  We employ the relative entropy as a measure to quantify the difference of
eigenmodes between Hermitian and non-Hermitian systems in elliptic optical
microcavities. We have found that the average value of the relative entropy in
the range of the collective Lamb shift is large, while that in the range of
self-energy is small. Furthermore, the weak and strong interactions in the
non-Hermitian system exhibit rather different behaviors in term of the relative
entropy, and thus it displays an obvious exchange of eigenmodes in the elliptic
microcavity.
