"The degree of control over light really is quite shocking," comments photonics expert Eli Yablonovitch at the University of California, Los Angeles. If the effect can be harnessed, it will revolutionise a range of fields- turning heat into light, for example, or prized terahertz rays.
Right now, the only way to shift the frequency of a light beam involves sending an extremely intense light pulse- with a power of many megawatts or even gigawatts- along next to it. This interacts with the first beam and alters its frequency, but the technique is expensive, requires high-power equipment, and is generally pretty inefficient.
But when Joannopoulos and his colleagues Evan Reed and Marin Soljacic investigated what happens when shock waves pass through a device called a photonic crystal, they discovered a completely unexpected effect. Photonic crystals, which are made by sandwiching together layers of material that bend light in different ways, can be designed to reflect some frequencies while letting others through. They are used to steer light through circuits in the same way that electronic circuits direct electric current.
From computer simulations, the team found that shock waves passing through a crystal alter its properties as they compress it. For example, a crystal that normally allows red light through but reflects green light might become transparent to green light and reflect red light instead.
The researchers worked out that if a photonic crystal is designed in a certain way, incoming light can get trapped at the shock wave boundary, bouncing back and forth between the compressed part of the crystal and the uncompressed part, in a "hall of mirrors" effect (see Graphic).
Because the shock wave is moving through the crystal, the light gets Doppler shifted each time it bounces off it. If the shock wave is travelling in the opposite direction to the light, the light's frequency will get higher with each bounce, while if it travelling in the same direction, the frequency drops.
After 10,000 or so reflections, taking a total of around 0.1 nanoseconds, the light can shift dramatically in frequency- from red up to blue, for example, or from visible light down to infrared. By changing the way the crystal is built up, it is possible to control exactly which frequencies can go into the crystal and which come out. "We ought to be able to do things that have never been possible before," Joannopoulos told New Scientist.
The technique can even focus a wide range of frequencies into a narrow band, something no other known method can do, says Joannopoulos. Normal colour filters merely let through the desired frequencies and chop the others away, so much of the energy is lost.
The team is now collaborating with researchers at Lawrence Livermore National Laboratory to demonstrate the effect. Initially they will generate shock waves by shooting bullets at photonic crystals. This would destroy the crystal, but not before the light has had time to shift. Eventually, sound waves should do the job just as well, they say. "It's really practical, and potentially even easier to do than with actual shock waves," says Reed.
The work is impressive, says materials chemist Michael Sailor at the University of California, San Diego, whose team has developed flexible, biodegradable photonic crystals. He says he now plans to test the phenomenon for himself.
Besides making devices such as light bulbs and solar cells more efficient, the method would also help to keep optical telecommunications networks moving. At the moment, many light frequencies are bounced down optical fibres simultaneously. If a particular frequency is being used to capacity, then optical switches could shift light beams to a frequency where there is still capacity to spare.
Another benefit of pushing the frequency of light downwards would be the ability to make terahertz radiation. Terahertz rays, in the range between microwaves and infrared, hold great promise for medical imaging, as they are easier to focus and less damaging than X-rays (New Scientist, 14 September 2002, p 34). But they're not yet widely used as they have been too difficult to produce.
Author: Charles Choi
New Scientist issue: 24 MAY 2003
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