In principle, nonlinear multi-photon microscopy is straight forward to attain increased imaging resolution by √2 or more, in which the signal is only generated at the very center of the focal spot of laser. Label-free and nonlinear CARS and SRS microscopies are applicable to this spot reduction effect. However, their excitation laser wavelengths are limited to near infrared (NIR), partially because NIR is the wavelength only commercially available for typical femtosecond laser. Thus, the potential improvement in spatial resolution is completely compromised by longer wavelengths adopted for imaging. To fully utilize nonlinear advantage to defeat resolution limit, we reduced the wavelengths of our femtosecond lasers to visible region and demonstrate hyperspectral blue SRS microscopy with resolution about ~100 nm. Moreover, the electronic pre-resonance condition was reported to enhance sensitivity of SRS imaging as the absorption of NIR dyes match the laser frequencies. In our concept, we gained SRS sensitivity by actively tuning the wavelengths of pump and Stokes lasers to near resonant to electronic transition of endogenous biomolecules (e.g. DNA and proteins). Additionally, to achieve specificity to biomolecules, we developed single optical fiber based spectral focusing technology, and demonstrated high-resolution hyperspectral SRS imaging of intact tissues.
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