  The realization of negative refraction in atomic gases requires a strong
magnetic response of the atoms. Current proposals for such systems achieve an
enhancement of the magnetic response by a suitable laser field configuration,
but still rely on high gas densities. Thus further progress is desirable, and
this requires an understanding of the precise mechanism for the enhancement.
Therefore, here we study the magnetic and electric response to a probe field
interacting with three-level atoms in ladder configuration. In our first model,
the three transitions are driven by a control field and the electric and
magnetic component of the probe field, giving rise to a closed interaction
loop. In a reference model, the coherent driving is replaced by an incoherent
pump field. A time-dependent analysis of the closed-loop system enables us to
identify the different contributions to the medium response. A comparison with
the reference system then allows one to identify the physical mechanism that
leads to the enhancement. It is found that the enhancement occurs at so-called
multiphoton resonance by a scattering of the coupling field and the electric
probe field mode into the magnetic probe field mode. Based on these results,
conditions for the enhancement are discussed.
