Interplay of Coulomb and nonparabolicity effects in the intersubband absorption of electrons and holes in quantum wells

Abstract

Quantum well intersubband absorption with contributions from both valence and conduction subbands is computed with a nonequilibrium Keldysh Green's function formalism, assuming an optical pump and probe scheme in undoped samples. The coupling between conduction and valence bands leads to contributions to the TE mode from the electrons, which are enhanced due to Coulomb corrections and may be resolved even in the presence of the dominating hole contributions. A strong contrast in the evolution of absorption spectra with increasing carrier density is predicted between TM and TE polarizations. The influence of strongly k-dependent dipole moments in combination with many-body effects is analyzed for intersubband transitions, including the evolution of the spectra with increasing excitation power. For the TE case, extra features appear in the spectra, due to interplays between band structure and Coulomb effects which are not present in the TM mode. The spectral evolution on both polarizations, broadening, number, and relative strength of the resolved peaks are in strong contrast with free-carrier results. Numerical results are given for four different structures.

Valence intersubband gain without population inversion

“…Moreover many-body effects (electron-electron scattering resonances) have been observed in recent experiments for THz QCLs operating in the Quantum Hall Regime [23]. Thus in the theoretical limit analyzed in this paper, the dephasing is due only to electron-electron mechanisms [24] Consideration of other scattering channels will be the subject of future research. Fig.…”

Intersubband gain without global inversion through dilute nitride band engineering

“…Finally, absorption and gain are given by the solution of the integro-differential equation for ͑k , ͒ obtained from the carriers Green's function in linear response. 9,[22][23][24] The numerical matrix inversion technique used to solve this equation is similar to the interband method used in Ref. 25.…”

Coupling of THz radiation with intervalence band transitions in microcavities

“…This avoids the usual need of complicated cascaded structures since even in isolated QWs, there are intersubband THz resonant with the TE mode. The microscopic approach presented here unique features not found in the literature so far: (i) The valence bands are investigated opening new possibilities for polaritonic devices; (ii) Intervalence-band transitions in the TE mode are investigated in contrast to the majority of work in the literature, focused on TM-mode inter-conduction band transitions(iii) Full nonequilibrium many body solutions for the intersubband susceptibility problem with electron-electron scattering included beyond the Hartree-Fock approximation [12,13] and valence band nonparabolicity are used as starting point and are adjusted to a simple model that can be used by researchers not trained in the numerical solution of microscopic nonequilibrium Green's functions methods. This gives a high potential for impact of this work for research in different research fields and applicability to new materials.…”

“…We follow the method developed in Refs [12,13]. with the main difference that here the system is globally out of equilibrium but the electrons are assumed to be independently thermalised within only one subband with occupation functions characterised by temperatures.…”