The THz quantum cascade laser (QCL) [1] is a semiconductor laser based on intersubband transitions in quantum wells. It presently covers a wide spectral window ranging from 4.9 THz to 1.39 THz: possible applications in imaging, spectroscopy and astronomy push the extension of the frequency range towards lower frequencies. A control over the in-plane degree of freedom of the electron may be an essential element to obtain population inversion and optical gain at very low THz frequencies, where the broadening of the states becomes very close to the photon energy (hv< 5 meV).
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