The Laser

Question 1: Absorption
At any given time, the number of photons inputted into the cavity must be equal to the number that have passed through the cavity without exciting an atom plus the number still in the cavity plus the number of excited atoms. Verify this conservation law by stopping the simulation and counting photons.

This simulation proves that the conservation law holds true. On this snapshot we see that there are 12 input photons. There are 9 excited atoms and 3 still in the cavity, that is a total of 12 photons.

Question 2: Direction of Spontaneous
During spontaneous emission, does there appear to be a preferred direction in which the photons are emitted?

No, they are emitted at random directions.

Question 3: Lifetime of Excited State
Does there appear to be a constant amount of time in which an atom remains in its excited state?

No, it seems like there is not constant time. They can be emitted at any moment.

Question 4: Stimulated Emission
Carefully describe what happens when a photon interacts with an excited atom. Pay careful attention to the phase and direction of the subsequent photons. (Can you see why this is called stimulated emission?)

As a photon interacts with an excited atom it stimulates the excited atom and a photon is emitted. More photons can be emitted from other excited atoms as these photons keep interacting.

Question 5: Pumping
Approximately what pumping level is required to achieve a population inversion? Remember, a population inversion is when the number of atoms in the excited state is at least as great as the number of atoms in the ground state.

On the simulation we see that pumping level should be about 10 to achieve a population inversion.

Question 6: Photon Emission
Although most photons are emitted toward the right in the simulation, occasionally one is emitted in another direction. Are the photons emitted at odd directions the result of stimulated or spontaneous emission?

The simulation shows that the photons emitted at odd directions are the result of spontaneous emission.