  Axions and axion-like particles (ALPs) are leading candidates for dark
matter. They are well motivated in many extensions of the Standard Model and
supported by astronomical observations. We propose an iterative transformation
of the existing facilities of the gravitational-wave detector and technology
testbed GEO600, located near Ruthe in Germany, into a kilometre-scale upgrade
of the laser-interferometric axion detector LIDA. The final DarkGEO detector
could search for coincident signatures of axions and ALPs and significantly
surpass the current constraints of both direct searches and astrophysical
observations in the measurement band from $10^{-16}$ to $10^{-8}$ $\text{eV}$.
We discuss realistic parameters and design sensitivities for the configurations
of the different iteration steps as well as technical challenges known from the
first LIDA results. The proposed DarkGEO detector will be well suited to probe
the parameter space associated with predictions from theoretical models, like
grand-unified theories, as well as from astrophysical evidence, like the cosmic
infrared background.
