Fig. 4

Controlling the emission spectrum. a–c Experimental SPP emission spectra at three different biases (1.4–1.6 V, width of patch is around 128 nm, period of patch is 400 nm, spectra intensity have been normalized by the maximum power with the bias of 1.6 V); d–f theoretical normalized SPP emission spectra with same bias voltage as the figures (a–c). The emission spectrum shows two spectral peaks due to the excitation of two resonant modes of the antenna. The peaks are broadened due to the inhomogeneity of the antenna width. The simulations result from an average over seven different widths varying from 116 to 140 nm to account for width fluctuations along the wires in the experiments. The dotted line in each figure shows the corresponding normalized spectrum from a planar junction with the same voltage bias (intensity of the spectra has been multiplied by a factor of 500). All the theoretical emission spectra are corrected by the quantum efficiency of the spectrometer responsible for the decay at low energies. g, h Theoretical magnetic field distribution⁵⁴ (H_y in the X–Z plane) of the two gap modes of a 132-nm-wide antenna corresponding to the lower photon energy at 1.2 eV (g) and the higher photon energy at 1.5 eV (h)