Fred-photons ahoy at Caltech.
Ken S. Tucker
http://www.sciencecentric.com/news/article.php?q=09121738-caltech-scientists-film-photons-with-electrons
By analogy the very brief laser pulse, works like an ultra fast
shutter speed does in photography, that reduces blurring of
fast moving objects, further the article implies to me the
quantized scattering in proportion to the time the socalled
EM wave (field intensity) reacts with the electron.
Still gropping.
Regards
Ken
+++++++++++++++++++++++++++
Techniques recently invented by researchers at the California
Institute of Technology (Caltech) - which allow the real-time, real-
space visualisation of fleeting changes in the structure of nanoscale
matter - have been used to image the evanescent electrical fields
produced by the interaction of electrons and photons, and to track
changes in atomic-scale structures.
Papers describing the novel technologies appear in the 17 December
issue of Nature and the 30 October issue of Science.
Four-dimensional (4D) microscopy - the methodology upon which the new
techniques were based - was developed at Caltech's Physical Biology
Centre for Ultrafast Science and Technology. The centre is directed by
Ahmed Zewail, the Linus Pauling Professor of Chemistry and professor
of physics at Caltech, and winner of the 1999 Nobel Prize in
Chemistry.
Zewail was awarded the Nobel Prize for pioneering the science of
femtochemistry, the use of ultrashort laser flashes to observe
fundamental chemical reactions occurring at the timescale of the
femtosecond (one-millionth of a billionth of a second). The work
'captured atoms and molecules in motion,' Zewail says, but while
snapshots of such molecules provide the 'time dimension' of chemical
reactions, they don't give the dimensions of space of those reactions
- that is, their structure or architecture.
Zewail and his colleagues were able to visualise the missing
architecture through 4D microscopy, which employs single electrons to
introduce the dimension of time into traditional high-resolution
electron microscopy, thus providing a way to see the changing
structure of complex systems at the atomic scale.
In the research detailed in the Science paper, Zewail and postdoctoral
scholar Aycan Yurtsever were able to focus an electron beam onto a
specific nanoscale-sized site in a specimen, making it possible to
observe structures within that localised area at the atomic level.
In electron diffraction, an object is illuminated with a beam of
electrons. The electrons bounce off the atoms in the object, then
scatter and strike a detector. The patterns produced on the detector
provide information about the arrangement of the atoms in the
material. However, if the atoms are in motion, the patterns will be
blurred, obscuring details about small-scale variations in the
material.
The new technique devised by Zewail and Yurtsever addresses the
blurring problem by using electron pulses instead of a steady electron
beam. The sample under study - in the case of the Science paper, a
wafer of crystalline silicon - is first heated by being struck with a
short pulse of laser light. The sample is then hit with a femtosecond
pulse of electrons, which bounce off the atoms, producing a
diffraction pattern on a detector.
Since the electron pulses are so incredibly brief, the heated atoms
don't have time to move much; this shorter 'exposure time' produces a
sharp image. By adjusting the delay between when the sample is heated
and when the image is taken, the scientists can build up a library of
still images that can be strung together into a movie.
'Essentially all of the specimens we deal with are heterogeneous,'
Zewail explains, with varying compositions over very small areas.
'This technique provides the means for examining local sites in
materials and biological structures, with a spatial resolution of a
nanometre or less, and time resolution of femtoseconds.'
The new diffraction method allows the structures of materials to be
mapped out at an atomic scale. With the second technique - introduced
in the Nature paper, which was coauthored by postdoctoral scholars
Brett Barwick and David Flannigan - the light produced by such
nanostructures can be imaged and mapped.
The concept behind this technique involves the interaction between
electrons and photons. Photons generate an evanescent field in
nanostructures, and electrons can gain energy from such fields, which
makes them visible in the 4D microscope.
In what is known as the photon-induced near-field electron microscopy
(PINEM) effect, certain materials - after being hit with laser pulses
- continue to 'glow' for a short but measurable amount of time (on the
order of tens to hundreds of femtoseconds).
In their experiment, the researchers illuminated carbon nanotubes and
silver nanowires with short pulses of laser light as electrons were
being shot past. The evanescent field persisted for femtoseconds, and
the electrons picked up energy during this time in discrete amounts
(or quanta) corresponding to the wavelength of the laser light. The
energy of an electron at 200 kilo-electron volts (keV) increased by
2.4 electron volts (eV), or by 4.8 eV, or by 7.2 eV, etc.;
alternatively, an electron might not change in energy at all. The
number of electrons showing a change is more striking if the timing is
just right, i.e., if the electrons are passing the material when the
field is at its strongest.
The power of this technique is that it provides a way to visualise the
evanescent field when the electrons that have gained energy are
selectively identified, and to image the nanostructures themselves
when electrons that have not gained energy are selected.
'As noted by the reviewers of this paper, this technique of
visualisation opens new vistas of imaging with the potential to impact
fields such as plasmonics, photonics, and related disciplines,' Zewail
says. 'What is interesting from a fundamental physics point of view is
that we are able to image photons using electrons. Traditionally,
because of the mismatch between the energy and momentum of electrons
and photons, we did not expect the strength of the PINEM effect, or
the ability to visualise it in space and time.'
Source: California Institute of Technology
FrediFizzx
news:02c0af77-9872-4a92...@v30g2000yqm.googlegroups.com...
> I think I can wrap my mind around this, (article copied below +++)
> http://www.sciencecentric.com/news/article.php?q=09121738-caltech-scientists-film-photons-with-electrons
>
> By analogy the very brief laser pulse, works like an ultra fast
> shutter speed does in photography, that reduces blurring of
> fast moving objects, further the article implies to me the
> quantized scattering in proportion to the time the socalled
> EM wave (field intensity) reacts with the electron.
> Still gropping.
> Regards
> Ken
HI Ken,
Check out the LCLS site at SLAC. "Probing the very small; Capturing the
ultrafast".
http://lcls.slac.stanford.edu/
They are doing the same basic thing only using photons instead of
electron pulses. I'm hoping that they are eventually going to try to
test the Schwinger mechanism.
Best,
Fred Diether