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Slowing and switching of light in semiconductor quantum wells

Murat Yıldırım

Published Dec 1, 2010• 0 citations
Publisher

Abstract

After light became a part of the telecommunication system, the manipulation of light, especially the control of the speed of light, became a focus of research. By using the different approaches of the interaction between light and matter, there have been many successful attempts to slow down the speed of light, even to several meters per second. In the first part of my thesis, I studied pulse propagation in absorptive media (GaAs/AlGaAs multiple quantum well structures-MQWs) to gain insight into the linear interaction between matter and light. Systematic and fundamental measurements of pulse propagation in GaAs/AlGaAs multiple quantum well structures will be presented. These measurements are simulated by treating MQWs as a slab. These measurements and simulations will be compared while using a newly introduced general formalism.\nIn the second part of my thesis, I will present another aspect of manipulating light: all-optical switching. Eric Gansen demonstrated all-optical switching in GaAs/AlGaAs MQWs at cryogenic temperatures. I will demonstrate that the same design of all-optical switch, which is based on polarized virtual carriers, can work without any serious loss of performance at near-room temperatures.\nAnother material used in all-optical switches is Bragg-spaced-quantum-well structures (BSQWs) which are grown by molecular beam epitaxy. The growth of BSQW is challenging because of the associated unusually long growth times. In the last part of this dissertation, I will discuss the challenges involved in MBE growth of 210 period long GaAs/InGaAs BSQWs. By presenting basic characterization measurements of the successful and unsuccessful attempts of BSQW growths, I will demonstrate the challenges of BSQW growth and how they can be addressed.

DOI
10.17077/etd.yjgjwjcd

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After light became a part of the telecommunication system, the manipulation of light, especially the control of the speed of light, became a focus of research. By using the different approaches of the interaction between light and matter, there have been many successful attempts to slow down the speed of light, even to several meters per second. In the first part of my thesis, I studied pulse propagation in absorptive media (GaAs/AlGaAs multiple quantum well structures-MQWs) to gain insight into the linear interaction between matter and light. Systematic and fundamental measurements of pulse propagation in GaAs/AlGaAs multiple quantum well structures will be presented. These measurements are simulated by treating MQWs as a slab. These measurements and simulations will be compared while using a newly introduced general formalism.\nIn the second part of my thesis, I will present another aspect of manipulating light: all-optical switching. Eric Gansen demonstrated all-optical switching in GaAs/AlGaAs MQWs at cryogenic temperatures. I will demonstrate that the same design of all-optical switch, which is based on polarized virtual carriers, can work without any serious loss of performance at near-room temperatures.\nAnother material used in all-optical switches is Bragg-spaced-quantum-well structures (BSQWs) which are grown by molecular beam epitaxy. The growth of BSQW is challenging because of the associated unusually long growth times. In the last part of this dissertation, I will discuss the challenges involved in MBE growth of 210 period long GaAs/InGaAs BSQWs. By presenting basic characterization measurements of the successful and unsuccessful attempts of BSQW growths, I will demonstrate the challenges of BSQW growth and how they can be addressed.
There is a wide variety of proposals for implementations of slow-light effects, and many of them are described in the various chapters of this book. The physical implementations vary from system to system, and so do the mathematical formalisms used to describe the specific physical systems. However, it is also possible that one has two completely different physical slow-light systems, yet the mathematical framework describing the slowing and stopping of light is very similar. In this chapter, we present an example falling into this category. The two physically different slow-light systems are (1) semiconductor multiple quantum wells (QWs) (more specifically: Bragg-spaced multiple QWs, or BSQWs [1-16]) and (2) optical resonators (more specifically: side-coupled integrated spaced sequence of resonators with two waveguide channels, or two-channel SCISSORs [17,18]). In Figure 13.1, BSQWs are schematically shown as a sequence of thin semiconductor QWs, spaced by a distance a with a dielectric of refractive index nb between the wells. SCISSORs are sequences of ring-shaped dielectric optical microresonators coupled to two waveguides (Figure 13.2). Clearly, these are very different physical systems, and so onemay wonder in which sense slow-light concepts (i.e., the concepts for delaying, stopping, storing, and releasing light) can be similar or almost identical in these systems. The answer to this question lies in the fact that both systems can be viewed as physical realizations of one-dimensional resonant photonic bandgap structures (RPBGs). RPBGs differ from conventional (nonresonant) photonic bandgap structures in that each unit cell in the photonic lattice exhibits an optical resonance with a resonance frequency that is identical or at least close to the Bragg frequency characterizing the optical lattice. In QWs, the optical resonance is taken to be the heavy-hole exciton resonance, while in SCISSORs we take one of the optical resonances of the microrings.
Epitaxial methods allow the fabrication of nonlinear interferomethc structures which include the mirrors and the GaAs Multiple Quantum Wells (MQW) nonlinear medium in a single crystal. This results in very compact nonlinear microcavities with typically 5im overall thickness. Optical bistability is observed in these structures at mW optical power. We review our latest experimental results on these structures including the observation of high contrast switching in the reflection mode the direct measurement of dispersive optical nonlinearities in MQWs evidencing their saturation behavior at high intensities. Finally we report external-beam switching experiments which allow to assess the potential of these structures as all-optical gates.
This dissertation presents research on the propagation and manipulation of excitons in GaAs. There are three main aspects to be addressed. In the first part, we provide a comprehensive understanding of the slow- and fast-light propagation based on excitonic resonances. Propagation (or transit) times of optical pulses through a medium near an absorptive resonance with and without spatial dispersion are studied and contrasted. We show that, when the broadening of the resonance (i.e., dephasing rate) is below a critical value, a frequency range exists near resonance where the transit times are determined by interference between co-propagating exciton-polaritons and deviate strongly from expectations based on the group velocities of the individual exciton-polariton branches. Our theory puts the well-known slow- and fast-light effects in systems without spatial dispersion into a broader context by interpreting them as a limiting case of systems with spatial dispersion. An important ingredient of the exciton theory is the light-matter coupling that can be expressed either in terms of the dipole or the momentum matrix elements. We re-examine the validity of a frequently used relation between the interband momentum and dipole matrix elements ('p-r relation') in semiconductors. An additional correction term was obtained when we applied the 'p-r relation' to finite-volume crystals treated with periodic boundary conditions. The correction term does not vanish in the limit of infinite volume. We illustrate this with numerical examples for bulk GaAs and GaAs superlattices. For bulk GaAs, the correction term is found to be always important; while for a GaAs superlattice, the importance of the correction term for the transition depends strongly on the origin of the unit cell.As an example for manipulation of excitons, we consider mechanical deformations of GaAs nanomembranes. The nanomembranes with lateral sizes much larger than their thicknesses exhibit great flexibility in non-planar deformations and thus promise applications in flexible electronic and photonic devices. These non-planar deformations do not fit in the well-established theory for planar deformations induced by lattice mismatch. Our theory relates the general mechanical deformations (planar or non-planar) of nanomembranes to their excitonic spectra, and is numerically evaluated within the average-strain approximation.
Controlling the group velocity of light has great potential applications in future optical communication, photonic computing, and microwave signal processing. These applications have recently triggered immense experimental and theoretical interest in the research community. Numerous studies report large optical delays while sacrificing many important aspects of engineering feasibility: bandwidth, device compactness, room temperature operation, ease of integration, and minimized signal insertion loss. Slow and fast light in the quantum-dot and quantum-well semiconductor optical amplifiers (SOAs) address many of these characteristics. First, this dissertation presents variable slow light in quantum-dot SOAs using population oscillation in an absorptive medium. Bandwidth tuning of slow light is observed with forward and reverse bias of quantum-dots. Second, the study explores four-wave mixing and population oscillation in a quantum-well gain medium. Control of optical advance and bandwidth is achieved by the medium's electrical bias or the optical pump power. Moreover, the fast light in the cascade of N number of quantum-well SOAs is experimentally and theoretically investigated. The cascaded scheme uses SOA-to-SOA attenuators to achieve active delay-bandwidth control. Finally, a large delay-bandwidth scheme is presented in which slow-to-fast light is observed by means of absorption to gain switching. More than a half cycle of tunable delay at 1 GHz bandwidth is achieved at optical frequencies.
Semiconductor quantum wells exhibit numerous nonlinear optical effects that are potentially useful in electro-optical or all-optical devices. In general, the underlying physical mechanisms are complex many-particle processes and can be viewed as interesting research subjects in their own right. One example of such a device is the all-optical polarization switch that has been demonstrated [1]. In that device, conventional (i.e. non-strained) GaAs quantum wells are used in conjunction with two polarizers to switch the throughput of a weak probe pulse with a strong pump pulse. The switching is due to an effective polarization rotation of the probe light by the pump light. The basic idea underlying this effective polarization rotation can be based on a simplified picture of two spin-degenerate heavy-hole exciton transitions. A linearly polarized probe pulse, which consists of right ("+") and left ("-") circularly polarized components, would not undergo polarization rotation, because it couples to both transitions in the same way. If, however, a circularly polarized pump beam modifies, for example, the transition frequency of the "+" transition, the absorption of the "+" component of the probe light differs from that of the "-" component. This leads to an effective polarization rotation.