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

Abstract

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 ea 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 ea ect is modelled with a resonance involving three atomic levels interacting with three electromagnetic ® elds, two optical and one RF, in a triangular ordelta' con® guration. While the ordinary optical- RF two-® eld resonance is limited by spin inhomogeneous width, the simul- taneous 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. We have observed a peculiarly narrow resonance structure in the course of optical-RF double resonance experiment of rare earth ions in solids. The reso- nance is characterized by a sharp doublepeak structure in the absorption spectrum as the RF frequency is scanned across the hyper® ne transition, with the few kilohertz width of the transparency hole at least an order of magnitude narrower than the spin inhomogeneous width. The ea ect is evident only when the laser bandwidth is larger than the hyper® ne level splittings. A theoretical model is presented based on the assumption that the optical ® elds of a noisy laser simul- taneously couples an optically excited state with two hyper® ne levels of the ground state, which in turn are acted on by the RF ® eld. A quantum interference ea ect among the three coupled transitions creates sharp fringes in the frequency domain that are not averaged out by ensemble average over the inhomogeneousbroadening of the optical or spin transitions. The model is found to be consistent with the main features of the experimental observations. Various quantum interference ea ects in atomic transitions have recently been research topics of great interest, because of the rich and often counter-intuitive