This Conceptual Design Report describes LUXE (Laser Und XFEL Experiment), an experimental campaign that aims to combine the high-quality and high-energy electron beam of the European XFEL with a powerful laser to explore the uncharted terrain of quantum electrodynamics characterised by both high energy and high intensity. We will reach this hitherto inaccessible regime of quantum physics by analysing high-energy electron-photon and photon-photon interactions in the extreme environment provided by an intense laser focus. The physics background and its relevance are presented in the science case which in turn leads to, and justifies, the ensuing plan for all aspects of the experiment: Our choice of experimental parameters allows (i) field strengths to be probed where the coupling to charges becomes non-perturbative and (ii) a precision to be achieved that permits a detailed comparison of the measured data with calculations. In addition, the high photon flux predicted will enable a sensitive search for new physics beyond the Standard Model. The initial phase of the experiment will employ an existing 40 TW laser, whereas the second phase will utilise an upgraded laser power of 350 TW. All expectations regarding the performance of the experimental set-up as well as the expected physics results are based on detailed numerical simulations throughout.

We image the ultraslow exciton-polariton propagation in a WSe2 nanoslab with extreme spatiotemporal resolution (50 nm, <45 fs).

Upconversion imaging, where mid-infrared (IR) photons are converted to visible and near-IR photons via a nonlinear crystal and detected on cheap and high-performance silicon detectors, is an appealing method to address the limitations of thermal sensors that are expensive, often require cooling, and suffer from both limited spectral response and limited spatial resolution as well as poor sensitivity. However, phase matching severely limits the spectral bandwidth of this technique, therefore requiring serial acquisitions in order to cover a large spectrum. Here, a novel upconversion imaging scheme covering the mid-IR based on adiabatic frequency conversion is introduced. The study presents mid-IR multicolor imaging and demonstrates simultaneous imaging on a complementary metal-oxide-semiconductor (CMOS) camera of radiation spanning a spectrum from 2 to 4 µm. This approach being coherent and ultrafast in essence, spectrally resolved spatiotemporal imaging is further demonstrated that allows spatially distinguishing the temporal evolution of spectral components.

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