Single-particle excitations and many-particle interactions in quantum wires and dots

We have investigated electronic excitations in GaAs-${\mathrm{Al}}_x{\mathrm{Ga}}_{1-x}\mathrm{As}$ quantum wires, and dots by resonance Raman spectroscopy. We find for quantum dots and quantum wires three types of excitations; single-particle, spin-density, and charge-density excitations (SPE's, SDE's, and CDE's, respectively). Our experiments show that SPE's are enhanced under conditions of extreme resonance, which indicates that energy-density fluctuations are responsible for the excitation of unscreened SPE's. The high intensity of the SPE's allows a detailed analysis of collective effects in the zero- and one-dimensional electron systems.