Nonlocality in the Two-Dimensional Plasmon Dispersion

Two-dimensional plasmons have been investigated in AlGaAs-GaAs heterostructures with transmission spectroscopy. In a magnetic field $B$, we observe an interaction of the plasmon excitation with harmonics of the cyclotron-resonance frequency ${\omega }_c=\frac{\mathrm{eB}}{m}$, which arises from nonlocal effects on the two-dimensional-plasmon excitation and is governed by the parameter ${\left(\frac{q{v}_{\mathrm{F}}}{{\omega }_c}\right)}^2$ ($q$ is the plasmon wave vector; $v_{\mathrm{F}}$ is the Fermi velocity). The dispersion and the excitation strengths of the combined resonances are in excellent quantitative agreement with a quasiclassical nonlocal theory.