Top quark pair production at photon-photon colliders is studied in low scale
quantum gravity scenario. From the dependence of the cross sections on the spin
configuration of the top quark and anti-quark, we introduce a new observable,
top spin asymmetry. It is shown that there exists a special top spin basis
where with the polarized parent electron beams the top spin asymmetry vanishes
in the standard model but retains substantial values with the large extra
dimension effects. We also present lower bounds of the quantum gravity scale
MS from total cross sections with various combinations of the laser,
electron beam, and top quark pair polarizations. The measurements of the top
spin state (t↑tˉ↓) with unpolarized initial beams are
shown to be most effective, enhancing by about 5% the MS bounds with respect
to totally unpolarized case.Comment: 18 pages, 4 figures, ReVTe