At the moment, planar periodic structures such as metasurfaces and photonic crystal slabs attract particular attention, since they can support surface waves and guided modes with unusual dispersion properties designed on-demand. Once the sample is fabricated, the measured dependence of both propagation constant and propagation length on the wavelength is of great importance. In this work, we propose an experimental approach allowing to retrieve the full complex dispersion of evanescent waves lying deeply beneath the light line, in arbitrary planar structures. The method is based on back focal plane microspectroscopy combined with a solid immersion lens separated from the sample by a precisely controlled nanoscale air gap. Varying the gap between the prism and the sample allows for extracting spectral dependence of both real and imaginary parts of the wavenumber of surface waves propagating in arbitrary in-plane direction. Our approach can be implemented for the development of various on-chip nanophotonic devices.

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