229ThF4 Thin Films for Solid-State Nuclear Clocks

Inorganic Non-Metallic Materials Materials Science Quantum Physics Physics Nuclear Physics
nuclear clock 229Th thin films laser spectroscopy solid-state materials

Research on Solid-State Nuclear Clocks Based on 229ThF4 Thin Films

Academic Background

Nuclear clocks, based on nuclear transitions, offer extremely high precision and stability. In recent years, nuclear clocks based on the thorium-229 (229Th) nuclear isomer transition have garnered significant attention. The 229Th nuclear isomer transition has an energy of approximately 8.4 electron volts (eV), falling within the vacuum ultraviolet (VUV) range, making it suitable for precise measurement via laser spectroscopy. Compared to existing optical atomic clocks, 229Th-based nuclear clocks are expected to be more robust and potentially outperform current standards, while also enabling tests of new physics beyond the standard model.

However, the scarcity and radioactivity of 229Th make the growth and handling of high-concentration 229Th-doped crystals extremely challenging. Previous studies required large amounts of 229Th material, and the high radioactivity levels limited the widespread application of nuclear clocks. Therefore, finding a scalable solution to reduce the use of 229Th material and mitigate radioactive hazards has become a critical challenge in the field.

Source of the Paper

This paper, titled “229ThF4 thin films for solid-state nuclear clocks,” was authored by Chuankun Zhang, Lars von der Wense, Jack F. Doyle, Jacob S. Higgins, Tian Ooi, Hans U. Friebel, Jun Ye, and others from JILA, NIST, and the University of Colorado. It was published in the journal Nature from December 19 to 26, 2024.

Research Process and Results

1. Preparation and Characterization of 229ThF4 Thin Films

The research team employed physical vapor deposition (PVD) technology to fabricate 229ThF4 thin films. PVD involves evaporating the material from a hot crucible and condensing it onto a substrate, enabling the production of 30-100 nm thick films using only micrograms of 229Th material. These films are intrinsically compatible with photonics platforms and nanofabrication tools, while significantly reducing radioactivity levels—up to three orders of magnitude lower than traditional 229Th-doped crystals.

The team first dissolved 229Th in its nitrate form in ultrapure water, then precipitated 229ThF4 by adding excess hydrofluoric acid (HF). The precipitate was loaded into a glassy carbon crucible and heated above 1000°C in a vacuum environment, allowing 229ThF4 to evaporate and deposit onto the substrate. Using platinum (Pt) masks, the team successfully fabricated small-area films with diameters as small as 50 micrometers, further reducing 229Th consumption.

Detailed characterization of the films’ physical and chemical properties was conducted using atomic force microscopy (AFM) and X-ray photoelectron spectroscopy (XPS). The results confirmed that the films primarily consisted of thorium and fluorine, with high VUV transmittance, making them suitable for nuclear clock spectroscopy.

2. Nuclear Laser Spectroscopy Experiments

The team performed nuclear laser spectroscopy experiments on 229ThF4 films using a VUV laser system. The laser was generated via four-wave mixing, with its frequency locked to a two-photon transition in xenon (Xe). The laser beam was directed at the films at a 70° angle, and fluorescence signals were detected using photomultiplier tubes (PMTs).

The experimental results showed that the nuclear isomer transition frequency was 2020406.8(4)stat(30)sys GHz on an MgF2 substrate and 2020409.1(7)stat(30)sys GHz on an Al2O3 substrate. These results are consistent with previous measurements in crystals, confirming that the nuclear transition in 229ThF4 films can be successfully excited and detected.

Additionally, the team measured the isomer lifetime, finding it to be 150(15)stat(5)sys seconds on Al2O3 and 153(9)stat(7)sys seconds on MgF2. These lifetimes are significantly shorter than those measured in 229Th:CaF2 and 229Th:LiSrAlF6 crystals, likely due to the high refractive index of the films and quenching effects from the host material.

3. Projected Nuclear Clock Performance

Based on density functional theory (DFT) calculations, the team predicted the performance of a nuclear clock based on 229ThF4 films. The calculations revealed that 229ThF4 crystals contain two non-equivalent thorium sites with different electric field gradients (EFGs) and energy level splittings. By selecting specific nuclear transitions, the team estimated that the fractional instability of the clock could reach 5×10^-17 at 1 second, demonstrating its potential for precision measurements.

Conclusions and Significance

This study successfully fabricated 229ThF4 films using PVD technology and, for the first time, achieved laser excitation and spectroscopic measurement of the 229Th nuclear isomer transition in thin films. This breakthrough lays the foundation for the future scalable production of low-radioactivity, integrated solid-state nuclear clocks. Compared to traditional 229Th-doped crystals, 229ThF4 films not only reduce material consumption but also significantly mitigate radioactive hazards, offering new possibilities for the widespread application of nuclear clocks.

Furthermore, the high nuclear emitter density of 229ThF4 films provides a new platform for quantum optics research, particularly in areas such as nuclear superradiance and coherent nuclear forward scattering. The team also suggested that future improvements in crystallinity and nuclear transition participation fraction could be achieved through annealing and fluorination processes, further enhancing clock performance.

Research Highlights

  1. Material Innovation: The use of PVD technology to fabricate 229ThF4 films significantly reduces 229Th material consumption and radioactive hazards.
  2. Spectroscopic Breakthrough: The first successful laser excitation and spectroscopic measurement of the 229Th nuclear isomer transition in thin films, validating its feasibility for nuclear clock applications.
  3. Performance Prediction: DFT-based predictions of the performance of 229ThF4-based nuclear clocks, showcasing their potential in precision measurements.
  4. Quantum Optics Platform: The high nuclear emitter density of 229ThF4 films provides a new experimental platform for quantum optics, particularly in nuclear superradiance and coherent nuclear forward scattering.

Summary

Through innovative material fabrication techniques and precise spectroscopic measurements, this study successfully addressed key challenges in the preparation of 229Th nuclear clock materials, paving the way for the future scalable production and application of low-radioactivity, integrated solid-state nuclear clocks. This achievement not only advances nuclear clock technology but also provides a new research platform for quantum optics and precision measurement.