  We study two-photon excitation using biphotons generated via the process of
spontaneous parametric down-conversion in a nonlinear crystal. We show that the
focusing of these biphotons yields an excitation distribution that is
essentially the same as the distribution of one-photon excitation at the pump
wavelength. We also demonstrate that biphoton excitation in the image region
yields a distribution whose axial width is approximately that of the crystal
thickness and whose transverse width is that of the pump at the input to the
crystal.
