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Physical Review B

Infrared-induced dark states and coherent population trapping of excitons in quantum well structures

Seyed M. Sadeghi, W. Li

Published Jan 22, 2004• 6 citations
Publisher

Abstract

We study enhancement of the $2s$ state emission intensity of the e1-hh1 excitons via generation of dark states. This process is shown for a ${\mathrm{In}}_{0.43}{\mathrm{Ga}}_{0.57}\mathrm{A}\mathrm{s}/\mathrm{I}\mathrm{n}\mathrm{P}$ quantum well structure interacting with a single infrared laser near resonance with the transition between e1 and e2. Via generation of strong nonparabolic hole dispersion, the tensile strain of this structure enhances the dipole moments of the nonallowed transitions between the $2s$ states of the e1-hh1 and the $1s$ states of e2-hh1 excitons. We show that, due to the simultaneous coupling of these transitions and those between the $1s$ states of the e1-hh1 and e2-hh1 excitons (allowed transitions), the infrared field can generate dark states, coherently pumping the $2s$ states of e1-hh1 excitons and dramatically increasing their emission intensities. We also show that when the allowed and nonallowed transitions are driven by two resonant infrared field beams, coherent population trapping of excitons of the e1-hh1 excitons occurs.

DOI
10.1103/physrevb.69.045311

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We study enhancement of the 2s2s2s state emission intensity of the e1-hh1 excitons via generation of dark states. This process is shown for a In0.43Ga0.57As/InP{\mathrm{In}}_{0.43}{\mathrm{Ga}}_{0.57}\mathrm{A}\mathrm{s}/\mathrm{I}\mathrm{n}\mathrm{P}In0.43​Ga0.57​As/InP quantum well structure interacting with a single infrared laser near resonance with the transition between e1 and e2. Via generation of strong nonparabolic hole dispersion, the tensile strain of this structure enhances the dipole moments of the nonallowed transitions between the 2s2s2s states of the e1-hh1 and the 1s1s1s states of e2-hh1 excitons. We show that, due to the simultaneous coupling of these transitions and those between the 1s1s1s states of the e1-hh1 and e2-hh1 excitons (allowed transitions), the infrared field can generate dark states, coherently pumping the 2s2s2s states of e1-hh1 excitons and dramatically increasing their emission intensities. We also show that when the allowed and nonallowed transitions are driven by two resonant infrared field beams, coherent population trapping of excitons of the e1-hh1 excitons occurs.
We theoretically propose coherent control of the nonradiative decay of excitons in infrared-coupled asymmetric quantum wells. This is done by (i) inducing intraband multiphoton quantum coherences between exciton states associated with three conduction subbands (E1, E2, and E3), and (ii) adjusting the contributions of these coherences using competition between one- and two-photon coupling of these states. The intraband excitonic transitions are caused by one or two infrared laser beams resonant with the intersubband transitions between E1, E2, and E3. We show that by tuning these beams to allow coupling of the E1--HH1, E2--HH1, and E3--HH1 excitons with different configurations one can coherently manipulate the quantum efficiency of exciton emission. In an asymmetric double ${\mathrm{G}\mathrm{a}\mathrm{A}\mathrm{s}/\mathrm{A}\mathrm{l}}_{x}{\mathrm{Ga}}_{1\ensuremath{-}x}\mathrm{As}$ quantum well this could lead to nearly complete destruction of the emission spectrum, or to the immunity of excitons from enhancement of their nonradiative decay rates via the intraband transitions. We discuss these effects in terms of coherent population trapping of excitons and show how field-coherent destruction effects influence the multiphoton excitonic coherences.
We show theoretically that infrared multisubband dressing of exciton states combined with their extensive damping rates allow one to coherently make these states immune against intense resonant infrared lasers. Such coherently frozen states occur as one detects bound exciton states associated with a broad conduction subband (e3)(e3)(e3) when this subband is dramatically damped by emission of longitudinal optical phonons and optically coupled to a relatively narrow conduction subband (e1)(e1)(e1) via a two-photon coupling process. It is shown that under such conditions the exciton states are optically dressed and become stationary without any energy shift and significant amplitude change. We also use multisubband dressing of exciton states to propose a two-field electromagnetically induced transparency where enhanced quantum interference in the excitonic transitions significantly removes contributions of the bound exciton states near the band edge.
We theoretically and experimentally investigate the non-linear optical properties of undoped and n-doped quantum wells (QW's) in the presence of intense infrared lasers. These lasers are polarized along the QW growth directions and near resonance with the transitions between the conduction subbands. In n-doped QW's by inclusion of electronelectron scattering and dynamic screening we study for the first time: (i) the effects of the frequencies and intensities of the infrared fields on the dephasing rates associated with the intersubband transitions, (ii) the generation of various nonlinear gain processes, and (iii) coherent and optical processes such as coherent population trapping, dark line and laser induced transparency effects. In undoped QW's by presenting a consistent theory we show how one can use infrared coupling of the conduction subbands of a QW to coherently manipulate absorption and emission spectra of excitons. We discuss these effects in.terms of multi-photon coupling of the excitons or electrons, quantum interferences, and by presenting a new model to treat laser-matter interaction. Following these developments we put some of our own results to test by observing the emission dynamics of excitons in the presence of an intense CO2 laser. We report the first observations of infrared enhancement of the effective non-radiative decay rates of excitons and the effects of multi-level transition processes in the emission of QW's. This thesis also includes an extensive analysis of nonlinear effects in atomic systems. While the primary motivation for this analysis was to understand and design nonlinear optics of QW's, in the process some original contributions were made to the study of nonlinear optics of atomic systems.
Quantum interference processes are investigated in the diagonal and nondiagonal excitonic emission spectra of quantum well structures coupled by single infrared fields polarized along their growth directions and resonant with the first-to-second-conduction-subband transitions. This investigation predicts generation of the Autler - Townes doublet in the emission spectrum of the coupled ground exciton state (diagonal transition) and development of a dynamic emission hole followed by a dark line in that of the higher coupled exciton states (nondiagonal transition). The characteristic limits determining these processes are determined and reported experimental results are explained successfully.
A quantum well (QW), with only three transitions, which are all dipole allowed because of the QW asymmetry, is investigated when driven coherently by two infrared optical fields. One field couples the $\ensuremath{\mid}1⟩\ensuremath{-}\ensuremath{\mid}2⟩$ and $\ensuremath{\mid}2⟩\ensuremath{-}\ensuremath{\mid}3⟩$ transitions simultaneously, while a second field couples the $\ensuremath{\mid}1⟩\ensuremath{-}\ensuremath{\mid}3⟩$ transition. The absorption linear response is computed allowing the illustration of some features due to quantum coherence. Without the second field, the absorption of a weak probe is that of a strongly saturated system. As the coupling strength of the second field is increased the absorption strength of a weak probe increases, by over an order of magnitude factor in absorption strength compared to the case in the absence of the second field. The results are shown to be due to an optically mediated, coherent population trapping effect.
Summary form only given.Recent experiments show signatures of exciton Bose-Einstein condensation in semiconductors. Thus a device that creates a coherent population of excitons while being pumped by a thermal source may be possible. Such an exciton matter laser could consist of a quantum well, possibly embedded in an optical microcavity. In it hot excitons slightly below the bandgap are created by a thermal distribution of free electrons and holes (generated by electrical injection or optical pumping). The excitons interact with (LA) phonons. At low densities excitons are bosonic, and their scattering is affected by statistical enhancement into states with large occupation. Under certain conditions the excitons can condense into a single state, with statistics similar to a coherent state. Light emitted by radiative recombination has the coherence properties of the condensate and is similar to laser light. However, the electronic population in the device does not need to be inverted. We consider 1S heavy-hole excitons in a two-dimensional harmonic trap, realized by a quantum well where interface roughness inevitably localizes low-energy excitons, or by strain fields in lattice mismatched structures. We treat the lowest two trap levels quantum mechanically, and the higher lying states act as a reservoir. Our model is valid at low density, where phase space filling and collisions are negligible. While simplistic, the model displays the main features of an exciton matter laser. Our model is described by the master equation.