Raman Heterodyne Detection of NMR

Abstract

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[1]. 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 ω_E (solid arrow), and the other at the rf frequency ω_H (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 ω_E + ω_H (dashed arrow). In addition, a Stokes field at the difference frequency ω_E − ω_H (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).