Photon-Gated Hole Burning

The hole burning mechanism in MgS: Eu systems takes place in the 4f⁷-4f⁶5d electronic transitions and implies two-photon excitation process. Initially, a photon with energy E₁ excites the electron from the ground state to the excited state. A second photon with energy E₂ further excites the electron to the conduction band where it becomes delocalized and can be captured by a nearby trap. This mechanism is known as Photon Gated Hole Burning (PGHB). An  interesting  feature of this mechanism is the fact that Eu³⁺ ions are acting as electron traps. This is because the action spectrum for hole erasure follows the 4f⁷-4f⁶5d absorption profile of Eu²⁺, which would be the photoproduct of Eu³⁺ electron trap.

Thus, the site a  Eu²⁺  ions are selectively excited by the narrow band laser and  the site b  Eu²⁺  are randomly distributed over the inhomogeneous line. As shown in the above schematic diagram, the source for the first photon(E1) is a very high resolution laser and the source for the gating photon (E2) is a Tungsten lamp, and phase sensitive detection is used to detect the spectral holes.