PUT DOWN that fork. Show your coleslaw a little more respect. For sliced
vegetables, and even the creamy mayonnaise dolloped on the side of your plate,
could help to unlock some of the strangest secrets of the Universe.
If you don鈥檛 believe it, just take a peek into Hiroshi Taniguchi鈥檚 lab at
Iwate University in Japan. He has turned to thick chunks of carrot and potato to
unravel some pretty indigestible physics. It may sound crazy that a fresh salad
could make anything more than a low-calorie lunch, but these aren鈥檛 any old
vegetables鈥攖hey are vegetable lasers.
Taniguchi鈥檚 recipe is simple. Take your freshly sliced vegetables and dunk
them in his special sauce鈥攁ctually a fluorescent dye鈥攂last them with
a laser beam of just the right wavelength and watch the slices glow.
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According to Taniguchi鈥檚 research, potatoes work well, but so do carrots,
green peppers and pumpkins. He has even used grains of rice. But any vegetable,
freshly prepared, should be well-suited to life as a guiding light. They shine
so brightly, says Taniguchi, that you don鈥檛 even need to darken the lab to pick
them out. And although no one is exactly sure how these vegetable lasers work,
one day they could provide the clues that astronomers need to decode mysterious
broadcasts from watery clouds in far-off galaxies.
When you think of a laser, sliced vegetables are probably not the first thing
that comes to mind. Lasers usually contain less palatable things, a chunk of
ruby or a glass cylinder of argon gas, for instance, in a cavity between two
highly polished and precisely aligned mirrors. Trigger the atoms in the cavity
with light from a flashlamp and they emit more of the same. As the photons
bounce back and forth between the mirrors, the light is amplified until, in a
fraction of a second, the cavity floods with laser light.
Taniguchi鈥檚 vegetable lasers are far less sophisticated. Instead of ruby or
argon, they rely on molecules of his dye that cling to the vegetables after
dunking. The dye absorbs light and re-emits it as fluorescence. Rather than
mirrors, the cavity is formed by the walls of the tiny, randomly-oriented plant
cells that make up the vegetable tissue. And instead of a flashlamp, the power
for Taniguchi鈥檚 vegetables comes from another laser beam.
When his vegetables give off their ghostly glow, Taniguchi believes that the
fluorescing photons inside them become 鈥渃oherent鈥�. In other words, they stop
behaving as individuals and take on the characteristics of laser light in which
the wavelength and phase of every photon is identical. He thinks he has created
something called a multiple light scattering laser鈥攐r random laser.
Physicists have argued for decades that it should be possible to create this
new kind of laser using photons that follow random paths. Shine light into
something that scatters light efficiently, so the theory goes, and if they are
deflected often enough in random directions, the chances are that some of them
will follow repetitive, circular paths. If they are amplified as they ricochet
around, and if the wavelength of light matches the lengths of these random
loops, the photons will lock their wavelengths and phases together. In theory,
the material should switch on like a light bulb, lit up by laser beams.
This year, physicist Hui Cao and her colleagues at Northwestern University in
Illinois turned this theory into practice. In March, they published a report in
Physical Review Letters (vol 82, p 2278) that showed they had turned a
powder into a random laser. They laid an ultrafine powdered mixture of zinc
oxide and gallium nitride onto a piece of glass and fired rapid bursts of bright
blue laser light at it. The zinc oxide is fluorescent so it acted like the dye
in Taniguchi鈥檚 vegetables. The tiny particles of zinc oxide and gallium
nitride鈥攅ach a mere 100 nanometres across鈥攁re very efficient
scatterers, so the photons changed direction after travelling only very short
distances. 鈥淣ow there鈥檚 a chance that the light is going to come back on
itself,鈥� says Cao. Her powder forms billions of minuscule 鈥渞ing cavities鈥� that
amplify the light, just like the cavity in an ordinary laser.
With this in mind, Taniguchi isn鈥檛 too surprised that it鈥檚 possible to build
lasers from little more than sliced vegetables. 鈥淲e knew that almost all
vegetables have the continuously disordered fine structures required for random
lasers,鈥� he says.
And back in his lab, Taniguchi has discovered something else: you don鈥檛 even
need to throw away the vegetable waste left over from preparing your lasers. He
extracts a pigment from radish leaves and uses it to create a fluorescent dye.
When he injects the dye into an emulsion of biological fats that scatters
light鈥攕omething exactly like mayonnaise, for example鈥攊t too behaves
as a laser. The meal is complete: inject the mayonnaise with dye, turn on the
laser and your coleslaw or potato salad becomes a truly light lunch.
If you鈥檙e worried by the prospect of being blinded by the beam from a carrot
laser, fear not. Random lasers will probably never make high intensity sources:
the light is entirely unfocussed and comes out in all directions鈥攚hich
explains why Taniguchi鈥檚 veg have such a ghostly glow. But Cao is looking at the
random laser as a possible new display technology. 鈥淎lthough the light can go in
all different directions, it鈥檚 got pretty high efficiency,鈥� she says. 鈥淚t鈥檚 like
a light-emitting diode.鈥�
Light-emitting coleslaw or a potato shining like a full moon may not be the
most practical of devices, but Taniguchi believes they will be useful
nonetheless. There鈥檚 loads these lasers can teach us about multiple light
scattering, he says. And luminous comestibles may have other uses: they could
offer physicists the tool they need to develop new techniques for identifying
molecules or particles in highly scattering environments.
Physicists already use light scattering to look inside things such as tissue,
blood or suspensions of fats. But these measurements are tricky: try to beam
light into something like milk and most is scattered straight back out before it
can reach whatever it is you鈥檙e looking for. If you want to probe more than a
few centimetres beneath the surface, you need an incredibly bright light, and a
large helping of luck.
So why not inject your sample with micrometre-diameter spheres containing a
fluorescent dye. Now you can use a laser to excite the dye and create a random
laser inside your sample. With the extra light this creates, scattering studies
should be far easier. Perhaps one day this approach could make it simpler to
identify rogue bugs inside vats of stout, or cancerous cells lurking deep inside
living tissue.
But those most likely to benefit from a plateful of glowing vegetables will
be astronomers. For laser salads have cosmic potential for shedding light on the
workings of strange lasers in distant galaxies.
Deep in the heart of galaxy NGC4258, for instance, is a cloud of water vapour
which blasts out radiation with astonishing power. Researchers believe that,
somehow, energy is being pumped into the water molecules, stimulating them and
turning them into a gigantic microwave laser or 鈥渕aser鈥�. The power for this
maser, astronomers believe, comes from energetic X-rays beamed out of a
supermassive black hole that lurks near the centre of the galaxy.
Understanding the way the maser works could tell us useful things about the
black hole, such as its rate of growth, which could in turn tell us something
about the first moments of the Universe. And the Universe is peppered with
billions of similar masers. Their radiation could also provide clues to the way
stars form and die鈥攊nformation that cosmologists would find
invaluable.
The problem, says Harvard University astronomer James Moran, is that masers
in galaxies such as NGC4258 are constantly changing, which makes them difficult
to understand. The clouds of vapour are moving across the galaxy at thousands of
kilometres per hour, and their shapes keep changing. With the pattern of maser
emission altering constantly, understanding how they work is difficult. 鈥淭he
details aren鈥檛 understood very well because masers are finicky,鈥� Moran says.
Enter the humble vegetable. The random orientation of plant cells that turns
a salad into a laser may mimic the random arrangement of water molecules in a
shimmering maser cloud. 鈥淚t could be similar,鈥� Moran admits.
A terrestrial random laser might be just the tool for bringing the subject
down to Earth. Cao believes her study of random lasers鈥攂ased on scattering
mechanisms similar to galaxy masers鈥攃ould also help. And her piles of
semiconductor powder seem to be lasing in exactly the same way as the sliced
vegetables in Taniguchi鈥檚 laboratory. So it鈥檚 not inconceivable the tangled
history of the Universe could be unravelled by the light from a glowing carrot.
Now there鈥檚 food for thought.