It’s not everyday you see photonic research news on slashdot, and this looks pretty interesting. A ring resonator is waveguide made into a ring shape, and light can be confined inside if it resonate with the ring cavity. If it is made of active materials, it becomes ring laser.

Another straight waveguide built on the side is used to coupled light into and out of the ring resonator. So from an off state, with no photons inside the cavity, it can be turned on coupling in some light. To turn it off again, you just do the same, couple in some light. When more photons are coupled into the cavity, the photon density changes. At some point, the refractive index changes induced by the photon density will shift the resonant frequency of the cavity.

At this point, the cavity does not match the light inside, and photons dissipate out, leaving an empty cavity or off state. In short:

• off -> put light in -> on
• on -> put light in -> off

Of course everything has to be just right for the switching to be working as it should, but it’s possible. From what I see, the problem will be to pack all these into small spaces, because one ring resonator can get pretty big. It is definitely much larger than the state of the art CMOS transistor now. But if this proves a viable concept, I’m sure there will be ways to miniaturize it. (using visible wavelength or even UV, for examples)