  We demonstrate electromagnetically induced transparency (EIT) in a four-level
cascade-like system, where the two upper levels are Rydberg states coupled by a
microwave field. A two-photon transition consisting of an off-resonant
microwave field and an off-resonant optical field forms an effective coupling
field to induce transparency of the probe light. We characterize the Rabi
frequency of the effective coupling field, as well as the EIT microwave
spectra. The results show that microwave assisted EIT allows us to efficiently
access Rydberg states with relatively high orbital angular momentum $\ell=3$,
which is promising for the study of exotic Rydberg molecular states.
