Interplay of Collective Excitations in Quantum-Well Intersubband Resonances

Intersubband resonances in a semiconductor quantum well (QW) display fascinating features involving various collective excitations such as Fermi-edge singularity (FES) and intersubband plasmon (ISP). Using a density matrix approach, we treated many-body effects such as depolarization, vertex correction, and self-energy consistently for a two-subband system. We found a systematic change in resonance spectra from FES- to ISP-dominated features, as QW width or electron density is varied. Such an interplay between FES and ISP significantly changes both line shape and peak position of the absorption spectrum. We found that a cancellation of FES and ISP undresses the resonant responses and recovers the single-particle features of absorption for semiconductors with a strong nonparabolicity such as InAs, leading to a dramatic broadening of the absorption spectrum.

Figure 1

Intersubband absorbance—$\mathrm{I}\mathrm{m}[\chi (\omega )]$—for GaAs QWs with varying width as a function of detuning $\hslash \omega -E_{21}^{(0)}$. Electron density is $1.25\times {10}^{12}{\mathrm{c}\mathrm{m}}^{-2}$. Dotted lines: free-carrier; dashed lines: XSE; dot-dashed lines: ISP only; long dashed lines: FES; solid lines: full many-body effects.

Figure 2

Intersubband absorbance for different width InAs QWs. Electron density is $1.0\times {10}^{12}{\mathrm{c}\mathrm{m}}^{-2}$. The same line styles are used here as in Fig. 1. All dotted and dashed lines are multiplied by a factor of 2, as indicated in the top row.