Canadian scientists have announced a new technology for measuring the electronic energy band structure of solid materials. The new method does not require the sample to be placed in a vacuum chamber, but it can also detect a large number of samples, something that some other techniques can not do. The group believes its new method is well suited for extreme research conditions, including diamond anvils under very high pressures.
Angle-resolved emission spectroscopy (ARPES) is an important method based on the use of lasers to study the electronic band structure of solid materials. The photons of the material presented from the sample have sufficient energy to eject the electrons, which in turn measure the energy and momentum of the electrons they emit. Suppressing the energy and momentum of their internal electrons, which reveal the structure of their electronic band.
Angle-resolved emission spectroscopy (ARPES) has been used extensively by physicists in materials research, including semiconductors and superconducting materials, but this technique has some important limitations. The measurement must be done at a very high vacuum (UHV) because the emitted electrons are dispersed and absorbed by the air. In addition, ARPES probes only a thin layer of material on the surface of the material because electrons can not escape deeper in the material.
Now, Paul Corkum and his colleagues, at the National Institute of Science and the National Research Council of Canada at the University of Ottawa, have developed new all-optical technologies to study solid energy band structures that overcome these problems.
The technique involves exposing the sample to intense laser pulses, but the photon energy is much lower than the energy of the electrons ejected from the material. Associated with such a pulse is a very large electric field that causes an electron to pass through the quantum tunnel from the top of the valence band to the bottom of the conduction band, thus creating holes in the conduction band. Electrons and holes are driven by the electric field to reach high momentum in the opposite direction. The electric field itself is oscillating, and as the direction of field changes, both electrons and holes undergo inversion and aggregation. At this point, electrons and holes recombine, emitting a photon escape material and for detection. The energy of a photon equals the energy gap between the valence band and the conduction band at the recombination point.
To measure the momentum of the electrons when recombining, Corkum and colleagues used darker, differently colored laser pulses to illuminate the sample at the same intensity for the same intensity. By measuring the emitted light intensity as a function of the phase between two laser pulses and the emitted light, the team can calculate the momentum of the electrons recombining the emitted photons.
The electron-hole recombination process occurs very fast, and the combination of very short laser pulses means that the technique can be used to study the band structure that changes in a very short period of time.
Corkum said that this technique can prove particularly useful for studying materials under greater stress on diamond anvils because diamond is transparent relative to the laser pulses used for the measurements. This method can be used to observe how the energy band structure of a material changes during catalysis and other chemical processes, and this can not be studied under super-vacuum. In the study, the material was carried out under a very high magnetic field, which may also deflect the electrons of ARPES.
This technique is published in Physical Review Letters.
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