Lasing without inversion

For zero detuning, however, the two frequencies thus emitted cannot be reab jl sorbed. That is, contrary to the emission process, the reabsorption process would lead to the same final state [ )), and there would be no way we can tell which pathway was chosen by the system (see Fig. 9.9). Thus, the resultant can cell atiprfj of reabsorption of the two emitted frequencies in the LWI case is seen to be a specia case of the bichromatic control scenario (Section 3.1.1). 

The net result of the phenomenon just described is the possibility of having lasini without the need for population inversion [303-306]. We can envision a pumpi process that transfers some population to state [ ,). This state can emit to the 1 

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Fig. 9.9 Simplified four-level model for lasing without inversion. Shown is a i in which a) corresponds to sublevels P p, F = 2), c) = P3/,2, F=2), l sublevels of ground state b) = S1/2, F = 1) and bf) = 51/2, F = 2). Driv tuned to the Z>2 resonance from the F = 1 ground state to the F — 2 of excited field wa couples the same sublevels of the ground state with levels having F -excited state. Polarizations of fields are described in text. (After Fig. 1(a), Rfif l...

It was realized early on [109], following fhe analysis of Fano [3], that the interference between overlapping resonances can give rise to "dark states." Such dark states, which are characterized by the vanishing of photo-absorpfion, were found experimenfally a few years lafer [111, 112]. They feafure very highly in many applications in coherenf optics and in particular in "lasing without inversion" [13,113, 114] and adiabatic passage (AP) phenomena [20-23,115]. 

A quantum-beat laser can be initialized by using lasing without population inversion. The mechanism for lasing without inversion essentially is CPT/EIT absorption is reduced while stimulated emission remains unaffected [50-56]. At the same time, the coherence leads to noise squeezing [57-61]. The fluctuations of the laser intensity are suppressed up to or more than 50% below the shot noise. We also note that noise squeezing is possible even when neither initial coherence nor external coherent driving is used. This is caused by dynamic noise reduction... 

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Mompart J, Corbalan R Lasing without inversion Journal of Qptics B-quantum and Semiclassical Optics 2000 Jun 2 (3) R7-R24. 

Haibin Wu, Min Xiao, J. Gea-Banacloche. Evidence of lasing without inversion in a hot rubidium vapor under electromagnetically-induced-transparem conditions. Physical Revew 2008 Oct23 A 78(4) 041802(4). 

Yifu Zhu, Min Xiao. Amplitude squeezing and a transition from lasing with inversion to lasing without inversion in a four-level laser. Physical Revew A 1993 Nov 48(5) 3895 - 3899. 

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M.O. Scully, S.-Y. Zhu, A. Gavridiles Degenerate quantum-beat laser Lasing without inversion and inversion without lasing. Phys. Rev. Lett. 62, 2813 (1989)... 

O. Kocharovskaya, R. Kolesov, Yu. Rostovtsev Lasing without inversion a new path to gamma-ray laser. Laser Physics 9, 745 (1999)... 

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