Scanning Near-field Optical Microscopy (SNOM) technique enables to overcome Abbe diffraction limit of far-field optics as well as to obtain simultaneously optical and topographical images. While the optical resolution of the method is limited by the aperture size and is typically 50 - 100 nm, an excellent spatial resolution in a topography channel can be realized. Naturally, we need a convenient and precise method to control the distance between the tip and sample for the successful operation of any SNOM device. Nowadays, the most popular method of the SNOM tip-sample distance control is the shear force - based feedback employing a glass fiber attached to the quartz tuning fork (TF). However, the shear-force distance control method is far from the ideal one. The crosstalk between optical and topographical image can warp the results. The forces between the tip and sample are high and in many cases might be destructive. We report the realization of a new approach to the problem: bent sharpened glass optical fibers with carefully controlled sizes of the bent part and the radius of the curvature of the bending were prepared and experimentally exploited as SNOM probes. Detailed analysis of fiber vibration modes shows that realization of truly tapping mode of the probe dithering requires extreme caution. In case of using the second resonance, mode probes vibrate mostly in the shear-force mode unless the bending radius is rather small (0.3 mm) and the probe's tip is short. The probe

Photonic crystal fibers (PCFs) provide a versatile platform for various applications, thanks to the flexibility with which light guiding can be customized by modifying the fiber geometry. We realize a PCF with guided modes produced by photonic band structure topology rather than conventional mode-trapping mechanisms. The design, which is compatible with the stack-and-draw fabrication process, consists of a cross-sectional photonic topological crystalline insulator with a disclination. A bulk-defect correspondence produces degenerate topological modes, lying below the cladding light line. We use various theoretical methods to confirm their topological origins, including a spectral localizer that makes minimal assumptions about the band structure. Our experiments on the fabricated fiber show it transmitting visible to near-infrared light with low losses of 10 to 20 decibels per kilometer, which do not increase substantially when the fiber is bent. A comparable solid-core PCF of conventional design exhibits substantially higher bending losses. Optical fibers based on topological modes hold promise for improved performance and versatile functionalities. status released display-pdf yes is-olf no is-manuscript no is-preprint no is-journal-matter no is-scanned no is-retracted no Received 2025 Apr 10; Accepted 2025 Oct 10; Collection date 2025 Nov 14. INTRODUCTION Photonic crystal fibers (PCFs) ( 1 – 9 ) are a subset of the broader class of photonic crystals: structures that use wavelength-scale modulations to manipulate light ( 1 , 10 ). Although photonic crystals have been used in high-performance lasers ( 11 ) and solar cells ( 12 ), arguably their most important applications are in PCFs, including high-power light delivery ( 5 ), supercontinuum light generation ( 13 ), and sensing ( 14 ). Previous theoretical proposals ( 27 – 31 ) for topological photonic crystal fibers (TPCFs) have been hampered by incompatibility with existing fiber fabrication methods, including the lack of mechanical stability in preform stacking, reliance on delicate structural features or precise index modulation in glass, and other issues. Many other designs for implementing topological photonics in PhC slabs are difficult to adapt to PCFs for similar reasons. The air holes are formed from glass capillaries of two different radii, stacked within a glass jacket with major gaps filled by additional solid glass rods, as shown in the inset of Fig. 1C . This stacking arrangement is derived from the configuration of Fig. 1A by a jam-packing procedure. The preform is drawn into a fiber cane ( Fig. 1C ), which is then further drawn into a fiber with a diameter of 310 μm. A scanning electron microscope image of the TPCF’s end face is shown in Fig. 1D , showing that the drawing process has filled in most of the interstitial air holes while also slightly deforming the main air holes. Further details about the fabrication procedure are given in text S1. 1A (see text S2) reveal the existence of guided modes that are strongly localized to the center of the sample ( Fig. 1E ). When light is coupled into the fabricated TPCF, we observe a spatially localized output profile at the end face ( Fig. 1F ). These optical microscope and infrared camera images are taken using a 100-m-long TPCF with a supercontinuum laser source coupled to the opposite end. Further details about the experimental setup are given in Materials and Methods. The light is concentrated at five high-index (glass) regions placed symmetrically around the central air hole, closely matching the prediction of Fig. 1E . Their intensities are strongly localized to five high-index regions surrounding the central air hole, similar to the previous ideal case ( Fig. 1E ). Moreover, with increasing k z , they exhibit rising in-plane quality ( Q ) factors and decreasing mode area ( Fig. 2, C and D ). The location of the GTDMs below the cladding light also ensures that they have the lowest losses among all the guided modes of the TPCF. As a consequence, when modes are launched in the fiber through standard butt-in coupling (as discussed in the next section), the other modes will be rapidly damped with propagation distance, so that only the GTDMs remain nonnegligible. Furthermore, by extending the spectral localizer analysis using a generalized local gap measure ( 47 ), we are able to show that the results remain valid even if the TPCF is bent along an arbitrary direction (which deform the structure in a manner that need not preserve the mirror symmetry S , as explained in the next section); for details, see text S5. Characterization of fiber properties Having established the existence of GTDMs in TPCFs and their topological origins, we show that their properties are well suited for waveguiding applications. Over much of the operating wavelength range, we find that the transmittance is only slightly reduced relative to the straight TPCF ( Fig. 3E ). For comparison, we fabricate a solid-core PCF with similar core size and an air hole radii/pitch ratio of 0.38 ( Fig. 3F , inset). For each fiber type, we measure the difference in output power (in dBm) between the straight and bent fiber (again using a two-loop bend with a radius of 1 cm). The solid-core PCF is found to have much stronger bending losses, by up to 25 dBm, particularly in the 600- to 800-nm range ( Fig. 3F ). By adjusting the 3D stage on which the fiber head is mounted, we locate a setting in which the output intensity is concentrated on one of the five high-index regions ( Fig. 3A , inset), and we then use this to obtain the results in Fig. 3 (B to D) . The reference angle θ 2 is arbitrarily chosen but is fixed during all subsequent measurements. The intensities are directly extracted from the beam profiler. In the bending loss experiment ( Fig. 3, E and F ), we place a 90-m-long fiber on a bending base with a preset bending radius. The other experimental procedures are as previously stated. Acknowledgments We