Direct observation of a highly forbidden optical transition in Sm:SrF$_2$

TL;DR

This paper reports the direct observation of a highly forbidden optical transition in Sm:SrF$_2$, revealing the longest-lived excited state ever observed in a solid, through laser-induced fluorescence at cryogenic temperatures.

Contribution

The study provides the first direct observation and measurement of a forbidden intra-configuration transition in Sm:SrF$_2$, including lifetime and excitation cross section data.

Findings

Longest-lived excited state observed in a solid

Direct detection of a forbidden optical transition

Measurement of excited state lifetime and cross section

Abstract

The $4f^6$ $^{7}F_0$ $\to 4f^6$ $^{5}D_0$ intra-configuration transition in Sm:SrF$_2$ is forbidden for Sm$^{2+}$ ions in the octahedrally symmetric substitution sites in SrF$_2$. We report the direct observation of this transition using laser-induced fluorescence at cryogenic temperatures, and measurements of the excited state lifetime and the excitation cross section. To the best of our knowledge, this optical transition has the longest lived excited state ever observed in a solid.