Looping X-rays to produce higher quality laser pulses

A proposed device could expand the reach of X-ray lasers, opening new experimental avenues in biology, chemistry, materials science and physics.BY ALI SUNDERMIER

Ever since 1960, when Theodore Maiman built the world’s first infrared laser, physicists dreamed of producing X-ray laser pulses that are capable of probing the ultrashort and ultrafast scales of atoms and molecules.

This dream was finally realized in 2009, when the world’s first hard X-ray free-electron laser (XFEL), the Linac Coherent Light Source (LCLS) at the Department of Energy’s SLAC National Accelerator Laboratory, produced its first light. One limitation of LCLS and other XFELs in their normal mode of operation is that each pulse has a slightly different wavelength distribution, and there can be variability in the pulse length and intensity. Various methods exist to address this limitation, including ‘seeding’ the laser at a particular wavelength, but these still fall short of the wavelength purity of conventional lasers.

Read more on the SLAC National Accelerator Laboratory website

Image: Schematic arrangement of the experiment. The researchers send an X-ray pulse from LCLS through a liquid jet, where it creates excited atoms that emit a pulse of radiation at one distinct color moving in the same direction. This pulse is reflected through a series of mirrors arranged in a crossed loop. The size of this loop is carefully set so that the pulse arrives back at the liquid jet at the same time as a second X-ray pulse from LCLS. This produces an even brighter laser pulse, which then takes the same loop. The process is repeated several times, and with each loop the laser pulse intensifies and becomes more coherent. During the last loop, one of the mirrors is quickly switched allowing this laser pulse to exit.

Credit: (Greg Stewart/SLAC National Accelerator Laboratory)