a crystalline bar is doped with a rare earth . the bar is transparent to ration , such as from a laser , at a light wavelength which excites the rare earth ions in the bar and the light is modulated at the frequency at which the bar mechanically resonates . the excited ion decay radiatively by light or non - radiatively by phonons . as the phonons decay , they excite the bar to resonate mechanically . the mechanical resonance may be coupled by a piezoelectric material and measured to give information respecting the phonon activity of the excited rare earth ions .