Rotational Dipole Plasmon Mode in Semiconductor Nanoparticles

We analyze a new type of the plasmon mode in nanosize semiconductor crystals. Rotational dipole plasmon resonance, in which only angular degrees of freedom are excited, prevails in the optical spectra of photodoped nanocrystals. Such a collective mode differs basically from surface plasmon resonances in typical photoabsorption spectra of metallic nanoclusters and can be described as an excited state in a finite Fermi system as well as a rotational motion of a quantum liquid. We demonstrate that such dipole oscillations are harmonic, which makes it possible to identify them as a plasmon resonant excitation.