Measurements of optical phase variations using interfering multiphoton ionization processes

We have used interfering multiphoton ionization processes to measure changes in the relative phase between two optical laser beams. In this work, the phase variation is due to the {π} phase shift of a focused Gaussian beam as it propagates through the focal region. An array of linear collection electrodes is used to measure the multiphoton ionization rate as a function of the distance from the beam waist when two laser fields are resonant with an atomic transition, one through a linear process, the other through a three-photon process.