16 August 2011

Stimulating photons

Is it possible that some things are conceivable only in a certain language; that they have no description in the wrong tongue? In trying to understand how lasers work I keep butting up against the fact that they are “triggered” by photons of a suitable energy passing by, and the resulting stimulated emission occurs with the same frequency, plane, energy etc  as the input photon. But why is this so? Why does a passing blip elicit the release of an identical one? Is this phenomenon comprehensible in words? Or pictures? Or is maths the only form in which it can be conceived and communicated?

One explanation, by SM Sze in his afore mentioned “Semiconductor Devices” (1985) states (p. 254):
“When a photon of energy h v12 impinges on an atom while it is in the excited state, the atom can be stimulated to make a transition to the ground state and gives off a photon of energy  h v12, which is in phase with the incident radiation. This process is called stimulated emission. The radiation from stimulated emission is monochromatic because each photon has an energy of precisely h v12 and is coherent because all photons are emitted in phase.”

I do not mean to criticize Mr Sze but although that sounds like an explanation it does not explain to me why stimulated emission occurs, and what it is that is actually occurring. Further reading leads me to Raman scattering and in “Infra-red physics” by Houghton and Smith (1966) am not reassured by the comment with regard to Raman lines (p. 53): “Because the wave-mechanical description of a perturbed system involves all states of the molecule, the intensity of a Raman transition will be determined by a sum over states of products of the transition moments between states. This is too complicated for calculation even in simple cases. ...” 

Time to return to the drawing board, which in this instance is actually a zinc plate since the drawing is an etching. I submit to the frustrations of literal incomprehension, working instead in the realm of visual misrepresentation, and accept, for the moment, that maybe some photons are simply more stimulating than others.

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