Optical ring resonators have been investigated for a number of interesting devices, including dye lasers and
sensors. However, in general, these devices can only operate on liquid samples with a low refractive index
(RI) because the whispering gallery modes (WGMs) are bound in the resonator through total internal
reflection at the resonator/sample boundary. We recently introduced a new opto-fluidic ring resonator
(OFRR) that uses a thin-walled capillary to deliver the sample through an array of ring resonators contained
within the circular cross-section of the capillary. Thus, in the OFRR, the WGM is bound at the outer
surface while the evanescent field interacts with the sample at the inner surface. Therefore, the OFRR can
operate on samples of lower and higher RI than the capillary material. This unique feature, in combination
with the OFRR's practical fluidic delivery design and its simplicity make it an attractive opto-fluidic device
for sensors, lasers, and other applications.
We analyze the OFRR's capability to support WGMs that are excited externally through fiber tapers and
that interact with the sample inside. Using a quantum mechanical analogy, we show that for liquid cores
with a higher RI than the capillary material, two coupled propagating waves exist that enable WGMs inside
the liquid core to be excited by a fiber taper outside the OFRR, across a few microns. We experimentally
verify our analysis by demonstrating refractometric sensors and dye lasers with core RIs lower and higher
than the capillary.