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Two Light Beams Make One Photonic Crystal?

“Photonic crystals have been one of the hottest topics in optics in the last 10 years. This research is born out of a desire to have precise control over light—similar to the semiconductor industry’s control over electrons. The current favorite way to do this is to construct devices that have a refractive index that varies on the scale of the wavelength of light.

These metamaterials are made by combining two different materials in a very precise way, leading to all sorts of fabrication headaches. This may change in the near future because a pair of researchers have shown that you might be able to create temporary photonic crystals in a single material simply by shining a couple of light fields on it.

As a starting point, the researchers noted that the refractive index experienced by light changes dramatically when the material it’s passing through can absorb the light—that is, the light frequency is coincident with an atomic resonance. So, if you choose a light field that has a frequency that is slightly higher than an atomic resonance frequency, it will experience a lower refractive index.

On the other hand, if you choose light with a frequency slightly lower than the atomic resonance, it will experience a larger refractive index and some absorption. This doesn’t appear to help though—to get a useful difference between the two refractive indices, the light field has to be very nearly resonant with the atomic resonance. Over very short distances, the light is lost to absorption.

In a fit of creativity, and, seemingly, ignoring reality, the researchers asked themselves what would happen if a light field were simultaneously at a frequency that was both higher and lower than an atomic resonance? Of course, this can’t be the same atomic resonance, that would be too bizarre. Instead, the light is sandwiched between two neighboring resonances. One atomic resonance provides gain and the other absorption, with the two balancing exactly. Under these circumstances, the refractive index could be made to vary strongly.”

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