Doctor of Philosophy (Ph.D.)
Degree Granting Department
Andreas Muller, Ph.D.
Myung Kim, Ph.D.
Dmitri Voronine, Ph.D.
Theresa Evans-Nguyen, Ph.D.
Chemical identification, External cavity diode laser, Fabry-Perot microcavities, Purcell effect, Raman spectroscopy
The continual increase in production and use of chemicals in an ever-growing field of applications naturally brings forth the necessity to accurately and efficiently measure molecular composition. Spontaneous Raman scattering is a reliable technique which can optically identify molecules based on their intrinsic rotational-vibrational energy structure. The Raman emission from a substance can be spectrally analyzed to detect molecular species simultaneously and with isotopic sensitivity using a single laser source. However, even though the process is non-invasive and effective, the rate at which the emission occurs is notoriously low due to a weak scattering cross-section. Therefore, research into the development of novel methods to enhance Raman emissions is a critical component for practical applications intrace gas detection.
The work presented here focuses on the investigation of Raman enhancement techniques that have shown promise for potential applications in optical trace gas sensing. Fundamental properties were characterized and studied in regards to their ability to further benefit miniaturization, versatility, simplicity and overall Raman enhancement. Microscopic optical cavities with high finesse were implemented to combine resonant pump recirculation with the Purcell spontaneous emission enhancement effect from cavity quantum electrodynamics. An improved compact cavity design (<1 cubic inch) was created with which gases could be introduced at pressures of up to 35 bar. The result is an approach based on ultra small sample volume for which Raman emission involving only several hundred thousand molecules was achieved. Pressurization was uniquely studied in the regime where cavity linewidth exceeds the Raman linewidth and thereby foregoes pressure broadening limitations. Additional characterization determined incident power limitations through optical bistability causing cyclical cavity length changes mediated by photothermal refraction and expansion nonlinear dynamics.
In an alternate effort aimed at bypassing the difficulties associated with the stringent resonance conditions arising in resonant optical cavities, multipass cavity configurations were explored. Multipass optical cavities also recirculate light and can create overlapped focal regions of high power, but without resonance conditions or the need for piezoelectric actuators, servo loops and other stabilization constraints of high finesse resonator operation. A cost effective multimode laser diode was operated with feedback which reduced the spectral bandwidth to ≈3.5 cm−1. A collinear Raman collection system was tested and found to provide novel capabilities such as the detection of ambient hydrogen in air (limit of detection ≈40 ppb H2). The limit of detection for ambient methane was found to be lower by several orders of magnitude compared to similar approaches and the enhancement through this optical system is such that spectral analysis can easily be done with an uncooled CMOS camera.
Scholar Commons Citation
Gomez Velez, Juan Sebastian, "Spontaneous Raman Scattering Enhancement with Microcavities and Multipass Resonators for Trace Gas Detection" (2020). Graduate Theses and Dissertations.