Optical near-field interference in the excitation of a bowtie nanoantenna

We show experimentally as well as in simulation that the phase-sensitive superposition of different plasmonic modes leads to a spatially controllable enhancement of the near-field inside and in the vicinity of a metallic nanostructure. Multiphoton photoemission electron microscopy maps the local near-field distribution. By controlling the relative phase $\Theta$ between two orthogonally polarized light pulses the spatial distribution of the near-field is manipulated on the basis of the interference of the near-field modes. This demonstration of optical near-field control is corroborated by finite integral time domain calculations.

Figure 1

Schematic of the experimental setup and the near-field interference mechanism. The local fields ${\mathbf{E}}_{\mathrm{loc},\mathrm{TE}}$ and ${\mathbf{E}}_{\mathrm{loc},\mathrm{TM}}$ generated by the TE and TM far-field components are no longer orthogonal.

Figure 2

Comparison of bowtie structure and one-photon photoemission pattern. (a) Scanning electron microscope image of the bowtie nanostructure. (b) One-photon photoemission (1PPE) pattern for UV excitation (unpolarized cw excitation with a cutoff energy of 4.9 eV).

Figure 3

Simulated near-field response ($z$ component) recorded at a single corner of the left nanoprism for TE (red dashed line) and TM (black dash dotted line) polarized excitation of a bowtie nanoantenna. The blue dotted line shows the response at the gap corner of the right nanoprism. The shadowed area shows the bandwidth (FWHM) of the laser pulses used with a maximum at λ $=$ 795 nm (yellow solid line). The inset sketches the points of interest.

Figure 4

Photoemission pattern from the bowtie antenna. (a) 1PPE pattern (off-resonant) obtained under UV excitation using a mercury lamp. (b) 3PPE pattern under TM-polarized laser pulse excitation reveals strong photoemission from the antenna gap and a weak photoemission at the corners of the left nanoprism. (c) 3PPE pattern under TE-polarized laser pulse excitation shows weak photoemission from the corners of the left nanoprism. The laser beam comes from the right side and is indicated by the parallel k vector component ${\mathrm{k}}_{\parallel }$.

Figure 5

Simulated field distribution on the surface of the gold nanostructure at a wavelength of λ $=$ 795 nm. The illumination geometry is as in Fig. 1, with the light coming from the right side as indicated by the arrow denoted with ${\mathrm{k}}_{\left|\right|}$. (a) Relative amplitude of the local electric field and (b) its z component for TM-polarized illumination. (c) and (d) Relative amplitude and z component for TE-polarized illumination. (e) and (f) Sixth power of the simulated field (E${}_{\mathrm{z}}$ component) in logarithmic scale to qualitatively compare the distribution with PEEM images from a 3PPE process. The incident field has an amplitude of 1 V/m.

Figure 6

(a) Control scheme for manipulation of the near-field distribution with two cross-polarized laser pulses. (b) Switching of the photoemission maxima at the outer corners of the left nanoprism. (c) Simulated 3PPE pattern in logarithmic scale. The images show the resulting intensity distribution after a linear superposition of the calculated field distributions for TM- and TE-polarized excitation with a relative phase of $+$π/2 and −π/2, respectively, i.e., left and right circular polarized light.