Raman Heterodyne Detection of NMR
A novel coherent Raman effect induced by an optical and a radio frequency (rf) field is used to detect nuclear magnetic resonance (NMR) in solids. The technique, which employs heterodyne detection, is capable of monitoring coherent spin transients or nuclear resonances under cw conditions, both in ground and excited electronic states and with high sensitivity and precision. We illustrate the versatility of the method in a dilute rare earth impurity ion crystal, Pf3+: LaF3, where Pr3+ spin echoes of nuclear quadrupole transitions are detected not only in the 3H4 ground electronic state but also for the first time in the 1D2 excited state. From the cw spectrum, the Pr3+ hyperfine splittings in these electronic states as well as the magnetically broadened inhomogeneous lineshapes and widths are determined with kilohertz precision, about a five-fold improvement over earlier measurements. In Fig. 1, the Pr3+ electronhyperfine energy level diagram reveals the basic stimulated Raman process where two coherent fields, one at the optical field ωE (solid arrow), and the other at the rf frequency ωH (squiggle arrow) drive two coupled transitions resonantly. The electric dipole allowed optical transition, and the magnetic dipole allowed quadrupole transition to combine in a two-quantum process to generate a coherent optical field at the sum frequency ωE + ωH (dashed arrow). In addition, a Stokes field at the difference frequency ωE − ωH (not shown), due to the second resonant packet, accompanies the above anti-Stokes field. To predict the characteristics of the Raman heterodyne beat signal, we have performed a new three-level perturbation calculation, which yields a dispersive or absorptive near-Gaussian lineshape (opencircles, Fig. 1) in close agreement with experiment (solid curves).