Strumming on the nano-banjo

August 21, 2012 by Angela Herring
Assistant professor Matteo Rinaldi received a DARPA Young Faculty Award to develop infrared and terahertz radiation detection devices. Photo by Brooks Canaday.

When you pluck a banjo string, you trigger a vibra­tion that res­onates at a fre­quency unique to the geom­etry and mate­rial of the string. We can dis­tin­guish that fre­quency as a par­tic­ular pitch, our ears acting like incred­ibly sen­si­tive detectors.

Matteo Rinaldi, an assis­tant pro­fessor of elec­trical and com­puter engi­neering at North­eastern Uni­ver­sity, has recently received a Young Fac­ulty Award from the Defense Advanced Research Projects Agency to develop detec­tion devices that work in a sim­ilar manner—but at a bil­lionth of the size.

In the case of the banjo string, the mechan­ical energy needed to actuate the vibra­tion comes from your finger. But in the case of Rinaldi’s tiny devices, the mech­a­nism of action is bit more com­pli­cated: Elec­trical energy is employed to actuate vibra­tion through a process called piezo­elec­tric transduction.

Rinaldi’s devices resemble two over­lapped ultra-​​thin sheets of paper sta­pled at either end. The top sheet absorbs energy in the form of infrared (IR) and ter­a­hertz (THz) elec­tro­mag­netic radi­a­tion, causing the whole device to heat up. This changes the stiff­ness of the tiny res­o­nant device, which causes it to vibrate at a new fre­quency. The change in fre­quency can be detected with unprece­dented res­o­lu­tion and speed.

The interest for IR and THz tech­nolo­gies has been steadily growing over the last few years because of their poten­tially rev­o­lu­tionary appli­ca­tions span­ning from night vision to med­ical diag­nos­tics. In par­tic­ular, THz radi­a­tion, which falls between the infrared and radio wave regions on the elec­tro­mag­netic spec­trum, has long eluded sci­en­tists in terms of both sources and detec­tion. Rinaldi’s devices rep­re­sent one of the first sys­tems to over­come the detec­tion barrier.

A few detec­tion sys­tems are com­mer­cially avail­able, but they are com­pli­cated and require resource-​​heavy cooling sys­tems to func­tion well. A minia­tur­ized detec­tion tech­nology that can operate at room tem­per­a­ture and dis­tin­guish indi­vidual IR and THz wave­lengths still does not exist.

Rinaldi’s will explore the use of carbon nan­otube forest as the mate­rial in the top sheet. But this is not ideal when looking for spec­tral selec­tivity, because, as Rinaldi said, “carbon nan­otubes are a broad­band absorber.”

With the help of the DARPA YFA award, Rinaldi will also focus on making devices with alter­na­tive top layer com­po­nents. By varying both the mate­rial and the way it is pat­terned, he can create devices that absorb only spe­cific wave­lengths of radiation.

The DARPA pro­posal was Rinaldi’s first after joining the North­eastern fac­ulty in the spring. “It’s a great oppor­tu­nity,” he said. “It has already given me a lot of vis­i­bility. I’ve had the chance to interact with pro­gram man­agers at DARPA and experts in the field.”

The DARPA YFA kick-​​off meeting pro­vided sev­eral exam­ples of how the tech­nolo­gies that have emerged from DARPA grants have made it into the mil­i­tary set­ting, Rinaldi said, giving him the oppor­tu­nity to see how his own research could be applied to solve real world problems.

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