  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 non-trivial. Here, we measure the
PLDOS dependence on the size parameter in a waveguide using a new technique -
nanowaveguide coupled cathode luminescence (CL). 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 open new avenues for spectroscopy based on evanescently coupled CL.
