Multifrequency excitation and detection scheme in apertureless scattering near-field scanning optical microscopy.

Near-field scanning optical microscopy has revolutionized the study of fundamental physics, as it is one of very few label-free optical noninvasive nanoscale-resolved imaging techniques. However, its resolution remains strongly limited by the poor discrimination of weak near-field optical signals from a far-field background. Here, we theoretically and experimentally demonstrate a multifrequency excitation and detection scheme in apertureless near-field optical microscopy that exceeds current state-of-the-art sensitivity and background suppression. We achieved a twofold enhancement in sensitivity and deep subwavelength resolution in optical measurements. This method offers rich control over experimental degrees of freedom, breaking the ground for noninterferometric complete retrieval of the near-field signal.