Mid-IR Plasmonics: Near-Field Imaging of Coherent Plasmon Modes of Silver Nanowires

Chemically synthesized metallic nanostructures can exhibit a strong local optical field enhancement associated with their high degree of crystallinity and well-defined geometry-dependent surface plasmon resonances. The extension of the plasmon modes into the mid-IR spectral range (3-30 mu m) is shown for micrometer-sized nanowires with high aspect ratios available in the form of pentagonally twinned Ag crystallites as grown by polyol synthesis. Using scattering-scanning near-field optical microscopy, the associated IR plasmon modes are identified, and their underlying spatial distribution and enhancement of the optical polarization density is measured via phase, amplitude, and polarization resolved optical vector-field mapping. The transition from dipolar to multipolar resonances is observed and described by modeling the Ag wires using a modified cylindrical waveguide theory. For 10.6 mu m excitation, dipole antenna resonances are observed at a resonant length of L = lambda(eff)/2 with lambda(eff) approximate to 10.6 mu m/(1.8 +/- 0.5) approximate to 6 +/- 2 mu m. This effective wavelength scaling is the result of electronic damping, despite the high aspect ratios of the wires of order 1:10 to 1:200. With the optical cycle period pi(IR) being comparable to the Drude relaxation time of tau similar to 40 fs the mid-IR defines the low-energy limit of the coherent plasmon regime (tau(IR) less than or similar to tau) at the transition to purely geometric antenna resonances (tau(IR) > tau).