Wireless communication system via nanoscale plasmonic antennas

Present on-chip optical communication technology uses near-infrared light, but visible wavelengths would allow system miniaturization and higher energy confinement. Towards this end, we report a nanoscale wireless communication system that operates at visible wavelengths via in-plane information transmission. Here, plasmonic antenna radiation mediates a three-step conversion process (surface plasmon → photon → surface plasmon) with in-plane efficiency (plasmon → plasmon) of 38% for antenna separation 4λ0 (with λ0 the free-space excitation wavelength). Information transmission is demonstrated at bandwidths in the Hz and MHz ranges. This work opens the possibility of optical conveyance of information using plasmonic antennas for on-chip communication technology.

show the efficiency as a function of the antenna arm length ( ). Figure S2. Calculated efficiency dependence on arm length .
Far-field radiation pattern. In order to determine the main direction of the radiation produced by the nWCS, it was calculated the far-field radiation pattern. Figure S3a shows the E-plane component of the far-field radiation pattern when the substrates used are air (black line) and glass (red line). Figure S3b shows the H-plane component of the far-field radiation pattern following the same conditions as shown in a. Excitation wavelength. Once the optimized parameters were obtained, it was studied the efficiency as function of the wavelength operation. Figure S4 shows the dependence of calculated efficiency wherethe red arrow represents the excitation wavelength used. The periodic behavior is due to the finite size of the broadcast/receiver regions where the SPs can resonate. Figure S4. Dependence of calculated efficiency on the operation wavelength.

Experimental results
Surface plasmon wavelength and propagation length. The interaction between the incoming and reflected SPs on the receiver region produces a standing wave that is observed in the nearfield intensity. The particular case of an inter-antenna distance of = 1.5 0 is shown in Figure   S4a. The color represents a linear intensity scale with red and black showing the maximum and minimum respectively, while scale bar is 1 m. An intensity transverse cut was made in the dashed line on Figure S4a is shown in Figure S4b in order to calculate the power spectrum of the resulting intensity profile ( Figure S4c). It is clear that the main peak of the power spectrum agrees correctly with half of the SP wavelength as shown by the red arrow, i.e. 320 nm. By measuring the SPP propagation on the sample surface, it was found that the propagation length was 4.4 m. This value is certainly much shorter than the theoretical one (80 m) due to the high roughness and consequently to the strong radiation losses. In this case, all the data manipulation was done in WSxM 1 . Near-field intensity at different distances. The normalized near-field intensity registered in the complete set of samples is shown in Figure S6a, where the inter-antenna distance is shown in each image at the top-left. In all the cases, the displayed area is 5x5 m 2 In order to make an easier comparison between the different images the intensity scale has been adjusted in a factor shown at the top-right of each image. As it is well-known the NSOM is capable to imaging the 5 near-field intensity at the same time than the topography of the sample. Taking advantage of this, it is possible to localize the position of the interactions held in the sample surface. Figure S6b shows a three-dimensional representation of the sample surface topography with the color corresponding to the near-field intensity measured simultaneously when =1.5 0 (red frame in S6a). It is clear that the SP is reflecting on the receiver slit. The data manipulation was done using WSxM 1 .

Figure S6. (a) Complete set of near-field intensities for the different inter-antenna distances. (b)
Three dimensional representation of the sample topography.

Surface plasmon propagation in the absence of receiver slit. An expected result is that when
the receiver slit is not present, the surface plasmons will propagate with no perturbation, meaning the interference pattern shown in Figure S5 is not visible. Figures S7a and S7b show the surface 6 topography of the receiver region and normalized near-field intensity, respectively, in the case of an inter-antenna distance of 4 0 . A three-dimensional representation of the sample surface topography with the color corresponding to the near-field intensity is shown in Figure S7c. It is clear the high intensity in the antenna junction to the flat surface (red arrow) due to strong farfield scattering. The data manipulation was done using WSxM 1.  Supplementary Video. In the images associated with this video, the symmetry of the system was mirrored in the horizontal direction with respect to that shown in the main text of this work.
A brief description of the Supplementary Video is made as follows: