Direct observation of the effective χ^(2) grating in bulk glasses encoded for second-harmonic generation

The effective chi((2)) grating responsible for second-harmonic generation in bulk glasses was spatially resolved with a charge-selective etching technique. We observed a spatially varying structure with a period equal to the coherence length of the material. The measured period of the grating showed excellent agreement with the expected value. In addition, cross-sectional etching patterns show that in this regime of encoding intensities the f lux responsible for the intense electric field is not unidirectional as predicted by most models.