Direct observation of below-diffraction-limited optical spot induced by nonlinear saturable absorption of Ag-doped Si nanofilms

In this work, using the scanning near-field optical fiber probe method, we carry out a direct observation of below-diffraction-limited optical spot induced by nonlinear saturable absorption characteristic of Ag-doped Si nanofilms. The experimental results indicate that the squeezed spot size decreases with laser power increase, and the smallest spot can be squeezed to 68% of the diffraction-limited focusing spot size, which is very useful and can be applied to superresolution optical recording, optical lithography, and optical imaging.