The "band structure" of a solid, or more simply the energy of electrons within the solid that controls its fundamental material properties such as conductivity, transparency, and even exotic states of matter such as superconductivity, is historically measured using angle-resolved photoemission spectroscopy (ARPES). Unfortunately, ARPES—wherein high-energy ultraviolet or x-rays bombard a solid, kicking out electrons that are then studied to reveal band structure—requires vacuum conditions that prevent measurement of ambient states of solids and can only see a thin layer of material, as deeper electrons remain buried.
Alternatively, a new technique from the University of Ottawa (Ottawa, ON, Canada) studies the high-energy photons created through high-harmonic generation within a solid that is illuminated with intense, low-energy infrared (IR) laser beams. These IR beams generate electrons and holes, and are set on a collisional path. The addition of a dimmer, higher-frequency optical laser field perturbs the path and induces a spectral signature on the high-harmonic light emitted upon collision. This signature is then used to dictate what band structure produced the result. Unlike ARPES, this IR laser field technique can study bulk solids, materials in high magnetic fields, and potentially even materials enclosed in highly pressurized diamond-anvil cells. Reference: G. Vampa, SPIE Newsroom (Aug. 31, 2016); doi:10.1117/2.1201608.006643.
