Low-emittance ultrafast electron sources
An important figure of merit for an electron source is its emittance, which determines how well the electron beam can be collimated and focused. Emittance is also related to the degree of coherence of the electron beam, which quantifies fringe contrast in an interference experiment. The highest resolution electron microscopes use low-emittance sources based on nanoemitters–sharpened metal wires ending in radii of curvature of 10-100 nm. We use ultrafast lasers to trigger electron emission from nanoemitters, which results in a low-emittance ultrafast electron source.
P. Hommelhoff, C. Kealhofer, M. A. Kasevich, Phys. Rev. Lett. 97 247402 (2006)
Electron microscopy is a powerful tool for studying matter, with an enormous variety of imaging modalities and contrast mechanisms. High resolution transmission electron microscopy can provide sufficient spatial resolution to directly observe the positions of atoms in a material, and simpler techniques such as electron diffraction also provide information about how the atoms are arranged.
When the electrons in a solid are excited with a short laser pulse, this can launch a change in the position of the atoms. Does the motion of the atoms lead or follow changes in the other physical properties (such as electrical and magnetic properties)? Observing what happens can provide insight into the interactions between electrons, nuclei, and spins in a material. This interplay is at the core of many difficult problems in condensed matter physics, including the quest to understand exotic materials like high-temperature superconductors.
Studying ultrafast processes is technologically challenging, in part because detectors respond orders of magnitude too slowly. This is like to trying to photograph a moving object with a shutter speed that is too slow, as illustrated in the first picture of Emily and Iona juggling. [Thank you to Ellery Galvin for taking the pictures!].
Pump-probe techniques can get around the slow-detector problem by using an ultrafast probe pulse, which only illuminates sample for a short time. An ultrafast pump pulse initiates dynamics in the system and is followed by an ultrafast probe pulse which interacts with the system and is then detected. Even though the detector measures for a long time, the probe only interacts for a short time so that the measured data reflects the state of the sample at the time it was probed.