Mapping local optical densities of states in silicon photonic structures with nanoscale electron spectroscopy

Relativistic electrons in a structured medium generate radiative losses such as Cherenkov and transition radiation that act as a virtual light source, coupling to the photonic densities of states. The effect is most pronounced when the imaginary part of the dielectric function is zero, a regime where in a nonretarded treatment no loss or coupling can occur. Maps of the resultant energy losses as a sub-5 nm electron probe scans across finite waveguide structures reveal spatial distributions of optical modes in a spectral domain ranging from near infrared to far ultraviolet.