Measurement of the thin nonlinearity profile of poled silica by the Maker fringe technique has been impossible because of total internal reflection (TIR) at the back surface of the sample. We demonstrate that this limitation can be removed by placing a prism against each face of the sample, thus avoiding TIR. This novel technique allows, for the first time to our knowledge, the nonlinearity profile of a thin film to be inferred by the Maker fringe technique. Applied to a silica sample thermally poled under standard conditions (275 °C and 5.3 kV for 30 min), it suggests a Gaussian profile with a width of and a maximum of 0.34 pm/V.
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