With four moving rows of magnets, the Delta undulator can create circularly polarized, or spiraling, light. Credit: SLAC National Accelerator Laboratory
A new device at the Dept of Energy’s SLAC National Accelerator Lab allows researchers to explore the properties and dynamics of molecules with circularly polarized, or spiraling, light. The use of polarized light is important in the study of many molecules and processes that affect our everyday lives. It can be used to tell the difference between chiral molecules with LH and RH variations, which affects everything from sense of smell and taste – eg difference between oranges and lemons, or spearmint and caraway seeds – to life-altering drugs such as thalidomide, in which one version helps ease nausea, but the other can cause abnormal limb growth in unborn children.
With the new Delta undulator, the Linac Coherent Light Source (LCLS) X-ray laser can now be tailored to look at changes in magnetic materials happening faster than a trillionth of a second, as well as fleeting processes that involve chiral compounds central to areas of biological and chemical research.
LCLS generates extremely short, bright pulses of X-ray laser light by sending an electron beam through what’s called an undulator. The undulator contains pairs of magnets that force the electrons to wiggle. This motion gives off energy in the form of X-rays, which interact with the electron beam to form laser pulses that can be used for experiments. Before Delta, the light delivered to experimental stations was always linearly polarized ie restricted to one direction. But circularly polarized light vibrates in 2 directions, producing a pattern like a corkscrew.
With the Delta, 4 rows of strong magnets shift to polarize X-rays in a linear, elliptical, or circular fashion. Scientists can use the spiraling light to reveal the orientation of molecules in certain materials, and even provide subatomic details as fine as electron distribution and spin. Researchers in Italy recently extended this ability into the extreme ultraviolet regime, using the FERMI Free Electron Laser. The beam at LCLS now opens doors to experiments using X-rays, which are able to probe matter in wholly new ways.
There are several types of experiments made possible by circularly polarized light. People who study magnetic storage for computing, for example, use spiraling light to watch magnetization changes to develop new methods and materials for faster and more compact storage devices. Now, with the power of the world’s strongest X-ray laser, the spiraling light can be delivered in extremely short and intense pulses over a wide range of energies.
The researchers can also gather the needed data quickly. The spiraling light produced by the Delta is nearly 100% polarized and orders of magnitude brighter than light produced by any other type of X-ray source with such short pulses. This enables measurement of ultrafast magnetism with unprecedented accuracy and speed.
This light can be used to study how X-rays trigger precise, fleeting changes in chiral molecules like amino acids, and researchers can create snapshots of how radiation damages the molecular building blocks of our bodies.
Research and development is underway for multiple Delta-II undulators that will produce spiraling light compatible with the beam of LCLS-II, the next generation of LCLS. LCLS-II will be 10,000 times brighter, on average, than LCLS, enabling high precision studies of even finer aspects of ultrafast magnetism and chirality. The Delta team will develop even more ways to manipulate polarized light. One scheme involves delivering X-rays of different energies and polarizations in a single experiment. https://www6.slac.stanford.edu/news/2016-06-15-spiraling-light-slac-x-ray-laser-offers-new-glimpses-molecules.aspx
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