Inverted crossover resonance within one Zeeman manifold

We carry out investigations of inverted crossover resonances (ICRs) in π-driven four-level systems where $\Delta F$ can be zero. Through the use of sub-Doppler frequency modulation spectroscopy of the $\left(6s^2\right)$ ${}^1{S}_0$ − $\left(6s6p\right)$ ${}^3{P}_1$ transition in ¹⁷¹Yb the resonance becomes manifest. The centre frequency is inherently insensitive to first-order Zeeman shifts and equates to the two-level resonance frequency in the absence of a magnetic field. A rate equation model is used to help validate the nature of the resonance. Optical frequency measurements of the $F^{\prime }=1/2$ hyperfine line recorded over two months demonstrate a statistical uncertainty of 2 × 10⁻¹¹. The ICR found with the $F^{\prime }=3/2$ line is used for 556 nm laser frequency stabilisation, which is an alternative means when applied to magneto-optical trapping of ¹⁷¹Yb. (paper)