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-opt ical double resonance experiments of rare-earth-doped solids, when the freq uency of the RF field is scanned across the hyperfine transitions while mon itoring the resonant optical absorption of a CW laser. The effect is observ ed only when the laser spectral width is broad compared to the hyperfine st ructure. The observed line widths are apparently free of the inhomogeneous widths of hyperfine levels and the line shape has peculiar double peak stru cture. The effect is modelled with a resonance involving three atomic level s interacting with three electromagnetic fields, two optical and one RF, in a triangular or 'delta' configuration. While the ordinary optical-RF two-f ield resonance is limited by spin inhomogeneous width, the simultaneous exc itation of three coupled transitions leads to narrow and highly nonlinear r esonance structures that are not averaged by the inhomogeneous distribution of hyperfine transition.