Probing complex phase of the photon-photon-Higgs vertex at photon   colliders
  We study the effects of heavy Higgs bosons on the top-pair production process at photon linear colliders. The interference patterns between the resonant Higgs-production amplitudes and the continuum QED amplitudes are examined. The patterns tell us not only the CP nature of the Higgs bosons but also the phase of the $\gamma\gamma$--Higgs vertex which gives new information about the Higgs couplings to new charged particles. 
Eri Asakawa