Fig. 4: Extraction of the three polaritonic components with pseudoheterodyne s-SNOM.

a Amplitude (A) and phase (φ) of the s-SNOM second harmonic signal close to the flake edge. These values can be combined and represented in a complex plane (Re, Im) = (A cosφ, A sinφ). b In this representation, the material component produces a distribution of close-together values; the direct and the roundtrip components instead contribute with values that are distributed along the arms of two spirals. From this distribution of values, each single component can be retrieved as described below. c, d Spatial average (along y) profiles for amplitude, phase, the real and imaginary parts of the polaritonic wave are obtained from the maps in a. e Complex Fourier transform that was obtained from the profiles that were reported in c and d. This spectrum shows two peaks, which correspond to the single and double periodicities of the direct and roundtrip components. Peaks appear only in the positive-wavenumber region, which directly corroborates the description of both the direct and roundtrip components as propagating waves that propagate in the same direction (away from the edge). f Inverse Fourier transform of e after applying two bandpass filters to restrict the spectrum region (dashed areas of the spectrum in e). g, h show the phasor evolution (spiral) of the electric field that results from the direct and roundtrip components, respectively