Bigger on the Inside? Scientists Trap Light in “Whispering Gallery” Microspheres

In a vacuum, light moves fast enough to travel all the way around the earth in about a tenth of a second. In recent years, though, scientists have found ways to slow and even stop light in its tracks by using new states of matter and other specially engineered materials. Now, researchers in France are reporting that they’ve devised a new way to tackle the challenge, one which circumvents many of the technical difficulties associated with previous techniques.

The new method, as described in a paper recently accepted by Physical Review Letters, works by sending light into microscopic glass spheres that have been infused with atoms of erbium, a fluorescent rare earth metal with a long history of applications in optics. The spherical geometry, together with the erbium ions, creates a path for the photons that’s more than a million times longer than the sphere itself.

Optical whispering-gallery modes in a glass sphere.
Light is pumped into the sphere from the optical fiber
at the right. Once inside the sphere, fluorescent
particles make the whispering-gallery effect visible.
Image Credit: NASA JPL

The effect is reminiscent of a “whispering gallery”, where sound waves can bounce repeatedly around the rim of a circular chamber, allowing for the creation of standing waves. Oftentimes arising as an accidental result of cylindrically-symmetrical architecture, the whispering gallery phenomenon can allow someone to be heard on the other side of a vast room without raising their voice.

Here, though, the effect is anything but accidental. Taking advantage of the same phenomenon with light waves, scientists can localize light for milliseconds at a time as infrared photons are passed around in a circle, handed off between the electrons of erbium atoms in different states of excitation. Milliseconds may not sound like a very long time, but it’s more than enough for a computation to take place in a computer processor, which is where the team hopes this technique will find application. Besides, keeping light in one place for that long is a feat in itself, considering it should travel upwards of a hundred miles in that amount of time, even at its comparatively slow speed in glass.

If the technology can be applied in a practical engineering context, as the paper’s authors predict, it would be a boon to the emerging field of optical computing and data storage, which many see as the first step toward proper quantum computers. Whether or not this brings us closer to having “Isolinear Optical Chips” like in Star Trek, it’s a fantastic advance in science’s mastery of light.

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