Comment on "Saturation of the All-Optical Kerr Effect"
  Br\'ee, Demircan, and Steinmeyer [Phys. Rev. Lett. 106, 183902 (2011)] recently calculated higher-order Kerr effect (HOKE) coefficients using the nonlinear Kramers-Kronig relations. We show that the saturated and negative nonlinear index of refraction obtained by Br\'ee et al. is an artifact of their use of a perturbative theory of ionization in an intensity region where it breaks down. 
J. K. Wahlstrand and H. M. Milchberg
physics.optics physics.atom-ph physics.plasm-ph