Thulium Impurities in Argon Reveal Crystal Field Levels via Laser Spectroscopy

Thulium atoms embedded within solid argon present a promising platform for nanoscale sensing and potential quantum technologies. Jason Marfey, Anthony Semenova, and Colin V. Parker, all from the Georgia Institute of Technology, investigate the unique optical properties of thulium within this icy host. Their research reveals that the arrangement of argon atoms around the thulium significantly influences its behaviour, splitting its energy levels and creating a magnetic-electric response. This discovery is important because it demonstrates the possibility of detecting tiny magnetic fields using only light, and provides valuable insight into optimising the creation of these materials for future applications in sensitive magnetometry and information storage. The team’s findings pave the way for all-optical detection schemes and more informed strategies for crystal growth.
The search for increasingly sensitive and miniaturised magnetic field sensors drives innovation across diverse fields, from medical diagnostics to materials science. Recent research presents a promising new approach: utilising thulium atoms embedded within a solid argon matrix to create an all-optical magnetic field sensor at the nanoscale. This innovative system leverages the unique properties of thulium and the inertness of solid argon to achieve exceptional sensitivity and stability.

The core of this advancement lies in the ability to isolate individual thulium atoms within a crystal of solid argon, creating a remarkably stable environment that preserves delicate quantum properties. Researchers focus on a specific transition within the thulium atom – a shift in its internal energy levels – which is particularly resilient to environmental disturbances. Crucially, this transition splits into multiple components due to the surrounding argon matrix, creating distinct energy levels sensitive to external magnetic fields.

Through detailed spectroscopic analysis, the team identified at least two stable “trapping sites” for the thulium atoms within the argon crystal, each exhibiting a unique energy structure determined by the arrangement of surrounding argon atoms. The interaction between the thulium atom and the argon matrix alters how light is absorbed and emitted, effectively mixing magnetic and electric properties, a key element of the sensor’s functionality. By precisely measuring shifts in these energy levels under the influence of even weak magnetic fields – on the scale of millitesla – the researchers demonstrate the potential for all-optical detection, eliminating the need for complex radio frequency equipment.

This thulium-argon system offers a potentially simpler and more stable platform than existing sensors, free from issues related to sensor proximity and surface noise. Results demonstrate that all-optical detection schemes for direct current (DC) magnetic fields are currently achievable with thulium-doped argon, establishing a pathway towards utilising optical methods for precise magnetic field measurement within material science applications. The current sensitivity achieved is 330 μT/ √ Hz, representing the first detection of small fields in matrix-isolated atoms without requiring microwave or radio frequency radiation.

The magnetic sensitivity is currently limited by the ensemble optical linewidth, achieved through careful sample annealing. Reducing this broadening through additional cooling is expected to improve sensitivity, with a predicted exponential scaling with inverse temperature. Furthermore, detailed knowledge of the trapping sites revealed in this study allows for more sophisticated investigations of annealing dynamics and potential improvements to inhomogeneous linewidths.

Future work may benefit from employing selective bleaching methods or light-induced trapping site transfer to isolate a single crystal axis and enable vector magnetometry. Although currently less sensitive than some existing nanoscale magnetometers, the ability to detect magnetic fields directly from changes in the optical spectrum, combined with the long-term stability of the solid argon matrix, promises a new paradigm for sensitive and miniaturised magnetic field sensing with applications ranging from materials characterisation to advanced medical imaging. This research also opens up exciting possibilities for monitoring and improving the processes of crystal growth and annealing, paving the way for enhanced control and optimisation of crystal quality through real-time feedback during fabrication.