Plasmon O’Clock

Scientists in Singapore have carved nanoscopic clock faces into thin layers of gold on a quartz substrate to create surfaces with hitherto unobserved plasmonic properties (Nano Lett. 2014, DOI: 10.1021/nl501997z). Light shining on such nanopatterned metallic surfaces can drive waves in the metal’s sea of conduction electrons. These excited modes, called plasmons, generate strong electric fields in their host nanostructures, making them useful tools to probe bound molecules. Nanofabricated metallic features can thus boost the sensitivity of optical analytical techniques such as surface-enhanced Raman spectroscopy. These technologies typically rely on plasmon dipole modes, but higher-order modes, such as quadrupoles, could further improve device sensitivity and resolution. Researchers led by Hailong Liu and Nikolay I. Zheludev of Nanyang Technological University found they could controllably excite nearly any mode from dipoles up to 32 poles—and in simulations as high as 128 poles—in arrays of nanoscopic clock faces showing different times. The team milled the nanoclocks using a focused ion beam. That approach is not well-suited for patterning the square-centimeter areas needed for sensors, but Liu thinks it’s only a matter of time before they overcome that hurdle with scalable nanofabrication techniques.