Highly sensitive nanoparticle sensing based on a hybrid cavity in a freely suspended microfiber

We report a hybrid cavity structure based on a suspended microfiber with a diameter of 1.7 μm applied to nanoparticle sensing in water. The proposed hybrid cavity is composed of two symmetrical reflectors with a slotted Au layer in the middle. After being characterized by the finite-difference time-domain method, the obtained strong reflection and sufficiently wide band gap provide the potential to realize an ultrasmall mode volume, which can improve sensitivity and lower loss. In addition, an ultrahigh Q/V ratio of 8.2 × 10⁶ (λ/n)^-3 and high resonance transmittance of T = 0.53 can be obtained through optimization analysis. After analyzing the trapping force and resonance shift caused by the change in local electric field, it is proven that our proposed cavity exhibits a high sensitivity and offers a convenient and stable method for particle sensing in water.