Intersubband plasmons in semiconductor quantum wires

Intersubband plasmons in semiconductor quantum wires are studied in the harmonic-confinement-potential model and in the random-phase approximation. For a system with M population subbands, we find there exist M different modes of the intersubband plasmon, each of which represents an electronic collective transition from one subband to its adjacent subband. When the coupling between the modes is neglected, an analytic expression for the frequencies of these M modes is derived. Also, we have included the couplings in a numerical calculation. Our calculation shows that, when one increases the magnitude of the gate voltage [such as in the experiments of Hansen et al., Phys. Rev. Lett. 58, 2586 (1987) and Brinkop et al., Phys. Rev. B 37, 6547 (1988)], which would effectively increase the subband separation and decrease the Fermi energy, the largest frequency of these multiple intersubband plasmon modes displays a quantum-oscillation behavior but with an overall increasing trend. In addition, the average value over the associated oscillation period for this mode falls in a range that is in good agreement with the experimental observations. Finally, the damping of the intersubband plasmon is analyzed.