A tiny ladderlike beam of silicon converts light into vibrations and vice versa with extremely high efficiency, physicists report. That may seem like an esoteric result, but the finding could open the way to new physics and someday serve as a key element in optical microcircuits akin to the electronic microcircuits in computer chips.
Although the effect is ordinarily very small, light exerts forces on the things it strikes or flows through. In recent years, physicists have exploited those forces to set micrometer-sized objects aquiver. For example, 4 years ago, a team led by Kerry Vahala of the California Institute of Technology (Caltech) in Pasadena showed that light leaking out the side of a nearby optical fiber could make a tiny disk of glass vibrate. And 2 years ago, Daniel Gauthier of Duke University in Durham, North Carolina, and colleagues showed that light traveling down a fiber could make the fiber itself shake. In fact, they stored a pulse of light as a vibration and released it nanoseconds later.
Now, Oskar Painter, Matt Eichenfield, and colleagues at Caltech have taken these efforts a big step forward. Along with Vahala, the applied physicists have designed a gadget that increases the strength of the interaction of light and vibrations by orders of magnitude, potentially opening the way to optical microchips in which low-frequency vibrations or microwaves control high-frequency optical signals or vice versa. The device combines two different but related fields: photonics and phononics.
For more than a decade, physicists have been developing so-called photonic crystals. These are samples of glass or other light-transmitting materials filled with regular patterns of holes, which alter the way light waves can travel--in much the same way that the array of atoms in a crystal affects the way electrons can move through it. In such photonic crystals, light of certain wavelengths cannot propagate, as the waves overlap and interfere to cancel themselves out. However, light of such a wavelength can be trapped within the crystal if the spacing of the holes is changed in one spot to allow it to exist there. Sound also consists of waves, so similar holey structures can be used to make phononic crystals that affect sounds in much the same way.
Eichenfield, Painter, and colleagues fashioned a hybrid photonic/phononic crystal out of a tiny bridge of silicon less than a micrometer wide and about 20 micrometers long. They etched rectangular holes into the beam to make a ladderlike structure, with several of the holes in the middle slightly closer together to trap light and vibrations of the same wavelengths but vastly different frequencies. The researchers then fed light into the beam with an optical fiber and measured the light reemerging from it. At predictable wavelengths, the amount of light coming back out dipped, showing that some of the light was getting trapped in the beam.
The researchers also looked at the total range of frequencies of the light coming out and found that some of it had been transferred to microwave frequencies--the exact frequencies of trapped vibrations, the team reports online this week in Nature. That shift proved that the light was making the beam vibrate and that the jiggling was then affecting the light and converting some of it to microwaves. In fact, each photon pushes on the beam with 10 times the force of gravity, Painter estimates.
"It's an incredibly exciting piece of work," says Duke's Gauthier. That's because the conversion of light to vibration is so much stronger than it has been in previous experiments. "People tend to use meters of optical fiber and milliwatts of laser power, whereas they have used a micrometer-sized device and microwatts of power." The device could have numerous applications, says physicist John Page of the University of Manitoba in Winnipeg, Canada. Specifically, the advance could pave the way to using vibrations or microwaves to control optical signals and to fashion switches, filters, or mixers in optical circuits on microchips.