We present a detection scheme to search for QCD axion dark matter, that is based on a direct interaction between axions and electrons explicitly predicted by DFSZ axion models. The local axion dark matter field shall drive transitions between Zeeman-split atomic levels separated by the axion rest mass energy m a c 2. Axion-related excitations are then detected with an upconversion scheme involving a pump laser that converts the absorbed axion energy (~hundreds of μeV) to visible or infrared photons, where single photon detection is an established technique. The proposed scheme involves rare-earth ions doped into solid-state crystalline materials, and the optical transitions take place between energy levels of 4f N electron configuration. Beyond discussing theoretical aspects and requirements to achieve a cosmologically relevant sensitivity, especially in terms of spectroscopic material properties, we experimentally investigate backgrounds due to the pump laser at temperatures in the range 1.9 − 4.2 K. Our results rule out excitation of the upper Zeeman component of the ground state by laser-related heating effects, and are of some help in optimizing activated material parameters to suppress the multiphonon-assisted Stokes fluorescence.

We report on a scheme for particle detection based on the infrared quantum counter concept. Its\ud operation consists of a two-step excitation process of a four level system, which can be realized\ud in rare earth-doped crystals when a cw pump laser is tuned to the transition from the second to\ud the fourth level. The incident particle raises the atoms of the active material into a low lying,\ud metastable energy state, triggering the absorption of the pump laser to a higher level. Following a\ud rapid non-radiative decay to a fluorescent level, an optical signal is observed with a conventional\ud detector. In order to demonstrate the feasibility of such a scheme, we have investigated the emission from the fluorescent level 4S3=2 (540 nm band) in an Er3þ-doped YAG crystal pumped by a\ud tunable titanium sapphire laser when it is irradiated with 60 keV electrons delivered by an electron gun. We have obtained a clear signature that this excitation increases the 4I13=2 metastable\ud level population that can efficiently be exploited to generate a detectable optical signal

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