  We analyze the influence of dipole-dipole interactions in an
electromagnetically induced transparency setup at high density. We show both
analytically and numerically that the polarization contribution to the local
field strongly modulates the phase of a weak pulse. We give an intuitive
explanation for this local field induced phase modulation and show that it
distinctively differs from the nonlinear self-phase modulation a strong pulse
experiences in a Kerr medium.
