April 27, 2009--Researchers at Yale University (New Haven, CT) have demonstrated silicon nano-scale cantilevers, smaller than the wavelength of light, that operate on photonic principles. The structures eliminate the need for electric transducers and expensive laser setups, the researchers say, and enable a new generation of tools for ultra-sensitive measurements at the atomic level.
The work is reported in an advance online publication of Nature Nanotechnology, which demonstrates how nanoelectromechanical systems (NEMS) can be improved by using integrated photonics to sense cantilever motion. In NEMS, the cantilevers act as nano-scale diving boards that "bend" when molecules "jump" on them and register a change that can be measured and calibrated. The Yale system can detect as little deflection in the nano-cantilever sensors as 0.0001 Angstroms (one ten thousandth of the size of an atom).
To accomplish this, the Yale team devised a photonic structure to guide the light wave through a cantilever. After exiting from the free end of the cantilever, the light tunnels through a nanometer gap and is collected on chip. "Detecting the lightwave after this evanescent tunneling," says Hong Tang, the paper's senior author, "gives the unprecedented sensitivity."
"The system we developed is the most sensitive available that works at room temperature. Previously this level of sensitivity could only be achieved at extreme low temperatures," said Tang, assistant professor of electrical and mechanical engineering in the Yale School of Engineering and Applied Sciences.
Tang's paper also details the construction of a sensor multiplex--a parallel array of 10 nano-cantilevers integrated on a single photonic wire. Each cantilever is a different length, like a key on a xylophone, so when one is displaced it registers its own distinctive "tone." According to postdoctoral fellow Mo Li, the lead author of the paper, "A multiplex format lets us make more complex measurements of patterns simultaneously--like a tune with chords instead of single notes."
At the heart of this breakthrough is the novel way Tang's group "wired" the sensors with light. Their technique is not limited by the bandwidth constraints of electrical methods or the diffraction limits of light sources. "We don't need a laser to operate these devices," said Wolfram Pernice, a co-author of the paper. "Very cheap LEDs will suffice." Futhermore, the LED light sources--like the million LED pixels that make up a laptop computer screen--can be scaled in size to integrate into a nanophotonic-chip--an important feature for this application.
"This development reinforces the practicality of the new field of nanooptomechanics," says Tang, "and points to a future of compact, robust and scalable systems with high sensitivity that will find a wide range of future applications--from chemical and biological sensing to optical signal processing."
For more information, see the paper, Broadband all-photonic transduction of nanocantilevers, published by Nature Nanotechnology. See also the page for Hong Tang's lab at Yale.
Posted by Barbara G. Goode, [email protected], for Laser Focus World.