Photonics ‘Wire’ Nanosensors

Silicon-based nanocantilevers smaller than the wavelength of light and operating on photonic principles have been demonstrated by a team of researchers at Yale University. The discovery eliminates the need for electric transducers and expensive laser setups, ushering in a new generation of tools for ultrasensitive measurements at the atomic level.

In nanoelectromechanical systems (NEMS), cantilevers are the most fundamental mechanical sensors. These tiny structures – fixed at one end and free at the other – act like nanoscale diving boards that “bend” when molecules “jump” on them and register a change that can be measured and calibrated. This paper demonstrates how NEMS can be improved by using integrated photonics to sense the cantilever motion.

This image shows the electronmicroscopic image of array (top) and simulation of light waves through array (bottom). Image courtesy of Li, Pernice,Tang / Yale.

“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 senior author of the study Hong Tang, assistant professor of electrical and mechanical engineering in the Yale School of Engineering and Applied Sciences.

The researchers’ system can detect as little deflection in the nanocantilever sensors as 0.0001 Angstroms – one ten-thousandth the size of an atom

To detect this tiny motion, 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 light wave after this evanescent tunneling gives the unprecedented sensitivity,” Tang said.

Tang’s paper also details the construction of a sensor multiplex – a parallel array of 10 nanocantilevers integrated on a single photonic wire. Each cantilever is a different length, such as a key on a xylophone, so that, when one is displaced, it registers its own distinctive “tone.”

“A multiplex format lets us make more complex measurements of patterns simultaneously – like a tune with chords instead of single notes,” said postdoctoral fellow Mo Li, the lead author of the paper.

At the heart of this breakthrough is the novel way Tang’s group “wired” the sensors with light. The 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 study. “Very cheap LEDs will suffice.”

The LED light sources – such as 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 nano-optomechanics 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,” Tang said.

Funding for the research was from a Yale Institute for Nanoscience and Quantum Engineering seed grant, a National Science Foundation career award and the Alexander von Humboldt postdoctoral fellowship programs.

For more information, visit: www.yale.edu