# LASER effect

Qubix
Wikipedia said:
An external electromagnetic field will affect an atom's state - changing the quantum mechanical variables mentioned above. Specifically, the atom will act like a small electric dipole which will oscillate with the external field. One of the consequences of this oscillation is that it encourages electrons to decay to the lowest energy state. When this happens due to the presence of the electromagnetic field from a photon, a photon is released in the same phase and direction as the "stimulating" photon, and is called stimulated emission.

I did not quite understand the "stimulation" part. How does the photon coming near the excited atom force this atom to emit another, identical (from the physical properties point of view) photon? More precisely, how do the stimulating photon and the atom interact?

Gold Member
If you have studied Quantum theory, you'd know that stationary states are stationary because they don't change. If I prepared an electron in the n=2 l=0 m=0 state, it would never change the state that it is in. What is required for that electron to change state is some sort of (time-dependent) perturbation of the Hamiltonian so that the stationary state is no longer stationary. The electron can then decay to a lower energy state and emit a photon.

One such possible perturbation would be another photon. If a photon comes in to interact with the atom, its EM field will perturb the atom (changing the Hamiltonian for the system) and stimulate the electron in that atom to decay to a lower energy state. To fully understand this process, one must study time-dependent perturbation theory.

Another such possible perturbation is the slight vacuum perturbations of the background EM field. This could also prompt the electron to decay to a lower energy state, releasing a photon.

If a photon is the source of the perturbation, we call this "stimulated emission". If background fluctuations are the source of the perturbation we call this "spontaneous emission".

If you have studied Quantum theory, you'd know that stationary states are stationary because they don't change. If I prepared an electron in the n=2 l=0 m=0 state, it would never change the state that it is in. What is required for that electron to change state is some sort of (time-dependent) perturbation of the Hamiltonian so that the stationary state is no longer stationary. The electron can then decay to a lower energy state and emit a photon.

One such possible perturbation would be another photon. If a photon comes in to interact with the atom, its EM field will perturb the atom (changing the Hamiltonian for the system) and stimulate the electron in that atom to decay to a lower energy state. To fully understand this process, one must study time-dependent perturbation theory.

Another such possible perturbation is the slight vacuum perturbations of the background EM field. This could also prompt the electron to decay to a lower energy state, releasing a photon.

If a photon is the source of the perturbation, we call this "stimulated emission". If background fluctuations are the source of the perturbation we call this "spontaneous emission".

One point to add here is that for stimulated emission, the probability that it will occur is highest when the frequency of the perturbing photon matches the energy gap between the excited state and a lower-lying state accessible by a dipole allowed transition. That fact, coupled to the existence of a population inversion (higher pop. in excited state than lower state), is how gain is generated in a lasing medium.

Qubix