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March 11, 2010

Optically driven nanoelectromechanical oscillators

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An illustration of the nanoelectromechanical oscillator, with the cantilever on the far right. The inset is a tilted 3-D profile of the structure, which shows the silicon dioxide posts.

Cornell researchers are a step closer to creating extraordinarily tiny sensors that can instantly recognize harmful substances and bacteria in air or water.

The researchers, led by professor of applied and engineering physics Harold Craighead, made a device just 200 nanometers thick and a few microns long with an oscillating cantilever hanging off one end. (A nanometer is one-billionth of a meter; a micron is one-millionth of a meter.) They identified exactly how to tune its sensitivity -- a breakthrough that could lead to advanced sensing technologies.

"The big purpose is to be able to drive arrays of these things all in direct synchrony," said first author Rob Ilic, a research associate at the Cornell NanoScale Science and Technology Facility. "They can be functionalized with different chemistries and biomolecules to detect various pathogens -- not just one thing."

The cantilever is like a diving board that resonates at distinct frequencies. In past research, the team has demonstrated that by treating the cantilever with different substances, they can tell what other substances are present. For example, E. coli antibodies attached to the cantilever can detect the presence of E. coli in water.

Journal of Applied Physics - Theoretical and experimental investigation of optically driven nanoelectromechanical oscillators




The actuation of biologically functional micro- and nanomechanical structures using optical excitation is an emerging arena of research that couples the fields of optics, fluidics, electronics, and mechanics with potential for generating novel chemical and biological sensors. In our work, we fabricated nanomechanical structures from 200 and 250 nm thick silicon nitride and single crystal silicon layers with varying lengths and widths ranging from 4 to 12 μm and 200 nm to 1 μm, respectively. Using a modulated laser beam focused onto the device layer in close proximity to the clamped end of a cantilever beam, we concentrate and guide the impinging thermal energy along the device layer. Cantilever beams coupled to chains of thermally isolated links were used to experimentally investigate energy transport mechanisms in nanostructures. The nature of the excitation was studied through steady-periodic axisymmetric thermal analysis by considering a multilayered structure heated using a modulated laser source. Results were verified by finite element analysis, which was additionally implemented for the solution of steady-periodic and transient thermal, as well as steady thermoelastic problems. These theoretical investigations, coupled with our experimental results, reveal that the complex dynamics underpinning optical excitation mechanisms consists of two disparate spatial regimes. When the excitation source is focused in close proximity to the structure the response is primarily thermal. We show that as the source is placed farther from the clamped end of the structure, the thermal response progressively fades out, indicating the possibility of mechanical wave propagation. Understanding the excitation mechanisms may be useful for applications including compact integration of nanophotonic elements with functionalized nanomechanical sensors for ultrasensitive biochemical analysis.


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