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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. http://physicsweb.org/article/news/4/7/7/1
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. http://physicsweb.org/article/news/5/8/14/1
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.
http://www.nasatech.com/Briefs/Sept00/NPO20687.html
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.http://murl.microsoft.com/LectureDetails.asp?1041
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.
http://physicsweb.org/article/news/4/9/18/1
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. http://physicsweb.org/article/world/16/5/3/1
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.
http://www.uni-konstanz.de/quantum-optics/nano-optics/singlemol.htm
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.
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