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Physical review. B, Condensed matter

Coherent control and enhancement of refractive index in an asymmetric double quantum well

Seyed M. Sadeghi, Henry M. van Driel, James M. Fräser

Published Dec 15, 2000• 65 citations
Publisher

Abstract

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 ${\mathrm{M}\mathrm{W}/\mathrm{c}\mathrm{m}}^{2}$. 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.

DOI
10.1103/physrevb.62.15386

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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 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 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 propose a new class of intersubband lasers and amplifiers that achieve net gain without population inversion. The laser scheme is based on a unipolar semiconductor double quantum- well structure where gain occurs at a transition between conduction band subbands. In order to achieve net gain without inversion, we utilize Fano-type interferences. The semiconductor laser scheme that we are considering is analogous to the atomic lambda system that has been extensively analyzed in the context of electromagnetically induced transparency and lasing without inversion. A coherent coupling field however, is not required in the present scheme. The electronic coherence necessary for Fano-type interferences is established by resonant tunneling. For nonlinear optics applications, the asymmetry of the structure allows for (chi) (2) processes and therefore higher conversion efficiency or parametric gain.
This paper presents the current results of field-induced quantum interference in semiconductor quantum wells. Three-level systems with two conduction subbands in single and double quantum wells coupled by a resonant field are studied. We investigate effects of the Coulomb and field-induced electronic renormalizations of the energy subbands and steady eigenstates of electrons. The random-phase and ladder approximations have been used to calculate the linear interband and intersubband optical absorptions and refractive indices. The effect of collective dipole moment on the nonlinear susceptibility has been incorporated into the study by using a local-field approach. Lasing without population inversion, electromagnetically induced transparency, and enhanced nonlinearity with reduced absorption inside the intersubband-coupled single quantum well and dc-field coupled double quantum wells are found.
We investigate the conduction intersubband linear response of a n-type quantum well driven by one or two infrared optical fields with arbitrary strengths. The fields couple the third subband to the second energy subband and the ground subband, forming a Λ system. We investigate the dependences of the absorption spectrum on the pump field strengths. The results for the equal Rabi frequencies can be understood in terms of quantum coherence processes of the intersubband transitions, such as coherent population trapping, and electromagnetically induced transparency. The effects of different Rabi frequencies on the absorption spectrum due to fields are also discussed.
We study quantum interference and nonlinear optical processes in the conduction band of an n-doped quantum well driven by one or two infrared fields. The fields couple a broad upper subband to a lower energy subband and the ground subband, forming a \ensuremath{\Lambda} configuration. We show, including the effect of electron-electron scattering and dynamic screening, that this configuration can lead to coherent control of the intersubband transitions, such as coherent population trapping. We also study quantum interference processes, such as laser-induced transparency and dark-line effects in the linear absorption and gain spectra of the system. The effects on these processes of differing field intensities and of electron-electron scattering are also discussed.
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.
It is shown theoretically that the interface morphology of a quantum well structure and coherent control of its optical properties can be used to generate an amorphous two-dimensional optical lattice. This is done by considering the interaction of an nnn-doped double-quantum-well structure containing three conduction subbands with an infrared laser beam responsible for the generation of quantum interference in the transitions between these subbands. We show that when a well/barrier interface in this structure contains large-scale monolayer growth islands the lateral variation of the conduction subband energies changes the effects of quantum coherence in these transitions along the quantum well plane. In the presence of electron tunneling in the double-quantum-well structure this leads to lateral modulation of the complex susceptibilities of these transitions, allowing the infrared laser beam to coherently suppress or enhance refractive indices of these transitions in specific regions in this plane while they become transparent. In other regions, the same laser field generates large amount of gain or absorption with different refractive indices. It is shown that for a signal field propagating along the quantum well plane these processes can generate an amorphous optical lattice with a morphology determined by the roughness of the quantum well interfaces and the frequency and intensity of the infrared laser. In the absence of such an infrared laser this plane is transparent to the signal field and has a uniform refractive index.