  The linear driving for a single-mode optical field in a cavity can result
from the external driving of classical field even when the coupling between the
classical field and the cavity is weak. We revisit this well known effect with
a microscopic model where a classical field is applied to a wall of the cavity
to excite the atoms in the wall, and re-combination of the low excitations of
the wall mediates a linear driving for the single-mode field inside the cavity.
With such modeling about the indirect driving through the quantum excitations
of the wall, we theoretically predict several non-linear optical effects for
the strong coupling cases, such as photon anti-bunching and photon squeezing.
In the sense, we propose the most simplified non-linear quantum photonics
model.
