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Heterodyne Detection for the Extraction of the Probe-Scattering Signal in Scattering-Type Scanning Near-Field Optical Microscope

We propose that heterodyne detection is useful for the scattering-type scanning near-field optical microscope (SNOM) to effectively remove the scattering from various sources on the sample which causes deterioration of the image resolution. First, we show in a simple model that this sample scattering cannot be completely removed by ac-mode detection of the SNOM signal for samples with high reflectivity, while only the probe-scattering signal can be extracted by heterodyne detection. Next, we show experimental results which compare the ac-mode and heterodyne images, and demonstrate the effectiveness of heterodyne detection.