Laser (Doppler) Cooling
The method of laser cooling was independently proposed by Hänsch and Schawlow for the cooling of free atoms and by Wineland and Dehmelt for the cooling of trapped ions. This technique exploits the momentum carried by the photons in a laser beam to slow down the motion of atomic particles.
Laser cooling is achieved by using a laser tuned slightly below the centre frequency of a strong (allowed) transition in the atom or ion to be cooled. The radiation is therefore Doppler-shifted into resonance with the transition frequency when the atom is moving towards the laser beam, and out of resonance when it is travelling in the opposite direction. This means that the atom preferentially absorbs photons when it is moving in the opposite direction to the laser beam, and each photon absorbed reduces its momentum. The photons resulting from spontaneous emission to the ground state, however, can be emitted in any direction, and so over many scattering events have no effect on the atom's momentum. The net effect is that the atom is slowed down (cooled). The loss in the kinetic energy of the atom comes about because the average energy of the photons increases: the mean energy of the absorbed photons is below the resonance energy, while the mean energy of the re-emitted photons corresponds to the line centre of the transition. With many tens of thousands of scattering events, the ion quickly loses energy and sub-Kelvin temperatures can be attained.
In many of the atomic species suitable for use as trapped ion optical frequency standards, the transitions used for laser cooling lie in the blue or UV regions of the spectrum, and so the specialised laser sources required represent one of the challenges in the development of these standards. However, for ions, the use of laser cooling allows the ion to be confined to a region of dimensions less than a wavelength of the light used to probe the reference transition, giving a spectrum free of the first order Doppler effect. For neutral atoms, Doppler cooling can reduce temperatures to the few-millikelvin regime, and in certain cases to the microkelvin regime.
Being able to both cool and confine the ion or atoms we wish to use in a frequency standard can significantly reduce the systematic errors in our measurements. Ions, once cooled in this way, can be held for long times in a variety of ion traps based on oscillating electric fields for confinement. Neutral atoms cannot be trapped in this way due to their lack of charge, and need a different type of trapping potential. Two useful methods are the magneto-optical trap (MOT), which combines laser cooling and magnetic fields, and dipole traps, which use the dipole force to confine the atoms.
- T. W. Hänsch and A. L. Schawlow, Opt. Commun. 13, 68 (1975)
- D. J. Wineland and H. G. Dehmelt, Bull. Am. Phys. Soc. 20, 637 (1975)