Light and general radiation laws. Coherence and incoherence. Emission, absorption and amplification of radiation. Units and physical constants

Inverse Population Levels

The process of transferring the system to the inverse state is called pumping. Pumping can be carried out by optical, electrical, injection and other methods. When optically pumped, the atoms, absorbing radiation, go into an excited state. When electrically pumped (for example, in a gaseous medium), the atoms become excited due to inelastic collisions of atoms with electrons in the gas discharge.

The inverse state is sometimes called a "negative temperature" state. The origin of this formal name can be explained using the Boltzmann formula, which determines the relative population of the energy levels of a system in thermodynamic equilibrium. In this case, as follows from (2.5),
(2.6)
It follows that for n₂> n₁ (since E₁ -E₂ <0) there is a place T <0. This does not mean that the temperature of the system is lower than absolute zero. "Negative temperature" formally expresses the case n₂> n₁. The environment with such an inverse population is called active.

In Fig. 2.4 As an example, the distribution of atoms at energy levels as a function of temperature is shown. The number of atoms is conventionally taken equal to four. As can be seen from the figure for n₂ 1, T> 0 takes place. If n₂> n₁ (since E₁ -E₂ <0), then T <0. For the equality n₁ = n₂, T = ± .

From simple considerations it follows that for an two-level system, an inverse population can not be obtained by optical pumping. Indeed, according to the Boltzmann distribution, for thermodynamic equilibrium always n₂ 1, and since B₁₂ = B₂₁, the number of transitions per unit time from the state E₁ to the state E₂ and vice versa will be the same. Consequently, the change in the number of atoms in the ground and excited states due to forced transitions does not occur, i.e. what was the ratio n₁ / n₂, so it remains in the interaction of light with the atom, if, of course, we are dealing, as it was said, with two energy levels of the atom - the main E1 and the excited E₂. Thus, in order to obtain an inverse population, it is necessary to use three energy levels of the activ of the medium or more.

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