TABLE OF CONTENTS This chapter should not be treated as detailed analysis of the principles of the quantum electronics. The reader can find them in [1 9]. Below there is given only some information related to the dynamical laser behaviour.
There are three problems, which led to the advent of quantum electronics: lowering the noise threshold of amplifiers, raising the stability of oscillators, and generating millimetre and shorter waves. In solving these problems the strategies of classical electronics encountered fundamental difficulties; many of them were overcome by using stimulated emission in systems of bound particles.
The possibility of enhancing electromagnetic fields by quantum systems is based on the induced (stimulated) emission of radiation. Under the influence of incident radiation a quantum system such as an atom, molecule or a crystal is capable to pass to a lower energy state by emitting a photon. The induced emission is fully identical to the incident radiation. The inverse of induced emission is the absorption of a photon when the quantum system makes a transition to an upper energy level.
In quantum electronics we deal with a medium composed of a large number of molecules rather with a single molecule. In thermal equilibrium the higher the energy of molecular levels the lower their population. Since downward and upward induced transitions have equal probabilities, media in thermal equilibrium are net absorbers. An excess of transitions with emission over those with absorption can be achieved only in nonequilibrium...
