Figure 1

(Color online) Measured PL spectra (black lines) of the microtube for (a) TE and (b) TM polarizations. The excitation power density was about $4\mathrm{kW}∕{\mathrm{cm}}^2$. The spectrum in (a) is approximated by the sum of Lorentzian curves for the resonant modes (below the measured spectrum) and a Gaussian curve for the leaky modes (gray shaded). $Q$ factors have been obtained by fitting. The left inset shows a nonscaled cross-sectional sketch of the microtube together with the excitation and collection configuration; the right inset shows a SEM image of the microtube.Reuse & Permissions

Figure 2

PL spectra of the unstructured sample obtained with different excitation power densities: (a) $4\mathrm{kW}∕{\mathrm{cm}}^2$ and (b) $0.3\mathrm{kW}∕{\mathrm{cm}}^2$. The spectra in (b) (vertically shifted for clarity) were obtained in one and the same measurement for slightly different spatial positions within the area of the laser spot.Reuse & Permissions

Figure 3

(Color online) (a) SEM picture of a part of the microtube. Both the inside and outside edges are visible. (b) Undispersed PL image of the sample shown in (a). Both images are equally scaled; the positions of the borders (b), the inside edge (ie), and the outside edge (oe) are marked. [(c)–(e)] Spectrally analyzed PL emission at the (c) inside edge, (d) radial position of the laser spot, and (e) outside edge. In the upper panels, the spatial information along the tube $\left(z\right)$ axis is retained. The lower panels show spectra obtained at the $z$ position on the level of the laser spot (marked by the dotted horizontal line). The excitation power density was about $5\mathrm{kW}∕{\mathrm{cm}}^2$.Reuse & Permissions

Figure 4

(Color online) (a) SEM picture of a part of the microtube. Inside and outside edges are marked with broken lines for clarification. The tube wall is thicker between these edges (see sketch in Fig. 1). (b) Scanning micro-PL spectra show three dimensionally confined optical modes (excitation power density of about $4\mathrm{kW}∕{\mathrm{cm}}^2$). The white broken lines represent calculated mode energies for $m=59$ to 56. (c) Calculated effective refractive index vs position. Horizontal broken lines mark the positions of some pronounced localized modes.Reuse & Permissions