The Study of Exciton-Polariton Phase Transitions Through Spontaneous Vortices and First-Order Correlation

Phase transitions are among the most fascinating phenomena discovered in the last century, with collective molecular dynamics leading to novel observations such as superconductivity and superfluidity. Yet even in the highly-ordered state of such systems, defects could still form and persist. Investigating the behavior of such defects is not just a matter of fundamental interest, but also valuable in applications where macroscopic order is important. Our studies of order and defects make use of an exciton-polariton (polaritons hereafter) system. These are quasi-particles with part-matter and part-light components, with a bosonic nature that allows for observation of macroscopic quantum phenomena typically seen in conventional atomic condensates. Their low effective mass allows for such observations at relatively high temperatures (10K - 300K), and the photonic component allows for experimental access using standard table-top optical techniques. These factors have generated much interest in the field of polaritonics over the past two decades. In a two-dimensional system like the polaritons, defects appear in phase transitions as spontaneously-formed vortices. While such vortices have been observed, their reported behaviors were influenced by sample properties or pump configurations. Formation and behaviors in which phase-transition mechanisms and polariton hydrodynamics are primary contributing factors have yet to be observed. Such observations would allow for comparisons with analogous results in atomic condensates and allow for deeper studies in universality. In this thesis, I present efforts taken towards the realization of such observations. I will propose the design of a Compact Mirroring Mach-Zehnder interferometer (CoMMZI) for the detection of photonic orbital angular momentum (OAM) states. The observation of such states would be an unequivocal indication of spontaneously-formed quantum vortices. I will demonstrate that the proposed interferometer is capable of detecting OAM states with a low number of photons, thereby making it suitable for the detection of moving vortices in a single-shot realization of a polariton condensate. I will then discuss how the interferometer was tested with vortex states formed with a spatial light modulator and continuous-wave lasers. I will show that the interferometer is capable of detecting vortex phases and present techniques to increase the chances of successful detection. I will also show preliminary experiments with a polariton condensate formed with non-resonant pump configurations. OAM states within an optically-induced ring trap have been detected. Finally, I will show spectrometric and temporal first-order correlation functions for polaritons within an optically-induced ring trap, a potential system for the observation of vortices. I will show the presence of three population fractions with coherence times spanning three orders of magnitude and briefly discuss possible implications for vortex-detection experiments. The efforts in this thesis demonstrate the possibilities and challenges associated with the detection of spontaneously-formed vortices within a single instance of a polariton condensate, providing insight for attempts at experimental realizations.PHDPhysicsUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/163142/1/lhofai_1.pd

Ambipolar Charge Photogeneration and Transfer at GaAs/P3HT Heterointerfaces

Recent work on hybrid photovoltaic systems based on conjugated polymers and III–V compound semiconductors with relatively high power conversion efficiency revived fundamental questions regarding the nature of charge separation and transfer at the interface between organic and inorganic semiconductors with different degrees of delocalization. In this work, we studied photoinduced charge generation and interfacial transfer dynamics in a prototypical photovoltaic <i>n-</i>type GaAs (111)B and poly­(3-hexyl-thiophene) (P3HT) bilayer system. Ultrafast spectroscopy and density functional theory calculations indicate the coexistence of electron and hole transfer at the GaAs/P3HT interface, leading to the generation of long-lived species and photoinduced absorption upon creation of hybrid interfacial states. This opens up new avenues for the use of low-dimensional III–V compounds (e.g., nanowires or quantum dots) in hybrid organic/inorganic photovoltaics, where advanced bandgap and density of states engineering may also be exploited as design parameters