Theses and Dissertations at Montana State University (MSU)
Permanent URI for this collectionhttps://scholarworks.montana.edu/handle/1/733
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Item Thermalization and exciton localization in 2D semiconductors(Montana State University - Bozeman, College of Letters & Science, 2023) Strasbourg, Matthew Christopher; Chairperson, Graduate Committee: Nick Borys; This is a manuscript style paper that includes co-authored chapters.2D semiconductors are a promising class of materials to investigate for applications in the next generation of photonic devices. They can be used to generate quantum light and also exhibit correlated many-body phenomena. Many of the novel optoelectronic properties of 2D semiconductors are associated with strongly-bound hydrogen-like states known as excitons. Excitons in 2D semiconductors have binding energies on the order of 100s of meV and are stable at room temperature. At low temperatures, higher-order excitonic states such as charged excitons and biexcitons--multiple-bound excitons that are like hydrogen molecules-- and localized excitons that emit quantum light are also observed. Whether excited optically or electronically, a diversity of high-energy excitons and free carriers are produced directly after excitation. The relaxation and thermalization of these initial states influence the formation of excitons, biexcitons, and localized excitons. Here, I present work that (i) investigates the thermalization of excited states in a prototypical 2D semiconductor, monolayer (1L-) WSe2, and reports the discovery that the generation of charged biexcitons is enhanced with increasing photoexcitation energy, (ii) shows the emergence of quantum emitters (QEs) in a new 2D QE platform: 1L-WSe2 nanowrinkle arrays induced by Au nano stressors, and (iii) uses a novel method to classify the excited-state dynamics of 2D QEs and differentiate emitter populations. A suite of low-temperature energy- and time- resolved optical spectroscopies are used to conduct this work. This work shows how excited state thermalization affects the formation of exciton and biexcitons and investigates the optical properties of an emergent class of 2D quantum light emitters.Item The development of superresolution spectroscopic techniques and characterization of microscale exciton diffusion in organic semiconducting polymers(Montana State University - Bozeman, College of Letters & Science, 2018) Massaro, Eric Stephen; Chairperson, Graduate Committee: Erik Grumstrup; Andrew H. Hill and Erik M. Grumstrup were co-authors of the article, 'Superresolution structured pump-probe microscopy' in the journal 'ACS Photonics' which is contained within this thesis.; Andrew H. Hill, Casey L. Kennedy and Erik M. Grumstrup were co-authors of the article, 'Imaging theory of structured pump-probe microscopy' in the journal 'Optics Express' which is contained within this thesis.; Erik M. Grumstrup was a co-author of the article, 'Label-free saturated structured excitation microscopy' in the journal 'Photonics' which is contained within this thesis.; Erik M. Grumstrup was a co-author of the article, 'Exceptionally fast nanoscale exciton diffusion in donor-acceptor polymer thin films' which is contained within this thesis.; Erik M. Grumstrup was a co-author of the article, 'Toward direct imaging of sub-10 nm carrier diffusion lengths by differential detection pump-probe microscopy' which is contained within this thesis.Disordered semiconducting materials offer cost effective, solution processable alternatives to highly crystalline semiconducting materials for utilization in a variety of optoelectronic devices. However, characterization of these complex materials systems using bulk spectroscopic methods is heavily influenced by chemical and morphological heterogeneity inherent to the material. The experiments described in this thesis are designed to improve the fundamental understanding of the photophysical processes in disordered solution processed semiconducting materials by developing and utilizing high spatial resolution spectroscopic methods. Chapters 2-4 will outline the experimental and theoretical development of two superresolution spectroscopic techniques. First (chapters 2 & 3), structured pump-probe microscopy (SPPM) utilizes a structured excitation profile along with a diffraction limited probe pulse to achieve ~100 nm spatial resolution. Using SPPM it is also possible to collect time resolved spectroscopic data from a sub-diffraction limited volume. Second (chapter 4), label-free saturated structured excitation microscopy (LF-SSEM) is theoretically developed. LF-SSEM is experimentally similar to SPPM but exploits the saturation of the absorption process to achieve even greater resolution enhancement. Here, simulated LF-SSEM is shown to achieve ~33 nm spatial resolution. Chapter 5 demonstrates the utilization of PPM to investigate exciton transport in the organic semiconducting polymer (OSP), poly [N-9''-hepta-decanyl-2,7-carbazole-alt-5,5-(4',7'-di-2-thienyl-2',1',3'-benzothiadiazole)] (PCDTBT). Although OSPs have shown great promise for use in a variety of optoelectronic applications, much remains un-known about their excited state dynamics. The data reported here represents a significant contribution to the rapidly growing wealth of knowledge pertaining to OSP systems. Specifically, the microscale exciton diffusivity observed in PCDTBT thin films using PPM is found to reach 3.2 cm 2/s. Chapter 6 examines a technique in the early stages of development and optimization that is able to detect excited state carrier diffusion with increased sensitivity and accuracy compared to PPM. Differential detection pump-probe microscopy (DDPPM) uses two probe pulses to selectively eliminate the signal of carriers that have not diffused beyond the boundaries of the initial excitation. The experiments described within this dissertation are diverse, yet the common goal is to increase and improve the knowledge of photophysical properties in disordered semi-conducting materials. This goal takes two forms in the development of novel spectroscopic methodology and the characterization of complex materials using PPM. The singular result is the advancement of basic science pertaining to complex semiconducting materials systems.Item Theoretical analysis and experimental design of dual-beam optical trap for large particles(Montana State University - Bozeman, College of Letters & Science, 2018) St. John, Demi Rose; Chairperson, Graduate Committee: Wm. Randall BabbittUltra-high sensitivity acceleration and gryometric sensors have been proposed as optically levitated particles in ultra-high vacuum (UHV). Larger particles (10- 30 microns in diameter) provide higher sensitivity, but they are difficult to trap in UHV without particle loss. To overcome the radiometric forces that lead to particle loss, rare earth (RE) ion dopants can be incorporated into the particles to enable solid-state laser cooling of the particles internal temperature. This thesis theoretically and experimentally explores development of optical traps designed for trapping and internally laser cooling large particles. The analysis focuses on dual-beam horizontal traps and the development of code to analyze dual-beam trap potentials and particle loading dynamics. Tolerance analysis, improved particle loader designs, and monitoring and automation of the loading process are investigated. The thesis provides a road map for achieving efficient optical trapping, cooling, and control of large particles.Item Demonstration of normalized differential detection using smart pixels with smart illumination(Montana State University - Bozeman, College of Letters & Science, 2000) Chen, Xiaofang