We investigate an approach to measuring harmful gases by enhanced Fano resonance generated in a universal multimode waveguide-microcavity model. Dynamic Fano resonance is theoretically described and experimentally proved to be associated with the phase shift between two waveguide modes as well as their amplitude ratio and coupling coefficient. The spectra can be engineered to form a Lorentz dip, various Fano lineshape, and Lorentz peak by controlling the coupling point in both microbottle and surface nanoscale axial photonics platforms. In principle, the model can be applied to any class of whispering gallery mode microcavity device. With sharp asymmetric lineshape, our model can improve the sensitivity by 51.5 times in theory when measuring the harmful gas refractive index change, which may open up opportunities for advancements in the harmful gas leakage detection applications.
With the finite difference time domain (FDTD) method, whispering gallery modes (WGM) in a microsphere coated with
three layers of high, low, and high refractive index (RI) are simulated. In the simulation, the coupling system includes a
coating microsphere, a waveguide and a nanoscale gap separating the waveguide and the microsphere. A pulse with
ultra-wide bandwidth that spans over several resonant modes of the resonator is used for simulation. Via waveguide
coupling, the relative intensity spectra of the three layers and the transmission spectrum of the coupling system are
obtained. We investigate the effects of the waveguide RI and the thickness of the low-RI layer on resonance
characteristics. It is found that each of the two high-RI layers can sustain its own WGM if the values of RI and thickness
of the three layers are appropriate. Furthermore, the effect of the RI of the surrounding medium on resonance
characteristics is also studied. The simulation results show that a RI change of the surroundings will only change the
resonance wavelength of the outer layer, and will not affect the WGM of the inner layer. Such property makes it feasible
for a potential application in high-precision RI and temperature sensing.
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