Continuous-wave nuclear laser absorption spectroscopy of Thorium-229

TL;DR

This study demonstrates the excitation of Thorium-229 nuclear resonance using a continuous-wave laser at 148 nm, enabling faster detection via absorption and advancing the development of a solid-state nuclear clock.

Contribution

We show that a continuous-wave laser with sub-nanowatt power can excite Thorium-229 nuclear resonance, improving detection speed and stability for nuclear clock applications.

Findings

Nuclear resonance excited with <1 nW CW laser at 148 nm

Resonance detected via absorption, not fluorescence

Identified a high-symmetry Th-center with minimal electric field gradient

Abstract

A low-energy nuclear transition in the isotope thorium-229 has been excited in thorium-doped crystals with laser light. This opens the perspective towards a highly stable and robust solid-state optical nuclear clock. The required laser radiation at 148 nm wavelength has so far been produced using pulsed laser systems where only a small fraction of the incident photons has been resonant with the narrow nuclear transition. Here we show that the nuclear resonance can be excited with a continuous-wave laser source with a power of less than 1 nW, and that the resonance signal can be detected in absorption rather than in fluorescence. This eliminates the slow nuclear fluorescence decay from the detection process and offers a considerable advantage for clock operation through fast signal acquisition. The laser is based on three sequential frequency doublings, starting from a diode laser at 1187 nm that is well suited for linewidth narrowing and for frequency comparisons with optical atomic clocks. We use absorption spectroscopy for the quantitative characterization of two different Th-centers in calcium fluoride and measure the isomeric shift between them. One of the centers shows a very small static electric crystal field gradient < 0.1V/$\r{A}^2$, to be compared to gradients in the range of 100 V/$\r{A}^2$ observed earlier. This indicates a center with high symmetry of the ions surrounding the Th nucleus, promising nuclear resonance lines that are less sensitive to the lattice spacing.