We investigate the conduction intersubband linear response of a n-type quantum well driven by one or two infrared
optical fields with arbitrary strengths. The fields couple the third subband to the second energy subband and the
ground subband, forming a Λ system. We investigate the dependences of the absorption spectrum on the pump
field strengths. The results for the equal Rabi frequencies can be understood in terms of quantum coherence
processes of the intersubband transitions, such as coherent population trapping, and electromagnetically induced
transparency. The effects of different Rabi frequencies on the absorption spectrum due to fields are also discussed.
Considering the Coulomb many-body interactions, we theoretically investigate the terahertz optical gain spectrum in an asymmetric step quantum well by employing the intersubband semiconductor-Bloch equations (SBEs). We show the effects of various many-body interactions on the THz gain spectrum. Because of the pump-induced population redistribution, optical gain saturation at larger pump intensities is predicted. The dependence of optical gain on electron density, temperature is also explored in detail.
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