Associated production of Intermediate Higgs or Z-boson with $t \bar t$   pair in $\gamma \gamma$ collision
  Photon-photon linear colliders can be realized by laser back-scattering technique on the next generation linear $e^+e^-$ colliders. Here the associated productions of an intermediate mass Higgs (IMH) or Z-boson with $t \bar t$ pair in $\gamma\gamma$ collisions are studied. Since IMH is very unlikely to decay into $t\bar t$ pair, $t\bar tH$ production is the only direct channel to probe the Higgs-top Yukawa coupling in case of an IMH. $t\bar tZ$ production can be a potential background to $t\bar tH$ if the Higgs mass is close to $m_Z$. As an alternative to its parent $e^+e^-$ collider, $\gamma\gamma \to t\bar tH(Z)$ productions are compared with the corresponding productions in the $e^+e^-$ collisions. 
Kingman Cheung