On-chip integration of optical microbottles for biosensing

In the last decades, coupling strategies of optical microresonators have been intensively explored to develop highly sensitive and label-free miniaturized biosensors. This work presents an innovative semi-automatic assembly approach for glass microbottles on a photonic integrated circuit (PIC) with single-mode waveguides. Microbottles are extraordinary whispering- gallery-mode structures with additional axial confinement of the light along the bottle shape. A high dense spectrum of resonances varying along the bottle curvature is typically observed. To excite these resonances, the evanescent field of waveguides is used, as it provides direct evanescent interaction, integration of multiple structures and mass production. Initial coupling tests in air yielded a Q factor of 10⁴ at 1550 nm by employing an active alignment setup and a customized gripping tool. Lateral coupling tolerances of Δx = ±50 μm and Δy = ±2 μm for a bottle diameter of 180 μm were also found. An existing assembly machine including a visual system, alignment system, high precision glue dispenser and UV light was used for the identification, placement and fixation of microbottles. A highest Q factor of 10⁵ was determined after the attachment of a microbottle. Similar results were obtained with bio-chemical modified samples. A laser cutting method was also applied for reducing the fiber length of the microbottle. In this way the hybrid PIC can be compatible with microfluidics. The dedicated assembly process is a promising tool to bring optical resonators into practical use for label-free biochemical sensing but also for other applications such as quantum sensing and communication.