We developed a system for performing angle-resolved laser photoemission. This is based around a frequency quadrupled Ti:sapphire oscillator running at 100 MHz. The high repetition rate is very helpful for obtaining a high signal to noise while keeping the instantaneous electron emission rate low. This last aspect is critical for keeping the electronic response of the sample in the linear regime and to minimize space-charge and other spurious effects.
* By this we mean ARPES in which the dispersion of an electronic state can be directly studied.
Bulk Sensitivity of Laser ARPES
One of the major advantages of Laser-ARPES is the decreased surface sensitivity of the spectroscopy, which can potentially open the technique up to the study of many materials systems on which one can not prepare perfect surfaces. The surface sensitivity of conventional photoemission is determined by the electron mean-free-path, which is the mean distance a photoelectron can travel between scattering events as it tries to leave the sample. The photons typically travel in much much deeper than the electron mean free paths and so are irrelevant to the surface sensitivity.
A compilation of electron mean free paths for many different materials is shown in the figure below. Our new laser-ARPES experiments at the low kinetic energy side (near 6 eV) have an increase in the bulk sensitivity of up to an order of magnitude over the standard ARPES range of 20-50 eV (the exact increase depends somewhat upon the material under consideration). We also note that much effort has recently been put into performing ARPES at the high kinetic energy side, although these experiments suffer greatly from reduced signal as well as reduced energy and momentum resolution.
Pump-Probe ARPES for electron dynamics
Another advantage of using femtosecond lasers for ARPES is the ability to get the electron dynamics through pump-probe experiments, in which the first photon excites an electron to an unoccupied state and the second photon ejects this electron before it has a time to decay. By varying the time delay between the pump and probe photons one can directly access the decay lifetimes of the excited state electrons. Pump-probe experiments are now fairly common in optics fields, especially after Ahmed Zewail of Caltech received the Nobel prize in Chemistry for his pioneering studies which could watch the chemical bond form in real-time. These experiments typically are performed by studying changes in the optical constants (e.g. through reflectivity of a photon beam). The way in which the optical constants vary with electronic excitation are very complex and so it is often hard to deconvolve the relevant electronic physics. ARPES gives much more detailed information about the electronic states, and so pump-probe ARPES experiments may be critical for solving the more difficult and subtle problems of electron dynamics.