  By extending the response function approach developed in nonlinear optics, we
analytically derive an expression for the non-Markovianity [Laine, et al, Phys.
Rev. A 81, 062115 (2010)] in the time evolution of a system in contact with a
quantum mechanical bath, and find a close connection with the directly
observable nonlinear optical response. The result indicates that memory in the
bath induced fluctuations rather than in the dissipation causes
non-Markovianity. Initial correlations between states of the system and the
bath are shown to be essential for a correct understanding of the
non-Markovianity.
