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Diffraction at a standing light wave
The second grating in the interferometer is used to diffract the molecules with the Kapitza-Dirac effect.
If we reflect a laser beam perpendicular at a mirror the counter propagating light beams form a standing wave, that can be used as a diffraction grating. Pjotr Kapitza and Paul Dirac predicted this effect back in 1933 for the diffraction of electrons at broad gratings (Bragg diffraction). Not before 2001 this effect could be realized in the group of Herman Batelaan in Lincoln, Nebraska. Another version of the Kapitza-Dirac effect was already observed in the 1980’s at MIT in the group of David Pritchard. They diffracted atoms at a standing light wave. We use a similar effect with molecules.
In the KDTLI the standing light wave acts as a phase grating. Each molecule can pass the grating and its wave function receives a position dependent phase shift. The rapidly oscillating electric light field induces a dipole moment in the polarizable molecule. It is therefore accelerated towards high laser intensity. Dependent of the intensity of the laser field the molecules are accelerated stronger or less strong.
If the molecules fly through the light grating they get a phase shift dependent on the position in the standing wave and the laser intensity.
In the applet the phase shift of the wave fronts is visualized. The modulated wave fronts overlap and form the interference pattern that we observe at the third grating.
We must not confuse the phase shift with the density pattern of the molecules at the 3rd grating. Only in our specific Talbot-Lau setup, with identical grating periods d1 = d2 the period of the light wave is identical with the distance between the maxima of the molecular density distribution at the position of G3.
Using light gratings is beneficial as the laser frequency and therefore the grating is very precisely defined; the theories of interactions are well understood and the experimental parameters can easily be changed. Additionally all molecules can pass the standing wave without losses.
Kapitza Dirac Effect
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