Doppler-free two-photon absorption
= Doppler-free two-photon absorption
{c}
{title2=$\Delta_{\rm Doppler}=(\mathbf k_1+\mathbf k_2)\cdot\mathbf v$}
Absorbing one <photon> from each of two opposing equal-frequency beams cancels the first-order Doppler shift through their nearly zero summed <wavevector>. Absorbing both <photons> from the same beam does not share this cancellation and can create a broad background.