Observation of sub-kilohertz resonance in Rf-Optical double resonance experiment in rare earth ions in solids

We have observed kilohertz and sub-kilohertz resonance structures in RF-optical double resonance experiments of rare-earth-doped solids, when the frequency of the RF ® eld is scanned across the hyper® ne transitions while monitoring the resonant optical absorption of a CW laser. The eå ect is observed only when the laser spectral width is broad compared to the hyper® ne structure. The observed line widths are apparently free of the inhomogeneous widths of hyper® ne levels and the line shape has peculiar double peak structure. The eå ect is modelled with a resonance involving three atomic levels interacting with three electromagnetic ® elds, two optical and one RF, in a triangular or ` delta’ con® guration. While the ordinary opticalRF two-® eld resonance is limited by spin inhomogeneous width, the simultaneous excitation of three coupled transitions leads to narrow and highly nonlinear resonance structures that are not averaged by the inhomogeneous distribution of hyper® ne transition.