Near-field absorption imaging by a Raman nano-light source
In aperture-less near-field scanning optical microscopy (NSOM), when the apex of a sharp silicon nano-tip is illuminated with an incident laser, the Raman scattered signal from silicon creates a nano-light source with a wavelength the same as the LO phonon mode of silicon. This Raman scattered light can be absorbed by a sample placed under the tip and thus enables us to study the optical properties of the sample at the nano-scale through the absorption of silicon Raman scattered light. In this study, we demonstrate nano-scale absorption imaging by measuring the intensity of silicon Raman signals generated at the apex of a near-field probe under a platform of conventional NSOM. We used two incident lasers of different colors, which allowed the absorption properties of the sample to be observed independent of topography, revealing the inherent optical properties of the sample. The present results demonstrate the flexibility of aperture-less NSOM, not only for vibrational spectroscopies, such as tip-enhanced Raman spectroscopy, but also for electronic energy state analysis, which is achieved by monitoring the intensity of the Raman nano-light source rather than the Rayleigh scattering. This new method of near-field imaging can extend the potential applications of aperture-less NSOM.