Towards a Novel Optical Spectroscopy Technique Using Photon Absorption Remote Sensing

Abstract

Optical spectroscopy has shown great promise in the field of biomedical research. For example, works employing traditional spectroscopy approaches have demonstrated that analyzing a sample’s optical response to incoming light can effectively differentiate between healthy and diseased tissue. However, these techniques suffer from limitations due to the fact that they typically capture signals from only a single light-matter interaction type, such as absorption, scattering or fluorescence. Therefore, many traditional methods are constrained in terms of the types of samples they can feasibly analyze, as well as, potentially, the depth of their sample characterization, as they do not focus on capturing relevant information from other interaction modalities. This work employs photon absorption remote sensing (PARS) to overcome these limitations.

PARS is a novel all-optical imaging technique capable of capturing radiative and non-radiative relaxation processes following electronic photon absorption. This thesis explores the initial development of the first PARS system specifically designed and optimized for optical spectroscopy applications, aimed at studying wavelength-dependent relaxation processes to characterize a wide range of liquid samples.

The first step of this work was to build a non-radiative PARS spectroscopy system capable of accurately capturing the thermal and acoustic relaxation processes that arise from different ultra-violet (UV) excitation wavelengths. These signals were processed and used to construct a non-radiative PARS absorption spectrum for each sample of interest. These spectra were benchmarked against the absorption data collected from a NanoDrop spectrophotometer, which served as the ground truth in this work. This study revealed that for certain samples, such as eumelanin, which is highly absorbent to UV light and relaxes almost all absorbed energy non-radiatively, the non-radiative PARS spectroscopy system is capable of generating highly accurate absorption spectra. However, this system did not generate as close to ground truth spectra for samples that do not have as strong UV absorbing tendencies and are not as non-radiative in nature.

The second step of this work was to integrate a radiative relaxation arm into the developed non-radiative PARS spectroscopy system. This pathway was configured to collect fluorescence emission spectra, which represent radiative sample relaxation, simultaneously with the collected non-radiative data. Radiative PARS absorption spectra were generated for each sample. In this way, the developed PARS system combines absorption (monitoring both relaxation pathways) and fluorescence emission spectroscopy onto a single bench-top system. The radiative PARS absorption spectra were compared to the ground truth, which revealed that molecules that are highly fluorescent in nature are more appropriately studied through the radiative relaxation arm than the non-radiative pathway. Total absorption spectra, which combine the non-radiative and radiative absorption data, were also generated, and it was determined that the absorption profiles of certain samples, such as NADH, are best studied using this approach.

The final step of this work was to use the collected total absorption and fluorescence emission data from the PARS spectroscopy system to identify the composition of different mixtures of craft red and blue ink samples. Traditional linear and generalized bilinear models were employed to perform this unmixing and the results from this study indicate that the combination of the absorption and fluorescence data collected on this system allows for a more accurate identification of a mixture’s components than either data source individually. This suggests that the PARS spectroscopy system provides an increased level of detail in sample characterization compared single-modality spectroscopy systems.

Ultimately, this research lays the groundwork for the development of a PARS spectroscopy system capable of being deployed in clinical settings to study samples and help inform diagnoses. This work demonstrates the feasibility of leveraging PARS for optical spectroscopy and presents a system design and framework that can be further iterated upon to enhance performance and enable a robust characterization of relevant and complex biological samples.