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Facilities
Laser ARPES
Laser-based ARPES represents the next step in the evolution of this venerable experimental technique. Traditionally, UV-ARPES has been performed with light from either a synchrotron source or from a helium lamp. By pushing to lower photon energies we are able to make many gains from an experimental point of view over the higher-energy sources. One distinct advantage is an inherently higher momentum resolution, as momentum uncertainty scales directly with electron kinetic energy. Also, these laser sources are an order of magnitude or more brighter than a similarly monochromatized line from a synchrotron source or He lamp, which in turn allows us to work at finer energy resolutions. Additionally, it has been shown that electron mean free paths increase dramatically with decreasing energy. This mean free path determines the depth which ARPES is able to probe in the sample being measured, therefore working at lower photon energies corresponds to making this traditionally highly surface sensitive measurement more bulk sensitive.
Spin Resolved ARPES
Besides its mass, an electron bears two fundamental properties: charge and spin. The world has witnessed the electronic revolution by utilizing its charge (electronics), so people are now trying to exploit its spin as well. Spin resolved PES enables us to detect the spin orientation of the photoelectron, thus its spin-state back in the specimen – this will greatly extend our understanding of the materials’ spin-properties and thus will facilitate us to find methods to use them in constructing spintronics. The general principle of the spin-resolved PES is illustrated in Fig.1 a, where we add a spin-polarimeter to a regular PES analyzer to analyze the spin polarization.
Time Resolved ARPES
Being able to observe the electronic and spin state of the electron in materials, we are now preparing to attack the ultimate goal of material physics - how matters form and how they transform; or in other words, the electron dynamics. To do so, we must advance into the atomic reaction time scale (femptosecond). With the development of the ultra-fast laser technology, we now have the capability to observe the real time electron dynamics directly. In time resolved PES (Fig.2 below), we first excite the specimen by a “pump” pulse, then detect the electron state by sending in a “probe” pulse after a time delay Dt. By relating the photoelectron’s property with Dt, we can extract the evolution of the electronic state with time, thus reconstruct the electron’s dynamics.
