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PL spectrum shows several high order axial resonant modes resolved for each azimuthal mode. The broad background of PL emission is fitted by a red curve. The inset depicts a sketch of a laser beam exciting the microtube at an end side region where is free-standing to the substrate. The free spectral range of azimuthal modes (FSR) is 45 meV.
(a) Color-coded (background-substracted) PL intensity map along the axis (z) of the tube which is shown in the optical image of (b). Efficient axial light confinement is found at the top end side (z between 18 and 27 μm) of the tube where the discernible azimuthal modes shift with arched traces as indicated by the dashed red curves. Week resolved axial modes are observed at the bottom end side of the tube where the azimuthal modes shift with slightly arched traces as indentified by dashed black curves. (c) The top panel shows a group of axial modes for a given azimuthal mode number measured at z = 21 μm. A localized parabolic variation is modelled to formulate the potential (bottom panel) which well fits the axial modes (solid triangles). The first eight field distributions are plotted together in the potential curve.
Background-subtracted PL spectra of an empty microtube (lower spectrum) and a tube filled with aqueous salt solution (upper spectrum). The azimuthal mode numbers as well as the calculated mode positions of the first axial resonances are indicated for M = 39–42. The inset shows the sketch of liquid core filling for optofluidic detection.
Average axial mode spacings (of spectra in Figure 3) for empty tube (empty circles) and solution-filled tube (red circles) as a function of the azimuthal mode number M. The dotted lines are guides to the eyes. The bottom left inset shows the sketch of the empty rolled-up tube while the upper right inset shows the case of the tube filled with salt solution.
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