Progress towards fabrication of 229Th-doped high energy band-gap crystals for use as a solid-state optical frequency reference

We have recently described a novel method for the construction of a solid-state optical frequency reference based on doping 229Th into high energy band-gap crystals [1]. Since nuclear transitions are far less sensitive to environmental conditions than atomic transitions, we have argued that the 229Th optical nuclear transition may be driven inside a host crystal resulting in an optical frequency reference with a short-term stability of 3 × 10−17 < Δf/f < 1 × 10−15 at 1 s and a systematic-limited repeatability of Δf/f ~2 × 10−16. Improvement by 102 – 103 of the constraints on the variability of several important fundamental constants also appears possible. Here we present the results of the first phase of these experiments. Specifically, we have evaluated several high energy band-gap crystals (Th:NaYF, Th:YLF, Th:LiCAF, Na2ThF6, LiSAF) for their suitability as a crystal host by a combination of electron beam microprobe measurements, Rutherford Backscattering, and synchrotron excitation/fluorescence measurements. These measurements have shown LiCAF to be the most promising host crystal, and using a 232Th doped LiCAF crystal, we have performed a mock run of the actual experiment that will be used to search for the isomeric transition in 229Th. This data indicates that a measurement of the transition energy with a signal to noise ratio (SNR) greater than 30:1 can be achieved at the lowest expected fluorescence rate.