Second-harmonic generation of nonlinear optical crystals in vacuum-ultraviolet and x-ray regions

In spite of high absorption, nonlinear optical crystals have a high nonlinear susceptibility in the vacuum-ultraviolet and the x-ray regions, providing a basis for second-harmonic generation. In the conventional transmission configuration, a crystal film {~}100 {} thick may generate sufficiently strong second harmonics with a few tenths conversion efficiency under a double-resonance condition. The phase-matching restriction is almost lifted, as a result of the tiny thickness. However, the reflection configuration is shown to be unique and promising under the double-resonance condition. The conversion efficiency may also reach a few tenths, while the crystals need not be thin films, and the problem of heat dissipation is relieved.