94 research outputs found
Thermoelectric properties of the bismuth telluride nanowires in the constant-relaxation-time approximation
Electronic structure of bismuth telluride nanowires with the growth
directions [110] and [015] is studied in the framework of anisotropic effective
mass method using the parabolic band approximation. The components of the
electron and hole effective mass tensor for six valleys are calculated for both
growth directions. For a square nanowire, in the temperature range from 77 K to
500 K, the dependence of the Seebeck coefficient, the electron thermal and
electrical conductivity as well as the figure of merit ZT on the nanowire
thickness and on the excess hole concentration are investigated in the
constant-relaxation-time approximation. The carrier confinement is shown to
play essential role for square nanowires with thickness less than 30 nm. The
confinement decreases both the carrier concentration and the thermal
conductivity but increases the maximum value of Seebeck coefficient in contrast
to the excess holes (impurities). The confinement effect is stronger for the
direction [015] than for the direction [110] due to the carrier mass difference
for these directions. The carrier confinement increases maximum value of ZT and
shifts it towards high temperatures. For the p-type bismuth telluride nanowires
with growth direction [110], the maximum value of the figure of merit is equal
to 1.3, 1.6, and 2.8, correspondingly, at temperatures 310 K, 390 K, 480 K and
the nanowire thicknesses 30 nm, 15 nm, and 7 nm. At the room temperature, the
figure of merit equals 1.2, 1.3, and 1.7, respectively.Comment: 13 pages, 7 figures, 2 tables, typos added, added references for
sections 2-
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
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
Superconducting nanowire single-photon detectors: physics and applications
Single-photon detectors based on superconducting nanowires (SSPDs or SNSPDs)
have rapidly emerged as a highly promising photon-counting technology for
infrared wavelengths. These devices offer high efficiency, low dark counts and
excellent timing resolution. In this review, we consider the basic SNSPD
operating principle and models of device behaviour. We give an overview of the
evolution of SNSPD device design and the improvements in performance which have
been achieved. We also evaluate device limitations and noise mechanisms. We
survey practical refrigeration technologies and optical coupling schemes for
SNSPDs. Finally we summarize promising application areas, ranging from quantum
cryptography to remote sensing. Our goal is to capture a detailed snapshot of
an emerging superconducting detector technology on the threshold of maturity.Comment: 27 pages, 5 figures, Review article preprint versio
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