If you take a closer look at a laser source and the laser beam, sooner or later you will inevitably encounter the term “stimulated emission.” This physical process plays the crucial role -because it ensures that light is amplified in the laser!
Stimulated emission is the emission of a photon, which is triggered by another photon. To explain this process, you have to get into quantum physics. Let’s first consider a quantum mechanical system, for example an atom. Energy is supplied to this system, either by absorption of a photon or collision with another atom. Through this process, the atom goes into an excited state. Now if a photon whose energy exactly matches the energy difference between the current state and a low-density energy level, hits the excited atom, an interesting process begins. The atom is now able to go into a state of low energy - and to deliver the energy difference as another photon in addition to the absorbed photon.
The new photon has the same energy as the absorbed photon, and therefore the same wavelength and frequency. The polarization direction and phasing are identical - it is, so to speak, a copy of the output photon that is emitted.
The relevance of the stimulated emission to the laser can be recognized just by looking more closely at the word laser. Laser ultimately stands for “light amplification by stimulated emission of radiation.” In a laser, the process of stimulated emission of photons is used in a similar way to a chain reaction. Special technology in the laser machine ensures that many similar atoms are in the excited state for an extended period of time. So there is ample opportunity for photons to trigger the stimulated emissions and thereby produce the actual laser beam.
The theoretical postulation of the stimulated emission is more than 100 years old. It was 1916 when Albert Einstein first proposed the hypothesis of an energy exchange by radiation in his paper “The Quantum Theory of Radiation.” Einstein presupposed that under the influence of radiation there always had to be a reversal process - the absorption. Einstein summarized this process under the term “change of state due to radiation” - however, he did not give a real name to the process at that time. It was more than 12 years before Rudolf Ladenburg succeeded in proving the stimulated emission for the first time in the context of gas discharges. Today, the stimulated emission is omnipresent in laser machines or in the laser beam emitted by machines - and yet one of the most fascinating physical processes of all.