Yukawa corrections to top pair production in photon-photon collision
  The $O(\alpha m_t^2/m_W^2)$ Yukawa corrections to top pair production in photon-photon collision are calculated in the standard model (SM), the general two-Higgs-doublet model (2HDM) as well as the minimal supersymmetric model (MSSM). We found that the correction to the cross section can only reach a few percent in the SM, but can be quite significant ($>$10\%) in the 2HDM and MSSM for favorable parameter values, which may be observable at the high energy $e^+e^-$ colliders. 
Chong Sheng Li, Jin Min Yang, Yun-lun Zhu and Hong Yi ZhU