Most ideal light radiates uniformly in all directions. It means that for each photon, the probability of each direction is the same for a single photon. The specific optical resonator design enables a laser beam to propagate in one definite direction. Could anyone tell me what modifications does the optical resonator offer in the state function of a photon? Is that possible to confine a plasma and let the plasma emit in one specific direction? Could anyone refer me to some literature in this field? What is the quantum mechanics descriptions of this modification?