Optical Modes in Semiconductor Microtube Ring Resonators

We demonstrate optical modes in $\mathrm{InGaAs}/\mathrm{GaAs}$ microtubes acting as optical ring resonators. Self-supporting microtubes were fabricated by optical lithography and wet-etching processes utilizing the self-rolling mechanism of strained bilayers. The optical modes were probed by the photoluminescence of InAs quantum dots embedded in the tube’s wall. In this novel microtube ring resonator we find a spectrum of sharp modes. They are in very good agreement with the theoretical results for a closed thin dielectric waveguide.

Figure 1

(a) Schematic sample structure. (b) Optical microscope image of a microtube bridge. The U-shaped part has a rolled-up beginning at the starting edge. (c) High resolution optical microscope image of the microtube bridge and its bearing. (d) Scaled schematic section of the self-supported part of the microtube.

Figure 2

Micro-PL spectra of a microtube bridge measured in TE (upper graph) and TM (lower graph) polarization configuration. The spectra a vertically shifted for clarity. The symbols indicate calculated mode positions (without any fitting) labeled with their azimuthal mode number $m$. The squares (circles) represent the waveguide (exact) approach.

Figure 3

(a) Gray scale plot of PL spectra measured at different positions on the microtube along its axis. Dark regions represent strong intensities. The spectrum marked with the arrow is depicted in (b).