MIT is hacking light rays to boost fiber-optic bandwidth and t-ray technology.
The simplest phenomena are sometimes the hardest to manipulate. Consider: Changing the wavelength of a ray of light – which consists of nothing more than massless photons – is a Herculean task. Even worse, it's tough to control exactly what frequency you end up with.
A group of MIT photonics researchers may have found a much better way to tweak light, a physics watershed that could reshape industries from medicine to defense. The researchers, led by postdoctoral associate Evan Reed, start with a photonic crystal – a light-bending microstructure that guides beams through fiber-optic circuits. Ordinarily, such a crystal repels light of a particular color while letting other colors pass. In computer simulations, Reed's team discovered that by pelting the crystal with shock waves, they could flip-flop its properties – make it welcome the colors it once warded off, and vice versa.
The process temporarily traps light inside the crystal, where it ricochets off the moving lattice thousands of times in less than a nanosecond. The reverberating light gets Doppler-shifted – it changes wavelength – in the process. For example, a visible blue beam shifts to an invisible ultraviolet ray when a shock wave is sent against the flow of light. The effect can be modified by altering the crystal's structure.
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Changing the Frequency of a Light Wave: 1) A shock wave reverberates through a photonic crystal. 2) A low-frequency (red) light wave enters the vibrating crystal and is temporarily trapped. 3) The wave ricochets inside the crystal, where it's Doppler-shifted to a higher frequency (changing its color). 4) The light wave (now blue) exits the crystal at a higher frequency and in the opposite direction.
This concept isn't merely some ivory-tower triumph; it could fundamentally alter telecommunications, says Reed, lead author of the Physics Review Letters paper that in May announced the MIT experiment. "Telcos have all these fiber optics to transmit signals," he says, "but you need to have some way of utilizing all the bandwidth." Too often, the potential of fiber-optics technology is wasted because all the information it carries is jammed into the same frequency, filling that portion of the network to capacity. Controlling signals at the crystal level would allow networks to shift data to less-congested frequencies. The only remaining limit on transmission speed would be the bandwidth of the cables themselves.
Proponents of alternative energy are also aglow over the technology, as cheap, easy frequency shifting could dramatically improve the efficiency of solar cells. Currently, a typical cell consists of four or five materials, each of which harnesses a different frequency of light. Individually, those frequencies represent only a small fraction of sunlight's energy potential. If all those rays could be converted into a single frequency, solar power might finally start to rival fossil fuels as a mainstream energy source.
Perhaps the most exciting possibility, though, is that frequency shifting will accelerate the development of terahertz-ray technology. Like x-rays, t-rays can be used to peer through walls, garments, or skin but are believed to be far less harmful than their higher-frequency counterparts. T-rays occupy the slice of electromagnetic spectrum between infrared and microwave.
The problem: They're maddeningly hard to produce, and the few t-ray devices that exist tend to emit multiple frequencies at once. "For the purposes of imaging, you really need a monochromatic source," says Reed. "But if you can do a frequency conversion, you could take a bright source and convert it into the terahertz region."
The next step for the MIT team is to test the computer-simulated method on real photonic crystals. To provide the necessary shock waves, they'll start by using bullets, which have the downside of destroying the crystal in the process. But they hope eventually to rely on nondestructive sound waves, perhaps created by speck-sized microelectromechanical systems – which should make fine-tuning optical frequencies no more difficult than flicking a light switch.
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