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Physical Review B

Amorphous two-dimensional optical lattices with coherently controlled morphologies in quantum well structures

Seyed M. Sadeghi

Published Sep 21, 2005• 3 citations
Publisher

Abstract

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 $n$-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.

DOI
10.1103/physrevb.72.125336

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•Jan 1, 2009•0 citations
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.
In this paper, quantum interference effects and atomically flat monolayer thickness fluctuations (growth islands) have been used in quantum well (QW) structures to propose a method to spatially modulate their refractive indexes and absorption/gain. The interference effects are caused by an infrared (IR) laser field, near resonantly driving the conduction intersubband transitions of an n-doped QW. The growth islands, on the other hand, laterally modulate these effects along the QW plane. It is shown that, when such a QW structure is properly designed, one can utilize these features to generate submicrometer regions (isles) of electromagnetically induced transparency (EIT) in the plane of the QW. This process occurs as the refractive indexes of these regions are coherently enhanced. The same laser field, in other regions of this plane, generates a large amount of gain or absorption with smaller refractive indexes. It is also shown that by varying the intensity and wavelength of the IR laser, one can dynamically switch over the EIT process from the lateral regions associated with a specific type of growth islands to other regions associated with either different types of islands or the nominal region of the QW. In the absence of this laser, such submicrometer-scale photonic structures do not exist, and the QW is transparent with a laterally uniform refractive index. These results can introduce a new trend for the generation of active photonic lattices, planar integrated optical circuits, and active photonic nanostructures
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 propose one-dimensional bright and dark optical lattices where Bragg scattering occurs, respectively, via periodic generation of gain without inversion and electromagnetically induced absorption in the absence of any refractive index perturbation. The absorption and gain processes are caused by quantum interference in the conduction intersubband transitions of a corrugated nnn-doped quantum well structure interacting with an intense infrared laser. The quantum well structure is embedded between two optical confinement layers, forming a periodic structure with a Bragg wavelength similar to that of the intersubband transition between the second and third conduction subbands of the quantum well. We show that in the absence of the intense infrared laser this transition is transparent, and the periodic structure forms a passive one-dimensional photonic gap generated by the background refractive index contrast of the corrugated region. In the presence of this laser, the intersubband transition is influenced by the quantum coherence effects, causing dramatic changes in the photonic band gap. We show that when one adjusts the intensity and wavelength of such a laser properly these effects can (i) destroy the one-dimensional photonic band gap, and (ii) form periodic regions of gain without inversion or electromagnetically induced loss, while the index perturbation is completely canceled out. Such bright and dark optical lattices can scatter a light at Bragg wavelength when its frequency is the same or close to that of the intersubband transition.
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 propose coherently generated photonic heterostructures using a functional photonic superstructure. In the absence of a laser field (control field), such a structure exhibits a conventional passive (off-resonant) photonic band gap. When a region(s) of such a structure is illuminated by the control field, coherent enhancement of refractive index increases the refractive index perturbations of that region, while electromagnetically induced transparency keeps it lossless. This forms a photonic heterostructure consisting of a passive (unilluminated region) and an active (illuminated region) photonic band gap structures. Using such a superstructure, we study a coherently generated photonic quantum well structure wherein two active photonic band gaps sandwich a passive region. We show that, since the active photonic band gaps are roughly twice wider than the passive band gap, they form two photonic barriers around the transparency band located at the longer wavelength side of the passive photonic band gap. This leads to formation of resonant transparency states or photonic subbands, similar to the conduction or valence subbands in electronic quantum well structures. We show that the energies and linewidths of such photonic subbands can be coherently controlled by just adjusting the control field beam.
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.