Multienergy Cu(+) ions with total dose of ~10(14) ions/cm(2) were implanted into LiNbO(3) crystals to form a nearly homogenous damage profile from the sample surface to the end of the ions' track. The extraordinary refractive-index enhancement was found in the damage region by observation of the dark mode. The extraordinary index showed an increasing trend until the ion dose reached a critical value. The largest index enhancement was ~0.0132. By use of low-dose ion implantation, we determined that the decreased spontaneous polarization caused the extraordinary index increase.