Tip-substrate interaction in optical near-field microscopy

We study the tip-substrate interaction in apertureless scanning near-field optical microscopy (aSNOM) with a three-dimensional model of both near and far fields. Starting from the analytically well understood two-sphere model, we obtain a realistic tip-substrate model by continuous deformation of the upper sphere into an elongated conical tip several wavelengths long with a hemispherical apex radius of just a few nanometers. The other sphere is enlarged to represent a locally flat substrate surface below the tip apex. This adiabatic transition toward the final system helps us to understand the origin of the different contributions. We find that dielectric tips, in spite of the absence of plasmonic resonances, are quite suitable for aSNOM. This is supported by experimental results. Working with the sharpest commercially available Si tips holds promise for pushing the routinely achievable lateral resolution down to the sub-10-nm range. At 1 nm tip-substrate distance, we obtain local-field enhancements of many tens. The instantaneous, elastically backscattered intensity is calculated for variable tip-substrate distances to study demodulation at higher harmonics of an oscillating probe. We confirm that the local optical information is discriminated even against strong background signals, provided the vibration amplitude is kept small. However, this discrimination scheme is rather sensitive to crosstalk from anharmonic motion and care must be exercised in choosing appropriate experimental parameters to achieve reliable optical contrast.