Abstract
Acquiring images of biological tissues and cells without the assistance of exogenous labels with a fast repetition rate and chemical specificity is what coherent anti-Stokes Raman Scattering (CARS) imaging offers. Nonresonant background (NRB) is one of the main drawbacks of the CARS microscopy technique because it limits the detection of weak Raman lines and the detection of low-concentration molecules. We show that a six-wave mixing process with two beams, which is a cascade effect of CARS, show better signal/NRB ratio and can be utilized for biological tissues imaging. The cascade CARS (CCARS) depends on chi-3 to the fourth power, instead of chi-3 squared as in the usual CARS signal; therefore, the contrast ratio with NRB is higher for CCARS than for CARS. We present analytic calculations showing that CCARS have better contrast over CARS in any situation. Comparison of the signals of both techniques generated on water-ethanol solutions confirm these results. Finally, we acquired CCARS images of fresh biological tissues, attesting that it is a useful tool for biological studies.
Highlights
Coherent Anti-Stokes Raman Scattering (CARS) is a four-wave mixing process showing resonance with Raman active vibrations and signal with orders of magnitude stronger than spontaneous Raman [1]
We demonstrate the generation of cascade CARS (CCARS) signal and its use to acquire images of biological tissues
The results are shown in figure Fig. 4(D), CCARS signal shows a stiffer curve than CARS, which is expected to Eq (2), because the CCARS intensity dependence with N4
Summary
Coherent Anti-Stokes Raman Scattering (CARS) is a four-wave mixing process showing resonance with Raman active vibrations and signal with orders of magnitude stronger than spontaneous Raman [1]. CARS process requires at least two laser beams at different frequencies, usually called pump, ωp, and Stokes, ωs, which are set at an energy difference matching a Raman active vibration, ωp - ωs = ΩR, in order to resonantly enhance CARS signal at 2ωp - ωs. The response CARS signal emerges at a different frequency from the two interrogation beams, pump and Stokes, making it easy to isolate the response beam from the excitation ones by using dichroic filters. CARS became a powerful tool for bioimaging after it had been successfully demonstrated that, on a tight focusing and with small interaction length of the incident beams, the phase matching conditions were eased [2]. Due to its chemical sensitivity, it has been widely used for imaging biological samples without the need of dye labeling [3,4,5] specially for lipids [6], an important characteristic for in vivo imaging.
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
