Solar power is great for converting light energy into electricity. But what about harnessing light energy directly? After all, photons—discrete packets of light energy—exert force themselves, albeit on a pretty small scale.
In a new study, a team of researchers from Yale University and the University of Washington reports doing just that, also on a pretty small scale—vibrating a tiny mechanical object physically by shining light through it.
Light-powered mechanics could form the basis for nanoscale components such as switches and routers, all operating off the grid, so to speak. “We can use light force to replace electrostatic force,” says Hong Tang, an assistant professor at Yale’s School of Engineering & Applied Science and co-author of the study published today in Nature. “You don’t need to apply voltage, you just need to pass light through it.”
The group’s experimental setup confines laser light in an on-chip silicon waveguide. The waveguide routes the light through a narrow section, 10 microns in length and just 110 nanometers thick, that resonates ever so slightly as the light passes through. (A micron is a millionth of a meter; a nanometer is a billionth of a meter.) “It’s a little bridge, a nanomechanical resonator,” Tang says. “It’s the simplest resonator you can find.”
Other approaches that have harnessed the force of light have primarily exploited the so-called radiation pressure force, a sort of direct hit that occurs when photons strike an object. But Tang’s team was able to move its resonator in a direction transverse, or crosswise, to the light’s path, using an effect called optical gradient forces. In other words, the horizontal flow of light through the resonator induces it to vibrate up and down.
Those vibrations are so tiny that Tang and his colleagues used a second laser to detect the motion. “When we talk about nanomachines, we cannot think of this nanomachine like it’s your hand moving around or some tools moving around—that’s the wrong picture,” Tang says. “Because they are small, the motion has to be small, too.”
Tang calls this demonstration a proof of principle, adding that his group will seek to increase the frequency of the vibrations by more than 100 times. In this study, the bridge’s resonant frequency was in the neighborhood of 10 megahertz, or millions of cycles per second. Tang would like to be able to get a similar device vibrating at much higher speeds, above the gigahertz range—in the billions of cycles per second.
Some observers see a bright future for such light-induced motion. “With this work, optical trapping ‘grows up,’” says Naomi Halas, a professor of electrical and computer engineering and of chemistry at Rice University. “Optical trapping has been so important in enabling new research approaches in atomic physics and biophysics, but with this work it gets implemented on a silicon chip, where it is clear … that it will prove to be a valuable approach in many technological applications.”
