Posted on 05/22/2003 11:25:37 AM PDT by sourcery
rare in the abstract of a reputable scientific paper. But the latest report by photonics crystal pioneer John Joannopoulos and his group at MIT, soon to be published in Physical Review Letters, does not disappoint.
The researchers document the ultimate control over light: a way to shift the frequency of light beams to any desired colour, with near 100 per cent efficiency. "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.
Hall of mirrors
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.
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 light¹s 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.
Shooting bullets
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. "It¹s 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 print edition, 14 September 2002, p 34). But they are not yet widely used as they have been too difficult to produce.
It's good that they are actually trying to produce this effect in the lab. Because sometimes those computer simulations are not as accurate as the real thing ...
Yellow and Cyan, specifically.
Vaporware alert. Lots of stuff looks good in simulation, only to fail in the real world.
The team is now collaborating with researchers at Lawrence Livermore National Laboratory to demonstrate the effect.
This should read: "to discover whether there really is any such 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. "It¹s really practical, and potentially even easier to do than with actual shock waves," says Reed.
I wonder why they don't try the sound method first. To bond a piezo to a photonic crystal, shine a laser on it, hook it up to a frequency generator and twiddle the knob sounds like the work of an afternoon.
That wouldn't give you the continuous emission spectrum we see in the CMBR. Where's the light coming from in the first place?
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