Optical Transmission Enhancement of Ionic Crystals via Superionic Fluoride Transfer: Growing VUV-Transparent Radioactive Crystals
Abstract
The 8 eV first nuclear excited state in 229Th is a candidate for implementing an nuclear clock. Doping 229Th into ionic crystals such as CaF2 is expected to suppress non-radiative decay, enabling nuclear spectroscopy and the realization of a solid-state optical clock. Yet, the inherent radioactivity of 229Th prohibits the growth of high-quality single crystals with high 229Th concentration; radiolysis causes fluoride loss, increasing absorption at 8 eV. We overcome this roadblock by annealing 229Th doped CaF2 at 1250x2103 in CF4. The technique presented here allows to adjust the fluoride content without crystal melting, preserving its single-crystal structure. Superionic state annealing ensures rapid fluoride distribution, creating fully transparent and radiation-hard crystals. This approach enables control over the charge state of dopants which can be used in deep UV optics, laser crystals, scintillators, and nuclear clocks.
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