  A method based on optical interferences for measuring optical nonlinearities
is presented. In a proof-of-principle experiment, the technique is applied to
the experimental determination of the intensity dependence of the
photoionization process. It is shown that it can also be used to control and
optimize the nonlinear process itself at constant input energy. The presented
strategy leads to enhancements that can reach several orders of magnitude for
highly nonlinear processes.
