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Author: Alexander Graham Winbow Publisher: ISBN: 9781124378299 Category : Languages : en Pages : 280
Book Description
Indirect excitons in GaAs coupled quantum well nanostructures are a versatile system for fundamental study of cold neutral bosonic gases and demonstration of novel optoelectronic devices based on excitons -- a bound electron-hole pair -- rather than electrons. Indirect exciton lifetimes range from nanoseconds to microseconds and cool rapidly after photoexcitation to the lattice temperature. Lithographically-patterned electrodes enable design of potential energy landscapes, and both energy and lifetime can be controlled in situ, rapidly, on timescales much shorter than the exciton lifetime. Such intrinsically optoelectronic devices can operate at speeds relevant to optical networks, and later be fabricated in other semiconductors for higher-temperature operation. Two different kinds of devices are demonstrated Photon storage -- an optical memory -- with 250 ps rise time of the readout optical signal and storage time reaching microseconds was implemented with indirect excitons in CQW. The storage and release of photons was controlled by the gate voltage pulse, and the transient processes in the CQW studied by measuring the kinetics of the exciton emission spectra. This control of excitons on timescales much shorter than the exciton lifetime demonstrates the feasibility of studying excitons in in situ controlled electrostatic traps. The Exciton Conveyer is a laterally moving electrostatic lattice potential for actively transporting excitons. Generated by laterally modulated electrodes, the potential velocity and depth are controlled in situ by frequency and voltage. We observed exciton transport characterized by average exciton cloud spatial extension over several tens of microns, and observed dynamical localization-delocalization transitions for the excitons in the conveyer : In the localization regime of deeper potentials and moderate exciton density, excitons are moved by the conveyer; in the delocalized regime of shallower lattice potential or high exciton density, excitons do not follow the conveyer motion. We explore conveyer velocities both slower and faster than phonon velocities. Realizing subnanosecond manipulations of exciton energy and lifetime required versatile control of pulsed and multiple AC RF electrical signals in optical, liquid helium cryogenic systems. Considerable detail is presented of design, construction, and test of flexible experimental apparatus.
Author: Alexander Graham Winbow Publisher: ISBN: 9781124378299 Category : Languages : en Pages : 280
Book Description
Indirect excitons in GaAs coupled quantum well nanostructures are a versatile system for fundamental study of cold neutral bosonic gases and demonstration of novel optoelectronic devices based on excitons -- a bound electron-hole pair -- rather than electrons. Indirect exciton lifetimes range from nanoseconds to microseconds and cool rapidly after photoexcitation to the lattice temperature. Lithographically-patterned electrodes enable design of potential energy landscapes, and both energy and lifetime can be controlled in situ, rapidly, on timescales much shorter than the exciton lifetime. Such intrinsically optoelectronic devices can operate at speeds relevant to optical networks, and later be fabricated in other semiconductors for higher-temperature operation. Two different kinds of devices are demonstrated Photon storage -- an optical memory -- with 250 ps rise time of the readout optical signal and storage time reaching microseconds was implemented with indirect excitons in CQW. The storage and release of photons was controlled by the gate voltage pulse, and the transient processes in the CQW studied by measuring the kinetics of the exciton emission spectra. This control of excitons on timescales much shorter than the exciton lifetime demonstrates the feasibility of studying excitons in in situ controlled electrostatic traps. The Exciton Conveyer is a laterally moving electrostatic lattice potential for actively transporting excitons. Generated by laterally modulated electrodes, the potential velocity and depth are controlled in situ by frequency and voltage. We observed exciton transport characterized by average exciton cloud spatial extension over several tens of microns, and observed dynamical localization-delocalization transitions for the excitons in the conveyer : In the localization regime of deeper potentials and moderate exciton density, excitons are moved by the conveyer; in the delocalized regime of shallower lattice potential or high exciton density, excitons do not follow the conveyer motion. We explore conveyer velocities both slower and faster than phonon velocities. Realizing subnanosecond manipulations of exciton energy and lifetime required versatile control of pulsed and multiple AC RF electrical signals in optical, liquid helium cryogenic systems. Considerable detail is presented of design, construction, and test of flexible experimental apparatus.
Author: Aaron Tynes Hammack Publisher: ISBN: 9781109764604 Category : Languages : en Pages : 168
Book Description
The study of excitons in semiconductors is a rich field that has seen numerous beautiful developments during the seven decades since they were proposed by Frenkel [1931]. The exciton states in dielectric materials and molecular complexes are fundamental to understanding the full electronic structure of materials and the complex interactions between light and matter. Of particular interest is the fact that excitons, bound electron-hole pairs, form composite bosons with very low effective masses. This opens up an incredibly rich opportunity to study the phase space of quantum degenerate Bose gases at relatively high temperatures. An overview of many of the rich features that have been observed in exciton gases is presented. The system of indirect excitons in coupled quantum wells (CQWs) provides for drastically increased and tunable exciton lifetimes, leading to improved thermalization. The present experimental study presents a full steady state and dynamic model for the transport, generation, recombination, and thermalization of indirect excitons in CQWs, as well as methods for in-situ manipulation of indirect excitons by both optical and electrical methods. Both the optical and electrical methods for exciton control provide for dynamic manipulation of excitons on timescales significantly shorter than the lifetimes of indirect excitons.
Author: Yuliya Yevgenyevna Kuznetsova Publisher: ISBN: 9781321451900 Category : Languages : en Pages : 67
Book Description
This dissertation explores studies of transport of indirect excitons (bosonic quasiparticles composed of bound pairs of an electron and a hole confined to spatially separated layers) in GaAs coupled quantum wells. The small mass and long lifetime of indirect excitons result in a relatively high quantum degeneracy temperature and efficient cooling, making indirect excitons a model system for studies of physics of cold bosons. In addition, indirect excitons are optically active, electronically controllable and have long enough lifetimes that their transport distances can be accommodated by lithography. These properties make indirect excitons a promising system for creating excitonic devices. The direction of the research presented in this dissertation is thus twofold: studying fundamental physics of excitons, including transport, thermalization, coherence, spin currents, and properties in high magnetic fields, and realization of optical excitonic devices, such as traps and transistors.
Author: Publisher: ISBN: Category : Languages : en Pages : 0
Book Description
We have used an external electric field to control important properties of semiconductor quantum wells. Specifically: (1) We have modified the exciton-exciton interaction. This interaction is determined by many-body effects, which depend on the overlap between the exciton's electron and hole wavefunctions, and is manifested in the energy difference between photoluminescence spectra with different polarizations. In time-resolved photoluminescence (PL) experiments with circular polarization we have observed that spectral difference, which we controlled by an electric field applied to GaAs-GaAlAs coupled quantum wells. Our results confirm the predictions of theory but also point out its limitations to fully explain our observations. (2) We have reversed the valence-band ordering in strained-layer quantum wells, thus 'undoing' the effects of strain. To prove the concept we have used strained InGaAs-InAlAs quantum wells in which the light-hole state is the ground state in the valence band. Photocurrent measurements under various fields have shown a change in the valence-band states that contribute to the fundamental (lowest-energy) transition, from light-hole states to heavy-hole states. This result opens the door to reversing the polarization of light emission in quantum wells, from TM to TE, which could find application in optical modulators. (3) We have shown the presence of low-temperature exciton-photon coupling in microcavities using PL spectroscopy. Because of thermalization until now it has been almost impossible to use PL to study at low temperature (T = 20K or less) the coupling of excitons and photons. We have determined the difference between the lowest-energy PL peak from a quantum well-microcavity system and that of an isolated quantum well, at various temperatures. This difference is constant with T when a field is applied but is T dependent when the field is suppressed. The maximum variation is a direct measure of the exciton-photon coupling.
Author: Joe Wilkes Publisher: ISBN: Category : Languages : en Pages :
Book Description
This thesis comprises a theoretical study of the dynamics of indirect excitons in coupled quantum wells at low lattice temperatures. The results of numerical simulations of the exciton photoluminescence pattern are presented and compared to available experimental data. The in-plane transport of quantum well excitons created by laser excitation is modeled using a non-linear drift-diffusion equation. Combined with a model of exciton relaxation thermodynamics, a complete description of the evolution of the exciton density and temperature is built. The optical decay of indirect excitons is included in the modeling. This is used to make predictions of the spatial photoluminescence patterns which have been observed experimentally. The transport of dipole orientated excitons via externally applied electrostatic potentials is also studied. The drift-diffusion equation is adapted to include the inplane electric field. This is done for some specific forms of the potential landscapes such as a linear potential energy gradient and a propagating lattice. These correspond to some recent experiments for which results are available. The combined theoretical and experimental studies reveal a deeper insight into the transport properties of indirect excitons. Finally, the external ring structure in the indirect exciton emission pattern is studied. Its formation is modeled using a set of coupled transport equations for electrons, holes and indirect excitons. The Coulomb interactions between all three species are incorporated in the model. It is shown that these interactions lead to an instability in the external ring and are responsible for its fragmentation into a periodic array of islands which has been observed experimentally.
Author: Sujaul Chowdhury Publisher: Grin Publishing ISBN: 9783668526051 Category : Languages : en Pages : 12
Book Description
Research Paper (postgraduate) from the year 2017 in the subject Physics - Quantum Physics, grade: 4.00, Shahjalal University of Science and Technology, course: Nanostructure Physics, language: English, abstract: We have investigated electron transmission through non-tunneling regime of a semiconductor nanostructure called Coupled Quantum Well (CQW). Oscillatory transmission coefficient as a function of energy is found to show spectacular waxing and waning in amplitude. Corresponding features are expected to be observed in optical (Physical Review B 54 (1996) 1541) and quantum transport (Physical Review Letters 58 (1987) 816) experiments and hence the results besides verifying the Physics will have impact on future devices based on CQW.
Author: Alexander Graham Winbow
Author: Alexander Graham Winbow
Author: Chih-Wei Eddy Lai
Author: Aaron Tynes Hammack
Author: Paul Harrison
Author: Sang Yup Kim
Author: Smadar Ben-Tabou de-Leon
Author: Yuliya Yevgenyevna Kuznetsova
Author: 
Author: Joe Wilkes
Author: Sujaul Chowdhury