anti-Stokes Raman Scattering Microscopy Imaging Research Articles

Coherent anti-Stokes Raman scattering microscopy imaging with suppression of four-wave mixing in optical fibers

We demonstrated an optical fiber delivered coherent anti-Stokes Raman scattering (CARS) microscopy imaging system with a polarization-based mechanism for suppression of four-wave mixing (FWM) signals in delivery fiber. Polarization maintaining fibers (PMF) were used as the delivery fiber to ensure stability of the state of polarization (SOP) of lasers. The pump and Stokes waves were coupled into PMFs at orthogonal SOPs along the slow and fast axes of PMFs, respectively, resulting in a significant reduction of FWM signals generated in the fiber. At the output end of PMFs, a dual-wavelength waveplate was used to realign the SOPs of the two waves into identical SOPs prior to their entrance into the CARS microscope. Therefore, it allows the pump and Stokes waves with identical SOPs to excite samples at highest excitation efficiency. Our experimental results showed that this polarization-based FWM-suppressing mechanism can dramatically reduce FWM signals generated in PMFs up to approximately 99%. Meanwhile, the PMF-delivered CARS microscopy system with this mechanism can still produce high-quality CARS images. Consequently, our PMF-delivered CARS microscopy imaging system with the polarization-based FWM-suppressing mechanism potentially offers a new strategy for building fiber-based CARS endoscopes with effective suppression of FWM background noises.

Hyperspectral coherent anti-Stokes Raman scattering microscopy imaging through turbid medium

Coherent Raman microspectroscopy imaging is an emerging technique for noninvasive, chemically specific optical imaging, which can be potentially used to analyze the chemical composition and its distribution in biological tissues. In this report, a hierarchical cluster analysis was applied to hyperspectral coherent anti-Stokes Raman imaging of different chemical species through a turbid medium. It was demonstrated that by using readily available commercial software (Cytospec, Inc.) and cluster analysis, distinct chemical species can be imaged and identified through a rather thick layer of scattering medium. Once the clusters of different chemical composition were distinguished, a phase retrieval algorithm was used to convert coherent anti-Stokes Raman spectra to Raman spectra, which were used for chemical identification of hidden microscopic objects. In particular, applications to remote optical sensing of potential biological threats and to imaging through a layer of skin tissue were successfully demonstrated.

Near-field effects on coherent anti-Stokes Raman scattering microscopy imaging

We introduce a numerical method, the finite-difference time-domain (FDTD) method, to study the near-field effects on coherent anti-Stokes Raman Scattering (CARS) microscopy on nanoparticles. Changes of the induced nonlinear polarization, scattering patterns, and polarization properties against different diameters of spherical nanoparticles are calculated and discussed in details. The results show that due to near-field effects, the induced nonlinear polarization is significantly enhanced at the water-particle interface, with 1.5-fold increase in intensity compared to that inside the particles, and the near-field enhancement increases with decreasing diameters of nanoparticles. The enhanced scattering dominates over the scattering contribution from the particles when the nanoparticle size decreases down to the scale of less than a half wavelength of excitation light. Further studies show that near-field effects make the induced perpendicular polarization of CARS signals being strictly confined within the nanoparticles and the particle-water interface, and this perpendicular polarization component could contribute approximately 20% to the backward scattering. The ratio values of the perpendicular polarization component to the total CARS signals from nanoparticles sizing from 75 nm to 300 nm in backward scattering are approximately 3 to 5 times higher than those in forward scattering. Therefore, near-field effects can play an important role in CARS imaging. Employing the perpendicular polarization component of CARS signals can significantly improve the contrast of CARS images, and be particularly useful for revealing the fine structures of bio-materials with nano-scale resolutions.