2025 Theses Doctoral

Stimulated Raman Scattering Microscopy: Theory and Applications in Nano Imaging

Gao, Xin

Stimulated Raman scattering (SRS) microscopy is an emerging chemical imaging modality that has gained tremendous attention in biomedical and material science due to its fast label-free imaging capabilities. Despite significant achievements, the field of SRS imaging is largely driving empirically, leaving many fundamental questions unanswered or controversial. In particular, there has been disagreement regarding the enhancement factor and detectability when compared to spontaneous Raman scattering.

In this thesis, an alternative framework is presented to quantitatively understand SRS microscopy. Starting from a phenomenologically-defined stimulated Raman cross section (𝛔_SRS), the intrinsically molecular Raman response is revealed. Unlike the traditional spontaneous Raman cross section 𝛔_Raman, 𝛔_SRS turns out to be strong and even exceeding the electronic counterparts. 𝛔_SRS is then connected with ?Raman through both a heuristic method and the full quantum electrodynamics derivation, which encompass both phenomena quantitatively in the same framework.

This new theory reveals a previously-unknown duality nature of Raman scattering, where both 𝛔_Raman and 𝛔_SRS can exhibit vastly different magnitudes for the same molecule, connected by the influence of vacuum zero-point fluctuations. This allows for the direct prediction of signal-to-noise-ratios (SNRs), vibrational population saturation, and photothermal effects. A mathematical model is built to discuss the fundamental detectability of both Raman techniques, which shows that SRS microscopy is almost always more sensitive than regular Raman microscopy. A diagrammatic approach reveals that SRS excels in high spatiotemporal regimes, explaining its advantage for microscopy applications.

Next, I will use the new theoretical frame to demonstrate the superiority of SRS microscopy in nano imaging, and apply the technique in nanoparticles. In particular, three examples will be presented, including solid lipid nanoparticles (Chapter 3), poly-lactic-glycolic-acid (PLGA) nanoparticles (Chapter 4), and polystyrene nanoparticles as an example of nanoplastics (Chapter 5). The imaging of these three types of nanoparticles points towards a strategy called generalized bio-orthogonal imaging, which fully utilizes the rich chemical information contained in Raman spectra for biomedical research.