Experimental Observation of Second Harmonic Generation at X-Ray Wavelengths
X-ray free-electron lasers operating in the angstrom regime have recently become available. These new systems, which deliver femtosecond-scale pulses with peak intensity exceeding 1018 W/cm2, open the path to new experiments in the field of nonlinear optics at x-ray wavelengths. Recently, we have observed phase-matched x-ray second harmonic generation of a pump beam at 1.7 Å. I will describe the experiment, and the theory of second order nonlinearity at x-ray wavelengths. I will discuss future directions, including the possibility of using second harmonic generation as a temporal correlator for diagnostics of ultrafast phenomena in the x-ray regime.