Probing collective oscillation of $d$-orbital electrons at the nanoscale

Here we demonstrate that high energy electrons can be used to explore the collective oscillation of $s$, $p$, and $d$ orbital electrons at the nanometer length scale. Using epitaxial AlGaN/AlN quantum wells as a test system, we observe the emergence of additional features in the loss spectrum with increasing Ga content. A comparison of the observed spectra with ab--initio theory reveals the origin of these spectral features is attributed to 3$d$--electrons contributed by Ga. We find that these modes differ in energy from the valence electron plasmons in Al$_{1-x}$Ga$_x$N due to the different polarizability of the $d$ electrons. Finally, we study the dependence of observed plasmon modes on Ga content, lending insight into plasmon coupling with electron--hole excitations.