May 15 2014
Doppler cooling is the laser based cooling of small particles, such as ions and atoms, which are trapped during the cooling process. It also involves the spontaneous emission of photons, which generate velocity-dependent light forces through the Doppler effect.
The minimum temperature achieved through Doppler cooling is set by the Doppler limit. Although cooling below the Doppler limit has been observed, this process is said to be Sisyphus cooling.
How Does Doppler Cooling Work?
In the process of Doppler cooling, the light frequency is adjusted with respect to the electronic transition state of the atom. As the light is detuned to a lower frequency during the transition, more photons are absorbed by the atoms as they move towards the light source due to the Doppler effect.
The application of light from two opposite directions, the atoms scatter more photons from the laser beam that points to the opposite direction of motion. The atom loses momentum in each scattering event, which is equal to the momentum of the photon.
As the loss of initial momentum is equal to the direction of motion, the momentum gain is in a random direction, thereby reducing the overall result of the emission and absorption processes.
The average speed and kinetic energy of the atom will be subsequently reduced if the emission and absorption is repeated a number of times. The temperature of a group of atoms is a function of the average random internal kinetic energy, which causes cooling of the atoms.
Applications of Doppler Cooling
The following are the major applications of Doppler cooling:
- Measurement of gravitational fields
- High-resolution spectroscopic measurements
- Metrological applications
- Lithographic techniques
References and Further Reading