Friday, January 30, 2009

Bragg diffraction of atoms by light standing wave

When a monochromatic electromagnetic wave incidents at some angle on a crystal lattice, it is reflected by the crystal lattice planes and a maximum intensity can be observed if the path difference of the reflected waves from the adjacent planes is an integral multiple of the wavelength of the wave. This phenomenon is called Bragg diffraction, after William Henry Bragg and his son William Lawrence Bragg, who were jointly awarded the 1915 Physics Nobel Prize for their services in the analysis of crystal structure by means of X-rays. Interestingly enough, diffraction can be observed when an atomic beam is directed through a light standing wave. A laser standing wave can be produced by reflecting a laser beam between two mirrors. When a beam of atom is passed through the standing wave, an atom absorbs a photon from a beam of laser and it does stimulated emission of a photon of the same momentum into the next beam, imparting a net momentum to the atom which is two times the momentum of a single photon. This is similar to the Bragg diffraction of electromagnetic waves as the laser standing wave is analogous to the crystal lattice planes.

Note: This posting is on progress.

No comments: