Kapitzadirac

Kapitzadirac, an electron sprite, flies through a grating made of nothing but light, and fans out into separate beams.

The Physics

Two laser beams travelling in opposite directions form a standing wave, with bright and dark bands half a wavelength apart. Pyotr Kapitza and Paul Dirac predicted in 1933 (“The reflection of electrons from standing light waves”, Proc. Camb. Phil. Soc. 29, 297) that electrons crossing such a wave would be diffracted, as if by a grating. The electrons are pushed away from the bright bands by the ponderomotive force, and the regular pattern of bands acts as a periodic grating.

d = λ / 2

• d: period of the light grating (m)
• λ: wavelength of the laser light (m)

The effect is tiny with ordinary light, so it waited for strong lasers. Daniel Freimund, Kayvan Aflatooni and Herman Batelaan reported the first clear observation with electrons in “Observation of the Kapitza–Dirac effect” (Nature 413, 142, 2001).

Honest caveat: The grating is the standing light wave itself, not a material grating, and it diffracts because the electrons behave as waves. Kapitza and Dirac described it as a reflection from the light wave, a picture equivalent to the diffraction description used today.

On the sticker

Kapitzadirac stands behind a glowing cyan bar grating of light as beams fan out through it. Printed text: KAPITZADIRAC with the line PERIOD λ/2. Pixel-art kiss-cut vinyl sticker, white, in 2″, 3″, 4″ and 6″ sizes.