  The strong Coulomb interaction in monolayer semiconductors represents a
unique opportunity for the realization of Wigner crystals without external
magnetic fields. In this work, we predict that the formation of monolayer
Wigner crystals can be detected by their terahertz response spectrum, which
exhibits a characteristic sequence of internal optical transitions. We apply
the density matrix formalism to derive the internal quantum structure and the
optical conductivity of the Wigner crystal and to microscopically analyze the
multi-peak shape of the obtained terahertz spectrum. Moreover, we predict a
characteristic shift of the peak position as function of charge density for
different atomically thin materials and show how our results can be generalized
to an arbitrary two-dimensional system.
