Dark state
In physics, a dark state is an energy level of an atom or molecule that is undesirable in certain applications in laser spectroscopy , such as laser cooling . These states are called dark because the atoms that are in this state become "invisible" for the desired application. However, there are exceptions in which the existence of dark states is explicitly used (e.g. with certain types of atomic clocks ), but then usually no longer speaks of dark states.
Physical basics
In laser spectroscopy, particles are excited by lasers and their emitted light is detected. Excited particles return to an energetically more favorable state by emitting a photon, either through spontaneous or stimulated emission . In the case of spontaneous emission it is possible that the particles do not fall back into the state in which they were before the laser excitation. Which transitions are possible is determined by the selection rules. As soon as a particle passes into such a state, it cannot be excited again by the laser - the particle is dark to the laser .
Countermeasures
The particles that are in a dark state can only be brought back into the desired state with another laser that is resonant with the dark state. Such a laser is called a "pump back laser" or "repumper".
Web links
- Precision dark resonance spectroscopy. What are dark resonances? Institute for Applied Physics - University of Bonn, archived from the original on February 6, 2012 ; accessed on June 28, 2016 .