A material's ultrafast behavior depends on the energy of the pump photons, which determines the initial energy of photoexcited electrons. In the Weber lab, we use 1.5-eV (800-nm) photons produced by a Ti:Sapph laser operating at a repetition rate of 80 MHz. Working with collaborators, we also do experiments using mid-infrared pump photons, which correspond to the energy of Dirac electrons in topological materials. 

The probe photons also matter: if the probe consists of X-ray or ultraviolet photons, then instead of time-resolved reflectivity an experiment may time-resolve diffracted X-rays (determining atoms' instantaneous positions) or photoemitted electrons (determining the electrons' instantaneous energy-vs-momentum dispersion). The Weber lab is using these and other elaborations of the pump-probe technique, described below.

If the X-rays come in a sufficiently short monochromatic pulse, then ultrafast X-ray diffraction is possible: the atoms' positions can be timed to precision better than 100 fs, allowing an ultrafast measure of their motion. Such pulses are produced by X-ray "free-electron lasers" (XFELs), large facilities of which just five have been built, beginning in 2009. 

The Weber lab seeks to use ultrafast X-ray diffraction to measure coherent phonons--the rapid, in-sync oscillation of atomic positions that can occur after exciting a material with a short laser pulse. As part of our project on optical control of topological materials (see Research Topics), we are exploring coherent phonons in SrMnSb2 and other Dirac and Weyl semimetals. It is predicted that, when a coherent phonon causes the atoms to move far enough, it may rapidly and reversibly change these materials' electronic properties.

Particles' random thermal motion tends to take them away from regions of high density and into regions where they are less dense, a process known as diffusion. (The famous Brownian motion is an example of diffusion.) A particle's diffusivity depends on both its speed and how frequently it suffers collisions, so a measurement of diffusivity can provide valuable microscopic insight into particles' motion. Transient-grating (TG) spectroscopy allows measurement of the diffusivity of photoexcited species like electrons, holes, phonons, or electrons' spins. 

TG spectroscopy is a variation on optical pump-probe spectroscopy, in which a sample is photoexcited by two pump pulses arriving simultaneously and obliquely. Where the pulses overlap, their interference gives rise to a sinusoidal pattern of bright and dark regions and a corresponding sinusoidal pattern in the population of photoexcited electrons--the eponymous "grating." As the electrons diffuse, they tend to lower the grating's peaks and fill in its valleys, reducing the grating's overall modulation. The rate at which the grating decays thus encodes information about the electrons' diffusivity.

TG spectroscopy is particularly useful in semiconductor spintronics (see Research Topics), because a simple variation in the experiment allows the excitation and measurement of a grating of the electrons' spin-orientation.