The researchers use a technique called high-harmonic generation (HHG). HHG was first discovered in the late 1980s, when researchers focused a powerful, ultra-short laser beam into a spray of gas. The researchers were surprised to find that the output beam contained a small amount of many different wavelengths in the ultraviolet region of the spectrum, as well as the original laser wavelength. The new ultraviolet wavelengths were created as the gas atoms were ionized by the laser.
"Just as a violin or guitar string will emit harmonics of its fundamental sound tone when plucked strongly, an atom can also emit harmonics of light when plucked violently by a laser pulse," adds Murnane. "The laser pulse first plucks electrons from the atoms, before driving them back again where they can collide with the atoms from which they came. Any excess energy is emitted as high-energy ultraviolet photons."
Like many phenomena, when HHG was first discovered, there was little science to explain it, and it was considered more a curious phenomenon than a potentially useful light source. After years of work, scientists eventually understood how very high harmonics were emitted, however there was one major challenge that most researchers gave up on--for most wavelengths in the X-ray region, the output HHG beams were extremely weak.
Murnane, Kapteyn and their students realized that there might be a chance to overcome that challenge and turn HHG into a useful X-ray light source--the tabletop-scale X-ray laser that has been a goal for laser science since shortly after the laser was first demonstrated in 1960.
"This was not an easy task," says Murnane. "Unlike a laser--which gets more intense as more energy is pumped into the system--in HHG, if the laser hits the atoms too hard, too many electrons are liberated from the gas atoms, and those electrons cause the laser light to speed up. If the speed of the laser and X-rays d
|Contact: Josh Chamot|
National Science Foundation