Optical Transmission Enhancement of Ionic Crystals via Superionic Fluoride Transfer: Growing VUV-Transparent Radioactive Crystals

TL;DR

This paper presents a novel annealing method in CF4 gas to grow high-quality, transparent, and radiation-hard $^{229}$Th-doped CaF2 crystals suitable for nuclear clock applications, overcoming radiolysis issues.

Contribution

It introduces a superionic fluoride transfer annealing process that preserves crystal quality and transparency in radioactive doped crystals, enabling advanced optical and nuclear technologies.

Findings

Crystals remain transparent and radiation-hard after annealing.

Fluoride content can be precisely controlled without melting.

Enhanced control over dopant charge states for optical applications.

Abstract

The 8 eV first nuclear excited state in $^{229}$Th is a candidate for implementing an nuclear clock. Doping $^{229}$Th into ionic crystals such as CaF$_2$ is expected to suppress non-radiative decay, enabling nuclear spectroscopy and the realization of a solid-state optical clock. Yet, the inherent radioactivity of $^{229}$Th prohibits the growth of high-quality single crystals with high $^{229}$Th concentration; radiolysis causes fluoride loss, increasing absorption at 8 eV. We overcome this roadblock by annealing $^{229}$Th doped CaF$_2$ at 1250$\unicode{x2103}$ in CF$_4$. 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.