Near-Field Imaging of Infrared Nanoantenna Modes Under Oblique Illumination

Abstract

Plasmonic nanoantennas provide powerful tools in enhancing light−matter interactions by linking propagating far fields with localized near fields. Microscopic understanding on the enhancements of light−matter interactions requires precise knowledges on the near-field distributions upon nanoantenna excitations. In this article, we study nearfield distributions of nanoantennas under oblique illumination. We acquire the amplitude-and phase-resolved images of the near fields on Au nanorods in the infrared range, by use of the scattering-type scanning nearfield optical microscopy and the interferometric homodyne detection. Asymmetric near-field distributions are experimentally visualized and theoretically reproduced. The asymmetry in the near-field distributions is attributed to the preferential excitation of the surface plasmon polariton in one direction and the finite loss of the surface plasmon polariton inside the cavity. Furthermore, we organize the excitation conditions of the nanoantenna modes under oblique illumination as two formulas, each of which illustrates the cavity resonance condition and the phase matching condition for surface plasmon polariton excitation, respectively. The findings will form the basis for novel near-field engineering with inhomogeneous or spatially structured illumination.

Gold Nanorods: The Most Versatile Plasmonic Nanoparticles

“…This assertion has been proven by many numerical studies. ,, The experimental methods for the investigation of the near-field optical properties of Au NRs are relatively limited compared to the far-field case. A few techniques have been demonstrated in the past few years for the direct assessment and measurements of the near-field enhancement induced by Au NRs, including photoelectron emission measurements (Figure d), ,− surface-enhanced Raman scattering, cathodoluminescence, , photoinduced force mapping, scanning near-field optical microscopy, , two-photon photoluminescence, and electron energy-loss spectroscopy . The near-field enhancement effect is highly dependent on the spatial position.…”

“…The optical response of the V-shaped dimers has been found to feature the resonances from both the SS and EE assemblies. While the relative intensities of the hybridized modes in the two basic configurations vary significantly as the angle formed by the two Au NRs is changed, their resonance wavelengths remain relatively unchanged. ,, The local EM field in the near-field experiences substantial enhancement in amplitude as plasmon coupling takes place. The enhancement in the gap region of Au NR dimers can be much greater than anywhere else. ,, However, when the antibonding mode in the EE Au NR dimer and trimer is excited, the EM field enhancement in the gap region is relatively small compared to that when the bonding mode is excited.…”

Vibrational Strong Coupling in Subwavelength Nanogap Patch Antenna at the Single Resonator Level

“…Such angle-independent characteristic of the polaritonic modes stems from the fact that the plasmon resonance is in principle determined by w and n GSP and has nothing to do with the incidence angle of reflection measurements. 21 Finite incidence angle may induce inhomogeneously (or retarded) excitation of the eigenmode, accompanied by imbalanced field distribution and reduced excitation efficiency, but it does not affect the resonance frequency of the eigenmode. The angle insensitive nature of the VSC is highly beneficial for the practical realization of molecular polaritonic-based devices.…”

Raman Spectroscopic Nanoimaging of Optical Fields of Metal Nanostructures with a Chemically Modified Metallic Tip

“…In a typical s-SNOM measurement, a metallic or dielectric nanotip is illuminated with a focused incident beam and the scattered light from the tip apex is collected for further analysis. s-SNOM has been demonstrated to probe optical field distributions of various plasmonic nanostructures with well-defined structures, including nanospheres, − nanotriangles, and nanowires. − However, s-SNOM with a single-frequency light source measures optical responses only for a single frequency, which is consistent with the frequency of the excitation light source, and hence only probes the field distribution for a single wavelength. Other important properties, such as the LSPR wavelength, are not detectable through s-SNOM with a single-frequency light source.…”