PORTLAND, Ore. Micro- and nanoscale mechanical structures have long been used to sculpt and channel optical signals, from waveguides to resonators, but lately the direction of influence has reversed.
Now optical signals are being used to manipulate these mechanical structures. Recently, researchers at both the Massachusetts Institute of Technology (Cambridge) and Cornell University (Ithaca, N.Y.) demonstrated new methods of using optical signals to control mechanical structures, at least one group of material scientists proposing to close the feedback loop.
The trend began many years ago with the invention of "optical tweezers" to manipulate living cells without damaging them. Now MIT engineers, professor Matthew Lang and doctoral candidate David Appleyard, have demonstrated next-generation technology: an optical tractor-beam that can manipulate both living cells and microelectromechanical systems (MEMS) structures as large as 20 microns. "We've begun applying optics to building structures on chips," said Lang.
Separately, Cornell University professors Michal Lipson and David Erickson, along with their graduate students Bradley Schmidt and Allen Yang, report harnessing the evanescent field surrounding solid-core optical fibers to attract and propel micro- and nano-scale particles through microfluidic devices. Lipson, a pioneering researcher who manages a team of EEs conducting silicon photonics research, collaborated with mechanical engineer Erickson to characterize the velocities that can be achieved for various particle sizes, reporting that speeds of 28 microns per second were achieved for three-micron-diameter polystyrene spheres using about 54 milliwatts of optical power down the fiber.
Back at MIT, in a separate lab, EE professor Erich Ippen teamed with physics professor Marin Soljacic and their graduate students Milos Popovic and Peter Rakich, to unify the influence of optics-on-mechanical with mechanical-on-optics by closing the feedback loop between the two. The researchers have crafted a control theory detailing how feedback from mechanically coupled optical cavities can be used to dynamically tune their resonance.
"We hope to eventually demonstrate working MEMS devices that can perform all-optical functions not possible today, from switching to adaptive dispersion and filter synthesis for applications like optical clock recovery," said Popovic.
The team is now crafting MEMS membranes and cantilevers that can perform signal processing operations presently requiring expensive translation to electrical signals and back to optical, such as resonators that can track communications signals across their entire free spectral range of about 4.5 THz.