Top quark threshold production in $\gamma\gamma$ collision in the   next-to-leading order
  The total cross section of the top quark-antiquark pair production near threshold in $\gamma\gamma$ collision is computed analytically up to the next-to-leading order in perturbative and nonrelativistic expansion for general photon helicity. The approximation includes the first order corrections in the strong coupling constant and the heavy quark velocity to the nonrelativistic Coulomb approximation. 
A.A.Penin and A.A.Pivovarov