| Microtubular resonator fabricated by release and roll-up of strained nanolayers guides light along the tube wall in the azimuthal direction. After few rotations performed from the pre-stressed nanolayers, the final tube˘s wall thickness is below the resonant wavelengths (or modes) supported by the resonator, as a consequence, the evanescent field of the resonant modes interacts with the media surrounding the microresonator, suggesting potential applications for on-chip components like filters and sensors. This interaction can be used to fine tune the resonant modes inside the microtube, which is demonstrated by stepwise one-by-one monolayer (ML) coating of Al2O3 on a SiO/SiO2 rolled-up microtube. As result, a controlable linear red shift is obtained in a wide spectral range, modulation also demonstrated by finite-difference-time-domain (FDTD) simulations. In addition, a new group of resonant modes emerge when the Al2O3 coating is thicker than 200 ML (~ 20 nm), which are associated to a set of modes (TE polarization) perpendicular to the previous group (TM polarization), as revealed by linear polarization analysis. Therefore, as the microresonator wall thickness increases, the difraction loss of the TE modes decrease, allowing the microresonator to simultaneously support both TM and TE resonant modes. FDTD shows a progressive increase of the microtube refractive index after the consecutive Al2O3 coating, which cause a higher contrast between the microtube and the sorrounding media resulting in the observed mode˘s shift. |
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