Doppler narrowing of EIT linewidth in closed-loop systems

We theoretically investigate a five-level closed-loop M-type atomic system and a three-level closed-loop $$\Lambda $$-system in hot atomic vapour. In contrast to closed-loop $$\Lambda $$-system, two unpopulated ground states of the M-system are coupled by a microwave field. We find that thermal averaging in the M-system causes many interesting modification of probe absorption lineshape including narrowing, splitting and absorption enhancement. In M-system, the linewidth of probe absorption after thermal averaging becomes remarkably narrow (100 times smaller) with respect to the linewidth of stationary atoms. On the contrary, the closed-loop $$\Lambda $$-system generates only 1.6 times smaller linewidth in thermal vapour. The absence of population transfer through the microwave field leads to this significant narrowing effect in the M-system which is unachievable in closed-loop $$\Lambda $$-systems. Hence, M-system has potential application in high-resolution spectroscopy, generation of ultra slow light, phase-dependent optical switching, and in microwave electro- and magnetometry.