Carrots make excellent Lasers

Guess what you can do with a plate of lovely vegetables? And it's got nothing to do with sex or slimming. Put down that fork. Show your coleslaw a little more respect. For Sliced carrots could help to unlock some of the strangest secrets of the Universe.

The tangled history of the Universe could be unravelled by the light from a glowing carrot. Now there's food for thought.

If you don't believe it, just take a peek into Hiroshi Taniguchi's 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't any old vegetables--they are vegetable lasers.

Taniguchi's recipe is simple. Take your freshly sliced vegetables and dunk them in his special sauce--actually a fluorescent dye--blast them with a laser beam of just the right wavelength and watch the slices glow.

According to Taniguchi's 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't 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's 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's vegetables comes from another laser beam.

When his vegetables give off their ghostly glow, Taniguchi believes that the fluorescing photons inside them become "coherent".  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--or 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's vegetables. The tiny particles of zinc oxide and gallium nitride--each a mere 100 nanometres across--are very efficient scatterers, so the photons changed direction after travelling only very short distances. "Now there's a chance that the light is going to come back on itself," says Cao. Her powder forms billions of minuscule "ring cavities" that amplify the light, just like the cavity in an ordinary laser.

With this in mind, Taniguchi isn't too surprised that it's possible to build lasers from little more than sliced vegetables. "We knew that almost all vegetables have the continuously disordered fine structures required for random lasers," he says.

And back in his lab, Taniguchi has discovered something else: you don't 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--something exactly like mayonnaise, for example--it 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're 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--which explains why Taniguchi's veg have such a ghostly glow. But Cao is looking at the random laser as a possible new display technology. "Although the light can go in all different directions, it's got pretty high efficiency," she says. "It's like a light-emitting diode."

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's 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're 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 "maser". 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--information 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. "The details aren't understood very well because masers are finicky," 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. "It could be similar," 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--based on scattering mechanisms similar to galaxy masers--could also help. And her piles of semiconductor powder seem to be lasing in exactly the same way as the sliced vegetables in Taniguchi's laboratory. So it's not inconceivable the tangled history of the Universe could be unravelled by the light from a glowing carrot. Now there's food for thought.

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