arXiv subjects

Lars von der Wense

Publications and source records attributed to Lars von der Wense.

At least 19 recordsLinked to original sources

arXiv2026

Generation of continuous-wave laser light at 148.4 nm using cavity-enhanced second harmonic generation in $BaMgF_4$

We experimentally investigate the potential of $BaMgF_4$ crystals to create a continuous-wave (CW) solid state laser at the vacuum ultraviolet (VUV) wavelength of 148.4 nm via cavity-enhanced second harmonic generation. This investigation is motivated by the development of a nuclear optical clock based on a transition between the ground and isomeric state in the $^{229}Th$ nucleus. For this purpose, a $BaMgF_4$ crystal was grown, optically polished and periodically poled. The crystal was inserted into a power-enhancement cavity, resonant at the fundamental wavelength of 296.8 nm and the generated laser light at 148.4 nm was characterized. Within this proof-of-concept investigation, a VUV output power of typically ($16\pm1$) pW is obtained. This marks the first time that this type of crystal is used to generate VUV laser light. The experimental findings are compared to theoretical expectations and provide a clear path for future improvements.

physics.optics↗

arXiv2025

A cryogenic Paul trap for probing the nuclear isomeric excited state $^{229\text{m}}$Th$^{3+}$

While laser excitation of the nuclear isomeric transition in $^{229}$Th has been recently achieved for thorium atoms embedded in large-bandgap crystals, laser excitation and characterization of the nuclear transition in trapped $^{229}$Th$^{3+}$ ions has not yet been accomplished. To address these experiments, a cryogenic Paul trap setup has been designed, built, and commissioned at LMU Munich. Here, we present the specifications of the new experimental platform and demonstrate its successful operation, showing the extraction, subsequent ion-guiding, mass-purification, and trapping of $^{229}$Th$^{3+}$ and $^{229\text{m}}$Th$^{3+}$ ions from a newly designed buffer-gas stopping cell as well as of $^{88}$Sr$^{+}$ ions from laser ablation of a solid target. Further, we show sympathetic laser cooling of $^{229\text{(m)}}$Th$^{3+}$ by Doppler-cooled $^{88}$Sr$^{+}$ ions and the formation of mixed-species Coulomb crystals.

physics.atom-ph↗

arXiv2025

Laser fluence-dependent production of molecular thorium ions in different charge states for trapped-ion experiments

Thorium ions and molecules, recognized for their distinctive nuclear and atomic attributes, are central to numerous trapped-ion experiments globally. Our study introduces an effective, compact source of thorium ions produced via laser ablation of microgram-scale, salt-based samples. We thoroughly analyze the variety of ion species and charge states generated at varying laser fluences. Utilizing 10$μ$g of thorium fluoride crystals and laser fluences between $1.00 - 7.00$ J$\cdot$cm$^{-2}$ we produce thorium molecular ions $^{232}$ThF$_x$$^{n+}$ (with $x= 0 - 3$ and charge states up to $n = 3+$), including ThF$^{2+}$ and ThF$^{3+}$. These species are particularly relevant for spectroscopy; ThF$^{3+}$ is valuable for its stable closed-shell configuration, while ThF$^{2+}$, which is isoelectronic to RaF, offers a unique probe for studying nuclear structure and fundamental symmetries due to its simple electronic structure with a single unpaired electron. Density functional theory calculations of the distribution of positive charge in the produced molecular cations and the simplicity of this setup indicate that this method is easily transferable to other actinide systems.

physics.atom-ph↗

arXiv2025

$^{229}$Th Nuclear Spectroscopy in an Opaque Material: Laser-Based Conversion Electron Mössbauer Spectroscopy of $^{229}$ThO$_2$

Here, we report the first demonstration of laser-induced conversion electron Mössbauer spectroscopy of the $^{229}$Th nuclear isomeric state, which provides the ability to probe the nuclear transition in a material that is opaque to light resonant with the nuclear transition. Specifically, we excite the nuclear transition in a thin ThO$_2$ sample whose band gap ($\sim$ 6 eV) is considerably smaller than the nuclear isomeric state energy (8.4 eV). As a result, the excited nucleus can quickly decay by internal conversion, resulting in the ejection of electrons from the surface. By collecting these conversion electrons, nuclear spectroscopy can be recorded. Unlike fluorescence spectroscopy, this technique is compatible with materials whose work function is less than the nuclear transition energy, opening a wider class of systems to study. Further, because ThO$_2$ can be made from spinless isotopes and the internal conversion decay process reduces the isomeric state lifetime to only $\sim$10 $μ$s, allowing $\sim$10$^8$ relative reduction in clock interrogation time, a conversion-electron-based nuclear clock could lead to a $\sim$10$^4$ reduction in clock instability.

physics.atom-ph↗

arXiv2024

$^{229}\mathrm{ThF}_4$ thin films for solid-state nuclear clocks

After nearly fifty years of searching, the vacuum ultraviolet $^{229}$Th nuclear isomeric transition has recently been directly laser excited [1,2] and measured with high spectroscopic precision [3]. Nuclear clocks based on this transition are expected to be more robust [4,5] than and may outperform [6,7] current optical atomic clocks. They also promise sensitive tests for new physics beyond the standard model [5,8,9]. In light of these important advances and applications, a dramatic increase in the need for $^{229}$Th spectroscopy targets in a variety of platforms is anticipated. However, the growth and handling of high-concentration $^{229}$Th-doped crystals [5] used in previous measurements [1-3,10] are challenging due to the scarcity and radioactivity of the $^{229}$Th material. Here, we demonstrate a potentially scalable solution to these problems by demonstrating laser excitation of the nuclear transition in $^{229}$ThF$_4$ thin films grown with a physical vapor deposition process, consuming only micrograms of $^{229}$Th material. The $^{229}$ThF$_4$ thin films are intrinsically compatible with photonics platforms and nanofabrication tools for integration with laser sources and detectors, paving the way for an integrated and field-deployable solid-state nuclear clock with radioactivity up to three orders of magnitude smaller than typical \thor-doped crystals [1-3,10]. The high nuclear emitter density in $^{229}$ThF$_4$ also potentially enables quantum optics studies in a new regime. Finally, we describe the operation and present the estimation of the performance of a nuclear clock based on a defect-free ThF$_4$ crystal.

physics.atom-ph↗

arXiv2024

Frequency ratio of the $^{229\mathrm{m}}$Th nuclear isomeric transition and the $^{87}$Sr atomic clock

Optical atomic clocks$^{1,2}$ use electronic energy levels to precisely keep track of time. A clock based on nuclear energy levels promises a next-generation platform for precision metrology and fundamental physics studies. Thorium-229 nuclei exhibit a uniquely low energy nuclear transition within reach of state-of-the-art vacuum ultraviolet (VUV) laser light sources and have therefore been proposed for construction of the first nuclear clock$^{3,4}$. However, quantum state-resolved spectroscopy of the $^{229m}$Th isomer to determine the underlying nuclear structure and establish a direct frequency connection with existing atomic clocks has yet to be performed. Here, we use a VUV frequency comb to directly excite the narrow $^{229}$Th nuclear clock transition in a solid-state CaF$_2$ host material and determine the absolute transition frequency. We stabilize the fundamental frequency comb to the JILA $^{87}$Sr clock$^2$ and coherently upconvert the fundamental to its 7th harmonic in the VUV range using a femtosecond enhancement cavity. This VUV comb establishes a frequency link between nuclear and electronic energy levels and allows us to directly measure the frequency ratio of the $^{229}$Th nuclear clock transition and the $^{87}$Sr atomic clock. We also precisely measure the nuclear quadrupole splittings and extract intrinsic properties of the isomer. These results mark the start of nuclear-based solid-state optical clock and demonstrate the first comparison of nuclear and atomic clocks for fundamental physics studies. This work represents a confluence of precision metrology, ultrafast strong field physics, nuclear physics, and fundamental physics.

physics.atom-ph↗

arXiv2022

Tunable VUV Frequency Comb for $^{\mathrm{229m}}$Th Nuclear Spectroscopy

Laser spectroscopy of the $^{\mathrm{229m}}$Th nuclear clock transition is necessary for the future construction of a nuclear-based optical clock. Precision laser sources with broad spectral coverage in the vacuum ultraviolet are needed for this task. Here, we present a tunable vacuum-ultraviolet frequency comb based on cavity-enhanced seventh-harmonic generation. Its tunable spectrum covers the current uncertainty range of the $^{\mathrm{229m}}$Th nuclear clock transition.

physics.optics↗

arXiv2020

The $^{229}$Th isomer: prospects for a nuclear optical clock

The proposal for the development of a nuclear optical clock has triggered a multitude of experimental and theoretical studies. In particular the prediction of an unprecedented systematic frequency uncertainty of about $10^{-19}$ has rendered a nuclear clock an interesting tool for many applications, potentially even for a re-definition of the second. The focus of the corresponding research is a nuclear transition of the $^{229}$Th nucleus, which possesses a uniquely low nuclear excitation energy of only $8.12\pm0.11$ eV ($152.7\pm2.1$ nm). This energy is sufficiently low to allow for nuclear laser spectroscopy, an inherent requirement for a nuclear clock. Recently, some significant progress toward the development of a nuclear frequency standard has been made and by today there is no doubt that a nuclear clock will become reality, most likely not even in the too far future. Here we present a comprehensive review of the current status of nuclear clock development with the objective of providing a rather complete list of literature related to the topic, which could serve as a reference for future investigations.

physics.atom-ph↗

arXiv2020

The theory of direct laser excitation of nuclear transitions

A comprehensive theoretical study of direct laser excitation of a nuclear state based on the density matrix formalism is presented. The nuclear clock isomer $^{229\text{m}}$Th is discussed in detail, as it could allow for direct laser excitation using existing technology and provides the motivation for this work. The optical Bloch equations are derived for the simplest case of a pure nuclear two-level system and for the more complex cases taking into account the presence of magnetic sub-states, hyperfine-structure and Zeeman splitting in external fields. Nuclear level splitting for free atoms and ions as well as for nuclei in a solid-state environment is discussed individually. Based on the obtained equations, nuclear population transfer in the low-saturation limit is reviewed. Further, nuclear Rabi oscillations, power broadening and nuclear two-photon excitation are considered. Finally, the theory is applied to the special cases of $^{229\text{m}}$Th and $^{235\text{m}}$U, being the nuclear excited states of lowest known excitation energies. The paper aims to be a didactic review with many calculations given explicitly.

nucl-th↗

arXiv2020

Concepts for direct frequency-comb spectroscopy of $^{229\text{m}}$Th and an internal-conversion-based solid-state nuclear clock

A new concept for narrow-band direct nuclear laser spectroscopy of $^{229\text{m}}$Th is proposed, using a single comb mode of a vacuum ultraviolet frequency comb generated from the 7th harmonic of an Yb-doped fiber laser system. In this concept more than $10^{14}$ $^{229}$Th atoms on a surface are irradiated in parallel and a successful nuclear excitation is probed via the internal-conversion (IC) decay channel. A net scanning time of 15 minutes for the most recent 1~$σ$ energy uncertainty interval of 0.34 eV appears to be achievable when searching for the nuclear transition. In case of successful observation, the isomer's energy value would be constrained to an uncertainty of about 100~MHz, which is a factor of $10^6$ of improvement compared to today's knowledge. Further, the comb mode could be stabilized to the nuclear transition using the same detection method, allowing for the development of an IC-based solid-state nuclear clock, which is shown to achieve the same performance as a crystal-lattice nuclear clock, however, with the advantage of a drastically simpler detection scheme. Finally, it is shown that the same laser system could be used to narrow down the isomer's transition energy by further six orders of magnitude during laser excitation of $^{229}$Th$^{3+}$ ions in a Paul trap and to drive nuclear Rabi oscillations, as required for the development of a nuclear clock based on a single $^{229}$Th$^{3+}$ ion.

physics.ins-det↗

arXiv2020

Electronic bridge excitation in highly charged Th-229 ions

The excitation of the 8 eV $^{229m}$Th isomer through the electronic bridge mechanism in highly charged ions is investigated theoretically. By exploiting the rich level scheme of open $4f$ orbitals and the robustness of highly charged ions against photoionization, a pulsed high-intensity optical laser can be used to efficiently drive the nuclear transition by coupling it to the electronic shell. We show how to implement a promising electronic bridge scheme in an electron beam ion trap starting from a metastable electronic state. This setup would avoid the need for a tunable vacuum ultraviolet laser. Based on our theoretical predictions, determining the isomer energy with an uncertainty of $10^{-5}$ eV could be achieved in one day of measurement time using realistic laser parameters.

physics.atom-ph↗

arXiv2019

Energy of the $^{229}$Th nuclear clock transition

The first nuclear excited state of $^{229}$Th offers the unique opportunity for laser-based optical control of a nucleus. Its exceptional properties allow for the development of a nuclear optical clock which offers a complementary technology and is expected to outperform current electronic-shell based atomic clocks. The development of a nuclear clock was so far impeded by an imprecise knowledge of the energy of the $^{229}$Th nuclear excited state. In this letter we report a direct excitation energy measurement of this elusive state and constrain this to 8.28$\pm$0.17 eV. The energy is determined by spectroscopy of the internal conversion electrons emitted in-flight during the decay of the excited nucleus in neutral $^{229}$Th atoms. The nuclear excitation energy is measured via the valence electronic shell, thereby merging the fields of nuclear- and atomic physics to advance precision metrology. The transition energy between ground and excited state corresponds to a wavelength of 149.7$\pm$3.1 nm. These findings set the starting point for high-resolution nuclear laser spectroscopy and thus the development of a nuclear optical clock of unprecedented accuracy. A nuclear clock is expected to have a large variety of applications, ranging from relativistic geodesy over dark matter research to the observation of potential temporal variation of fundamental constants.

nucl-ex↗

arXiv2018

Towards a precise determination of the excitation energy of the Thorium nuclear isomer using a magnetic bottle spectrometer

$^{229}$Th is the only known nucleus with an excited state that offers the possibility for a direct laser excitation using existing laser technology. Its excitation energy has been measured indirectly to be 7.8(5) eV ($\approx$160 nm). The energy and lifetime of the isomeric state make it the presently only suitable candidate for a nuclear optical clock, the uncertainty of the excitation energy is, however, still too large to allow for a direct laser excitation in a Paul trap. Therefore, a major goal during the past years has been an improved energy determination. One possible approach is to measure the kinetic energy of electrons which are emitted in the internal conversion decay of the first isomeric state in $^{229}$Th. For this reason an electron spectrometer based on a magnetic bottle combined with electrical retarding fields has been built. Its design, as well as first test measurements are presented, which reveal a relative energy resolution of 3 % and thus enable to measure the electrons' expected kinetic energy to better than 0.1 eV. This is sufficiently precise to specify a laser system able to drive the nuclear clock transition in $^{229}$Th.

physics.ins-det↗

arXiv2018

Towards a 229Th-based nuclear clock

An overview over the current status of the development of a nuclear clock based on the state of lowest known nuclear excitation energy in $^{229}$Th is presented. The text is especially written for the interested reader without any particular knowledge in this field of research. It is thus ideal as an introductory reading to get a broad overview over the various different aspects of the field, in addition it can serve as a guideline for future research. An introductory part is provided, giving a historic context and explaining the fundamental concept of clocks. Finally, potential candidates for nuclear clocks other than $^{229}$Th are discussed.

nucl-ex↗

arXiv2018

Laser spectroscopic characterization of the nuclear clock isomer $^{229m}$Th

The isotope $^{229}$Th is the only nucleus known to possess an excited state $^{229m}$Th in the energy range of a few electron volts, a transition energy typical for electrons in the valence shell of atoms, but about four orders of magnitude lower than common nuclear excitation energies. A number of applications of this unique nuclear system, which is accessible by optical methods, have been proposed. Most promising among them appears a highly precise nuclear clock that outperforms existing atomic timekeepers. Here we present the laser spectroscopic investigation of the hyperfine structure of $^{229m}$Th$^{2+}$, yielding values of fundamental nuclear properties, namely the magnetic dipole and electric quadrupole moments as well as the nuclear charge radius. After the recent direct detection of this long-searched-for isomer, our results now provide detailed insight into its nuclear structure and present a method for its non-destructive optical detection.

nucl-ex↗

arXiv2018

Lifetime measurement of the $^{229}$Th nuclear isomer

The first excited isomeric state of $^{229}$Th possesses the lowest energy among all known excited nuclear states. The expected energy is accessible with today's laser technology and in principle allows for a direct optical laser excitation of the nucleus. The isomer decays via three channels to its ground-state (internal conversion, $γ$ decay and bound internal conversion), whose strengths depend on the charge state of $^{229m}$Th. We report on the measurement of the internal-conversion decay half-life of neutral $^{229m}$Th. A half-life of 7 $\pm$ 1 $μ$s has been measured, which is in the range of theoretical predictions and, based on the theoretically expected lifetime of $\approx10^4$ s of the photonic decay channel, gives further support for an internal conversion coefficient of $\approx10^9$, thus constraining the strength of a radiative branch in the presence of IC.

nucl-ex↗

arXiv2017

Direct detection of the 229Th nuclear clock transition

Today's most precise time and frequency measurements are performed with optical atomic clocks. However, it has been proposed that they could potentially be outperformed by a nuclear clock, which employs a nuclear transition instead of the atomic shell transitions used so far. By today there is only one nuclear state known which could serve for a nuclear clock using currently available technology, which is the isomeric first excited state in $^{229}$Th. Here we report the direct detection of this nuclear state, which is a further confirmation of the isomer's existence and lays the foundation for precise studies of the isomer's decay parameters. Based on this direct detection the isomeric energy is constrained to lie between 6.3 and 18.3 eV, and the half-life is found to be longer than 60 s for $^{229\mathrm{m}}$Th$^{2+}$. More precise determinations appear in reach and will pave the way for the development of a nuclear frequency standard.

nucl-ex↗

arXiv2017

The extraction of 229Th3+ from a buffer-gas stopping cell

In the whole landscape of atomic nuclei, $^{229}$Th is currently the only known nucleus which could allow for the development of a nuclear-based frequency standard, as it possesses an isomeric state of just 7.6 eV energy above the ground state. The 3+ charge state is of special importance in this context, as Th$^{3+}$ allows for a simple laser-cooling scheme. Here we emphasize the direct extraction of triply-charged $^{229}$Th from a buffer-gas stopping cell. This finding will not only simplify any future approach of $^{229}$Th ion cooling, but is also used for thorium-beam purification and in this way provides a powerful tool for the direct identification of the $^{229}$Th isomer to ground state nuclear transition.

physics.ins-det↗