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Real-time monitoring of all-optical poling by two-beam second-harmonic generation

We demonstrate a simple method for monitoring all-optical poling in real time. The susceptibility pattern created by two collinear beams, a writing beam at the fundamental frequency and a seed beam at the secondharmonic frequency, is reconstructed by two-beam second-harmonic generation due to the writing beam and a non-collinear probe beam at the fundamental frequency. This leads to spatial separation of the seeding and signal beams, and allows easy detection of the signal during the seeding period. When the probe beam is sufficiently weak, it does not distort the collinear poling process. Furthermore, the method provides a significantly stronger signal than monitoring based on a probe beam alone.