Tunable Mid-Infrared Localized Surface Plasmon Resonances in Silicon Nanowires

We observe and systematically tune an intense mid-infrared absorption mode that results from phosphorus doping in silicon nanowires synthesized via the vapor–liquid–solid technique. The angle- and shape-dependence of this spectral feature, as determined via <i>in-situ</i> transmission infrared spectroscopy, supports its assignment as a longitudinal localized surface plasmon resonance (LSPR). Modulation of resonant frequency (740–1620 cm^–1) is accomplished by varying nanowire length (135–1160 nm). The observed frequency shift is consistent with Mie–Gans theory, which indicates electrically active dopant concentrations between 10¹⁹ and 10²⁰ cm^–3. Our findings suggest new opportunities to confine light in this ubiquitous semiconductor and engineer the optical properties of nontraditional plasmonic materials