Non-collinear optical parametric oscillator for stimulated Raman spectroscopy in real-time

Stimulated Raman spectroscopy is a powerful chemical analysis technique that provides information about the investigated material composition by selectively stimulating a molecular vibrational transition. Like a fingerprint, the Raman spectra of various molecules differ. In contrast to spontaneous Raman spectroscopy, which is based on inelastic scattering with low efficiency, stimulated Raman spectroscopy is a coherent process. The simultaneous irradiation of two laser beams under the condition that their difference frequency is exactly the same as the transition frequency of the molecule excites the vibrational state with high probability. The intensity change of the input radiation provides information about the strength of the Raman transition. To interrogate broad spectral regions, a light source with a tunable wavelength is required. In this dissertation, a non-collinear optical parametric oscillator (NOPO) is evaluated for stimulated Raman spectroscopy. The light source is characterized by its particularly fast and narrowband tuning behavior. It allows real-time recording of Raman spectra over a large wavenumber range of more than 2000 cm-1 in 8 ms. In this work, the required phase matching conditions in the nonlinear crystal, as well as the influence of the dispersion on the output spectra, are discussed in detail. Furthermore, the NOPO in combination with stimulated Raman spectroscopy allows real-time observation of mixtures of substances. For instance, the change of a water-alcohol mixture is monitored live. Furthermore, microplastic particles are detected. The tiny plastic particles are ubiquitous in our modern society. Nevertheless, there is a lack of reliable and consistent detection methods. The NOPO proves to be a promising light source to analyze samples for microplastics in real-time without extensive preparation. In this work, along with macroscopic plastic samples, moving 50 – 160 µm sized polyamide particles were detected directly in a water-filled cuvette. ; Die stimulierte Ramanspektroskopie ...