SEM image of a campanile probe.[1]SEM image of a campanile probe.[1]False-color SEM image of the Campanile near-field probe fabricated on the edge of an optical fiber using nanoimprint.[2]Comparison of photoluminescence maps recorded from amolybdenum disulfide flake using a campanile probe (top) and conventionalconfocal microscopy (bottom). Scale bars: 1 μm.[1]
Innear-field scanning optical microscopy thecampanile probe is a tapered optical probe with a shape of acampanile (asquare pyramid). It is made of an optically transparentdielectric, typicallysilica, and its two facets are coated with a metal, typicallygold. At the probe tip, the metal-coated facets are separated by a gap of a few tens of nanometers, which determines the spatial resolution of the probe. Such a probe design allows collecting optical signals, usuallyphotoluminescence (PL) orRaman scattering, with a subwavelength resolution, breaking thediffraction limit.[1][3]
The campanile probe is attached to anoptical fiber, which both provides a laser excitation of the studied sample and collects the measured signal. The probe is rastered over the sample with a standardscanning probe microscopy scanner, keeping the distance to the sample surface at a few nanometers.[1] Contrary to the traditional (circular) near-field probes, the campanile probe has no cut-off frequency and is insensitive to the spatial mode of the optical near field. Hence its application is not limited to thin-film samples.[3] Another advantage of the campanile probe is a high signal collection efficiency, which exceeds 90%.[4]
Campanile probes are typically fabricated as follows: a standard cylindricalsingle-mode optical fiber is etched withhydrofluoric acid to create a conical tip with a radius of ca. 100 nm. Then a square pyramid is carved on the tip usingfocused ion beam (FIB) milling, and its two facets are coated with a metal by shadow evaporation. A nanometer gap is then opened on the tip by FIB.[3] Alternative fabrication method usesnanoimprint lithography to replicate campanile pyramid from a mold. This approach significantly increases fabrication speed.[2]
