Effects of tunneling coupling on plasmon modes in asymmetric double-quantum-well structures

The collective charge density excitations in asymmetric double-quantum-well (DQW) structures with different tunneling strengths are systematically studied. In particular, the damping properties of the plasmon modes in various tunneling strengths are investigated in detail. It is shown that plasmon modes in asymmetric DQW structures are quite different from those in symmetric DQW systems. In weak tunneling regime, an intra-subband mode ω- with an acoustic-like dispersion relation which is damped in symmetric DQW structures arises and coexists with the optical-like mode ω+ while the inter-subband mode ω10 is highly damped. With the tunneling strength being increased, the ω10 branch gradually becomes undamped and emerges out of the (1-0) single-particle continuum, whereas the ω- branch gradually approaches the (0-0) single-particle continuum. In intermediate coupling regime, these three branches of modes coexist undamped. In strong tunneling regime, ω- enters the (0-0) single-particle continuum and becomes damped. Consequently, only the ω+ and ω10 modes exist in this regime.