In this thesis the strong coupling of excitons with intense THz radiation in GaAs/AlGaAs and InGaAs/GaAs multi-quantum wells (MQW) and the strong coupling of electrons to phonons in InAs/GaAs quantum dots (QD) are investigated. Experimental studies in the field of non-linear terahertz (THz) spectroscopy were carried out using the narrowband THz emission of a free-electron laser (FEL). In the first part intra-excitonic transitions are pumped with intense THz radiation.
The THz-pump–near-infrared(NIR)-probe experiments are analysed focusing on the behaviour of the Autler-Townes (AT) splittings with increasing THz field strength. Furthermore measurements of the temperature dependence up to room temperature are discussed. With the help of a microscopic theory the contribution of higher lying intra-excitonic states to the lineshape and splitting of the heavy-hole absorption line is analysed at low temperatures. The second part is about the lifetime and dephasing time of polarons in InAs/GaAs QDs that was measured for inter-sublevel excitation in the THz spectral region (below the Reststrahlen band). Single electrons inside QDs strongly interact with phonons and form quasi-particles called polarons. The temperature dependence of the dephasing behavior and the contribution of pure dephasing is discussed.:1 Introduction 1
2 Theoretical background 5
2.1 Semiconductor quantum structures 5
2.2 Selection rules for optical excitation 8
2.3 Linewidth of an optical transition 10
2.4 Excitons in a quantum well 10
2.4.1 Excitonic linewidth 13
2.4.2 The concept of exciton-polariton and exciton formation 15
2.5 Electron-phonon interaction in a quantum dot 18
2.5.1 Phonons in GaAs 18
2.5.2 Interaction of electrons with phonons 20
2.5.3 The phonon bottleneck 22
2.5.4 Anharmonicity and LO phonon disintegration 23
2.6 Light-matter interaction 27
2.6.1 The two-level system 27
2.6.2 Autler-Townes splitting 30
2.6.3 Optical Bloch Equations 33
2.6.4 Bloch sphere and photon echo 36
3 Experimental Methods 41
3.1 The Dresden free-electron laser FELBE 41
3.2 THz pump - NIR probe setup 43
3.3 QW samples 47
3.4 THz pump-probe and four-wave mixing setup 50
3.5 Thermally annealed QD samples 50
4 Intra-excitonic Autler-Townes effect in quantum wells 55
4.1 Experimental data 56
4.1.1 Resonant and detuned intra-excitonic excitation 56
4.1.2 Anti-crossing behavior 59
4.1.3 Temperature dependence . 60
4.2 Comparison of experimental data with microscopic theory 63
4.2.1 Exciton linewidth 64
4.2.2 Anti-crossing from microscopic theory 69
4.2.3 Rabi oscillations and polarization redistribution 69
4.3 Summary 72
5 Inter-sublevel coherence in InAs/GaAs quantum dots 73
5.1 Strong electron-phonon coupling 74
5.2 Dephasing above the Restrahlen band 77
5.3 Pump-probe and transient four-wave-mixing measurements below the
Reststrahlenband 80
5.4 Temperature dependence 80
5.5 Summary 84
6 Appendix 85
6.1 Appendix A - Autler-Townes splitting calculated in a three-level configuration 85
6.2 Appendix B - Microscopic theory 88
6.3 Appendix C - Coherent oscillations in the THz pump-probe signal 92
Bibliography 94
| Identifer | oai:union.ndltd.org:DRESDEN/oai:qucosa:de:qucosa:28344 |
| Date | 26 September 2014 |
| Creators | Teich, Martin |
| Contributors | Helm, Manfred, Dekorsy, Thomas, Technische Universität Dresden |
| Source Sets | Hochschulschriftenserver (HSSS) der SLUB Dresden |
| Language | English |
| Detected Language | English |
| Type | doc-type:doctoralThesis, info:eu-repo/semantics/doctoralThesis, doc-type:Text |
| Rights | info:eu-repo/semantics/openAccess |
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