Nanometric tip diameter fiber probes for photon scanning tunneling microscopes (PSTM) were fabricated by selective chemical etching of GeO2-doped optical fibers. The cone angle θ of the fiber probe tip was controlled by varying the doping ratio of the fiber core and the composition of the etching solution, and the standard deviation of the cone angle Δθ was within 0.5°. A fiber probe with θ=20° and tip diameter of less than 10 nm was fabricated. This is, to the authors' knowledge, the sharpest PSTM probe which has ever been reported.

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A T4 bacteriophage with a tail diameter of about 9 nm was observed for the first time in an atmospheric condition without a metal film coating, by means of a superresolution photon scanning tunneling microscope (PSTM) using a fiber probe. Fiber probes with cone angles of 25°-100° and their minimum curvature radii less than 10 nm were fabricated andthe angle variance was less than 0.5°. The normal resolution of such a PSTM was evaluated to be less than 1 nm, and the lateral resolution, limited by the curvature radius of the fiber probe, was estimated to be within 20 nm.

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Summary: A selective chemical etching was used to fabricate fiber probes for the photon scanning tunneling microscope (PSTM). The cladding diameter of the fiber probe was controlled by varying the first-step etching time. The cone angle of the fiber probe tip was controlled by varying the doping ratio of the fiber and the composition of the etching solution. A cladding diameter of 8 µm and a tip diameter of about 3 nm were fabricated. The smallest cone angle was 14°.