We present an optical near-field detection mechanism based on optical modulation of the image force between a semiconducting probe tip and a glass surface. The modulation stems from a phenomenon called surface photovoltage. The performance of the mechanism for near-field imaging is demonstrated by using a scanning force microscope over a standing evanescent light wave. The lateral resolution is found to be 170 nm (subwavelength) and a representative minimum detectable power is 0.1 pW/√Hz in air. We develop a simple theoretical model and discuss some possible applications.
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