Direct frequency comb spectroscopy in trapped 40Ca+ ions on a dipoleforbidden clock transition

Optical frequency combs (FCs) have revolutionised ultra-high precision metrology by providing a link between optical and microwave frequencies. While FCs are traditionally used for calibrating a CW probe-laser, also direct frequency comb spectroscopy (DFCS) is possible. We have recently demonstrated the versatility of DFCS performed in the environment of an ion trap, combining the broad spectral range of a Ti: sapphire FC with the feasibility of trapping and (sympathetic) cooling of different ion species. In the presented contribution it is shown that by employing single-ion imaging, this technique can be extended even to dipole-forbidden transitions, despite the low power per unamplified comb mode of ~0.1 μW.