Hyperspectral imaging with in-line interferometric femtosecond stimulated Raman scattering spectroscopy

Abstract

We present the hyperspectral imaging capabilities of in-line interferometric femtosecond stimulated Raman scattering. The beneficial features of this method, namely, the improved signal-to-background ratio compared to other applicable broadband stimulated Raman scattering methods and the simple experimental implementation, allow for a rather fast acquisition of three-dimensional raster-scanned hyperspectral data-sets, which is shown for PMMA beads and a lipid droplet in water as a demonstration. A subsequent application of a principle component analysis displays the chemical selectivity of the method.

Femtosecond Stimulated Raman Spectroscopy

“…This drawback can be improved by chopping only the actinic pump at 500 Hz (with Raman pump present), which results in an excited‐state difference spectrum being acquired using 100 % of the measurement time . For high‐repetition‐rate laser systems with low pulse energies, the choppers could also be replaced by acousto‐ or electro‐optic modulators . In any case, the choppers should be synchronized to the laser repetition rate to minimize artifacts caused by phase walk‐off.…”

“…However, it relies on a linear pump beam polarization, which inevitably leads to a baseline due to non‐resonant cross‐phase modulation with the probe. Therefore, II‐FSRS ranks in between FSRS and FRIKES by providing a good balance of acquisition speed and signal‐to‐noise ratio, which makes it an ideal tool for hyperspectral Raman imaging applications …”

“…The short Rayleigh length limits the nonlinear interaction to a small volume. On the one hand, this restricts the usable sample length to a few tens of microns, but on the other hand, provides the possibility of 3D confocal Raman imaging . Further improvements in spatial resolution are possible by careful experimental design.…”

“…who use a 200‐kHz Yb‐fiber laser, Gilch and co‐workers who use a 75‐MHz Ti:Sa oscillator and Yb‐fiber amplifier combination, Beier et al . who use an 80‐MHz Ti:Sa oscillator, Dobner et al . who use a homebuilt 1‐MHz Yb‐fiber amplifier, and the ULTRA instrument operating at 10 kHz, respectively.…”

“…Therefore, the most common approach is direct supercontinuum generation in transparent dielectrics based on the Kerr effect. Examples include solids like sapphire,, CaF2 and YAG crystals for Yb‐based laser systems, as well as liquids in flow cells, like water and CCl 4 . Liquids, in principle, allow higher fluences, but solids are much easier to use and produce more stable continua.…”