ActivPhysics Lasers

Purpose: To understand how lasers function and how they are practical applications of quantum mechanics.
We will use ActivPhysics simulations from the following website:
http://media.pearsoncmg.com/bc/aw_young_physics_11/pt2a/Media/AtomicPhysics/1803Laser/Main.html

Introduction: There are 20 particles in the unexcited states. Photons are coming in from the left with enough energy to excite the particles to an excited state (shown in red)
Observations:

Conservation of energy is observed by the fact that every photon that has entered the cavity  is accounted for either by remaining in the cavity, exiting the cavity or being absorbed by a particle. (excited to red)




This spontaneous emission mode shows initially all the excited particles going to a lower energy ground state by emitting a photon. The direction on the photon is random.
The time a particle remains excited seems to be random as well.


Stimulated emission mode sends in photons which "knock" a particle out of its excited state and (since it is  already excited) it shows 2 photons leaving, one that was incident and the other from lowering the energy. The emitted photon is in the same direction as the incident photon. They are also in phase.


In the laser mode it includes the excitation, spontaneous and stimulated emissions and you can control the amount of photons being put into the cavity. At low pumping levels the amount of photons exiting roughly equal the amount entering and a majority are in the unexcited state.
When the pumping level reaches at least 40 out of 100 a population inversion is seen (more particles in the excited state).

It is seen that there is an amplification of the number of photons once this population inversion is seen. Occasionally a photon no in the direction of incidence is observed. This occurs due to spontaneous emission that takes place before a photon can "knock" it to a lower state.