Inverse Fourier transform technique to determine second-order optical nonlinearity spatial profiles

A technique is described to measure the second-order nonlinearity spatial profile of nonlinear films. It consists of placing two identical samples back-to-back to create a symmetric nonlinearity profile and measuring the Maker fringes of this sample. Because the profile is symmetric, both its shape and location inside the film can be determined uniquely from the Maker fringe data. Applied to a sample of thermally poled silica, this method reveals a nonlinearity profile that is both deeper (∼25 μm) and stronger (peak d33=0.8 pm/V) than previously believed, with spatial features that could not be resolved with previous techniques. This powerful principle is applicable to many other physical problems in which the amplitude, but not the phase, of the Fourier transform can be measured.