Figure 1

Principle of the experiment. (a) Cartoon: launching, propagation, and reemission of TWTL modes. (b),(c) Simulated transverse modal profiles in an infinitely long TWTL on glass for the symmetric (b) and antisymmetric (c) eigenmodes. Instantaneous charge distributions are symbolized by $+$ and $-$.Reuse & Permissions

Figure 2

(a) SEM image of the plasmonic nanocircuit. (b) SEM image cross section of a TWTL (52° observation angle). (c),(d) Simulated far-field images (far-field projection [31]) for a pure symmetric (c) and antisymmetric (d) mode arriving at the mode detector. Pure modes are excited by illuminating the incoupling antenna (overall length 450 nm) with a focused beam polarized parallel (c) and perpendicular (d) to the TWTL. The intensity scale is normalized to the reflected intensity at the glass-air interface. To match the experiments the antenna reflections have been attenuated numerically using OD 2.2 [left of the vertical (green) line]. (e),(f) CCD images (detected power per pixel) showing the attenuated [OD 2.2, indicated by the (green) circle segment] reflected spot at the incoupling antenna position and the mode detector emission spots (100 nW excitation power). (c)–(f) The arrows indicate the polarization. A nanocircuit outline is superimposed as a guide to the eye. All images show background “fringes” from direct laser scattering which have no influence on the transmission and thus the outcome of the experiment.Reuse & Permissions

Figure 3

Simulated (lines, small symbols, ${\eta }_{\mathrm{in}}^s$) and experimentally obtained incoupling efficiency (large symbols with error bars, ${\eta }_{\mathrm{in}}^e$) vs total antenna length. Symmetric mode: Illumination parallel to the TWTL (red squares, dashed line). Antisymmetric mode: Illumination perpendicular to the TWTL (blue circles, solid line).Reuse & Permissions

Figure 4

Focal spot position dependent modal excitation efficiency maps (450 nm overall antenna length): (a)–(d) experiments, (e)–(h) simulations. Middle panel: pictograms indicating excitation polarization and emission position. When centered on the antenna the antisymmetric mode (d),(h) is more efficiently excited than the symmetric mode (a),(e) consistent with Fig. 3.Reuse & Permissions

Figure 5

(a) Emitted intensity scan (log scale, no attenuation of direct scattering) of the circuit using 45° polarized illumination ($\lambda =800\mathrm{nm}$) [29, 30]. (b) Group velocities of symmetric (red squares, dashed line) and antisymmetric mode contributions (blue circles, solid line) of ultrafast plasmon pulses determined for one of the arrays used in Fig. 3. Quantitative agreement between experimental data (symbols with error bars) and simulation results (horizontal lines) is obtained. Note that the values obtained here differ from those in Table  due to different wavelengths.Reuse & Permissions