Role of spontaneously transferred coherence in laser cooling

The well-known sub-Doppler polarization gradient cooling in a type-I transition (${F}_{\mathrm{e}}={F}_{\mathrm{g}}+1$) is caused by red-detuned lasers. On the other hand, in a type-II transition (${F}_{\mathrm{e}}\ensuremath{\le}{F}_{\mathrm{g}}$), sub-Doppler cooling takes place through blue-detuned lasers. This opposite behavior for the two types of transitions is due to spontaneously transferred coherence (STC). In the absence of STC, both types of transitions show blue-detuned cooling. In this paper, we experimentally and theoretically demonstrate blue-detuned cooling for both types of transitions in $^{87}\mathrm{Rb}$. For completeness, we compare the temperatures in various configurations.