Combining electrons and lasers to create designer beams for materials research

Tabletop-created designer beams of EUV and soft x-rays enable ultrafast spectroscopy measurements that are sensitive to the magnetic and electronic structure of materials. Ultrafast dynamics of energy flow (on the timescales of electron transitions and atomic lattice vibrations) are critical to understanding and discovering materials for energy-conserving next-generation electronics and information storage.

After a material is excited, the first electronic motions are fundamental processes that enable transport of energy and information. Snapshots of these ultrafast motions will further our understanding of the electronic structure of materials. However, probing the dynamics of electronic transitions and atomic-level energy flow requires using techniques with ultrafast time resolution. Here ultrafast is defined as the timescale of an electron transition or a single atomic vibration. Ultrafast responses can be probed with specialty pulsed beams.

The beams can both excite the system and probe it stroboscopically. Such ultrafast "pump and probe" experiments require light that ranges into the EUV and soft x-ray regime, with unique spectral shape and polarization (that is, the direction of electromagnetic forces that the light creates in relation to the direction of the beam). High-power lasers from tabletop systems have been employed to create designer beams. The key to the technique is to match the sequence of crests in the wave of the laser beam with the wave of the outgoing designer beam, as they both travel through a gas.

Two groups have recently been successful in doing just that: (1) One group created a pulsed beam with circular polarization that can detect the orientation of magnetic spins in a material through polarization dependent x-ray absorption. (2) The other created sharp pulses of energetic photons at a high repetition rate. These pulses can be used to measure photo-emitted electrons in small, repeated bunches before the electrons spread apart due to electrostatic repulsion. These new probes reveal information about the electronic structure in a material, such as the energy of electrons versus the momentum and direction traveled in a solid.

The new probe sources will enable important studies of materials on the timescale of the electronic motion (both magnetic and charge) after an excitation -- the fundamental processes that enable energy and information transport in solids.

This work was supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences; USA-Israel Binational Science Foundation, Israeli Centers for Research Excellence Program of the Planning and Budgeting Committee, and the Israel Science Foundation; the German Academic Exchange Service; the Deutsche Forschungsgemeinschaft; the Swedish Research Council; Air Force Office of Scientific Research-Defense University Research Instrumentation Program (laser system), and the National Science Foundation.