Beyond lock-in analysis for volumetric imaging in apertureless scanning near-field optical microscopy.

Conventional apertureless scanning near-field optical microscopy uses lock-in demodulation techniques to filter higher harmonics from the periodically modulated optical signal. On the one hand, this signal notoriously may contain contaminating mechanical contributions; on the other, it reduces the available data to one number per pixel. Realizing that the vertically oscillating near-field probe actually traverses a whole volume above the sample surface, we discuss a model to extend the data analysis in an attempt to better extract the optical information and to recover its three-dimensional spatial distribution.