Femtosecond written waveguides for evanescent excitation of resonant optical sensing devices

Optical resonant structures, such as circular disks and optical microbubble resonators (OMBRs), are crucial for high-resolution chemical and biochemical sensing. Both can be integrated into microfluidic systems: resonant disks can be fabricated within microfluidic channels, while OMBRs use thin silica capillary walls to confine fluid samples in a hollow-core cavity. Optical modes are typically excited using tapered optical fibers, which offer efficiency but lack robustness for functional devices. This work presents two femtosecond laser-written waveguide designs for exciting whispering gallery modes (WGMs) in these resonant structures. For resonant disks, suspended waveguides are fabricated tangentially between the microfluidic channel walls. For OMBRs, integrated waveguides are written on fused silica substrates to excite resonant modes. Both configurations provide stable and robust optical sensing solutions. The OMBR platform achieved a sensitivity of 45nm/RIU with a resolution of 4.4×10-5 RIU, while monolithically integrated disks reached 80nm/RIU with a resolution of 7.0×10-4 RIU. In both cases, the Q-factor exceeded 104 across the measurement range. These results confirm that femtosecond laser-written waveguides can efficiently excite resonant modes, offering promising platforms for chemical and biochemical sensing applications.