EDN, May 26, 2011

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pulse R Researchers claim extremely sensitive sensor based on Raman scattering esearchers at Princeton University claim to have developed the extremely sensitive D2PA (disk-coupled dots-on-pillar antenna-array) sensor employing Raman scat- tering, a phenomenon that physicist Chandrasekhara Raman discovered in the 1920s. Raman found that light reflecting off an object carries a signa- ture of its molecular composition and structure. The researchers say that the sensor opens new ways of detecting a range of substances—from those that indicate telltale signs of can- cer to those that detect hidden explosives. The researchers are touting the sensor as a major advance in the long search to identify materials using the phe- nomenon. Placing material—as small as one molecule—on a scattering of light enables the sensor to boost faint signals, allowing the identification of vari- ous substances depending on the color of light they reflect. "Raman scattering has enor- mous potential in biological and chemical sensing and could have many applications in industry, medicine, the military, and other fields," says Stephen Y Chou, professor of electrical engineering at Princeton, who led the research team. "But current Raman sensors are so weak that their use has been DILBERT By Scott Adams very limited outside of research. We've developed a way to sig- nificantly enhance the signal over the entire sensor, and that [technique] could change the landscape of how Raman scat- tering can be used." In Raman scattering, a beam of pure one-color light focuses on a target. The reflected light from the object contains two extra colors of light. The fre- quency of these extra col- ors is unique to the molecu- lar makeup of the substance, providing a potentially power- ful method of determining the identity of the substance. For many materials, according to the researchers, even the most sophisticated laboratory equip- ment cannot see the extra col- ors of reflected light. Czechoslovakian electro- chemist Martin Fleischmann and fellow researchers in 1974 discovered that the Raman signals became stronger if the team placed the substance it wanted to identify on a rough metal surface or on tiny par- ticles of gold or silver. The technique, SERS (surface- enhanced Raman scattering), has proved difficult to put to practical use. According to the research- ers, the small particles and gaps significantly boost the Raman signal. The cavities serve as antennas, trapping light from the laser so that it passes the plasmonic nanodots multiple times rather than only once to generate the Raman signal. The cavities also enhance the out- going Raman signal. The sensor E 200 nm Princeton researchers devel- oped this sensor for sensing Raman scattering (courtesy Stephen Y Chou). GOLD SILICON SILICON DIOXIDE SILICON Researchers at Princeton University claim to have developed the extremely sensitive D2PA (disk-coupled dots-on-pillar antenna-array) sensor employing Raman scattering (courtesy Stephen Y Chou). is a billion times more sensitive than is possible without SERS boosting the Raman signals and is uniformly sensitive, making it more reliable for use in sensing devices. The researchers claim that the technology would be cost-effective to produce using nanoimprint self-assembly tech- niques. Chou's team produces the sensors on 4-in. wafers and can scale the fabrication to much larger wafers. "This is a powerful method to identify molecules," says Chou. "The combination of a sensor that enhances signals far beyond what was previously possible, that's uniform in its sensitivity, and that's easy to mass-produce could change the landscape of sensor tech- nology and what's possible with sensing." Electrical engineering gradu- ate students Wen-Di Li and Fei Ding, postdoctoral fellow Jona- than Hu, and Chou published a paper on this technology (Reference 1). The research- ers received funding from the Defense Advanced Research Projects Agency. –by Suzanne Deffree ▷Princeton University, www.princeton.edu. REFERENCE 1 Li, Wen-Di; Fei Ding; Jon- athan Hu; and Stephen Y Chou, "Three-dimensional cavity nanoantenna coupled plasmonic nanodots for ultrahigh and uniform sur- face-enhanced Raman scat- tering over large area," Optics Express, Feb 28, 2011, pg 3925, http://bit.ly/ iCM2yi. 14 EDN | MAY 26, 2011

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