Tabletop X-rays light up : Nature News: The tabletop sources rely on a technique called high-harmonic generation, in which laser light is passed through a medium that converts it to light of shorter wavelengths and higher frequencies. Shine a ruby laser into a quartz crystal, for example, and a beam of ultraviolet light comes out — albeit dimmer, but still focused like a laser beam.
Murnane and Kapteyn have pushed high-harmonic generation to its limits, with a system that uses an infrared laser as the source and pressurized helium gas as the medium. The laser creates a strong electric field, which draws electrons away from the helium atoms, allowing the electrons to absorb energy from the electric field. When they slam back in to the helium atoms, they release that absorbed energy as shorter-wavelength photons — but only about one photon comes out for every 5,000 infrared photons that are put in.