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Physical Review B

Multisubband electromagnetically induced transparency and coherently frozen exciton states in quantum well structures

Seyed M. Sadeghi, W. Li

Published Aug 19, 2005• 3 citations
Publisher

Abstract

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 $(e3)$ when this subband is dramatically damped by emission of longitudinal optical phonons and optically coupled to a relatively narrow conduction subband $(e1)$ 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.

DOI
10.1103/physrevb.72.075347

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Electromagnetically induced transparency in a subband quantum well system

Konstantin L. Vodopyanov, G. B. Serapiglia, Emmanuel Paspalakis +1 more

The phenomenon of electromagnetically induced transparency, which was discovered recently in atomic systems, is demonstrated for the first in a subband quantum well system. Applying strong coupling field (Rabi frequency is on the same order as the intersubband linewidth), which is two- photon-resonant with the 1 - 3 intersubband transition, produces dramatic change in the 1 - 2 intersubband absorption profile. This effect can be accounted for in terms of 1 - 2 and 2 - 3 dipoles being driven into coherence by a strong coupling field; it is not related to self- induced transparency or spectral hole-burning (the frequency of the coupling field is quite different from that of 1 - 2 resonance) and, similar to Fano-type interference, is a pure result of destructive interference of probability amplitudes.

Proceedings of SPIE, the International Society for Optical Engineering/Proceedings of SPIE•Apr 23, 2001•0 citations
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.
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.
A strong infrared field tuned in resonance with dipole-allowed intersubband transitions in quantum-well conduction bands can act to coherently mix or dress the resonant states. The effects of dressing two excited conduction subbands on the linear response of a weak probe field nearly resonant with one of the excited subbands and the partially occupied ground subband is theoretically treated to arbitrary order in the coupling-field strength. Three critical coupling-field strengths are identified that demark: (i) the minimum coupling required to observe changes in the linear response, (ii) the onset of a resolvable doublet in the response, and (iii) the strong-coupling limit in which the line doubling is fully developed. Analytic expressions for these three critical fields are obtained in terms of the Rabi frequency and the phenomenological damping parameters associated with states in the three subbands.
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 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 investigate the conduction intersubband linear response of an n-type quantum well driven by two coherent optical fields with arbitrary strengths. The optical fields couple two excited conduction subbands to a common excited subband with either higher energy (\ensuremath{\Lambda} configuration) or intermediate energy (\ensuremath{\Xi} configuration). The system is then monitored weakly from the uncoupled partially occupied ground conduction subband. We find that the driven \ensuremath{\Lambda} system supports two different coupling mechanisms: (i) indirect two-photon coupling of the two lower-energy-excited subbands through the third one (Raman coupling), and (ii) one-photon coupling of all three coupled excited subbands. It is shown that in contrast to the one-photon process, the two-photon process is immune to the higher-energy-excited subband damping rate. The same mechanisms are found for corresponding states of the \ensuremath{\Xi} system. The manifestations of these mechanisms are discussed and the detailed evolutions of the coupled systems in both high-and low-field intensity cases are investigated. Also, the infrared coupling of quantum wells with large conduction-band offsets with a single field are studied using the results for the \ensuremath{\Xi} configuration. The role played by the damping rate of each subband in the infrared coupling of the quantum-well conduction band is discussed. \textcopyright{} 1996 The American Physical Society.
The phenomenon of electromagnetically induced transparency, which was discovered recently in atomic systems, is demonstrated for the first in a subband quantum well system. Applying strong coupling field (Rabi frequency is on the same order as the intersubband linewidth), which is two- photon-resonant with the 1 - 3 intersubband transition, produces dramatic change in the 1 - 2 intersubband absorption profile. This effect can be accounted for in terms of 1 - 2 and 2 - 3 dipoles being driven into coherence by a strong coupling field; it is not related to self- induced transparency or spectral hole-burning (the frequency of the coupling field is quite different from that of 1 - 2 resonance) and, similar to Fano-type interference, is a pure result of destructive interference of probability amplitudes.