Laser Makes Memory Mechanical

Engineers at Yale University say they’ve invented a new type of mechanical memory device that is read from and written to by light. According to its creators, this development could lead to better sensors and new techniques in optical telecommunications.

The device is essentially a tiny piece of silicon that can be bent up or down by the light propagating inside a photonic circuit. Once the light is switched off, the piece remains in one of those states, representing the 1s and 0s of digital coding. The engineers from Yale who developed the device, which is called a "nanomechanical resonator," described it yesterday in the journal Nature Nanotechnology.

"We really can achieve control of the nanodevice at very high amplitude and repeatability," says Hong X. Tang, an associate professor of electrical engineering, who led the work.

To make the resonator, Tang and his colleagues started with a commercially available silicon-on-insulator wafer and created an oval-shaped waveguide on the wafer to act as an optical cavity. They etched away a bit of the wafer below the waveguide to create a strip of silicon 10 micrometers by 500 nanometers by 110 nm, so they ended up with a membrane of material across part of the waveguide, attached at both ends but free to fluctuate up and down in the middle. Because of stress put on the wafer during the initial process of attaching the silicon to the insulator, this strip naturally buckled a bit. So with no force applied, it would be stable when bent either upward or downward.

When the researchers fired laser light into the optical cavity at a frequency that was slightly higher than the resonant frequency of the cavity, the resonator started oscillating, bending up and down in rapid succession. "If you put extra energy into the cavity, the mechanical resonator will gain energy from the laser field," Tang explains. When the laser was turned off, the oscillation stopped, leaving the resonator in either the up or the down state—a 1 or a 0.

But to make the device effective as memory, the group wanted to be able to control whether the strip came to rest bent up or down, so they turned to laser cooling, the same technique used to slow atoms to a near motionless state. Injecting laser light with a lower frequency than the device’s resonant frequency damped the oscillations. Selecting one damping frequency made it more likely that the strip would settle into the buckle-up state; a different damping frequency made it probable it would stop at buckle-down.

"The two states [up and down] are separated by a huge energy barrier," Tang says. That means with the laser turned off, they stay put, making the memory nonvolatile. The device is also much less sensitive to stray radiation and heat effects that can sometimes switch a bit in electrical or magnetic memory, he says.

To read the memory, the researchers simply use a laser with an energy too low to flip the bits. The position of the resonator changes the refractive index of the optical cavity, so it’s easy to know whether it’s up or down by how the laser light bends.

The optical technique "may enable ultrahigh-speed manipulation" of a mechanical bit, says Pritiraj Mohanty, professor of physics at Boston University, who was not involved in the research.

It takes a relatively large amount of energy—a microjoule—to switch a bit, so Tang says the device as it stands is impractical for large-scale storage. But he says it could be useful for something like an optical router, which doesn’t have to switch too often. It might also be valuable in control circuitry; for example, it could provide timing on a computer chip. And it could make sensors for acceleration or trace gases more sensitive. In fact, the research was funded under a Defense Advanced Research Projects Agency grant for using optical control to improve sensors.