We present experimental and finite-difference time-domain simulation results on the tunability of optical resonant modes of spiral microtube cavities, rolled-up from square patterned SiO/SiO_(2) thin nanomembranes on glass substrates. The peak positions of resonant TM modes shift to lower energies by coating the microtube wall with Al_(2)O_(3) monolayers, which is well described by simulations. Moreover, a second group of tunable resonant modes appears beyond a certain critical thickness of the coated Al_(2)O_(3). The polarization of this group of modes is TE, as we find out by a detailed analysis of the polarization-dependent photoluminescence spectra.
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