Breaking the Stokes–anti-Stokes symmetry in Raman heterodyne detection of magnetic-resonance transitions

Coherent Raman scattering can generate Stokes and anti-Stokes fields of comparable intensities. When the Raman shift is due to a magnetic resonance transition (usually in the MHz to GHz range), the Raman fields are generally detected by optical heterodyne detection, using the excitation laser as the local oscillator. In this case, the two sidebands generate beat signals at the same frequency and are therefore indistinguishable. Separation of the two contributions becomes possible, however, by superheterodyne detection with a frequency-shifted optical local oscillator. We compare the two scattering processes, and show how the symmetry between them can be broken in ${\mathrm{Pr}}^{3+}{:\mathrm{Y}\mathrm{A}\mathrm{l}\mathrm{O}}_3.$