Abstract
The photodamage in coherent anti-Stokes Raman scattering (CARS) imaging of spinal tissues is featured by plasma-induced myelin splitting and shockwaves. When the excitation is tuned on resonance with the symmetric CH2 stretch vibration, the average point-scanning time to cause the photodamage is reduced by half. Similar Raman resonance-enhanced photodamage is also observed for a polymer film. The light-matter energy transfer in coherent Raman processes with both plane waves and focused excitation beams is analyzed to interpret this phenomenon. Our calculation indicates that at Raman resonance, a significant vibrational absorption in the material can be stimulated by the concomitant Raman gain and Raman loss processes due to high incident-field intensities under a tight-focusing condition. As a result, while the nonlinear damage induced by multiphoton absorption can be diminished in CARS microscopy owing to the use of near-infrared picosecond pulses, the coherent Raman-induced vibrational pumping is able to enhance the photodamage by assisting plasma generation in the material.
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