Abstract
This study constructs interference-based model of the apertureless scanning near-field optical microscopy (A-SNOM) heterodyne detection signal which takes account of both the tip enhancement phenomena and the tip reflective background electric field. The analytical model not only provides a meaningful explanation of the image artifacts and errors, but also suggests methods for reducing these effects. It is shown that the detection signal obtained in the heterodyne A-SNOM method has a significantly higher signal-to-background (S/B) ratio than in the homodyne method. It is also shown that the S/B ratio increases as the wavelength of the illuminating light source is increased or the incident angle is reduced. Finally, an inspection reveals two fundamental phenomena which may potentially be exploited to obtain further significant improvements, namely (1) the modulation depth parameter has certain specific values greater than 1; and (2) the AFM tip apparatus using a ramp function.
Highlights
Apertureless SNOM (A-SNOM) adopts a sharp vibrating tip supporting a sphere with a nanometer-scale radius achieves a local enhancement of the electric field and makes possible an optical resolution at the sub-10 nm scale [1,2]
The A-SNOM detection signal is contaminated by a complex interference between the background electric field and the nearfield electric field
The interference-based formulation developed for the S/B ratio in the current study has no points of discontinuity, and represents a more suitable means of analyzing the detection signal than the signal contrast formula presented by the current group in [3]
Summary
Apertureless SNOM (A-SNOM) adopts a sharp vibrating tip supporting a sphere with a nanometer-scale radius achieves a local enhancement of the electric field and makes possible an optical resolution at the sub-10 nm scale [1,2]. The current study develops a comprehensive interference-based model with which to analyze the amplitude and phase of the heterodyne detection signal at different harmonics of the tip vibration frequency. In constructing this model, the present study expands the model presented in [3] to take account of the electric field scattered directly from the AFM tip, and the tip scattering field reflected from the sample. The final electric field in the near-field region is that of the light scattered directly from the sample surface Since this electric field is not modulated by the AFM tip motion, it can be expressed as. Where ES and ΦS are the amplitude and initial phase of the scattering light, respectively
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