Second-harmonic generation (SHG) microscopy as a label-free imaging technique to study structure and function of cell and tissue has been gaining much interest and popularity. As with fluorescence microscopy techniques, there is a growing demand for enhancing spatial resolution of SHG microscopy to see more details. Considering the contrast mechanism of SHG microscopy, we extend the subtractive imaging method to SHG microscopy to enhance the spatial resolution and contrast. This method is based on the intensity difference between two images obtained with circularly polarized Gaussian and doughnut-shaped beams, respectively. By characterizing the intensity and polarization distributions of the two focused beams, we verify the feasibility of the subtractive imaging method in polarization dependent SHG microscopy. The resolution and contrast enhancement in different biological samples is demonstrated. A 1.3-fold resolution improvement is achieved and a significant contrast enhancement is obtained without any prior information of samples. And we demonstrate the possibility of further enhancement of spatial resolution and contrast using a partial aperture illumination strategy. The presented method can be easily adapted to other nonlinear microscopies, and will facilitate non-invasive in vivo imaging in biomedical studies.
|