Crystal Of Green Light Bends Matter

Jens Lönn writes: "The Kapitza-Dirac effect is the diffraction of a beam of particles, electrons in particular, by a standing wave of light. One can interpret it as waves of matter diffracted from "crystals" made of light, it's like matter and light swap roles. It was predicted in 1933 by a pair of future Nobel Prize winners, Russian Peter Kapitza (1894-1984) and Englishman P.A.M. Dirac (1902-84), but the technology needed to demonstrate it didn't exist at the time. It wasn't until April 11, 2001, when it was observed for the first time in Herman Batelaan's lab in the Behlen Laboratory for Physics at the University of Nebraska - Lincoln. This is the first time _ever_ that scientists have shown that light can bend matter, not just the opposite."

Comments

Lithography alternative?

[Christopher Thomas]

It seems that this suggests a couple of attractive long-term alternatives to photolithography. The experiments reported diffraction of argon atoms and of electrons, both of which have far shorter wavelengths than light. By using light-based "optics", you would in principle be able to either use imaged electrons to cure a photoresist, or deposit matter directly with direct masking and imaging of the matter stream, at far higher resolution than photolithography allows.

Problems with this:

• You'd need a standing wave pattern with cylindrical symmetry and a wavelength that _varies_ with radius to get a diffracting "lens". Varying wavelength in-flight is impossible in vacuum. There may be workarounds.
  
  
• For electron lithography, you'd be better off just using magnetic fields to focus electron images as with electron microscopes. Presumably there are problems that prevent this from being used, because it would beat the heck out of the scanning e-beam lithography that's currently used for bleeding-edge research.
  
  

Still interesting to think about, though.

Does anyone have information on why electron imaging isn't used for lithography now?

Re:Lithography alternative?

[jd]

Varying wavelengths might be a bit tough. But why not simply pack coherent waves together in a "sheet" to make a diffraction grating?

This would still be tough, but it seems like it should be within current technology. The problems are mechanical (ensuring that the waves are evenly spaced by a small enough distance, and that your sheet is extremely thin, for example), rather than theoretical.

You wouldn't be able to "focus" - it's not a lens - but I don't believe that's the problem. The problem, I suspect, is steering and directing the electrons, which this certainly could do.

I'm a little confused...

[hubie]

The U. Nebraska press release says that this is the first time this effect has been observed, but the post has a link to a Phys. Rev. A article (Dynamical diffraction of atomic matter waves by crystals of light) that was submitted in 1998 and published in July 1999 that talks about observing this effect.

There is also a 1986 PRL article, Diffraction of atoms by light - The near-resonant Kapitza-Dirac effect, which has as the abstract:

The Kapitza-Dirac effect is observed in the scattering of sodium atoms by a near-resonant standing-wave laser field. The data clearly show diffraction peaks of the atomic momentum transfer at even multiples of the photon momentum. Theoretical predictions for an off-resonant, adiabatic interaction with a two-state system are in reasonable agreement with the data.

It isn't clear whether a special case of the Kapitza-Dirac effect was first observed (e.g., the first time observed using an electron beam), but it seems that it wasn't the first time this effect was seen in the lab. (The press release also mentions that the basic physics demo of the double-slit experiment was Quantum Mechanics 101, when it really is High School Physics 101).

Re:I'm a little confused...

[hubie]

Young's DS experiment does work at extremely low light intensities which is why it is one of the classic examples demonstrating the wave/particle duality and/or uncertainty principle in QM. The wave-like interference is seen in the double slit experiment even if you turn the light intensity all the way down so that you are emitting single photons. I (or anyone else) can't explain it any better than Richard Feynman in his classic Feynman Lectures, but I can point you to the results of the DS experiment for very low intensities here.

By the way, diffraction gratings are completely explained within the particle/wave nature of matter, which is why confirmation of the Kapitza-Dirac effect is scientifically interesting but not unexpected. QM doesn't explain everything as waves, the wave/particle duality arises from the fact that all matter has an associated wavelength, the DeBroglie wavelength. Wavelike behavior becomes evident when matter is subjected to dimensions that are on the order of this wavelength (for instance, you won't see a diffraction pattern from light if the slit is too large, and in the case of the Kapitza-Dirac effect, standing waves from the laser create an appropriately spaced diffraction grating to act on the DeBroglie wavelength of the electrons they used).