Bright Coherent Ultrahigh Harmonics in the keV X-ray Regime from Mid-Infrared Femtosecond Lasers

From Long to Short

When you play a string instrument, you raise the frequency, or pitch, of the note by shortening the vibrating portion of the string: Drop the length in half, and you hear a harmonic at double the frequency. It is possible to do essentially the same thing with light waves by using selective excitation and relaxation processes of the electrons in crystals or high-pressure gases through which the beam of light is directed to produce light harmonics. Over the past decade, researchers have been optimizing the conversion of red light to the far edge of the ultraviolet, which corresponds to tens of harmonics. Popmintchev et al. (p. 1287 ) now show that mid-infrared light can undergo a process in high-pressure gas to generate ultrahigh harmonics up to orders greater than 5000 in the x-ray regime.