Question

(e) In the steady state and above threshold, why will the gain and carrier density inside a semiconductor laser be clamped (limited)to the threshold condition? How about photon density inside the cavity? If the driving current is time varying form, are gain and carrier density still kept in constant values? How about in AC driving conditions?

Answer 1

When this condition is satisfied, a large photon population can build up inside the
cavity starting from spontaneous emission and we have a laser (light amplification from stimulated
emission of radiation). z=0 z=L The simplest way to analyze and understand laser dynamics is using rate Consider a photon of wavelength ? which is sent to a cavity whose three dimensions, L, W, H, are not multiples of ?/2, see figure. Assume the cavity walls perfectly reflective.
What will happen with the photon? In the cavity the interference is destructive. But, the conservation energy doesn't allow the photon to disappear.Then the photon will not occupy the ground state, but an excited state in that box, assuming the spectrum allows its energy as one of the allowed levels; otherwise it can only be described as a superposition of energy eigenstates in the box, i.e. will not have a definite energy within the box. If the walls are perfectly reflective, why wouldn't the photon bounce around till it re-emerges from the cavity?
Do you need to specify in addition that the entrance gets closed after the photon has entered? (Unless a half-silvered mirror covering the entrance would be satisfactory, the cavity entrance would have to close extremely rapidly, for anything but a very large cavity.)