John Joannopoulos and his photonic crystal research group at MIT have discovered that a shock wave travelling thru a crystal will cause light to reflect off the location of the wave and that this can be used to shift the frequency of light up or down as the light moves thru the crystal.
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 lightıs 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.
It will eventually be possible to use non-destructive acoustic shockwaves in microelectromechanical systems (MEMS) devices to build crystals that will shift the frequency of light. It will also become possible to build devices that will take light that is spread over a wide range of frequencies and shift it to a narrower range.
One application of the ability to shift frequencies in this manner could be to increase the efficiency of photovoltaics. Many photovoltaic materials absorb only a narrow subset of the range of frequencies in natural light. If a photonic crystal could shift the natural light into a frequency band that the photovoltaics materials could convert to electricity then the same amount of photovoltaic material could generate more electricity.
A preprint of the article is available in PDF format. If you have Apple Quicktime installed an animation of the phenomenon is available for viewing.
|Share |||Randall Parker, 2003 May 27 01:42 AM Materials Advances|