Inverted crossover resonance within one Zeeman manifold

We carry out investigations of inverted crossover resonances (ICRs) in π-driven four-level systems where ΔF can be zero. Through the use of sub-Doppler frequency modulation spectroscopy of the (6s²)¹S₀-(6s6p) ³P₁-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'= 1/2 hyperfine line recorded over two months demonstrate a statistical uncertainty of 2 × 10^-11. The ICR found with the F'= 3/2 line is used for 556 nm laser frequency stabilisation, which is an alternative means when applied to magneto-optical trapping of ¹⁷¹Yb.