The key to the laser is that many atoms have one or more excited energy levels whose lifetimes are much longer (10-3 s) than those of most excited levels (10-8 s). Such relatively long-lived levels are called “metastable”. The first step in laser operation is to bring as many atoms as possible in an appropriate material to such metastable levels. (There are various ways to do this, depending on the material.) When one of the atoms in a metastable state spontaneously drops to a lower state, the photon that is emitted has just the right frequency to cause other atoms in metastable states to drop to the same lover state by emitting radiation of this frequency. The process is called induced emission and was predicted by Einstein in 1917. The result is an avalanche of photons, all of the same frequency and all coherent, which means that their electromagnetic waves are exactly in phase (in step) with one another. A coherent beam can deliver more power than an incoherent one whose waves have random phases. .In the first step of the laser process, electrons are promoted to a higher state by random collisions with high speed electrons of all different energies. Normally, the electrons would immediately return to the ground state; however, a metastable state with an energy slightly lower than the ordinary excited state also exists in the material, and the electrons move to that state instead. The mechanism that causes the electrons to quickly drop to the ground state does not exist for the metastable state, thus over time many atoms in the material end up ‘storing’ electrons in the metastable state. One process that does encourage electrons in the metastable state to return to the ground state is resonance, when radiation with the same frequency as the energy difference between the two states is present in the material. Once a single electron does manage to return to the ground state, other electrons also drop to the ground state, increasing the amount of emission frequency radiation present in the material, and the laser emission process continues like a chain reaction.

Questions

1. Explain why only particular materials can be used as a laser.
2. What is meant by coherence? Based on this animation, why is laser light coherent?
3. Why is laser light all of the same color? Why is laser light so much more intense compared to the same energy being released by electrons returning to the ground state from the ordinary excited state?