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

Abstract

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 microm) 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 microm excitation, dipole antenna resonances are observed at a resonant length of L = lambda(eff)/2 with lambda(eff) approximately 10.6 microm/(1.8 +/- 0.5) approximately 6 +/- 2 microm. 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 tau(IR) being comparable to the Drude relaxation time of tau approximately 40 fs the mid-IR defines the low-energy limit of the coherent plasmon regime (tau(IR) less, similar tau) at the transition to purely geometric antenna resonances (tau(IR) > tau).

Tunable plasmon modes in single silver nanowire optical antennas characterized by far-field microscope polarization spectroscopy

“…2,[16][17][18][19][20][21] In recent years, indirect far-eld techniques, such as overcoating the plasmonic nanowire with uorescent polymer or quantum dots (QDs) which emit under the surface plasmon near-eld excitation, have been developed. 9,14,15,[22][23][24][25][26][27][28][29][30] The emission intensity is proportional to the surface plasmon near-eld intensity (or its square for the two-photon excitation), and can be used to elucidate the surface plasmon modes in nanowire antennas.…”

Observation of the enhancement of electric fields normal to the surface using mid-infrared slot antennas and an atomic layer deposition technique

“…However, it is still necessary to evaluate the appropriateness of the obtained results in a practical manner, because FDTD simulation necessitates the frequency dispersive nature of media in an analytical domain and very fine discretization in space in order to correctly predict the electromagnetic distribution, particularly, in the case of optical nanoantennas including metals. The field enhancement of the antenna can be measured by observing a reflection or transmission spectra through a micro-Fourier transform infrared (FT-IR) spectrometer [7][8][9][10][11][12][13]. Local information on the field enhancement is also available if one uses a scattering near-field microscopy (s-SNOM) [14].…”

A Microwave Band Pass Filter with Suspended Diamond Slot Structure

“…The spoof SPPs device can produce deep sub-wavelength effect at microwave and terahertz frequencies, which can be employed to fabricate novel microwave compact devices [7]. The spoof SPPs microwave device, compared with traditional microwave devices, possesses special properties [8][9][10][11]. For example, it can confine the microwave field into sub-wavelength size, it possesses better resistibility to electromagnetic interference as well as possesses higher sensitivity and larger bandwidth.…”