Thulium atoms embedded in noble gas crystals for sensing applications

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Gaire, Vinod
Parker, Colin V.
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Narrow linewidth atomic transitions provide opportunities for the development of various quantum technologies. Thulium has an unfulfilled 4$f$ orbital with the electronic configuration similar to Yb$^{3+}$ which is used in several solid-state optical applications. The fine structure of thulium atom is split into ground state ${}^2\mathrm{F}_{7/2}$ and an excited state ${}^2\mathrm{F}_{5/2}$ which is at an energy of 8771 cm$^{-1}$ above the ground state. Because the 4\textit{f} orbital remains submerged underneath the fully filled 5\textit{s}, 5\textit{p} lying near the nucleus, the magnetic dipole transition at 1140 nm has very narrow linewidth and in previous works it was found that the transition was not broadened significantly when trapped in liquid and solid helium. Motivated by this fact that the narrow linewidth transitions observed in thulium coming from the inner shell transitions might have possible applications in building atomic sensors, thulium atoms are studied by trapping them in the solid crystals of argon and neon at cryogenic temperatures. An experimental setup is built to trap the thulium atoms in the ``\textit{matrix}'' of argon and neon, and the samples are prepared on a sapphire substrate and on the tip of a cold multimode fiber. With a home-built high resolution spectrometer for emission spectroscopy and the method of laser absorption spectroscopy, we demonstrated that the magnetic dipole transition is in fact split into multiple components because of the crystal field from argon/neon. In addition to that, we found that the thulium atoms are trapped in multiple trapping sites which are reproducible giving emission at different wavelengths. The experimental setup, methods of sample preparation and high resolution spectroscopy to study the internal structure of the thulium atoms in argon/neon will be discussed.
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