies. Here, we present an integrated
photonic quantum technology platform utilising the silicon-on-insulator
material system, where quantum interference and the manipulation of quantum
states of light are demonstrated in components orders of magnitude smaller than
in previous implementations. Two-photon quantum interference is presented in a
multi-mode interference coupler, and manipulation of entanglement is
demonstrated in a Mach-Zehnder interferometer, opening the way to an
all-silicon photonic quantum technology platform.Comment: 7 page
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