Numerical investigation of optical response from rolled-up microtube resonator and its application

We preformed a numerical investigation of the optical mode in rolled-up microtube resonators. The optical responses of the self-rolling semiconductor-based resonators are calculated by solving Maxwell’s equations using the finite-difference time-domain (FDTD) method. Observed mode patterns characterized by whispering-gallery-mode-like profile are presented with various sets of structural parameters (tube diameter, tube wall thickness, and the number of rotation). The degeneracy breaking results in splitting of the mode peaks with the same azimuthal mode index is observed and we confirmed that it causes by structural asymmetry of the rolled-up structure. For the rolled-up tube with thick tube wall, the quasi-whispering gallery mode pattern due to very high degree of structural asymmetry is shown. In this presentation, we also propose an application of the rolled-up microtube as a refractometric sensor. For this proposal, we experimentally fill the rolled-up tube with liquid and measure the mode peaks shift due to the change of refractive index inside the tube. A FDTD simulation with realistic structural parameters fits well with the experimental spectra. The field shows more leaky to the inner part of the tube due to the increase of the refractive index. High spectral sensitivity is obtained from the tube with thin-wall structure at the long wavelength peak with low azimuthal mode index. This is due to the large overlap between the leakage field and the liquid for the lower mode index. However, the quality factor considerably drops when the wall is very thin. Rolled-up tube with larger number of rotation has lower sensitivity as effectively similar to the tube with thicker wall.