Raman heterodyne detection of nuclear magnetic resonance

Abstract

A novel coherent Raman effect induced by a laser and a radio-frequency (rf) field is used to detect cw and pulsed nuclear magnetic resonance (NMR) in ground and excited electronic states. The effect is illustrated in the impurity-ion solid Pr3+: LaF3 at 1.6 K utilizing the Pr3+ optical transition H43(1)'D21(1). The laser field of frequency E and the rf field (H) induce a light wave at the sum E+H (anti-Stokes) and difference E-H (Stokes) frequencies, generating an absorptive or dispersive heterodyne beat signal (H) with the laser field at a photodetector. The theory of this effect is characterized in a new three-level perturbation calculation which requires, unlike the usual stimulated Raman effect, that all three transitions be electric- or magnetic-dipole allowed. Detailed predictions are confirmed by cw measurements of the Pr3+: LaF3 hyperfine splittings where the optical heterodyne signals are shot-noise limited. The Pr3+ nuclear quadrupole parameters are obtained for the H43 and D21 states where the line centers are determined with kilohertz precision. The corresponding wave functions show significant hyperfine-state mixing, as required for all three transitions to be dipole allowed. The cw line shapes are narrow (30-160 kHz), inhomogeneously broadened by nuclear magnetic interactions, and reveal either a Gaussian or an anomalous second-derivative like line shape. The spin-echo measurements for the H43 and D21 hyperfine transitions yield homogeneous line shapes which are Lorentzian, and rather surprisingly, linewidths in the narrow range 10-20 kHz, a result which tests current line-broadening theories. © 1983 The American Physical Society.