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Description
In this work we map qualitatively and quantitatively the sub-bandgap cathodoluminescence of optomechanical silicon nanobeam resonators with $10$ and $20$ nanometer resolution. By analyzing the symmetric and anti-symmetric optical modes both locally and spectrally in the far-field, we measure for the first time the sign-flip of the anti-symmetric mode in such one-dimensional cavities. To emphasize the role of the high density of states of these confined resonances we set the experimental condition to suppress the both incoherent and coherent electron-photon coupling, and find that the CL is dominated by a mixed term, where photons are generated by coherent mechanisms, albeit only from singly or multiply scattered electrons. Such single-mode fiber-coupled nanobeam resonators provide an attractive platform for investigating quantum interactions between light and charged particles. In turn, they may enable non-classical sensing in electron microscopes through quantum-optical operations on free electrons.