The photonic local density of states (PLDOS) determines the light-matter interaction strength in nanophotonic devices. For standard dielectric devices, the PLDOS is fundamentally limited by diffraction, but its precise dependence on the size parameter $s$ of a device can be nontrivial. Here, we measure the PLDOS dependence on the size parameter in a waveguide using direct detection of cathode luminescence (CL) through the modes of a nanowaveguide. We observe that, depending on the position within the waveguide cross section, the effective diffraction limit of the PLDOS varies and the PLDOS peak shape changes. Our results are of fundamental importance for optimizing coupling to nanophotonic devices and also opening avenues for applications based on evanescently coupled CL photons.

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• Cavity quantum electrodynamics
• Nanophotonics
• Photoemission