Shape dependent thermal effects in apertured fiber probes for scanning near-field optical microscopy

Metal-coated, pulled, and conically shaped fiber probes used in scanning near-field optical microscopy (SNOM) typically undergo a thermal expansion when injected with laser light, due to partial energy absorption by the metallic film. Here, we report investigations into the thermal behavior of fiber probes produced by selective chemical etching that in our experience provide high light throughputs (10(-3)-10(-4) vs 10(-6) for the pulled fibers). Unexpectedly, we find a shortening of such probes in response to high-power laser injection (>1 mW). Thermal stress due to prolonged high-power laser injection (similar to 9 mW at 325 nm; compared to powers < 1 mW often used in SNOM experiments) determines permanent alterations of the probes, after which their thermomechanical behavior reverts to the commonly observed elongation in response to laser injection. Scanning electron microscopy after high-power irradiation on such probes shows partial detachment of the metallic coating near the fiber termination. This, however, does not appear to compromise the probe's performance in terms of light confinement outside the aperture area, suggesting that the detachment only affects the coating over the fiber cladding and confirming the operational robustness of these probes. In comparison, tube-etched, conical probes display substantial damage of the coating, up to several microns from the apex, after being injected with a comparable high-power laser beam (>10 mW at 633 nm). Although the vertical feedback mechanism of the microscope can compensate for dilations/contractions of the probes, these findings are of general importance to the field. More specifically they are significant for the achievement of a detailed understanding of apertured-SNOM operation, for the selection and operation of near-field probes, and for preventing potential artifacts in imaging and lithography, due to uncontrolled alteration of the probe properties and/or light leakage from cracks of the opaque coating induced by thermal fatigue. In addition, our results demonstrate that it is important for probe design to also consider the probe's thermal regime during operation, so as to prevent cracks in the functional parts of the coating and thus spurious, undesired sample illumination from regions other than the probe intended aperture. (C) 2006 American Institute of Physics.