Gordon, A.A.GordonWang, C.Y.C.Y.WangDiehl, L.L.DiehlKärtner, F.X.F.X.KärtnerBelyanin, A.A.BelyaninBour, D.D.BourCorzine, S.S.CorzineHöfler, G.G.HöflerLiu, H.C.H.C.LiuSchneider, H.H.SchneiderMaier, T.T.MaierTroccoli, M.M.TroccoliFaist, J.J.FaistCapasso, F.F.Capasso2022-03-042022-03-042008https://publica.fraunhofer.de/handle/publica/21603310.1103/PhysRevA.77.0538042-s2.0-43449088843A theoretical and experimental study of multimode operation regimes in quantum cascade lasers (QCLs) is presented. It is shown that the fast gain recovery of QCLs promotes two multimode regimes: One is spatial hole burning (SHB) and the other one is related to the Risken-Nummedal-Graham-Haken instability predicted in the 1960s. A model that can account for coherent phenomena, a saturable absorber, and SHB is developed and studied in detail both analytically and numerically. A wide variety of experimental data on multimode regimes is presented. Lasers with a narrow active region and/or with metal coating on the sides tend to develop a splitting in the spectrum, approximately equal to twice the Rabi frequency. It is proposed that this behavior stems from the presence of a saturable absorber, which can result from a Kerr lensing effect in the cavity. Lasers with a wide active region, which have a weaker saturable absorber, do not exhibit a Rabi splitting and their multimode regime is governed by SHB. This experimental phenomenology is well-explained by our theoretical model. The temperature dependence of the multimode regime is also presented.en667530journal article