Question

There are three ways that radiating light can interact with matter. (a)List those 
processes and Illustrate each processes using simple two level system. (b) which 
one is the basis of lasing? (c) In that (lasing) process, explain why emitted photon 
has the same phase, polarization and direction as the exciting photon.

Answer 1

(a) The three processes are absorption, reflection and transmission.

Absorption can be illustrated in the following way. Consider a 2-level system with an electron (say) in the lower state (energetically). Then, an incoming photon can interact with the electron, and if enough energy is available to the electron due to the interaction (and if a few other constraints are matched), then it would move to the higher state. The photon gets "absorbed" in this process.

In reflection, the electron can "absorb" the incoming photon, and not jump to the higher state (possibly because it does not have enough energy). Then the electron can re-emit the electron to reduce it's energy. This is the principle of reflection.

For transmission (or emission) , there are 2 types. One is spontaneous, where an electron absorbs a photon, moves to a higher state, and then emits another photon and comes back to the lower state. The other is stimulated, where an electron is already in the higher state, and then a photon is introduced, which causes the electron to emit another photon of the same phase and polarization and come back to the lower state.

(b) Stimulated emission is used as the basis of lasing.

(c) Photons are bosons and thus follow Bose-Einstein Statistics. Most photons in a system would tend to be in the same state rather than different states due to the statistics. Thus given that there exists a photon in the system (that "stimulates"), the emitted photon would "want" to be in the same state, and thus but QED we have that they have the same phase, polarization and direction.