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We review X-ray parametric down-conversion, one of the second order nonlinear optical process, into the optical region, focusing on both the fundamental and the application sides. First we see that the local optical property determines the second order nonlinear susceptibility and how to measure it. Then we reconstruct the microscopic structure of the second order nonlinear susceptibility, and discuss what it means. Finally, we consider possible applications at synchrotrons and X-ray free-electron lasers. |