PHYSICS NEWS UPDATE
The American Institute of Physics Bulletin of Physics News
Number 791 September 6, 2006  by Phillip F. Schewe, Ben Stein,
and Davide Castelvecchi        www.aip.org/pnu

LASER OPTICAL ANTENNAS represent a relatively new approach to
getting around the old diffraction limit characterizing conventional
optics, namely the inability of a lens to focus light for imaging
purposes to any better than about half the wavelength of the light
being used.  Like a rooftop antenna which grabs meter-sized radio
waves and turns them (courtesy of a tuned circuit) into signals far
smaller in physical extent, so the optical antenna converts visible
light into an illuminating beam of much higher resolving power.  For
example, 800-nm light can produce images with a spatial resolution
of no better than about 400 nm.  A new device, built by the groups
of Ken Crozier and Federico Capasso at Harvard, producing spot sizes
as small as 40 nm using 800-nm light, is the first optical antenna
to be fully integrated (laser and focusing apparatus on one
platform) and the first to prove (by directly measuring light
intensities) the narrowness of the focused spot of light.  Their
method combines two proven techniques---plasmonics, in which light
waves, striking a metal surface, can create plasmons, which are a
sort of electromagnetic disturbance (see
http://www.aip.org/pnu/2006/split/770-1.html for background) with a
wavelength less than that of the incoming light; and near-field
microscopy, in which the diffraction limit is avoided by placing the
specimen very close to the imaging device.  In the Harvard setup the
antenna consists of two gold patches (130 nm long by 50 nm wide)
separated by a 30 nm gap.  Light falling on the gold strips (which
sit right on the facet of an ordinary commercial laser diode)
excites a huge electric field in the gap.  A specimen located
beneath this gap sees it as a 30-nm wide burst of light (although at
this stage in the work the spot size is more like a 40 nm x 100 nm
rectangle).  In many forms of subtle microscopy, power is sometimes
feeble, but here, in pulsed operation, the antenna can generate a
robust peak intensity of more than a gigawatt/cm^2.  (For comparison
images recorded with a force microscope, an electron microscope, and
the new laser antenna, see http://www.aip.org/png/2006/266.htm ).
Crozier  (kcrozier{at}deas.harvard.edu, 617-496-1441) says that spot
sizes of 20 nm should be possible and that likely applications for
their laser antenna will be found in the areas of optical data
storage (where 3 terabytes of data could be stored on a CD),
spatially-resolved chemical imaging, and near-field scanning optical
microscopy (NSOM).  (Cubukcu et al., Applied Physics Letters, August
28, 2006; lab website at www.deas.harvard.edu/crozier ; see also
http://www.aip.org/pnu/2004/split/701-1.html)
                                        
ARTIFICIAL MUSCLES FOR LIFELIKE COLOR DISPLAYS. Adjustable
diffraction gratings made of tiny artificial muscles could bring
more lifelike colors to TVs and computer displays, physicists at ETH
Zurich show in the September 1 issue of Optics Letters.  In ordinary
displays such as TV tubes, flat-screen LCDs, or plasma screens, each
pixel is composed of three light-emitting elements, one for each of
the fundamental colors red, green, and blue. The fundamental colors
in each pixel are fixed, and only their amounts can change--by
adjusting the brightness of the color elements---to create different
composite colors. That way, existing displays can reproduce most
visible colors, but not all. For example, current displays do not
faithfully reproduce the hues of blue one can see in the sky or in
the sea, says Manuel Aschwanden (aschwanden{at}nano.mavt.ethz.ch,
+41-44-632-08-04). Aschwanden and his colleague Andreas Stemmer
figured that one can overcome such limitations by changing the
fundamental colors themselves, not just their brightness, using a
tunable diffraction grating.
In their setup, white light hits a 100-micron wide, gold-coated
artificial muscle membrane that's been molded into a shape that
resembles microscopic pleated window shades. The artificial muscle
is made of a polymer that contracts when voltage is applied. When
white light hits a diffraction grating, different wavelengths fan
out at different angles. "It's like when you hold a CD in direct
sunlight, and you rotate it," Aschwanden says. Like the microscopic
tracks on a CD surface, the grooves on the artificial muscle split
white light into a rainbow of colors. But instead of rotating the
surface to obtain different colors, the ETH team adjusts the
diffraction angle by applying different voltages to the artificial
muscle. As the membrane stretches or relaxes, the incoming light
"sees" the grooves spaced closer or tighter. All the angles of
reflection change, so the entire fan of wavelengths turns as a
whole. The desired color can then be isolated by passing the light
through a hole: As the hole stays fixed, different parts of the
spectrum will hit it and go through it.
To obtain composite colors, every pixel would use two or more
diffraction gratings. By this method, a display could produce the
full range of colors that the human eye can perceive, Aschwanden
says. Tunable diffraction gratings are routinely used in
applications such as fiberoptic telecommunications and video
projectors, but existing technologies are based on hard,
piezoelectric materials rather than artificial muscles, limiting
their stretchability to less than a percentage point. By contrast,
artificial muscles can change their length by large amounts.
Getting a full range of colors requires a source of "true" white
light to begin with -- rather than a mere combination of red, green
and blue that looks like white light to the human eye. For that
purpose, the technology could exploit a new generation of white LED
lights that have recently been developed, Aschwanden says (see PNU
772, http://www.aip.org/pnu/2006/split/772-3.html). )

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