High-Resolution Infrared Spectroscopy of CO2-Kr and water-CO2 Complexes in the 3.5-4.3 μm Region &amp; Finite Difference Time Domain Analysis of Grating Coupled Silicon-on-Sapphire Microring Resonators
Creators: name: Gartner, Travis
Publisher: name: Graduate Studies
Year: 2023
Subjects: subject: Spectroscopy
subject: Photonics
subject: carbon dioxide
subject: water
subject: krypton
subject: mid-infrared spectroscopy
subject: Education--Sciences

description: This thesis is comprised of two parts, the first being a spectroscopic study of the CO2-Kr complex in the 4.3 μm region with consideration for isotopic splitting and broadening alongside a spectroscopic study of water-CO2 complexes in the 3.5-4.3 μm region with consideration for tunnelling splitting due to internal rotation. The second part comprises a finite difference time domain study of grating coupled silicon-on-sapphire microring resonators for use at 4.28 μm. The infrared spectrum of the CO2-Kr complex is studied in the region of the carbon dioxide ν3 fundamental vibration (~2350 cm-1). Tunable IR radiation from an OPO is employed to record absorption spectrum of the complex generated in a pulsed supersonic slit jet. The spectrum exhibits broadening and splitting of transitions due to mass dependence of the five Kr isotopes with natural abundances exceeding 2%. Good simulation of the spectrum is achieved by scaling the vibrational and rotational parameters. This scaling model is particularly important for the combination band involving the intermolecular bending mode where many isotope splittings are observed. As with other CO2-rare gas complexes, we also observe weak hot bands of CO2-Kr transitions corresponding to the hot band originating in the CO2 intramolecular bend. From these we determine a splitting of 1.418 cm-1 between the in-plane and out-of-plane bend of CO2, due to the neighboring Kr atom. Spectra of water - CO2 dimers are also studied using a tunable mid-infrared source to probe a pulsed slit jet supersonic expansion. H2O-CO2 and D2O-CO2 are observed in the CO2 3 fundamental region (~2350 cm-1), D2O-CO2 is also observed in the D2O 3 fundamental region (~2790 cm-1), and HDO-CO2 is observed in the HDO O-D stretch fundamental region (~2720 cm-1), all for the first time in these regions. Analysis of the spectra yields excited state rotational ii parameters and vibrational shifts. Most importantly, they also yield the first experimental values of the ground state internal rotation tunneling splittings for D2O-CO2 (0.003 cm-1) and HDO-CO2 (0.0234 cm-1). The latter value is a direct determination made possible by the reduced symmetry of HDO-CO2. These results provide stringent and easily interpreted tests for theoretical water - CO2 potential energy surface calculations. A finite difference time domain (FDTD) study of silicon-on-sapphire (SoS) ring resonators is conducted. In this study a modal analysis is conducted in COMSOL Multiphysics which resolves the effective mode distribution of varying geometries of air clad SoS waveguides. This modal analysis comprises the calculation of the effective mode index of the waveguides, group index, evanescent field ratio, and interaction parameter in consideration for the waveguide’s suitability as a trace gas sensor for carbon dioxide. Three-dimensional ring resonator structures are simulated in COMSOL and the resulting resonance spectrum is presented with quality factors as high as ~51,000. The ring resonator is deigned to overlap CO2 absorptions near the 4275 nm mark. Fully etched grating couplers on the SoS wafer platform are designed and optimized in the Ansys Lumerical software suite by apodizing both the period and fill factor of the gratings; optimization of the structure is carried out by means of a python-based minimization gradient algorithm. The resulting structures have a theoretical coupling efficiency of ~40% for vertically incident free space light. A mask layout is generated by means of a python based script which places grating optimized structures from Lumerical onto a 9mm x 9mm chip area. An initial manufacturing run was conducted by Applied Nanotools and the resulting chip is currently undergoing laboratory testing and characterization efforts.