Radial order dependence of the gas sensing sensitivity of whispering gallery mode microspheres

Theoretical Analysis of the Refractometric Sensitivity of a Coated Whispering Gallery Mode Resonator for Gas Sensing Applications

“…In Figure 2 , the dependence of on d is shown for n c = 1.4 which would correspond to a porous silica layer with a porosity of 9%, a reasonable value for sol-gel porous silica [ 13 , 14 ]. It is important to note that in this case n c < n s since in the opposite case very different behavior is observed, as will be discussed later.…”

“…Microring resonators have been produced achieving sensitivities of 70 nm/RIU with Q = 2 × 10 5 [ 17 ] and 212 nm/RIU with Q = 1.8 × 10 3 [ 18 ]. The results reported in Zhivotkov et al [ 13 , 14 ] show higher sensitivity than microbubble, microring and uncoated microspheres while still retaining high Q-factors. However, a detailed theoretical study on the refractometric sensitivity of a microsphere coated with a porous material is still lacking in literature.…”

“…This severely limits the refractometric sensitivity, although this is somewhat offset by the fact that the high quality of the resonators makes possible the tracking of very small shifts of the WGM frequency. Recently, a new approach to sensing using WGM microresonators has been proposed which is particularly suitable for gas sensing [ 9 , 10 , 11 , 12 , 13 , 14 ]. When a microresonator is coated with a thick porous layer, the electric field of the WGM can be engineered to be confined inside the coating.…”

“…The same group also managed to detect ammonia vapors using a porous silica-coated microsphere [ 10 , 11 , 12 ]. More recently, Zhivotkov et al [ 13 , 14 ] managed to increase the refractometric sensitivity of the porous silica sol-gel-coated microsphere to more than 1000 nm/RIU, leading to ammonia detection sensitivity of more than 100 nm/ppm while retaining very high Q-factor (between Q = 10 5 –10 6 ). This value of 1000 nm/RIU is among the highest available in literature so far for resonator-based refractometric sensors.…”