Integrated All-In-Silica Optofluidic Platform Based on Microbubble Resonator and Femtosecond Laser Written Surface Waveguide

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Abstract
Optical microbubble resonators (OMBRs) – understood as localised thin wall bulges induced in silica microcapillaries – are gaining an ever-growing interest in microfluidic sensing applications due to their capability to sustain whispering gallery modes (WGMs) and confine the fluidic sample within their own hollow-core microcavity. Currently, most applications use an external tapered optical fibre for coupling light to the resonator. This arrangement is known to be fragile and prone to vibrations. In this work an alternative approach, based on coupling OMBR with a femtosecond (fs) laser written optical waveguides, integrated at the surface of fused silica substrate, is proposed. In this configuration, a stable and robust final structure is accomplished by gluing the two ends of the microcapillary, on which the OMBR is made, to the substrate. The OMBR quality factors, measured at the excitation wavelength of 1540 nm, show values close to 104 in the case of a water-filled cavity, with a maximum coupling efficiency up to 6.5%. Finally, the operation of the integrated optical devices as refractometres is demonstrated by delivering different solutions with successively increasing concentrations of NaCl inside the OMBR. An average sensitivity of 45 nm/RIU is obtained, yielding a resolution of 4.4×10 -5 RIU, creating the potential for this platform to be applied in chemical/biochemical sensing.
Funding Information
  • Fundação para a Ciência e a Tecnologia (SFRH/BD/128795/2017)
  • Regione Toscana (688735)
  • SAFE WATER project
  • European Union through the Next Generation Program (ECS00000017 by Italian MUR)

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