Ultra-Sensitive Extinction Measurements of Optically Active Defects in Monolayer MoS.

We utilize cavity-enhanced extinction spectroscopy to directly quantify the optical absorption of defects in MoS generated by helium ion bombardment. We achieve hyperspectral imaging of specific defect patterns with a detection limit below 0.01% extinction, corresponding to a detectable defect density below 1 × 10 cm.

Energy of the Th nuclear clock transition.

Owing to its low excitation energy and long radiative lifetime, the first excited isomeric state of thorium-229, Th, can be optically controlled by a laser and is an ideal candidate for the creation of a nuclear optical clock, which is expected to complement and outperform current electronic-shell-based atomic clocks. A nuclear clock will have various applications-such as in relativistic geodesy, dark matter research and the observation of potential temporal variations of fundamental constants-but its development has so far been impeded by the imprecise knowledge of the energy of Th. Here we report a direct measurement of the transition energy of this isomeric state to the ground state with an uncertainty of 0.

Preparing an Isotopically Pure 229Th Ion Beam for Studies of 229mTh.

A methodology is described to generate an isotopically pure Th ion beam in the 2+ and 3+ charge states. This ion beam enables one to investigate the low-lying isomeric first excited state of Th at an excitation energy of about 7.8(5) eV and a radiative lifetime of up to 10 seconds.