Comment on 'Counterintuitive Dispersion Violating Kramers-Kronig   Relations in Gain Slabs'
  In this Comment, we refute conclusions made in Phys. Rev. Lett. 112, 233601 (2014) by L.-G. Wang, L. Wang, M. Al-Amri, S.-Y. Zhu, and M. S. Zubairy. These conclusions stem from the use of the linear theory, which is not applicable to systems having poles of the reflection and transmission coefficients in the upper half of the complex frequency plane. 
D. G. Baranov, A. A. Zyablovsky, A. V. Dorofeenko, A. P. Vinogradov,   and A. A. Lisyansky
cond-mat.other