Diffuse reflectance-based femtosecond stimulated Raman spectroscopy of opaque systems

Raman spectroscopy is a non-invasive and non-destructive technique that obtains vibrational information of a system but is notorious for producing small signals. By measuring the vibrational spectrum of a system through femtosecond stimulated Raman spectroscopy (FSRS), we can measure the detailed ultrafast structural dynamics of the system upon photoexcitation. Additionally, stimulated Raman scattering (SRS) is known to greatly increase the magnitude of detected signal relative to traditional spontaneous Raman spectroscopy. However, typical FSRS experiments require transparent samples. To overcome this limitation, we adjusted the collection optics used in traditional FSRS experiments to collect a diffusely reflected probe pulse as it scattered off opaque, turbid samples. We collected the ground-state stimulated Raman spectra of 12.5% (v:v) ethanol in a 1% intralipid solution and cyclohexane-intercalated poly(tetrafluoroethylene) microbeads. Further, we characterized the SRS signal obtained by comparing the signal intensity and incident pump-probe polarization dependence of the data collected using both traditional FSRS and diffuse reflectance-based FSRS (drFSRS). While the initial drFSRS experiments focused on obtaining the ground-state Raman spectrum of several opaque systems, improvements to drFSRS could allow for the measurement of ultrafast structural dynamics of opaque systems.