Inverted crossover resonance aiding laser cooling of ${}^{171}$Yb

Abstract

We observe an inverted crossover resonance in $\pi$-driven four-level systems, where $F^{\prime }-F=0,+1$. The signal is observed through saturated absorption spectroscopy of the $(6s^2)$ ${}^1{S}_0$ $-$ $(6s6p)$ ${}^3{P}_1$ transition in ${}^{171}$Yb, where the nuclear spin $I=1/2$. The enhanced absorption signal is used to generate a dispersive curve for 556 nm laser frequency stabilisation and the stabilised light cools ${}^{171}$Yb atoms in a two-stage magneto-optical trap, achieving temperatures of 20 $\mu$K. The Doppler-free spectroscopy scheme is further used to measure isotopic frequency shifts and hyperfine separations for the intercombination line in ytterbium.