Spinning the (X-ray) Light Fantastic

Delta undulator provides researchers with a new tool to probe chiral materials with widespread utility in agriculture and pharmaceuticals.

X-ray
The X-rays polarization is controlled by precisely adjusting the position of four longitudinally moving rows of magnets. Image courtesy of SLAC National Accelerator Laboratory

The Science

Scientists can now measure changes in magnetic materials that occur faster than a trillionth of a second, as well as fleeting processes that involve chiral compounds—“left-handed” and “right-handed” variations that affect a compound’s interactions with its environment. The measurements come thanks to researchers at SLAC National Accelerator Laboratory’s Linac Coherent Light Source (LCLS) who installed a new device called the Delta undulator. In this device, X-rays vibrating in a corkscrew-like pattern as they travels forward, known as “circularly polarized” light, allow researchers to take snapshots of magnetization dynamics for individual chemical elements.

The Impact

For the first time, X-ray scientists have access to wavelength-tunable circularly polarized free-electron laser pulses in the range between 280 and 1200 eV. Several types of experiments can benefit from the short and intense pulses ranging from studies of ultra-fast magnetization to chiral molecules , with relevance in understanding the function of many substances in biological and chemical research, including certain amino acids and sugars, pharmaceuticals, and pesticides.

Summary

At the LCLS, researchers have demonstrated the generation of X-ray pulses with polarization control at photon energies from 280 eV to 1200 eV, by using a new device called the Delta undulator. In the Delta undulator, four rows of strong magnets shift to control the wiggling motion of the electrons and, therefore, the polarization of the produced X-rays. The LCLS was optimized to produce hundreds of microjoules of circularly polarized X-rays. The pulses last for tens of femtoseconds and are fully circularly polarized within the accuracy of the measurement. Scientists can use circularly polarized X-rays to investigate the magnetization dynamics for individual chemical elements. The short durations allow taking snapshots for ultra-fast processes. The team also demonstrated two-color pulses for X-ray pump/X-ray probe experiments, with polarization control of the probe pulse. They controlled photon energy separation up to 2.4  percent and with time delays up to 950 femtoseconds. In this configuration, the first X-ray pulse selectively triggers fleeting changes in a chiral molecule and the second pulse explored the induced dynamics.