Silicides for infrared surface plasmon resonance biosensors

Pt-, Pd-, Ni-, and Ti-silicide films on silicon were evaluated as conducting hosts for surface plasmon polaritons (SPP) in proposed long-wave IR (LWIR) attenuated total reflection biosensors. Original LWIR complex permittivity data was collected, from which SPP properties were determined and compared with those for noble metals. LWIR SPPs on silicide films were found to offer enhanced sensitivity to thinner biological entities than when usual metal films are used. INTRODUCTION Thin dielectric films on a conducting surface strongly affect the bound electromagnetic waves known as surface plasmon polaritons (SPP), providing a means for real-time label-free sensing and monitoring of biological entities from molecules to cells. Surface plasmon resonance (SPR) biosensors have become a sensitive label-free method to study biological interactions, 1-3 e.g. those marketed by Biacore or Texas Instruments. Established sensors are based on wavelength and angle dependent resonances in attenuated total reflection (ATR) devices using visible/near-infrared light. We propose LWIR operation using silicon-based materials because of a number of potential advantages. The LWIR is defined roughly as the 8-12 micron wavelength range, which corresponds to a band of high atmospheric transmission. Large changes for the refractive index are expected near the characteristic LWIR vibrational frequencies of biomolecules, giving potentially better specificity than can be achieved at visible wavelengths. Semiconductor quantum cascade lasers have become available throughout this range. Silicon is highly transparent here, and silicon-based devices offer benefits of integrated manufacturing, miniaturization, micro-fluidic sampling. Because the ATR method works in the regime of total internal reflection, silicon allows observation of larger index values for the sample of interest, up to ~3.4, than can be observed in the visible with glass prisms (n ~1.5). This paper considers conducting silicides as IR surface plasmon hosts as opposed to the usual noble metals. Silicides may be grown by standard processing procedures directly on the polished surfaces of Si prisms or wafers. The lower carrier concentration and plasma frequency of silicides relative to metals pushes the surface plasmon dispersion curve farther from the light line in the IR, which is advantageous for observing resonances in the ATR configuration. Surface plasmons on silicides offer tighter mode confinement to increase the sensitivity to thin Mater. Res. Soc. Symp. Proc. Vol. 1133 © 2009 Materials Research Society 1133-AA10-03