Lasers have been used for photoemission for many years, especially for the study of electron dynamics at surfaces.  Reviews of this field exist, such as that by R. Haight of IBM.  This type of work typically has been done with low repetition-rate laser systems (kHz or less) which give low counting efficiency, and they have mostly been carried out with electron analyzers which were not capable of measuring the band dispersion of an electronic state, i.e. they measured the angle-integrated photoemission spectrum.  While still very powerful, angle-integrated photoemission does not allow one to uncover the E vs. k (energy vs. momentum) dispersion relation of an electronic band state - information which is typically considered a starting point for understanding the electronic behavior of a solid. 

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.

We have recently made the first demonstration of true* Laser-ARPES (Angle Resolved Photoemission Spectroscopy) at our home lab in Colorado.  The data below shows the measured dispersion (Energy vs. momentum) for electronic states from the high T_c superconductor Bi₂Sr₂CaCu₂O₈ along the "nodal" direction of the Brillouin zone.  The two right panels were taken at the synchrotron (Advanced Light Source, Berkeley) using their most powerful undulator beamlines.  The left panel was taken using our laser-ARPES system and shows dramatically sharper spectral peaks.  This increase in the spectral resolution is due to improvements in the energy and momentum resolution of the experiment, but also may be due to other issues such as a decrease in the final-state electron broadening effects and an improved bulk sensitivity (see below).  The increase in the spectral resolution can give a much greater window into the inherent electronic physics of these exotic materials, allowing us to measure the details of the "quantum dance" that the electrons are undergoing in these materials.  Ultimately we hope this kind of information will enable us to uncover the interactions responsible for the superconductivity in these amazing materials.  Click here for a preprint of this work, or go to http://arxiv.org/abs/cond-mat/0508404.

    * 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.