An Ultrafast Photo-Electron Diffractometer.

Abstract

Ultrafast laser pulses - optical pulses shorter than a picosecond - result in rapid processes occurring at both the surface and the interior of solid materials. Understanding these processes requires ultrafast probes; optical probes (reflectivity, spectral) are suitable for some surface studies, but the tracking of structural changes are well suited to x-ray and electron diffraction. An ultrafast photo-electron diffractometer is a tool for tracking structural changes such as thermal expansion, melting and super-heating, crystal phase changes, ionization, and more.

The design and operation of an ultrafast photo-electron diffractometer is detailed, and its successful operation is demonstrated by sub-picosecond recording of strain in a free-standing polycrystalline platinum film of 9 nm thickness subjected to a fluence of 2 mJ/cm2 from 150 fs laser pulses. The temporal profile of the relative change of strain is used to determine corresponding temperatures changes; for the (311) peak an increase of 70 K is noted within 10 ps. The increase in temperature takes place at a very nearly linear 7 K/ps. The (111) peak heats more rapidly, reaching 84 K in 6 ps, and is also nearly linear at 14 K/ps. A temporal relationship is found which connects the phonons in different directions with energy transport: the rate of change of temperature per phonon oscillation period is the same in both directions, indicating that thermalization of phonons in polycrystalline platinum is coupled to the actual vibration rate.

Reflectivity data shows rapid, coherent oscillations, but slower than acoustic phonons. These appear to be connected to the nanoparticle network structure of the ultrathin film; further work is planned to unravel these unexpected results.

A new, in-situ method for the determination of time-zero - when the pump and probe pulses are temporally coincident at the sample - is demonstrated, and shown to be quick, reliable, and precise to within half a picosecond.