  Within the model of rectangular potentials and different effective masses of
electrons in different elements of plane two-barrier resonance tunnel structure
there is developed a theory of spectral parameters of quasi-stationary states
and active conductivity for the case of mono-energetic electronic current
interacting with electromagnetic field. It is shown that the two-barrier
resonance tunnel structure can be utilized as a separate or active element of
quantum cascade laser or detector. For the experimentally studied
In$_{0.53}$Ga$_{0.47}$As/In$_{0.52}$Al$_{0.48}$As nano-system it is established
that the two-barrier resonance tunnel structure, in detector and laser regimes,
optimally operates (with the biggest conductivity at the smallest exciting
current) at the quantum transitions between the lowest quasi-stationary states.
