Investigating exciton correlations using coherent multidimensional optical spectroscopy

Abstract

The optical measurements described in this thesis reveal interactions among bound electron-hole pairs known as excitons in a semiconductor nanostructure. Excitons are quasiparticles that can form when light is absorbed by a semiconductor. Exciton interactions gained prominence in the 1980s when unexpected signals were observed in studies of carrier dynamics. The presence of exciton interactions in semiconductors motivated an ongoing, focused research effort not only because the materials had valuable commercial applications but also because the interactions could be used to test fundamental theories of many-body physics. Laser light provides a coherent electric field with a well defined phase. In linear spectroscopy, an electric field that is resonant with an exciton transition will induce coherent oscillations of electronic charge density. The charges will oscillate at the transition frequency with a well defined phase, and these oscillations will radiate a signal that has an amplitude proportional to the incident field amplitude and has the same direction as the incident light. If the laser light is intense, its field may induce a high density of excitons, and the field can interact with those excitons to induce transitions to higher-energy states composed of multiple interacting excitons. Many-body interactions among the excitons can predictably modify--or unpredictably scramble--the quantum phase of the exciton. The interactions can produce signals that have amplitudes proportional to high powers of the incident field amplitude, and the signal fields often propagate in directions different than the incident field. The signal fields contain information--often encoded in their phases--that can reveal the nature of the higher-energy states and the many-body interactions that produced them. Thus, many-body interaction studies rely on measurements of exciton phases that are reflected in the optical phases of coherent signals. These measurements require a tool that can detect optical coherence before the exciton phases are scrambled by the environment. Coherent ultrafast optical spectroscopy is that tool. The spectra displayed in this work were measured by an experimental apparatus that separates the electric fields as needed into different laser beams with controllable directions; it controls the optical phase, arrival time, and polarization of the femtosecond light pulse(s) in each of those beams; it then recombines all of the beams at the 5 sample to generate the signal field; and finally it measures the signal field, including its phase. Using this instrument, we isolated--with a high degree of selectivity--signals that arose from different numbers of field interactions and from different microscopic origins using various beam geometries and pulse timing sequences. In this thesis, we present electronic spectra measured at varying orders in the electric field to isolate and measure the properties of excitons and their many-body interactions. As the number of electric fields is increased and the resulting higherorder signals are generated, interactions involving increasing numbers of particles can be measured. The vast majority of previous work focused on the interactions manifest in third-order signals. This thesis not only includes new insights gained from third-order signals, but also includes new phenomena observed in fifth-order and seventh-order signals. We measure signals due to four-particle correlations in the form of bound biexcitons and unbound-but-correlated exciton pairs. We also measure signals due to six-particle correlations in the form of bound triexcitons. Although we searched for them, there were no signals due to eight-particle correlations, indicating that the set of multiexciton states truncates. We thus measured the properties and the extent of many-body interactions in this system. The spectra presented here reveal a large set of excitonic many-body interactions in GaAs quantum wells and answer questions about the many-body interactions posed decades ago. The optical apparatus constructed to perform these measurements will soon be used to measure correlations in a range of systems, including other semiconductors and their nanostructures, molecular aggregates, molecules, and photosynthetic complexes. Because future technologies such as entangled photon sources, advanced photovoltaics, and quantum information processing will rely on these types of materials and their many-body correlations, it is important to develop techniques to measure their microscopic interactions directly.