Vectorial Nanoscale Mapping of Optical Antenna Fields by Single Molecule Dipoles

Abstract

Optical nanoantennas confine light on the nanoscale, enabling strong light-matter interactions and ultracompact optical devices. Such confined nanovolumes of light have nonzero field components in all directions (x, y, and z). Unfortunately mapping of the actual nanoscale field vectors has so far remained elusive, though antenna hotspots have been explored by several techniques. In this paper, we present a novel method to probe all three components of the local antenna field. To this end a resonant nanoantenna is fabricated at the vertex of a scanning tip. Next, the nanoantenna is deterministically scanned in close proximity to single fluorescent molecules, whose fixed excitation dipole moment reads out the local field vector. With nanometer molecular resolution, we distinctly map x-, y-, and z-field components of the dipole antenna, i.e. a full vectorial mode map, and show good agreement with full 3D FDTD simulations. Moreover, the fluorescence polarization maps the localized coupling, with emission through the longitudinal antenna mode. Finally, the resonant antenna probe is used for single molecule imaging with 40 nm fwhm response function. The total fluorescence enhancement is 7.6 times, while out-of-plane molecules, almost undetectable in far-field, are made visible by the strong antenna z-field with a fluorescence enhancement up to 100 times. Interestingly, the apparent position of molecules shifts up to 20 nm depending on their orientation. The capability to resolve orientational information on the single molecule level makes the scanning resonant antenna an ideal tool for extreme resolution bioimaging.

Nanoscale Mapping and Control of Antenna-Coupling Strength for Bright Single Photon Sources

“…The fully out-of-plane z-molecule shows the dipolar two-hotspot pattern, similar to that seen for an in-plane molecule aligned along the antenna long axis (Fig. 3), however the coupling is even stronger due to the stronger z-fields between cavity and sample plane 23,24 . The in-plane y-molecule displays characteristic four-lobed pattern with weaker coupling (Fig.…”

“…Normalized dark-field (DF) scattering spectrum of a single aluminium nanoantenna (in red) and theoretical FDTD modelling scattering spectra (black) compared to fluorescence emission spectrum of the molecule (in blue).Scanning antenna microscope:All experiments were performed using a custom-built scanning probe microscope head, on top of an inverted confocal microscope with piezo-electric scanners capable to scan both sample and antenna, and a shear-force feedback mechanism to control antenna-sample distance, as previously described23 . Near-field intensity maps of the nanoantenna are acquired by scanning the nanoantenna with nanometre accuracy in the close vicinity (≈10 nm) of a fluorescent molecule, positioned in the diffraction-limited focus of the circularly polarized excitation laser beam (λ = 635 nm) focused using a 100x 1.3NA oil-immersion microscope objective.…”

Hybrid Plasmonic–Ferroelectric Architectures for Lasing and SHG Processes at the Nanoscale

“…In that respect, the lack of inversion symmetry of ferroelectric crystals offers the possibility of cationic lattice sites lacking of inversion symmetry, providing odd crystal field components that mix opposite parity configurations of RE 3+ ions. Consequently, forced electric dipole transitions between the Stark split levels are possible and the orientation of the electric dipolar moment of the RE 3+ emitter with respect to the electric field, plays a major role in the transition intensity …”

Large near-to-far field spectral shifts for terahertz resonances

“…These structures are interesting because they combine two distinct electromagnetic effects: the lightning rod effect due to the sharp metallic tips, which leads to high electric field accumulation in the gap, and resonant response to the driving field, which results from the size and shape of the bowtie. As a result, they do not only create highly enhanced and localized fields in the gap between the two triangles [13,[24][25][26][27] but also give precise spectral and spatial control over the response with relatively large bandwidths [28]. These structures have been used in the optical range to study effects like single molecule fluorescence [25], two-photon photoluminescence [24], surface-enhanced Raman scattering [29], or as sensors in the THz regime [12].…”