Investigation of Electronic Quantum Coherence in Semiconductor Materials using Time-resolved Non-Linear Optical Microscopy at Nanoscale Level

Abstract

Electronic coherence has a significant impact on the properties of semiconductor materials, with applications ranging from photovoltaic devices to quantum computing. In this context, the word coherence is related to electronic states wavefunctions, and it describes the degree of synchronization between electronic states wavefunctions. Upon photoexcitation, the coherence time reflects the timescale during which the ground and excited electronic states evolve with a fixed phase relationship. The goal of this thesis is to study the microscopic characteristics and ultrafast timescale of the coherent excited state dynamics in different type of semiconductor materials ranging from organic macromolecules to inorganic quantum dots to reveal the role of coherence in their electronic performance. However, probing the exciton state coherent dynamics of these materials is not a straightforward task owing to the ultrafast decay and short exciton transport length. In this dissertation work, sophisticated spectroscopy techniques were used to investigate these phenomena. Time-resolved near-field scanning optical microscopy (NSOM) has been implemented to investigate coherence in the solid phase, while three pulse photon echo spectroscopy method has been utilized to probe coherences in solution. In this thesis, particular attention will be given to the time-resolved NSOM which combines three different optical features: a near-field optical excitation, a two-photon induced emission detection, and a pulse pair phase-locked time-resolved capability. In this work, organic macromolecules for photovoltaic applications have been investigated in solution phase and in the solid phase. Interestingly, a more efficient coherent charge transport dynamics have been found for oligothiophene dendritic macromolecules in the solid phase compared to the analogous dendrimer macromolecules in solution phase. In this study, high-resolution photo-echo spectroscopy and interferometric NSOM techniques have been compared to investigate the ultrafast coherent dynamics. Differences in the excited state coherent dynamics of organic macromolecules have been found in the solid phase compared to the solution phase. Interestingly, the dendritic structure has shown higher charge delocalization in the solid phase due to stronger intra- and inter-molecular couplings. These findings could be used to improve the potential of this class of organic macromolecules for light harvesting applications. In addition, time-resolved NSOM was used to investigate coherent dynamics of semiconductor quantum dots at a single particle level. Single-particle excitation approach implemented in this work allows us to take out the effects of heterogeneity that are usually present in ensemble averaging measurements. Interestingly, the electronic coherence time of a single perovskite quantum dot (PQD) has been measured, showing a relatively longer lifetime at room temperature compared to other non-perovskite nanoparticles. Furthermore, the electronic coherent wavepacket in PQDs was found to be sensitive to both halide elements and cations (organic vs. inorganic). These differences are likely due to the weaker interaction between the organic cation and the inorganic moiety occurring in hybrid organic-inorganic PQDs. Additionally, PQDs have showed a significantly high two-photon absorption (TPA) cross section. This promising TPA response together with the extremely high photoluminescence quantum yield and long coherence time make these systems favorable for applications as light emitting diodes, solar cell devices and solid-state quantum computers. These interesting findings not only gives a better understanding of the coherent electronic properties in PQDs, but they also suggest the possibility of manipulating exciton coherence times by changing PQD-composition, which will be important in the design of highly efficient single photon emitters for quantum optical technologies and information processing applications.