The electron and phonon are two fundamental particles (quasi-particles) of condensed matter and their interplay in single crystals can create many interesting physical phenomena, such as polaritons, charge density waves and up-conversion fluorescence. In laser crystal, the electronic transitions of an active ion can be manipulated by its surrounding lattice vibrations, whereby the emitting photon energy gradually decreases or increases by the creation or annihilation of quantized phonons.
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Source: Phys.org