Collective excitations and far-infrared optical absorptions in parallel quantum-well wires

The intrasubband and intersubband plasmons in the long-wavelength limit are calculated numerically for GaAs/AlAs parallel quantum-well wires (PQWW) produced by the deposition of GaAs and AlAs fractional layers on (001) vicinal GaAs substrates. One intrasubband plasmon mode is found, the energy of which approaches zero as the in-plane wave vector q to 0. The dispersion relations of the intrasubband plasmons are anisotropic with h(cross) omega p =(( sigma 2 q x 2 + alpha 0 2 q y 2 )/q) 1 2/. The constant alpha 0 increases monotonically with the electron density n e of the PQWW, while a increases with n e when the Fermi energy E F is in the electron conduction subbands; it decreases with n e when E F is in the gaps of the subbands. The intersubband plasmons are found to be multi-branch structured with non-zero energies as q to 0. The calculations show that, for the GaAs/AlAs PQWW we consider where the lateral periods L x of the PQWW are much less than 200 AA, the far-infrared optical absorptions are dominated by single-electron direct transitions between different electron conduction subbands. For PQWW with L x larger than 300 AA, the absorptions are due to intersubband plasmon absorptions. Between these two regions, both single-electron direct transitions and intersubband plasmon absorptions can make important contributions to the absorptions, depending on the structures of the PQWW.