282 research outputs found
Shot noise in the chaotic-to-regular crossover regime
We investigate the shot noise for phase-coherent quantum transport in the
chaotic-to-regular crossover regime. Employing the Modular Recursive Green's
Function Method for both ballistic and disordered two-dimensional cavities we
find the Fano factor and the transmission eigenvalue distribution for regular
systems to be surprisingly similar to those for chaotic systems. We argue that
in the case of regular dynamics in the cavity, diffraction at the lead openings
is the dominant source of shot noise. We also explore the onset of the
crossover from quantum to classical transport and develop a quasi-classical
transport model for shot noise suppression which agrees with the numerical
quantum data.Comment: 4 pages, 3 figures, submitted to Phys.Rev.Let
Parscale - an open-source library for the simulation of intra-particle heat and mass transport processes in coupled simulations
We introduce the open-source library ParScale for the modeling of intra-particle transport processes in non-isothermal reactive fluid-particle flows. The underlying equations, the code architecture, as well as the coupling strategy to the widely-used DEM solver LIGGGHTS is presented. A set of verification cases, embedded into an automated test harness, is presented that proofs the functionality of ParScale. To demonstrate the capabilities of ParScale, we perform simulations of a non-isothermal granular shear flow including heat transfer to the surrounding fluid. We present results for the conductive heat flux through the particle bed for a wide range of dimensionless cooling rates and particle volume fractions. Our data suggests that intra-particle temperature gradients need to be considered for an accurate prediction of the
conductive flux in case of (i) a dense particle bed and (ii) for large cooling rates characterized by a critical Biot number of ca Bicrit ≈ 0.1
Determining the coefficient of friction by shear tester simulation
The flow behaviour of very dense particle regimes such as in a moving or fluidized bed is highly dependent on the inter-particle friction, which can be characterized by the coefficient of friction. Since only rough guide values for common material pairs are available in the literature, we determine the exact parameters by fitting numerical simulations to experimental measurements of a simplified Jenike shear tester [1, 2]. The open-source discrete-element-method code LIGGGHTS [3] is used to model the shear cell, which is built of triangulated meshes. In order to preload the bulk solid in the shear cell with a constant principal stress, the movement of these walls is controlled by a prescribed load. A comprehensive sensitivity study shows that the results are nearly insensitive to the spatial dimensions of the shear tester as well as all other material properties. Therefore, this set-up is applicable to determine the coefficient of friction. Furthermore, we calculate the coefficient of friction of glass beads showing very good agreement with literature data and in-house experiments. Hence, this procedure can be used to deduce material parameters for the numerical simulation of dense granular flows
Mirror-coupled plasmonic bound states in the continuum for tunable perfect absorption
Tailoring critical light-matter coupling is a fundamental challenge of
nanophotonics, impacting diverse fields from higher harmonic generation and
energy conversion to surface-enhanced spectroscopy. Plasmonic perfect absorbers
(PAs), where resonant antennas couple to their mirror images in adjacent metal
films, have been instrumental for obtaining different coupling regimes by
tuning the antenna-film distance. However, for on-chip uses, the ideal PA gap
size can only match one wavelength, and wide range multispectral approaches
remain challenging. Here, we introduce a new paradigm for plasmonic PAs by
combining mirror-coupled resonances with the unique loss engineering
capabilities of plasmonic bound states in the continuum (BICs). Our BIC-driven
PA platform leverages the asymmetry of the constituent meta-atoms as an
additional degree of freedom for reaching the critical coupling (CC) condition,
delivering resonances with unity absorbance and high quality factors
approaching 100 in the mid-infrared. Such a platform holds flexible tuning
knobs including asymmetry parameter, dielectric gap, and geometrical scaling
factor to precisely control the coupling condition, resonance frequency, and
selective enhancement of magnetic and electric fields while maintaining CC. We
demonstrate a pixelated PA metasurface with optimal absorption over a broad
range of mid-infrared frequencies (950 ~ 2000 1/cm) using only a single spacer
layer thickness and apply it for multispectral surface-enhanced molecular
spectroscopy in tailored coupling regimes. Our concept greatly expands the
capabilities and flexibility of traditional gap-tuned PAs, opening new
perspectives for miniaturized sensing platforms towards on-chip and in-situ
detection.Comment: Main text and supporting information, 31 pages, 5 Figures manuscript
+ 11 Supporting Figure
Continuous spectral and coupling-strength encoding with dual-gradient metasurfaces
Enhancing and controlling light-matter interactions is crucial in
nanotechnology and material science, propelling research on green energy, laser
technology, and quantum cryptography. Central to enhanced light-matter coupling
are two parameters: the spectral overlap between an optical cavity mode and the
material's spectral features (e.g., excitonic or molecular absorption lines),
and the quality factor of the cavity. Controlling both parameters
simultaneously is vital, especially in complex systems requiring extensive data
to uncover the numerous effects at play. However, so far, photonic approaches
have focused solely on sampling a limited set of data points within this 2D
parameter space. Here we introduce a nanophotonic approach that can
simultaneously and continuously encode the spectral and quality factor
parameter space of light-matter interactions within a compact spatial area. Our
novel dual-gradient metasurface design is composed of a 2D array of smoothly
varying subwavelength nanoresonators, each supporting a unique mode. This
results in 27,500 distinct modes within one array and a resonance density
approaching the theoretical upper limit for metasurfaces. By applying our
dual-gradient to surface-enhanced molecular sensing, we demonstrate the
importance of coupling tailoring and unveil an additional coupling-based
dimension of spectroscopic data. Our metasurface design paves the way for
generalized light-matter coupling metasurfaces, leading to advancements in the
field of photocatalysis, chemical sensing, and entangled photon generation.Comment: 5 figures, 5 supplementary notes, 14 supplementary figure
Mirror‐Coupled Plasmonic Bound States in the Continuum for Tunable Perfect Absorption
Tailoring critical light-matter coupling is a fundamental challenge of nanophotonics, impacting fields from higher harmonic generation and energy conversion to surface-enhanced spectroscopy. Plasmonic perfect absorbers (PAs), where resonant antennas couple to their mirror images in adjacent metal films, excel at obtaining different coupling regimes by tuning the antenna-film gap size. However, practical PA applications require constant gap size, making it impossible to maintain critical coupling beyond singular wavelengths. Here, a new approach for plasmonic PAs is introduced by combining mirror-coupled resonances with the unique loss engineering capabilities of plasmonic quasi-bound states in the continuum. This novel combination allows to tailor the light–matter interaction within the under-coupling, over-coupling, and critical coupling regimes using flexible tuning knobs including asymmetry parameter, dielectric gap, and geometrical scaling factor. The study demonstrates a pixelated PA metasurface with optimal absorption over a broad range of mid-infrared wavenumbers (950–2000 cm−1) using only a single gap size and applies it for multispectral surface-enhanced molecular spectroscopy. Moreover, the asymmetry parameter enables convenient adjustment of the quality factor and resonance amplitude. This concept expands the capabilities and flexibility of traditional gap-tuned PAs, opening new perspectives for miniaturized sensing platforms towards on-chip and in situ detection
“Switch-Off” of Respiratory Sinus Arrhythmia Can Occur in a Minority of Subjects During Functional Magnetic Resonance Imaging (fMRI)
A group of 23 healthy scanner naïve participants of a functional magnetic resonance imaging (fMRI) study with increased state anxiety exhibited 0.1 Hz oscillations in blood-oxygenation-level-dependent (BOLD) signals, heart rate (HR) beat-to-beat intervals (RRI) and respiration. The goal of the present paper is to explore slow oscillations in respiration and RRI and their phase-coupling by applying the dynamic “wave-by-wave” analysis. Five participants with either high or moderate levels of fMRI-related anxiety (age 23.8 ± 3.3y) were found with at least one bulk of consecutive breathing waves with a respiration rate between 6 to 9 breaths/min in a 5-min resting state. The following results were obtained: (i) Breathing oscillations with dominant frequencies at 0.1 Hz and 0.15 Hz displayed a 1:1 coupling with RRI. (ii) Inspiration time was significantly longer than expiration time. (iii) RRI minima (start of HR decrease) coincided with the early inspiration, and RRI maxima (start of HR increase) coincided with the late inspiration. (iv) RRI rhythm led over the respiratory rhythm. This phase-coupling pattern is quite contrary to typical respiratory sinus arrhythmia where HR increases during inspiration and decreases during expiration
Plasmonic Bound States in the Continuum to Tailor Light-Matter Coupling
Plasmon resonances play a pivotal role in enhancing light-matter interactions
in nanophotonics, but their low-quality factors have hindered applications
demanding high spectral selectivity. Even though symmetry-protected bound
states in the continuum with high-quality factors have been realized in
dielectric metasurfaces, impinging light is not efficiently coupled to the
resonant metasurfaces and is lost in the form of reflection due to low
intrinsic losses. Here, we demonstrate a novel design and 3D laser nanoprinting
of plasmonic nanofin metasurfaces, which support symmetry-protected bound
states in the continuum up to 4th order. By breaking the nanofins out-of-plane
symmetry in parameter space, we achieve high-quality factor (up to 180) modes
under normal incidence. We reveal that the out-of-plane symmetry breaking can
be fine-tuned by the triangle angle of the 3D nanofin meta-atoms, opening a
pathway to precisely control the ratio of radiative to intrinsic losses. This
enables access to the under-, critical-, and over-coupled regimes, which we
exploit for pixelated molecular sensing. Depending on the coupling regime we
observe negative, no, or positive modulation induced by the analyte, unveiling
the undeniable importance of tailoring light-matter interaction. Our
demonstration provides a novel metasurface platform for enhanced light-matter
interaction with a wide range of applications in optical sensing, energy
conversion, nonlinear photonics, surface-enhanced spectroscopy, and quantum
optics.Comment: 33 pages, 4 figures, 9 supplementary figure
- …