The large t behaviour of the helicity amplitudes of diffractive
photoproduction is estimated relying on models of the photon and meson
light-cone wave functions and on the double-logarithmic approximation to the
exchanged gluon interaction. The role of large-size colour dipole contributions
to the photon-meson transition impact factor is discussed.Comment: 13 pages LaTex, 3 figures, to appear in EPJ