Comparing Coherent and Spontaneous Raman Scattering Under Biological Imaging Conditions

Comparison of the sensitivity and image contrast in spontaneous Raman and coherent Stokes Raman scattering microscopy of geometry-controlled samples

“…The CSRS intensity is reduced by a factor of ∼2 when the two beams are temporally separated or when the pump beam is blocked. Notably, this result is similar to that reported by Cui et al 8 where the SpRS intensity was similar to the CSRS intensity when the specimen was illuminated by the same total power for each measurement. However, the full significance of this similarity is unclear because the experiments differ considerably with respect to the sample, optical scheme, and detection geometry.…”

“…We compare these results to the linear variant of Stokes Raman, spontaneous Raman scattering (SpRS). This provides for a direct comparison at an identical signal wavelength of the spatial resolution, scattering intensity, and image contrast of the coherent and noncoherent microscopy performance, as previ-ously shown by Pestov et al 7 for microspectroscopy and Cui et al 8 for microscopy configurations. We now extend this comparison to include the backscattered, epi, detection geometry in a microscope and, using a well-defined sample series, illustrate the differences in image contrast between these two imaging modalities.…”

“…They also reported that the signal from the nonresonant background is signifcantly suppressed by delaying the probe pulse. Cui et al 8 applied the spectral modulation technique to a single broadband laser source and created a "single-shot" three-color geometry. They demonstrated an improvement in spatial resolution for CSRS over SpRS in images of polystyrene beads with forward-scattering detection.…”

Label-Free Cellular Imaging by Broadband Coherent Anti-Stokes Raman Scattering Microscopy

“…All of these studies arrive at the same conclusion-CARS gives higher signal only when the product of laser power and analyte concentration is above some threshold. Cui et al (46) have recently discussed this issue specifically for cellular imaging, showing that, for laser powers conducive to biological imaging at 800 nm, CARS gives signal equivalent to or greater than spontaneous Raman only for analytes of relatively high local concentration (such as lipids). In the B-CARS works shown here, using 42 mW total excitation, which is below the physical damage threshold, we are able to obtain full spectra significantly faster than can be done with spontaneous Raman under comparable excitation powers (2,5), as evidenced by the spectra of Figs.…”

Supercontinuum Generation With Photonic Crystal Fibers and Its Application in Nano-imaging

“…In short, for n v pulse ∝ N 0 , the anti-Stokes signal strength is strictly linear and quadratic on concentration in the low and high concentration samples, respectively, which conflicts with the popular statements of CARS signal consistently quadratic concentration dependence. The linear concentration dependence is especially beneficial to quantitative analysis for low concentration samples [67][68][69][70]. In the cross area between the low and high concentration, the observed concentration dependence from the relative experiments is neither quadratic nor linear, i.e., n CARS ∝ N α , where 1 < α < 2.…”