Absorption, spontaneous emission, stimulated emission

Three ways light and matter interact

In 1917, long before the first laser, Einstein showed that an atom and a photon can interact in three ways. Two were already known; the third, which he predicted theoretically, is the key to the laser.

Consider a two-level atom: a lower level E1 and an upper level E2, separated by ΔE = E2 - E1 = hf.

1. Absorption

A photon of energy hf reaches an atom in the lower state. The atom absorbs it and jumps to the upper level. The photon disappears.

   AVANT              APRES
   ---- E2            -O-- E2   <- atome monte
    hf ~~>
   -O-- E1            ---- E1
   (photon absorbe, atome excite)

2. Spontaneous emission

An excited atom is unstable. After a random amount of time, it falls back on its own, emitting a photon hf. But this photon leaves in an arbitrary direction, at an unpredictable instant, with an arbitrary phase.

   AVANT              APRES
   -O-- E2            ---- E2
                            ~~> hf  (direction et instant ALEATOIRES)
   ---- E1            -O-- E1

This is what happens in a light bulb or a candle: a multitude of atoms each emit in their own corner, at random. The resulting light is disordered (incoherent), multicoloured, scattered in all directions.

3. Stimulated emission: the key to the laser

Here is Einstein's discovery. If a photon hf passes near an atom that is already excited, it can trigger its de-excitation before it happens spontaneously. The atom then emits a second photon — but not just any photon:

   AVANT                    APRES
   -O-- E2                  ---- E2
    hf ~~>  (photon           ~~> hf   (photon incident, inchange)
            incident)         ~~> hf   (photon EMIS, clone parfait !)
   ---- E1                  -O-- E1

The emitted photon is a perfect clone of the incident photon: same energy (same colour), same direction, same phase, same polarization. Where there was one photon, there are now two, rigorously identical.

The amplification effect

It is this cloning that makes amplification possible. One photon stimulates another, these two stimulate two more, and so on: an avalanche of photons that are all identical.

   1 photon  ->  2  ->  4  ->  8  ->  ...  (tous identiques)

   LASER = Light Amplification by Stimulated Emission of Radiation
           (amplification de lumiere par emission stimulee)

The word "LASER" is the exact acronym of this mechanism. All laser light — coherent, monochromatic, directional — comes from this cascade of cloning.

In summary

Three processes connect light and matter: absorption (the atom jumps up by swallowing a photon), spontaneous emission (it falls back on its own, emitting a photon in a random direction and at a random instant — disordered light), and stimulated emission (one photon triggers another, a perfect clone: same colour, direction, phase). It is this avalanche of cloning, predicted by Einstein, that underlies the laser.