Mapping the near fields of plasmonic nanoantennas by scattering‐type scanning near‐field optical microscopy

Abstract

Near-field optical microscopy techniques provide information on the amplitude and phase of local fields in samples of interest in nanooptics. However, the information on the near field is typically obtained by converting it into propagating far fields where the signal is detected. This is the case, for instance, in polarization-resolved scattering-type scanning near-field optical microscopy (s-SNOM), where a sharp dielectric tip scatters the local near field off the antenna to the far field. Up to now, basic models have interpreted S-and P-polarized maps obtained in s-SNOM as directly proportional to the in-plane (E x or E y ) and out-of-plane (E z ) near-field components of the antenna, respectively, at the position of the probing tip. Here, a novel model that includes the multiple-scattering process of the probing tip and the nanoantenna is developed, with use of the reciprocity theorem of electromagnetism. This novel theoretical framework provides new insights into the interpretation of s-SNOM near-field maps: the model reveals that the fields detected by polarization-resolved interferometric s-SNOM do not correlate with a single component of the local near field, but rather with a complex combination of the different local nearfield components at each point (E x , E y and E z ). Furthermore, depending on the detection scheme (S-or P-polarization), a different scaling of the scattered fields as a function of the local near-field enhancement is obtained. The theoretical findings are corroborated by s-SNOM experiments which map the near field of linear and gap plasmonic antennas. This new interpretation of nanoantenna s-SNOM maps as a complex-valued combination of vectorial local near fields is crucial to correctly understand scattering-type near-field microscopy measurements as well as to interpret the signals obtained in field-enhanced spectroscopy.

Real-Space Mapping of the Chiral Near-Field Distributions in Spiral Antennas and Planar Metasurfaces

“…Indeed, the phase images (Figure 2i) show constant phase on the spiral antenna (blue color) indicative of a standing wave pattern as typically observed with linear antennas. 45,53 Our images thus demonstrate that in spiral antennas, traveling waves (5-turn antenna) are the cause for a strong chiral optical response, while standing waves (1-turn antenna) are the cause for a weak chiral optical response in the near-field. As an interesting side aspect, we also observe the same, strong near-field contrast when we leave the illumination field fixed to LCP and compare the original right-wound 5-turn spiral antenna with its left-wound mirror image ( Figure 3).…”

“…In contrast, linear antennas radiate entirely in s-polarization normal to the antenna axis, thus the in-plane component of the field inside the gap is only contained in the s-component (assuming light detection normal to the antenna axis) and consequently a dark gap is observed when the p-component is measured. 45 The employed s-SNOM model, 53 which properly accounts for the complex-valued combination of near-field components, not only confirms the bright gap but also some specific features in the experimental near-field distributions (Figures 2e,f). In more detail, the model correctly predicts the stronger field enhancement on the wire edges than on the wire center (Figures 2e), the breaks (dark spots) in the ring-shape structure (Figure 2f, RCP) and the general asymmetric nature of the amplitude distribution observed in the experiment.…”

“…However, a recent study has revealed that the signal obtained in s-SNOM is given by a complex combination of the different local near-field components with both linear and quadratic dependencies. 53 For a dipole antenna, it was shown that the experimental near-field distribution can deviate substantially from the numerical calculated distribution of a single near-field component, an effect that can be expected to be even more pronounced in the case of more complex structures such as the presented chiral antennas and metasurfaces. We thus apply a recently developed model for the scattering process in s-SNOM 53 (see Methods) to calculate the theoretical s-SNOM amplitude and phase images (|E p |, φ p ) of the antenna near-field distribution.…”

“…This might be surprising and is in contrast to s-SNOM experiments with linear antennas, where detection of the pcomponent of the tip-scattered light typically yields a dark gap. 42,45,53 We explain the bright gap in our near-field images by a strong scattering of the antenna near fields by the tip via the antenna itself into the far field, which predominates over the direct scattering of the near fields by the tip. 53 Furthermore, the spiral antennas radiate with both s-and p-component as a result of the circular polarization of the 5-turn spiral antenna, and the tilted elliptical polarization of the 1-turn spiral antenna (Supporting Information Figure S2).…”

Full vectorial mapping of the complex electric near-fields of THz resonators

“…[32][33][34][35] These techniques offer a high spatial resolution but the detected field at different positions is a superposition of different field components. 36 Vectorial mapping of near electromagnetic fields at THz frequencies offers the potential to gather full information as local fields rather than intensity, which can be measured as a function of time. Several schemes have been developed for this purpose.…”