Mapping Near-Field Coupling Effects in Infrared Gap Antennas

P. ALONSO; P. ALBELLA; L. ARZUBIAGA; M. SCHNELL; J. CHEN; F. HUTH; F. GOLMAR; F. CASANOVA; L. HUESO; J. AIZPURUA; R. HILLENBRAND

Suntec, Singapore

Conferencia; ICMAT 2011; 2011

The vector near-field distribution of infrared gap antennas (linear dipole antennas coupled via a nanometric gap) is mapped by scattering-type scanning near-field microscopy (s-SNOM). The images provide direct experimental evidence of strong in-plane near-field localization inside a gap as small as 50 nm. By measuring the gap fields as a function of the total antenna length (near-field spectroscopy), we observe a clear resonance shift compared to uncoupled linear dipole antennas, thus verifying strong near-field coupling via the gap. We also find significant differences between near-field and far-field spectra of the antennas and discuss their implications. Vector near-field imaging of the infrared antennas [1] was carried out with s-SNOM where s-polarized laser light is used for antenna excitation. A dielectric Si tip scatters the local near fields of the antennas. Interferometric and polarization-resolved detection of the tip-scattered light yields amplitude E and phase φ of the in- (x) and out-of-plane (z) near-field components (Ex, φx) and (Ez, φz). Our results show that vector near-field mapping is a powerful tool for measuring spectral resonance shifts in the near field of infrared antennas, in both amplitude and phase. This enables detailed studies of near-field coupling signatures, including the mapping of strongly localized field enhancement (“hot spots”) and resonance shifts of near-field spectra, which are not accessible by far-field spectroscopy. [1] Schnell, M.; García-Etxarri, A.; Alkorta, J.; Aizpurua, J.; Hillenbrand, R. Nano Lett. 2010, 10 (9), 3524–3528.