| We investigate the optical resonance modes in circular and spiral microtube resonators by using a numerical finite-difference time-domain method. Due to the lower symmetry in the spiral structure, conventional whispering gallery modes observed in circular resonators are separated into two modes. In this work, we propose a scheme to use the spiral resonators for sensing the absolute angular position of nearby nano-/microparticles. The shifts and splittings of the optical resonance modes in the presence of particles at different positions are systematically studied. Influences of tube geometry and the size of particles are also presented. |
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