Guest Writer - Gastautor - Gast Schrijver


Sub-wavelength Imaging


Light comes in small chunks called photons, which generally act like waves. When a drop falls into a pool of water, one or more peaks surrounded by troughs move across the surface. It's easy to describe a single wave: the curvy shape between one peak and the next. Multiplem waves are just as easy. But what is the meaning of a fractional wave?

Chop out a thin slice of a wave and set it moving across the water: it would almost immediately collapse and turn into something else. For most purposes, fractional waves can't exist. So it used to be thought that microscopes and projection systems could not focus on a point smaller than half a wavelength. This was known as the diffraction limit.

There are now more than half a dozen ways to beat the so-called diffraction limit. This means that we can use light to look at smaller features, and also to build smaller things out of light-sensitive materials. And this will be a big help in doing advanced nanotechnology.

The wavelength of visible light is hundreds of nanometers, and a single atom is a fraction of one nanometer. The ability to beat the diffraction limit gets us a lot closer to using an incredibly versatile branch of physics—electromagnetic radiation—to access the nanoscale directly.

Here are some ways to overcome the diffraction limit:

There's a chemical that glows if it's hit with one color of light, but if it's also hit with a second color, it doesn't. Since each color has a slightly different wavelength, focusing two color spots on top of each other will create a glowing region smaller than either spot.

There are plastics that harden if hit with two photons at once, but not if hit with a single photon. Since two photons together are much more likely in the center of a focused spot, it's possible to make plastic shapes with features smaller than the spot.

Now this one is really interesting. Remember what we said about a fractional wave collapsing and turning into something else? Not to stretch the analogy too far, but if light hits objects smaller than a wavelength, a lot of fractional waves are created, which immediately turn into "speckles" or "fringes." You can see the speckles if you shine a laser pointer at a nearby painted (not reflecting!) surface. Well, it turns out that a careful analysis of the speckles can tell you what the light bounced off of—and you don't even need a laser.

A company called "Angstrovision" claims to be doing something similar, though they use lasers. They say they'll soon have a product that can image 4x12x12 nanometer features at three frames per second, with large depth of field, and without sample preparation. And they expect that their product will improve rapidly.

High energy photons have smaller wavelengths, but are hard to work with. But a process called "parametric downconversion" can split a photon into several "entangled" photons of lower energy. Entanglement is spooky physics magic that even we don't fully understand, but it seems that several entangled photons of a certain energy can be focused to a tighter spot than one photon of that energy.

A material's "index of refraction" indicates how much it bends light going through it. A lens has a high index of refraction, while vacuum is lowest. But certain composite materials can have a negative index of refraction. And it turns out that a slab of such material can create a perfect image—not diffraction-limited—of a photon source. This field is advancing fast: last time we looked, they hadn't yet proposed that photonic crystals could display this effect.

A single atom or molecule can be a tiny source of light. That's not new. But if you scan that light source very close to a surface, you can watch very small areas of the surface interact with the "near-field effects." Near-field effects, by the way, are what's going on while speckles or fringes are being created. And scanning near-field optical microscopy (SNOM, sometimes NSOM) can build a light-generated picture of a surface with only a few nanometers resolution.

Finally, it turns out that circularly polarized light can be focused a little bit smaller than other types. (Sorry, we couldn't find the link for that one.)

Some of these techniques will be more useful than others. As researchers develop more and more ways to access the nano-scale, it will rapidly get easier to build and study nanoscale machines.


Chris has studied nanotechnology and molecular manufacturing for more than a decade. After successful careers in software engineering and dyslexia correction, Chris co-founded the Center for Responsible Nanotechnology in 2002, where he is Director of Research. Chris holds an MS in Computer Science from Stanford University.

Copyright © 2004 Chris Phoenix


Chris Phoenix

CRN Director of Research