Applications of subwavelength-diameter optical fibres in nonlinear optics require precise knowledge of the submicrometre fibre waist diameter. We demonstrate a new technique for optical measurement of the diameter based on second- and third-harmonic generation with an accuracy of better than 2%. To generate the harmonic light, inter-modal phase matching must be achieved. We find that the phase-matching condition allows us to unambiguously deduce the fibre diameter from the wavelength of the harmonic light. High-resolution scanning electron microscope imaging is used to verify the results.
We review our recent progress in the production and characterization of
tapered optical fibers with a sub-wavelength diameter waist. Such fibers
exhibit a pronounced evanescent field and are therefore a useful tool for
highly sensitive evanescent wave spectroscopy of adsorbates on the fiber waist
or of the medium surrounding. We use a carefully designed flame pulling process
that allows us to realize preset fiber diameter profiles. In order to determine
the waist diameter and to verify the fiber profile, we employ scanning electron
microscope measurements and a novel accurate in situ optical method based on
harmonic generation. We use our fibers for linear and non-linear absorption and
fluorescence spectroscopy of surface-adsorbed organic molecules and investigate
their agglomeration dynamics. Furthermore, we apply our spectroscopic method to
quantum dots on the surface of the fiber waist and to caesium vapor surrounding
the fiber. Finally, towards dispersive measurements, we present our first
results on building and testing a single-fiber bi-modal interferometer.Comment: 13 pages, 18 figures. Accepted for publication in Applied Physics B.
Changes according to referee suggestions: changed title, clarification of
some points in the text, added references, replacement of Figure 13
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