• Refine Query
  • Source
  • Publication year
  • to
  • Language
  • 1
  • Tagged with
  • 1
  • 1
  • 1
  • 1
  • 1
  • 1
  • 1
  • 1
  • 1
  • 1
  • 1
  • 1
  • 1
  • About
  • The Global ETD Search service is a free service for researchers to find electronic theses and dissertations. This service is provided by the Networked Digital Library of Theses and Dissertations.
    Our metadata is collected from universities around the world. If you manage a university/consortium/country archive and want to be added, details can be found on the NDLTD website.
1

Non-linear THz spectroscopy in semiconductor quantum structures

Teich, Martin 26 September 2014 (has links)
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

Page generated in 0.0885 seconds