Materials: Plasmonics

As the Chemical, Microsystem, and Nanoscale Technologies Group developed its expertise in nanolithography and chemistry in the early 2000s, it embarked on a broader strategic direction of applying these capabilities to developing new devices and systems. In particular, it started exploring the highly publicized promise of the field of nanoplasmonics, in which the combination of controlled nanostructures and materials science has engendered new phenomena, mainly in the optical and infrared portions of the spectrum.

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Schematic diagram and scanning electron microscope image of a dense array of Au nanoparticles, which was formed by a convective assembly process on a prepatterned substrate using 157-nm interference lithography at a pitch of 90 nm. The interparticle gaps are nominally 10 nm, enabling high local electric field enhancement.

Since low-loss metal nanostructures have been shown to enhance the local electric field by orders of magnitude, applications ranging from nanophotonics to chemical sensing have been pursued. The group now has solid expertise in electromagnetic computational simulations and design; nanofabrication based on advanced optical and electron-beam lithography, anchored in the Microelectronics Laboratory; and highly sensitive optical and chemical diagnostics.

One of the applications that the group is exploring is surface-enhanced Raman spectroscopy (SERS). SERS holds the promise of extremely sensitive chemical sensing coupled with a high degree of selectivity when the analyte is adsorbed on the surface of a plasmonic device or is in its close proximity (within less than ~10 nm). However, some of the key challenges to wide-scale implementation revolve around the ability to scale up the production of such nanodevices while retaining the required nanometer-scale precision. In some of its first experiments, the group teamed up with a group at Northeastern University and demonstrated the scalability of preparing surfaces covered with uniformly placed gold nanospheres. More recently, the group has initiated a project to utilize aluminum as a plasmonic metal. This approach, besides having certain advantages with respect to the spectral location of the plasmonic resonance, would enable access to the most advanced silicon device fabrication technologies. Because of concerns of device contamination, leading-edge cleanroom facilities and equipment used in silicon device fabrication are usually not available to researchers employing the more common gold plasmonics. Such concerns do not exist with respect to the yet unexplored aluminum plasmonics, and therefore the whole infrastructure of silicon mass production becomes accessible to the plasmonics field, albeit still requiring application-specific process development.

A different application of nanoplasmonics is the utilization of local field enhancement to increase the probability of nonlinear absorption or scattering processes. The group has been exploring the underlying science and potential optimization paths of this effect, with the goal of enabling efficient optical limiting behavior based on plasmonic nanostructures, possibly coupled with graphene derivatives. Other applications of nanoplasmonics being pursued in the group include the incorporation of plasmonic nanoparticles in photovoltaic cells in order to enhance their performance and the combination of nanoantennas with liquid crystals to obtain electrically tunable, fast optical filters in the infrared. In all these programs, the Chemical, Microsystem, and Nanoscale Technologies Group has been applying rigorous computational analyses of nanostructures and has combined bottoms-up lithography with chemical synthesis of nanoparticles, thin-film processing, and optical metrologies, including spectroscopic ellipsometry, Raman microscopy, and angle-resolved scatterometry.

 

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