Surface-Plasmon Photonics for Nanofabrication and Bio-chemical Sensing
J. Todd Hastings
Electrical & Computer Engineering, Center for Nanoscale Science and Engineering, University of Kentucky
Date: October 5, 2007, Time: 3:30 pm, Location: W122-D3 Engineering Building 1, The University of Houston
Abstract:
Surface-plasmon polaritons are coupled charge-density/electromagnetic waves that exist at interfaces of two materials with real dielectric constants of opposite sign, i.e. a dielectric and certain metals such as gold or silver. Surface-plasmons (SP) can either be propagating (i.e. on the surface of metal films) or localized (i.e. around metal nanoparticles). This talk will address two applications of surface-plasmons: one well established (bio-chemical sensing based on propagating SPs) and one entirely novel (nanofabrication through localized SP excitation).
Surface-plasmon resonance (SPR) sensors use propagating surface-plasmon waves to detect refractive-index changes immediately adjacent to a thin metal film. This refractive index change most often results from binding of a target analyte to the functionalized surface of the sensor. SPR has become a widely used, label free technique to detect and study biological and chemical interactions. Nevertheless, a fundamental challenge remains unresolved: How does one differentiate between non-specific effects (temperature fluctuations, solution refractive index changes, non-specific binding of interferents, etc.) and detection of a target analyte? This problem currently limits the effectiveness of SPR in complex biological samples and for medical, environmental, food safety, and defense applications that require field deployable sensors.
To address this problem we have developed a novel approach to SPR sensing that uses two surface plasmon waves, long- and short- range plasmon-polaritons, propagating along the same region of a gold film. The two plasmon waves penetrate to different depths in the solution, and thus allow one to differentiate surface interactions and bulk refractive index changes. Such a sensor offers self-referenced measurements with no spatial separation between detection and reference regions; requires only a single channel spectroscopic interrogation system; and is simple to fabricate, functionalize, and calibrate because the sensor surface is composed of only one material. We present a sensor design that allows simultaneous coupling to both long- and short- range surface-plasmons and demonstrate its self-referencing capabilities by monitoring alkanethiol self-assembled monolayer formation and streptavidin-biotin binding.
Metallic nanoparticles also exhibit strongly enhanced optical absorption when illuminated at their surface plasmon resonance wavelength. This wavelength is determined by the nanoparticle's material, size, shape, and surrounding environment. In addition, metallic nanoparticles exhibit dramatically reduced surface and bulk melting temperatures. The combination of these two unique nanoscale phenomena allows for the selective optical excitation, heating, fusion, and even ablation of metallic nanoparticles especially when targeted by a nanoscale scanning probe. To explore these phenomena for nanofabrication applications we have investigated particle fusion and ablation in an atomic force microscope with laser illumination via total-internal reflection. Our experimental and theoretical results suggest that these processes can be carried out selectively in the presence of the probe tip and controllably based on optical exposure energy.
Biography:
Dr. J. Todd Hastings is an Assistant Professor of Electrical Engineering at the University of Kentucky. His current research focus areas include surface-plasmon photonics for sensing and nanoscale assembly, in-vivo electrochemical sensing, and electron-beam based nanofabrication techniques. Prior to his current appointment Dr. Hastings received the B.S. degree in physics from Centre College in Danville, Kentucky. He received the M.S. degree in electrical engineering from Purdue University where he investigated high-speed, high-efficiency infrared light-emitting diodes for short-range optical communications. Most recently, he received the Ph.D. degree in Electrical Engineering from the Massachusetts Institute of Technology. While a member of MIT's NanoStructures Laboratory he developed nanometer-precision electron-beam lithography techniques and silicon-on-insulator photonic devices.
