Improvements in Optical Spectroscopy: Rotationally Asymmetric Multipass Cells and Progress toward Optically Pumped Deep-UV Semiconductor Lasers

Material detection has widespread applications across many fields including industrial sensing, medical diagnostics, atmospheric and fundamental studies, and security and defense. Many different detection methods have been developed to service these applications, each with its own benefits. Optical detection methods are particularly useful because they are nondestructive, non-contact solutions, and can be used for standoff detection.

In this work we present our work towards increasing the sensitivity of optical detection methods. We address two approaches: optical absorption spectroscopy and Raman spectroscopy. To improve absorption spectroscopy, we aim to increase the attenuation of light by extending the optical path length through an absorbing material using an optical multipass cell, directly increasing the sensitivity. To improve sensitivity in Raman spectroscopy, our work focuses on the development of semiconductor lasers operating in the deep-ultraviolet (DUV) spectral region. This will allow Raman spectroscopy to take advantage of the lambda^-4 dependence of Raman scattering intensity to greatly increase the signal to noise ratio in Raman spectroscopy.

Our work on absorption spectroscopy focuses on improving the process for identifying long optical trajectories in rotationally asymmetric multipass cells (RACs). We simulate optical trajectories in RACs using a MATLAB script which models the shape and size of a beam propagating in the cell. An optical trajectory with a path length of 18 m is presented which maintains a beam small enough to exit the cell through a 3.8 mm^2 hole after 359 reflections. This trajectory is contained within a 68 cm^3 volume, yielding a path length to volume ratio of 26.6 cm^-2, higher than that of other state-of-the-art multipass cells such as the astigmatic Herriott cell (9 cm^-2). We also show that the stability of these cells is comparable to that of other state-of-the-art cells, as the solid angle of acceptance for trajectories of equal number of reflections is twice as large for an RAC as for an astigmatic Herriott cell.

We discuss the development, fabrication, and testing of optically-pumped DUV semiconductor laser structures. Several waveguide designs are simulated which maximize the optical confinement factor for emission around 270~nm while minimizing optical mode overlap with lossy regions. A waveguide based on a thin GaN/AlN quantum well active region is presented which has a confinement of 7.45%, and a waveguide using an AlGaN/AlN active region is simulated with a confinement factor of 27.3%. Some of these designs are grown by molecular beam epitaxy, and the optical gain of these structures is measured by the variable stripe length method. Gain measurement results are presented for ten samples with varying structures, and net modal gain values up to 35 cm^-1 are measured, which is sufficient for lasing when combined with high quality facets. We produce high quality facets through a combination of a dry etch process and focused ion beam milling. We discuss gain saturation in III-Nitride laser waveguides and its effect on lasing threshold. Finally, we measure and analyze the internal quantum efficiency of thin GaN quantum wells and the dependence of this efficiency on quantum barrier width.