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

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 dielectric grating and by imaging the fluorescence from individual rhodamine-6G molecules with a spatial resolution of ≊160 nm.