arXiv subjects

Ulrich Wahl

Publications and source records attributed to Ulrich Wahl.

5 recordsLinked to original sources

arXiv2026

Radiotracer photoluminescence for element-specific identification of color centers

We report on the implementation of a radiotracer photoluminescence spectroscopy setup at the ISOLDE radioactive ion beam facility at CERN, enabling element-specific identification of optically active defects in solids. The method combines radioactive ion implantation with optical spectroscopy, allowing the temporal evolution of photoluminescence signals to be correlated directly with nuclear decay. The setup is currently optimized for color centers in diamond and related wide-bandgap materials and enables room-temperature measurements. The system consists of an optical microscope coupled to a fiber-fed Czerny-Turner spectrometer with a liquid-nitrogen-cooled CCD detector, providing the stability required for long-duration measurements. As a proof-of-principle, radioactive $^{75}$Ga was implanted into diamond as a precursor to produce $^{75}$Ge impurities. The photoluminescence band extending from 600 nm, corresponding to the well-known GeV$^{-}$ center, exhibits an exponential decay with a half-life of $82.3^{+2.5}_{-2.3} \mathrm{min}$, in agreement with the known $\beta^{-}$ decay half-life of $^{75}$Ge of $82.78(4) \mathrm{min}$. This establishes a direct and unambiguous correlation between the observed spectral feature and its germanium origin. These results demonstrate the capability of the setup to perform element-specific optical spectroscopy and extend radiotracer methods to color centers in wide-bandgap materials, taking advantage of the uniquely broad range of radioactive isotopes available at ISOLDE.

physics.ins-det↗

arXiv2026

Toward nanophotonic platforms for solid-state $^{229}$Th nuclear clocks

While the $^{229}$Th nuclear isomer has recently been observed and laser-excited, converting optical nuclear manipulation into a chip-scale solid-state frequency standard remains an open challenge. Here, we present a nanophotonic platform to realize an all-solid-state nuclear clock based on the low-energy isomeric transition of $^{229}$Th embedded in high-$Q$ fluoride photonic resonators. By coupling ensembles of thorium nuclei to confined optical modes, we show that resonant field build-up in the cavity can substantially enhance the nuclear excitation rate, enabling optical interrogation at practical laser intensities. We model the nuclei-photon interaction dynamics and outline a technological roadmap toward addressing this challenge, including resonator fabrication in fluoride crystals, thorium implantation, nuclear excitation with integrated lasers, and on-chip detection of vacuum-ultraviolet photons. As an initial proof of concept, we implant a crystalline fluoride whispering-gallery-mode resonator with $^{229}$Th and assess the impact of implantation-induced damage on resonator performance. Our platform leverages recent advances in materials integration and nanophotonics to chart a realistic route toward compact and scalable nuclear frequency standards.

physics.optics↗

arXiv2024

Electron Emission Channeling for lattice location of radioactive isotopes in single crystals: Improvements from a Timepix3 quad detector and new PyFDD data analysis software

Electron Emission Channeling (EC) is a powerful technique for the investigation of the lattice location of radioactive isotopes implanted into single crystals. After implantation the isotopes occupy certain lattice locations in the crystal, which can in some cases be altered by annealing. Upon decay, the emission of a charged particle, typically a beta, may result in a channeling trajectory when its starting lattice location is aligned with major symmetry axes or planes of the crystal. By measuring the emission anisotropy in the direction of these axes for distinct annealing temperatures, the lattice location of the isotope can be determined with great precision and insightful information can be obtained on how annealing affects the occupied sites. This work reports on the installation of a Timepix3 quad detector and Katherine Gen2 readout in an experimental setup located at ISOLDE at CERN. The large increase in the number of pixels of the Timepix3, in comparison to previously used pad detectors, required more sophisticated tools for data treatment and fitting of channeling patterns. From this need, the PyFDD software was born. Its latest update features an intuitive graphical interface, with tools for noise masking, pattern visualization, simulations browsing, chi-square or maximum likelihood based fits and gamma background correction.

physics.ins-det↗

arXiv2022

Observation of the radiative decay of the ${}^{229}\mathrm{Th}$ nuclear clock isomer

The nucleus of the radioisotope thorium-229 (${}^{229}$Th) features an isomer with an exceptionally low excitation energy that enables direct laser manipulation of nuclear states. For this reason, it is a leading candidate for use in next-generation optical clocks. This nuclear clock will be a unique tool, amongst others, for tests of fundamental physics. While first indirect experimental evidence for the existence of such an extraordinary nuclear state is significantly older, the proof of existence has been delivered only recently by observing the isomer's electron conversion decay and its hyperfine structure in a laser spectroscopy study, revealing information on the isomer's excitation energy, nuclear spin and electromagnetic moments. Further studies reported the electron conversion lifetime and refined the isomer's energy. In spite of recent progress, the isomer's radiative decay, a key ingredient for the development of a nuclear clock, remained unobserved. In this Letter, we report the detection of the radiative decay of this low-energy isomer in thorium-229 (${}^{229\mathrm{m}}$Th). By performing vacuum-ultraviolet spectroscopy of ${}^{229\mathrm{m}}$Th incorporated into large-bandgap CaF${}_2$ and MgF${}_2$ crystals at the ISOLDE facility at CERN, the photon vacuum wavelength of the isomer's decay is measured as 148.71(42) nm, corresponding to an excitation energy of 8.338(24) eV. This value is in agreement with recent measurements, and decreases the uncertainty by a factor of seven. The half-life of ${}^{229\mathrm{m}}$Th embedded in MgF${}_2$ is determined to be 670(102) s. The observation of the radiative decay in a large-bandgap crystal has important consequences for the design of a future nuclear clock and the improved uncertainty of the energy eases the search for direct laser excitation of the atomic nucleus.

nucl-ex↗

arXiv2022

Magnesium-vacancy optical centers in diamond

We provide the first systematic characterization of the structural and photoluminescence properties of optically active defect centers fabricated upon implantation of 30-100 keV Mg+ ions in artificial diamond. The structural configurations of Mg-related defects were studied by the emission channeling technique for 27Mg implantations performed both at room-temperature and 800 °C, which allowed the identification of a major fraction of Mg atoms (~30-42%) in sites which are compatible with the split-vacancy structure of the MgV complex. A smaller fraction of Mg atoms (~13-17%) was found on substitutional sites. The photoluminescence emission was investigated both at the ensemble and individual defect level in a temperature range comprised between 5 K and 300 K, offering a detailed picture of the MgV-related emission properties and revealing the occurrence of previously unreported spectral features. The optical excitability of the MgV center was also studied as a function of the optical excitation wavelength enabling to identify the optimal conditions for photostable and intense emission. The results are discussed in the context of the preliminary experimental data and the theoretical models available in the literature, with appealing perspectives for the utilization of the tunable properties of the MgV center for quantum information processing applications.

cond-mat.mtrl-sci↗