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  • From: "Lawrence F. London, Jr." <lflj@intrex.net>
  • To: permaculture <permaculture@lists.ibiblio.org>
  • Subject: [permaculture] OT but fascinating scientfic breakthrough: Scientists trap light in nano-soup
  • Date: Mon, 17 Dec 2007 00:59:24 -0500


<http://www.rsc.org/chemistryworld/News/2007/December/10120701.asp>
RSC - Advancing the Chemical Sciences
Chemistry World
Scientists trap light in nano-soup
10 December 2007

Physicists at the Bhavnagar University in Gujarat, India, have demonstrated
how to trap and retrieve light using a soup
of micro- and nano-sized magnetic spheres.1 The unusual fluid, they say,
works at room temperature, holds photons for
far longer than other systems, and can also be tuned with a magnet to store
any wavelength of visible light. 'The
discovery could pave the way for lab-on-a-chip devices for processing optical
information,' Rasbindu Mehta, who led the
team, told Chemistry World.

For over a decade scientists have been working towards light-based computing:
where circuits control photons - particles
of light - in the same way that they currently manipulate electrons.
Photon-based computing should be faster than
electronics. It would also cut out the ungainly components used to convert
between the optical signals that transmit
data and the electrical signals that manipulate and store it.

Any microchip designed to process optical signals has to store photons,
perhaps by slowing or trapping light in
carefully designed crystals. Mehta's team coated micron-size magnetite
spheres with oleic acid and dispersed them
through a ferrofluid, which is a suspension of much smaller magnetic
nanoparticles (in this case held in kerosene). When
an external magnetic field was applied to the fluid, which was held in a
glass cell, laser light passing through the
medium was trapped inside. Photons escaped when the field was switched off.

Flash of red light
Red light passes through the fluid with no external magnetic field applied
(left-hand diffraction pattern); is stopped
by a particular magnetic field strength (middle); photons appear again when
the magnet is switched off (right).

'It is fantastic,' said Hema Ramachandran, who heads the photonics unit at
the Raman Research Institute in Bangalore,
and was one of several physicists who witnessed the demonstration. She said
she was planning some experiments with
Mehta's group.

'It was a chance discovery,' explained Mehta's colleague Rajesh Patel.
While investigating the optical properties of
their transparent fluid the researchers noticed that, in a certain magnetic
field range, light scattering - both forward
and backward - became zero. 'We thought the light got trapped inside,' said
Patel. 'So, we switched off the laser
[which was shining light through the system] and then the magnetic field, and
there it was - a flash of colour lighting
up our dark room.'

A rigorous theoretical explanation is yet to come, but the researchers
believe that the spheres are aligned by the
magnetic field and form microcavities - filled by the ferrofluid - in which
the photons get trapped, resonating back and
forth. Changing the external magnetic field alters the refractive index of
the cavities, by changing the dispersion of
both the ferrofluid and the larger spheres. This in turn decides which
wavelength of light is trapped by the system.
And what is more, said Mehta, photons can be stored for as long as the
magnetic field is switched on. 'This is the first
visual evidence of storage and retrieval of light for a long and controllable
duration - in all other reports, storage
time of photons is restricted to a few nanoseconds,' he said.

Physicist Ortwin Hess, at the Advanced Technology Institute in the University
of Surrey, said the results were 'very
interesting'. 'As I understand it, the spheres act as resonators (every
resonator stores light) for a particular
wavelength,' he told Chemistry World. 'The interesting fact is that via the
ferroelectric properties of the materials,
a particular wavelength can then be controlled and released by application of
a field.' Hess recently published a
scheme to slow down and eventually store light using artificially designed
solid metamaterials, but his idea is still
theoretical.2Other experiments have involved stopping light altogether by
using a gas of sodium or rubidium atoms
chilled to near absolute zero, though that system is not practical for
microchips.

Many photonic crystals also control light by being patterned on the nano and
micro-scale, and having regions of variable
refractive index. Pumping a second laser light at photonic crystals can even
alter their photon-trapping properties. But
Mehta's team seem to have chanced on a simpler fluid system with the easy
dynamic control for which photonic crystal
designers yearn.

Killugudi Jayaraman



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References

1 R V Mehta et al, Current Sci., 2007, 98, 1071

2 K L Tsakmakidis, A D Boardman and O Hess, Nature, 2007, 450, 397
Also of interest

Tunable photonics with high refractive index

Scientists in Hong Kong have created a polymer with a refractive index that
can be tuned as high as 1.81.
Related Links




  • [permaculture] OT but fascinating scientfic breakthrough: Scientists trap light in nano-soup, Lawrence F. London, Jr., 12/17/2007

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