Nonlinear Pulse Compression to Sub-40 fs at 4.5 μJ Pulse Energy by Multi-Pass-Cell Spectral Broadening

Johannes Weitenberg, Tobias Saule, Jan Schulte, and Peter Rußbüldt


We report on the pulse compression of an 18.5 MHz repetition rate pulse train from 230 fs to sub-40 fs by nonlinear spectral broadening in a multi-pass cell and subsequent chirp removal. The compressed pulse energy is 4.5 μJ, which corresponds to 84 W of average power, with a compression efficiency of 88%. This recently introduced compression scheme is suitable for a large pulse energy range and for high average power. In this paper, we show that it can achieve three times shorter pulses than previously demonstrated.

A Laser Excitation Scheme for 229mTh

Lars von der Wense, Benedict Seiferle, Simon Stellmer, Johannes Weitenberg, Georgy Kazakov, Adriana Pálffy, and Peter G. Thirolf


Direct laser excitation of the lowest known nuclear excited state in 229Th has been a long-standing objective. It is generally assumed that reaching this goal would require a considerably reduced uncertainty of the isomer’s excitation energy compared to the presently adopted value of (7.8±0.5)eV. Here we present a direct laser excitation scheme for 229mTh, which circumvents this requirement. The proposed excitation scheme makes use of already existing laser technology and therefore paves the way for nuclear laser spectroscopy. In this concept, the recently experimentally observed internal-conversion decay channel of the isomeric state is used for probing the isomeric population. A signal-to-background ratio of better than 104 and a total measurement time of less than three days for laser scanning appear to be achievable.

Laser spectroscopic characterization of the nuclear clock isomer 229mTh

Johannes Thielking, Maxim V. Okhapkin, Przemyslaw Glowacki, David M. Meier, Lars von der Wense, Benedict Seiferle, Christoph E. Düllmann, Peter G. Thirolf, Ekkehard Peik


The isotope 229Th is the only nucleus known to possess an excited state 229mTh 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 229mTh2+, 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, an important step towards the development of a nuclear clock.

Laser-induced electronic bridge for characterization of the 229mTh→ 229gTh nuclear transition with a tunable optical laser

Pavlo V. Bilous, Ekkehard Peik, Adriana Pálffy


An alternative method to determine the excitation energy of the 229mTh isomer via the laser-induced electronic bridge is investigated theoretically. In the presence of an optical or ultra-violet laser at energies that fulfill a two-photon resonance condition, the excited nuclear state can decay by transfering its energy to the electronic shell. A bound electron is then promoted to an excited state by absorption of a laser photon and simultaneous de-excitation of the nucleus. We present calculated rates for the laser-induced electronic bridge process and discuss the experimental requirements for the corresponding setup. Our results show that depending on the actual value of the nuclear transition energy, the rate can be very high, with an enhancement factor compared to the radiative nuclear decay of up to 10^8.

Multi-pass-cell-based nonlinear pulse compression to 115 fs at 7.5 μJ pulse energy and 300 W average power

Johannes Weitenberg, Andreas Vernaleken, Jan Schulte, Akira Ozawa, Thomas Sartorius, Vladimir Pervak, Hans-Dieter Hoffmann, Thomas Udem, Peter Russbüldt, Theodor W. Hänsch


We demonstrate nonlinear pulse compression by multi-pass cell spectral broadening (MPCSB) from 860 fs to 115 fs with compressed pulse energy of 7.5 μJ, average power of 300 W and close to diffraction-limited beam quality. The transmission of the compression unit is >90%. The results show that this recently introduced  compression scheme for peak powers above the threshold for catastrophic self-focusing can be scaled to smaller pulse energies and can achieve a larger compression factor than previously reported. Good homogeneity of the spectral broadening across the beam profile is verified, which distinguishes MPCSB among other bulk compression schemes.

Nubis et Nuclei: A study on noise and precision

Kerstin Ergenzinger, Thorsten Schumm, Simon Stellmer


This study sets out to explore the perception of noise, as well as the recovery of meaning or information that it might contain, in arts, science, and daily life. It is realized as an installation based on an arrangement of nitinol drums that create a sonic environment evolving in time and space. The nitinol drums are driven by digital random noise. The observer is free to explore the sonic environment, and will discover regions in time and space with a “meaningful” signal. This discovery of a clear signal in a noisy background holds strong analogies to the scientific search for a nuclear resonance performed in the nuClock project.

Feasibility Study of Internal Conversion Electron Spectroscopy of Th-229m

Benedict Seiferle, Lars von der Wense, Peter G. Thirolf


With an expected energy of 7.8(5) eV, the isomeric first excited state in Th-229 exhibits the lowest excitation energy of all known nuclei. Until today, a value for the excitation energy has been inferred only by indirect measurements. In this paper, we propose to use the internal conversion decay channel as a probe for the ground-state transition energy. MatLab-based Monte Carlo simulations have been performed to obtain an estimate of the expected statistics and to test the feasibility of the experiment. From the simulations we conclude, that with the presented methods an energy determination with a precision of better than 0.1 eV is possible.

Reduced transition probabilities for the gamma decay of the 7.8 eV isomer in Th-229

Nikolay Minkov, Adriana Pálffy


The reduced magnetic dipole and electric quadrupole transition probabilities for the radiative decay of the 229Th 7.8 eV isomer to the ground state are predicted within a detailed nuclear-structure model approach. We show that the presence and decay of this isomer can only be accounted for by the Coriolis mixing emerging from a remarkably fine interplay between the coherent quadrupole-octupole motion of the nuclear core and the single-nucleon motion within a reflection-asymmetric deformed potential. We find that the magnetic dipole transition probability which determines the radiative lifetime of the isomer is considerably smaller than presently estimated. The so-far disregarded electric quadrupole component may have non-negligible contributions to the internal conversion channel. These findings support new directions in the experimental search of the 229Th transition frequency for the development of a future nuclear frequency standard.

Optomechanically induced transparency of x-rays via optical control

Wen-Te Liao and Adriana Pálffy


Future photonic quantum networks will require interfaces between different photon frequency regimes. Here we envisage for the first time an optomechanical system that bridges optical photons and x-rays with the aim to control the latter and exploit their unique properties regarding large momentum transfer, focusability, penetration, robustness and detection efficiency. The x-ray-optical interface system comprises of an optomechanical cavity and a movable microlever interacting with both an optical laser and with x-rays via resonant nuclear scattering. We develop a theoretical model for this system and show that x-ray absorption spectra of nuclei can be tuned optomechanically. In particular, our theoretical simulations predict optomechanically induced transparency of x-rays.

Internal conversion from excited electronic states of Th-229 ions

Pavlo V. Bilous, Georgy A. Kazakov, Iain D. Moore, Thorsten Schumm, Adriana Pálffy


The process of internal conversion from excited electronic states is investigated theoretically for the case of the vacuum-ultraviolet nuclear transition of 229Th. Due to the very low transition energy, the 229Th nucleus offers the unique possibility to open the otherwise forbidden internal conversion nuclear decay channel for thorium ions via optical laser excitation of the electronic shell. We show that this feature can be exploited to investigate the isomeric state properties via observation of internal conversion from excited electronic configurations of Th+ and Th2+ ions. A possible experimental realization of the proposed scenario at the nuclear laser spectroscopy facility IGISOL in Jyväskylä, Finland is discussed.

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