  We present a general analysis of the influence of nonlinear optical
absorption on terahertz generation via optical difference frequency generation,
when reaching for the quantum conversion efficiency limit. By casting the
equations governing the process in a suitably normalized form, including either
two-photon- or three-photon-absorption terms, we have been able to plot
universal charts for phase matched optical-to-terahertz conversion for
different values of the nonlinear absorption coefficients. We apply our
analysis to some experiments reported to date, in order to understand to what
extent multiphoton absorption could have played a role and also to predict the
maximum achievable conversion efficiency at higher peak pump intensities.
