3,716 research outputs found
Enhancement of optical response in nanowires by negative-tone PMMA lithography
The method of negative-tone-PMMA electron-beam lithography is investigated to
improve the performance of nanowire-based superconducting detectors. Using this
approach, the superconducting nanowire single-photon detectors (SNSPDs) have
been fabricated from thick 5-nm NbN film sputtered at the room temperature. To
investigate the impact of this process, SNSPDs were prepared by positive-tone
and negative-tone-PMMA lithography, and their electrical and photodetection
characteristics at 4.2 K were compared. The SNSPDs made by negative-tone-PMMA
lithography show higher critical-current density and higher photon count rate
at various wavelengths. Our results suggest a higher negative-tone-PMMA
technology may be preferable to the standard positive-tone-PMMA lithography for
this application
Measuring thickness in thin NbN films for superconducting devices
We present the use of a commercially available fixed-angle multi-wavelength
ellipsometer for quickly measuring the thickness of NbN thin films for the
fabrication and performance improvement of superconducting nanowire single
photon detectors. The process can determine the optical constants of absorbing
thin films, removing the need for inaccurate approximations. The tool can be
used to observe oxidation growth and allows thickness measurements to be
integrated into the characterization of various fabrication processes
Optimized polar-azimuthal orientations for polarized light illumination of different Superconducting Nanowire Single-Photon Detector designs
The optimal orientations are determined for polarized substrate side
illumination of three superconducting nanowire single-photon detector (SNSPD)
designs: (1) periodic niobium-nitride (NbN) stripes standing in air with
dimensions according to conventional SNSPDs, (2) same NbN patterns below
~quarter-wavelength hydrogensilsesquioxane-filled nano-cavity, (3) analogous
NbN patterns in HSQ nano-cavity closed by a thin gold reflector. Numerical
computation results have shown that the optical response and near-field
distribution vary significantly with polar-angle, fi, and these variations are
analogous across all azimuthal-angles, gamma, but are fundamentally different
in various device designs. Larger absorptance is available due to p-polarized
illumination of NbN patterns in P-structure configuration, while s-polarized
illumination results in higher absorptance in S-structure arrangement. As a
result of p-polarized illumination a global maximum appears on absorptance of
bare NbN pattern at polar angle corresponding to NbN-related ATIR; integration
with HSQ nano-cavity results in a global absorptance maximum at polar angle
corresponding to TIR at sapphire-air interface; while the highest absorptance
is observable at perpendicular incidence on P-structures aligned below gold
reflector covered HSQ nano-cavity. S-polarized light illumination results in a
global absorptance maximum at TIR on bare NbN patterns; the highest absorptance
is available below HSQ nano-cavity at polar angle corresponding to ATIR
phenomenon; while the benefit of gold reflector is large and polar angle
independent absorptance.Comment: 24 pages, 7 figure
Timing performance of 30-nm-wide superconducting nanowire avalanche photodetectors
We investigated the timing jitter of superconducting nanowire avalanche
photodetectors (SNAPs, also referred to as cascade switching superconducting
single photon detectors) based on 30-nm-wide nanowires. At bias currents (IB)
near the switching current, SNAPs showed sub 35 ps FWHM Gaussian jitter similar
to standard 100 nm wide superconducting nanowire single-photon detectors. At
lower values of IB, the instrument response function (IRF) of the detectors
became wider, more asymmetric, and shifted to longer time delays. We could
reproduce the experimentally observed IRF time-shift in simulations based on an
electrothermal model, and explain the effect with a simple physical picture
Electrothermal feedback in superconducting nanowire single-photon detectors
We investigate the role of electrothermal feedback in the operation of
superconducting nanowire single-photon detectors (SNSPDs). It is found that the
desired mode of operation for SNSPDs is only achieved if this feedback is
unstable, which happens naturally through the slow electrical response
associated with their relatively large kinetic inductance. If this response is
sped up in an effort to increase the device count rate, the electrothermal
feedback becomes stable and results in an effect known as latching, where the
device is locked in a resistive state and can no longer detect photons. We
present a set of experiments which elucidate this effect, and a simple model
which quantitatively explains the results
Frequency pulling and mixing of relaxation oscillations in superconducting nanowires
Many superconducting technologies such as rapid single flux quantum computing
(RSFQ) and superconducting quantum interference devices (SQUIDs) rely on the
modulation of nonlinear dynamics in Josephson junctions for functionality. More
recently, however, superconducting devices have been developed based on the
switching and thermal heating of nanowires for use in fields such as single
photon detection and digital logic. In this paper, we use resistive shunting to
control the nonlinear heating of a superconducting nanowire and compare the
resulting dynamics to those observed in Josephson junctions. We show that
interaction of the hotspot growth with the external shunt produces high
frequency relaxation oscillations with similar behavior as observed in
Josephson junctions due to their rapid time constants and ability to be
modulated by a weak periodic signal. In particular, we use a microwave drive to
pull and mix the oscillation frequency, resulting in phase locked features that
resemble the AC Josephson effect. New nanowire devices based on these
conclusions have promising applications in fields such as parametric
amplification and frequency multiplexing
Electric circuit networks equivalent to chaotic quantum billiards
We formulate two types of electric RLC resonance network equivalent to
quantum billiards. In the network of inductors grounded by capacitors squared
resonant frequencies are eigenvalues of the quantum billiard. In the network of
capacitors grounded by inductors squared resonant frequencies are given by
inverse eigen values of the billiard. In both cases local voltages play role of
the wave function of the quantum billiard. However as different from quantum
billiards there is a heat power because of resistance of the inductors. In the
equivalent chaotic billiards we derive the distribution of the heat power which
well describes numerical statistics.Comment: 9 pages, 7 figure
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