arXiv subjects

S. Stengel

Publications and source records attributed to S. Stengel.

10 recordsLinked to original sources

arXiv2025

Quantum dots emission enhancement via coupling with an epsilon-near-zero sublayer

Quantum emitters operating at telecom wavelengths are essential for the advancement of quantum technologies, particularly in the development of integrated on-chip devices for quantum computing, communication, and sensing. Coupling resonant structures to a near-zero-index (NZI) environment has been shown to enhance their optical performance by both increasing spontaneous emission rates and improving emission directionality. In this work, we comparatively study emission characteristics of colloidal PbS/CdS (core/shell) quantum dots at telecom wavelengths on different substrates, where two different sets of quantum dots emitting within and outside the epsilon-near-zero region are deposited on both glass and indium tin oxide (ITO) substrates. Our results demonstrate that coupling quantum dots to the epsilon-near-zero spectral region results in a reduction of photoluminescence lifetime of 54~times, a 7.5-fold increase in saturation intensity, and a relative emission cone narrowing from 17.6{\deg} to 10.3{\deg}. These results underline the strong dependence of quantum dot emission properties on the spectral overlap with the epsilon-near-zero condition, highlighting the potential of transparent conducting oxides (TCOs), such as ITO, for integration into next-generation quantum photonic devices. Due to their CMOS compatibility, fabrication tunability, and high thermal and optical damage thresholds, TCO NZI materials offer a robust platform for scalable and high-performance quantum optical systems operating within the telecom bandwidth.

physics.optics↗

arXiv2025

Beam-Normal Single-Spin Asymmetry in $^{208}$Pb at low energy: discrepancy resolved or new kinematic puzzle?

A longstanding discrepancy between measured and predicted beam-normal single-spin asymmetries $A_{n}$ in elastic electron scattering off $^{208}$Pb has challenged our understanding of two-photon exchange (TPE) in heavy nuclei. We report a new measurement at 570 MeV and $Q^2$=0.04 $GeV^2$/$c^2$, yielding$A_n = (-9.1 \pm 2.1~\text{(stat)} \pm 0.7~\text{(syst)})~\mathrm{ppm}$. This nonzero value contrasts with previous results at higher energies and suggests a kinematic dependence of TPE effects not captured by current theory, prompting a reevaluation of earlier interpretations.

nucl-ex↗

arXiv2025

Measurement of the $^{40}$Ar(e,e$^{\prime}$) elastic scattering cross section with a novel gas-jet target

We report on a measurement of elastic electron scattering on argon performed with a novel cryogenic gas-jet target at the Mainz Microtron accelerator MAMI. The luminosity is estimated with the thermodynamical parameters of the target and by comparison to a calculation in distorted-wave Born approximation. The cross section, measured at new momentum transfers of 1.24 fm$^{-1}$ and 1.55 fm$^{-1}$ is in agreement with previous experiments performed with a traditional high-pressure gas target, as well as with modern ab-initio calculations employing state-of-the-art nuclear forces from chiral effective field theory. The nearly background-free measurement highlights the optimal properties of the gas-jet target for elements heavier than hydrogen, enabling new applications in hadron and nuclear physics.

physics.ins-det↗

arXiv2024

High-harmonic generation from subwavelength silicon films

Recent years have witnessed significant developments in the study of nonlinear properties of various optical materials at the nanoscale. However, in most cases experimental results on harmonic generation from nanostructured materials are reported without the benefit of suitable theoretical models and appropriate comparisons to assess enhancement of conversion efficiencies compared to the intrinsic properties of a given material. Here, we report experimental observations of even and odd optical harmonics generated from a suspended subwavelength silicon film, a dielectric membrane, up to the 7th harmonic tuned deep in the UV range at 210nm, which is the current limit of our detection system, using peak power densities of order 3TW/cm^2. We explain the experimental data with a time domain, hydrodynamic-Maxwell approach broadly applicable to most materials. Our approach accounts simultaneously for surface and magnetic nonlinearities that generate even optical harmonics, as well as linear and nonlinear material dispersions beyond the third order to account for odd optical harmonics, plasma formation, and a phase locking mechanism that makes the generation of high harmonics possible deep into the UV range, where semiconductors like silicon start operating in a metallic regime.

physics.optics↗

arXiv2023

Nuclear density dependence of polarization transfer in quasi-elastic ${\rm A}(\vec{e},e' \vec{p})$ reactions

The ratio of the transverse and longitudinal component of polarization transfer to protons in the quasi-elastic $(\vec{e}, e^{\prime} \vec{p}\,)$ reaction, $P^{\prime}_x/P^{\prime}_z$, is sensitive to the proton's electromagnetic form factor ratio, $G_E/G_M$. To explore density-dependent in-medium modifications, a comparison of polarization transfer ratios involving protons from distinct nuclear shells, each with different local nuclear densities, has been proposed. In this study, we present such comparisons between four shells, $1s_{1/2}$, $1p_{3/2}$ in $^{12}\mathrm{C}$ and $1d_{3/2}$, $2s_{1/2}$ in $^{40}\mathrm{Ca}$. In an effort to account for other many-body effects that may differ between shells, we use a state-of-the-art relativistic distorted-wave impulse-approximation (RDWIA) calculation and present the double ratios, $(P^{\prime}_x/P^{\prime}_z)_{\rm Data}/(P^{\prime}_x/P^{\prime}_z)_{\rm RDWIA}$ as well as the super ratios, $\left[(P^{\prime}_x/P^{\prime}_z)_{\rm A}/(P^{\prime}_x/P^{\prime}_z)_{\rm B}\right]_{\rm Data}/\left[(P^{\prime}_x/P^{\prime}_z)_{\rm A}/(P^{\prime}_x/P^{\prime}_z)_{\rm B}\right]_{\rm RDWIA}$, for chosen shells A and B, as a function of effective local nuclear densities. We find that double ratios for individual shells show a dependence on the probed effective nuclear densities. Studying the ratios, we observed a systematic variation between pairs of higher- and lower-density shells.

nucl-ex↗

arXiv2023

Measurement of polarization transfer in the quasi-elastic $^{40}{\rm Ca}(\vec{e},e' \vec{p})$ process

Polarization transfer to a bound proton in polarized electron knock-out reactions, $\mathrm{A}(\vec{e},e^{\prime}\vec{p})$, is a powerful tool to look for in-medium modification of the bound proton. It requires comparison to calculations which consider the many-body effects accompanying the quasi-free process. We report here measured components $P_x^{\prime}$, $P_z^{\prime}$, and their ratio $P_x^{\prime}/P_z^{\prime}$, of polarization transfer to protons bound in $^{40}\mathrm{Ca}$, which is described well by the shell model and for which reliable calculations are available. While the calculations capture the essence of the data, our statistical precision allows us to observe deviations which cannot be explained by simple scaling, including by varying the proton electromagnetic form factor ratio $G_E/G_M$. We further explore the deviations of the ratio of the polarization transfer components from that of a free proton, $(P_x^{\prime}/P_z^{\prime})_{\rm A}/(P_x^{\prime}/P_z^{\prime})_{\rm H}$, and its dependence on the bound-proton virtuality.

nucl-ex↗

arXiv2022

High accuracy synchrotron radiation interferometry with relativistic electrons

A high-precision hypernuclear experiment has been performed at the Mainz Microtron (MAMI) to determine the hypertriton Λ binding energy via decay-pion spectroscopy. A key element of this measurement is an accurate calibration of the magnetic spectrometers with the MAMI beam. For such an absolute calibration with small statistical and systematic uncertainties the undulator light interference method will be applied. In this contribution the basic principle of this method is discussed and the analysis status of the measured synchrotron radiation spectra is presented

physics.ins-det↗

arXiv2022

Low-$Q^2$ elastic electron-proton scattering using a gas jet target

In this paper, we describe an experiment measuring low-$Q^2$ elastic electron-proton scattering using a newly developed cryogenic supersonic gas jet target in the A1 three-spectrometer facility at the Mainz Microtron. We measured the proton electric form factor within the four-momentum transfer range of $0.01\le Q^2 \le 0.045(\text{GeV/c})^2$. The experiment showed consistent results with the existing measurements. The data we collected demonstrated the feasibility of the gas jet target and the potential of future scattering experiments using high-resolution spectrometers with this gas jet target.

nucl-ex↗

arXiv2021

Operation and characterization of a windowless gas jet target in high-intensity electron beams

A cryogenic supersonic gas jet target was developed for the MAGIX experiment at the high-intensity electron accelerator MESA. It will be operated as an internal, windowless target in the energy-recovering recirculation arc of the accelerator with different target gases, e.g., hydrogen, deuterium, helium, oxygen, argon, or xenon. Detailed studies have been carried out at the existing A1 multi-spectrometer facility at the electron accelerator MAMI. This paper focuses on the developed handling procedures and diagnostic tools, and on the performance of the gas jet target under beam conditions. Considering the special features of this type of target, it proves to be well suited for a new generation of high-precision electron scattering experiments at high-intensity electron accelerators.

physics.ins-det↗

arXiv2021

Electron beam studies of light collection in a scintillating counter with embedded fibers

The light collection of several fiber configurations embedded in a box-shaped plastic scintillating counter was studied by scanning with minimum ionizing electrons. The light was read out by silicon photomultipliers at both ends. The light yield produced by the 855-MeV beam of the Mainz Microtron showed a strong dependence on the transverse distance from the beam position to the fibers. The observations were modeled by attributing the collection of indirect light inside of the counter and of direct light reaching a fiber to the total light yield. The light collection with fibers was compared to that of a scintillating counter without fibers. These studies were carried out within the development of plastic scintillating detectors as an active veto system for the DarkMESA electron beam-dump experiment that will search for light dark matter particles in the MeV mass range.

physics.ins-det↗