Magneto-optical Trap - Atomic Structure Necessary For Magneto-optical Trapping

Atomic Structure Necessary For Magneto-optical Trapping

As a normal atom has many thousands of times the momentum of a single photon, the cooling of an atom must involve many absorption-spontaneous emission cycles, with the atom losing up to ħk of momenta each cycle . Because of this, if an atom is to be laser cooled, it must possess a specific energy level structure known as a closed optical loop, where following an excitation-spontaneous emission event, the atom is always returned to its original state. 85Rubidium, for example, has a closed optical loop between the 5S_1/2 F=3 state and the 5P_3/2 F=4 state. Once in the excited state, the atom is forbidden from decaying to any of the 5P_1/2 states, which would not conserve parity, and is also forbidden from decaying to the 5S_1/2 F=2 state, which would require an angular momentum change of −2, which can not be supplied by a single photon.

Many atoms that do not contain closed optical loops can still be laser cooled, however, by using repump lasers which re-excite the population back into the optical loop after it has decayed to a state outside of the cooling cycle. The magneto-optical trapping of rubidium 85, for example, involves cycling on the closed 5S_1/2 F=3 → 5P_3/2 F=4 transition. On excitation, however, the detuning necessary for cooling gives a small, but non-zero overlap with the 5P_3/2 F=3 state. If an atom is excited to this state, which occurs roughly every thousand cycles, the atom is then free to decay either the F=3, light coupled upper hyperfine state, or the F=2 "dark" lower hyperfine state. If it falls back to the dark state, the atom stops cycling between ground and excited state, and the cooling and trapping of this atom stops. A repump laser, which is resonant with the 5S_1/2 F=2 → 5P_3/2 F=3 transition is used to recycle the population back into the optical loop so that cooling can continue.