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Physical review. B, Condensed matter

Coherent control of the nonradiative decay of excitons in asymmetric quantum well structures

Seyed M. Sadeghi, Henry M. van Driel

Published Jan 9, 2001• 4 citations
Publisher

Abstract

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.

DOI
10.1103/physrevb.63.045316

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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 GaAs/AlxGa1\ensuremath−xAs{\mathrm{G}\mathrm{a}\mathrm{A}\mathrm{s}/\mathrm{A}\mathrm{l}}_{x}{\mathrm{Ga}}_{1\ensuremath{-}x}\mathrm{As}GaAs/Alx​Ga1\ensuremath−x​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 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 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 propose coherent control and enhancement of the refractive index with zero absorption in an n-type asymmetric double quantum well using intersubband transitions. These effects are caused by quantum coherence and interference whereby a strong infrared laser mixes upper conduction subbands of an intersubband transition with an auxiliary subband. In the ${\mathrm{G}\mathrm{a}\mathrm{A}\mathrm{s}/\mathrm{A}\mathrm{l}}_{x}{\mathrm{Ga}}_{1\ensuremath{-}x}\mathrm{As}$ system considered here, an approximately 10% enhancement of the refractive index accompanied with zero absorption occurs at 10.5 \ensuremath{\mu}m, when the system is driven by a 6.2-\ensuremath{\mu}m laser field with an intensity of \ensuremath{\sim}1 MW/cm2{\mathrm{M}\mathrm{W}/\mathrm{c}\mathrm{m}}^{2}MW/cm2. The interplay between quantum coherence and electron population dynamics not only causes an enhancement of refractive index similar to that in atomic systems, but can also induce both low positive and negative group velocities.
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.
We investigate theoretically coherent control of optical absorption and emission (optical gain) in semiconductor double wells by use of two coherent light sources. Our analysis is based on a microscopic theory of optical excitation in semiconductors which accounts for many-body effects. Specifically, we show that electron–hole pair (exciton) formation in an asymmetric double well can be controlled by intensity and phase of a dc microwave (MW) field if the pulse duration lies below the MW-induced Rabi period. Similarly, resonant MW fields are shown to allow control of intersubband absorption and emission spectra of semiconductor heterostructures.
A formalism is described that self-consistently treats the effects of infrared (ir) coupling fields on the interband quantum-well absorption spectrum when both the $e1\ensuremath{-}e2$ and $e2\ensuremath{-}e3$ conduction subband transitions are nearly resonant with the ir frequency. For the 8.5-nm-wide GaAs quantum-well samples typically considered for ir dressing studies, we show that the simultaneous two-photon coupling of the $e1\ensuremath{-}hh1$ and $e3\ensuremath{-}hh1$ excitons and the one-photon coupling of all three excitons produces significantly different results than those obtained when the $e2\ensuremath{-}e3$ coupling is neglected. In particular we find that the position and shape of the dominant $e1\ensuremath{-}hh1$ exciton feature each change nonmonotonically, and saturate with increasing ir intensity. This is in better agreement with existing experimental results than previous calculations based on simple $e1\ensuremath{-}e2$ coupling.