Role of spontaneously generated coherence (SGC) in laser cooling of atoms

The well-known sub-Doppler polarization gradient cooling in type-I transition (\(F_e=F_g+1\)) is caused by red-detuned lasers. On the other hand, in type-II transition (\(F_e\le F_g\)), sub-Doppler cooling takes place through blue-detuned lasers. This opposite behavior for the two types of transitions is due to SGC. In the absence of SGC, both types of transitions show blue-detuned cooling. In this work, we experimentally and theoretically demonstrate blue-detuned cooling for both types of transitions in \(^{\textrm{87}}\)Rb. For completeness, we compare the temperatures in various configurations.