We have measured the tune-out wavelength of lithium isotope
7Li at 671 nm. We have used our atom interferometer to measure the phase shift due to the dynamical Stark effect as a function of the laser frequency when a laser beam was focused on a single interferometer arm. The tune-out wavelength is a function of the hyperfine
F,mF sublevel, and we have prepared the atoms in the sublevel
F=2,mF=+2 or $\ensuremath{-}2$. We find that the tune-out frequency of the
F=2,mF=+2 sublevel is at
3388(8)MHz to the blue of the $^{2}S_{1/2},F=1\ensuremath{\rightarrow}^{2}P_{1/2},{F}^{\ensuremath{'}}=2$ transition, the accuracy of this measurement being limited by imperfect optical pumping. This measurement is in excellent agreement with its theoretical value, and, once corrected for different tensorial contributions, our measurement agrees with the more precise measurement of Copenhaver et al. [Phys. Rev. A 100, 063603 (2019)], who also used atom interferometry but different experimental conditions.
The longest tune-out wavelength for potassium atoms, ${\ensuremath{\lambda}}_{\mathrm{zero}}=768.9701(4)$ nm, was measured using an atom interferometer with a large irradiance gradient supported in a multipass optical cavity. Systematic errors in ${\ensuremath{\lambda}}_{\mathrm{zero}}$ measurements that arise from laser light, Doppler shifts, and the Earth's rotation are described. The ratio of oscillator strengths for the potassium
D2 and
D1 lines inferred from this ${\ensuremath{\lambda}}_{\mathrm{zero}}$ measurement is $\ensuremath{\rho}={f}_{D2}/{f}_{D1}=2.0066(11)$, and the ratio of line strengths is
R=SD2/SD1=1.9977(11).