Figure 6a illustrates the effect of the film thickness h in the transmission of the device consisting of the slit and corrugations as designed above. The results are shown using either η 0 or η as the criterion (note the different scales). They exhibit a typical Fabry-Perot-like variation of transmittance through a subwavelength-width metal-insulator-metal waveguide buy CP673451 of finite length h. With both criteria, the first maximum is obtained at h ≈ 180 nm; hence, this value was chosen for further simulations and experiments. Figure 6 Transmission efficiency. (a) Variation of the zero-order efficiency η 0 (red line, left-hand scale) and the total transmission efficiency η (black line, right-hand scale) as

a function of the Al film thickness h. (b) Angular dependence of transmission efficiency: surface corrugation optimized for SPP coupling (green line), the final design without (blue line) and with (black line) the TiO2 layers. To get an idea of the far-field radiation pattern of the probe, we plot in Figure 6b the angular distribution of transmission efficiency check details η(θ), which in FMM calculations means plotting the efficiencies η m  of all propagating orders of the superperiodic

grating. Comparison of the green and blue lines illustrates the improvement of transmission achieved by final optimization of the corrugation on the exit face of the Al layer. In the design process, the presence of the thin TiO2 layers shown in Figure 1 was ignored. The effect of including these layers in the analysis is illustrated by the black line, and it is seen to reduce η 0 slightly (in principle, the design could be improved slightly by optimization of the parameters in the presence of these layers). In all cases considered, however, the strong and narrow central peak, with half-width at half-maximum of approximately 3°, is surrounded by a wide

‘pedestal’ extending over the entire half-space. Hence, if the light efficiency of the system is Atezolizumab a critical CHIR-99021 concentration factor (which was not the case in our experiments), the use of high-numerical-aperture collection optics is recommended despite of the beaming effect being utilized in the design. Let us next consider in more detail the advantages gained by adding the corrugations on the rear side of the Al film. Field amplitude distributions |H y (x,z)| without and with corrugations are compared in Figures 7 and 8. The fields inside the probe and in its wavelength-scale neighborhood are illustrated in Figures 7a and 8a, where the regions 0 ≤ z ≤ 0.18 μm contain the Al film. A close inspection of these figures shows the interference of the reflected and the incident fields, the high intensity inside the slit, and the slight penetration inside all the metal surfaces. Also seen are the plasmon waves that propagate away from the slit; these are particularly apparent on the exit side.