Scanning near-field optical microscopy utilizing silicon nitride probe photoluminescence

We describe a simple method for performing high-resolution scanning near-field optical microscopy (SNOM). A commercial Si₃N₄ tip is illuminated by an intense light source, which causes the tip to emit redshifted (inelastically scattered) light. Part of the redshifted light passes through a sample, allowing transmission light microscopy. By simple modification of a commercial atomic force microscopes (AFM), we are able to image many different samples with high-resolution optical microscopy, achieving 20–30 nm lateral resolution for the best samples. The high resolution of the technique is not only due to the high curvature of the AFM tip, but also to the fact that the intensity of inelastically scattered light transmitted through the sample decays exponentially with the separation between the tip and the sample (decay length ∼100 nm). We envisage applications to transmission SNOM, spectroscopic imaging, and imaging of fluorescently labeled bioconjugates. The collection of the optical image does not interfere with the normal operation of the AFM, so deflection, height, or other modes of operation can be captured simultaneously.