Simultaneous Collection of Resonance Raman and Fluorescent Signatures using a 405 nm Excitation Source

Alberto Ruiz graduated from Harvey Mudd College in 2014 with a degree in Physics and is now working at ThorLabs. For his senior thesis, he explored the temperature dependence of laser emission wavelength and its utility in Shifted Excitation Raman Difference Spectroscopy (SERDS). Ruiz built an instrument that can tune its excitation wavelength to extract Raman signals from highly fluorescent samples.

His instrument collects two raw spectra of a sample, the first with the laser diode tuned, for example, to ~405.1 nm, and the second with the laser tuned to ~405. 2 nm. The Raman spectral signal shifts precisely with the shift in excitation while the fluorescence signal does not. Subtracting the two spectra produces a biphasic Raman signal and eliminates the fluorescence response. His system used a thermoelectrically cooled GlacierX spectrometer provided by Edmonds Optics using a 10µm slit and a 1800/500 diffraction grating.

Ruiz’s implementation of the Raman signal collection system.

The Raman system successfully generated Raman signatures for water, carbon tetrachloride (CCl4), benzene, and calcite even though it suffered from laser bleed-through because it lacked a shaping filter.

The water Raman signature collected by Ruiz (left) and presented in literature (right).
The CCl4 spectra collected by Ruiz (left) and presented in literature (right).
The Benzene Raman signal collected by Ruiz (left) and presented in literature (right).

Excitation sources close to the absorption bands of a sample can achieve a state known as resonance to increase the Raman signal output by several orders of magnitude. Unfortunately, this also increases the fluorescence signal. Shifted Excitation Raman Difference Spectroscopy (SERDS) is one technique that efficiently separates Raman signal from fluorescence, making the peaks sharper and the data easier to analyze. Ruiz’s device was able to collect these signals 100% more efficiently than previous works presented in literature.