Field-Mapping and Focal-Spot Tracking for S-SNOM

System and method for optical alignment of a near-field system, employing reiterative analysis of amplitude (irradiance) and phase maps of irradiated field obtained in back-scattered light while adjusting the system to arrive at field pattern indicative of and sensitive to a near-field optical wave produced by diffraction-limited irradiation of a tip of the near-field system. Demodulation of optical data representing such maps is carried out at different harmonics of probe-vibration frequency. Embodiments are operationally compatible with methodology of chemical nano-identification of sample utilizing normalized near-field spectroscopy, and may utilize suppression of background contribution to collected data based on judicious coordination of data acquisition with motion of the tip. Such coordination may be defined without knowledge of separation between the tip and sample. Computer program product with instructions effectuating the method and operation of the system.