Design and construction of a versatile scanning near-field optical microscope for fluorescence imaging of single molecules

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Abstract
A scanning near‐field optical microscope (SNOM) for fluorescence imaging of single molecules requires efficient optical signal detection and background rejection combined with long‐term stability and high spatial resolution. These requirements are dictated by the extremely low fluorescence signal of an individual dye molecule. We have built a SNOM that meets these requirements by combining a rigid and versatile near‐field optical scanner with confocal detection optics. The relevant design parameters are discussed in detail. The near‐field part of the microscope is based on a commercially available fiber aligner for coarse approaching the sample with respect to the tip. It also permits us to select a specific sample area in a range of 3 mm×3 mm with a nominal resolution of 10 nm. The tip–sample separation is probed by shear‐force detection using a fiber‐optical interferometer, which gives an excellent signal‐to‐noise ratio. The high versatility of this SNOM is demonstrated with measurements of a transparent dielectricgrating and by imaging the fluorescence from individual rhodamine‐6G molecules with a spatial resolution of ≊160 nm.