POLYMERIC PLATFORM BASED ON MICROBOTTLE RESONATORS

In the present work, we presented an integrated full polymeric platform with self-assembled bottle microresonator packaged in a stable structure. NOA81 resin (Norland products) based microbottle is fabricated and integrated with a SU-8 exciting waveguide in a fully planar optical configuration. Bulk sensing measurements are performed by using water:ethanol solutions and a bulk sensitivity of 120 nm/RIU is evaluated. Microbottle resonators have been widely investigated in the recent years due to their potential applications in optical sensing, microlasers and nonlinear optics. In particular, polymers are promising and attractive materials for microresonator fabrication thanks to its high transparency above 400 nm, excellent optical quality, biocompatibility and low cost [1]. Usually, polymeric microresonators fabricated without lithographic or molding processes are excited by evanescent coupling from a tapered silica fiber [2]. This approach is inherently fragile and also lacks of integration capability, which is very important for practical applications. In the proposed platform, the microbottle is fabricated onto an optical fiber stem acting as a mechanical support that is successively glued on a PMMA substrate which integrates the exciting waveguides. Two drilled holes in the substrate allow the rise of the glue at the ends of the fiber stem and the fixing of sensor on the substrate. This configuration allows for the stability to surrounding disturbance of the optical coupling between the microbottle resonator and the planar waveguide. The microbottle resonator is fabricated by a self-assembling process. NOA81 is dispensed with a syringe pump onto a 125 ?m diameter optical fiber stem, acting as mechanical support. The microbottle sizes is controlled by the amount of photoresist dispensed by a syringe pump equipped with appropriated syringe size. The dispensed polymer materials are gently placed on the glass fiber and are exposed to UV light at 360nm for crosslinking process and for fixing the microresonator to the fiber stem. In order to confer a sphere-shaped symmetry to the microresonator, the fiber is rotated at a constant frequency of 1Hz during the whole fabrication process. The microresonator is interrogated with a planar integrated waveguide fabricated in SU-8 by means of standard photolithography technique on 2mm thick PMMA substrate. The waveguides are 2.3 ?m wide and 0.7 ?m thick showing a single - mode behaviour at the operation wavelength of 770 nm. The photograph of the fabricated platform is shown in Figure 1. For the optical characterization of the microbottle, a fiber coupled tunable laser (emitting power ~35 mW) over a range of wavelengths 765-781nm, is used. The light coming from the tunable laser passes through a polarization controller and then is butt coupled to the planar waveguide by a polarization maintaining optical fiber. The output of the waveguide is butt coupled into a single mode optical fibre, sent to a photodetector and analysed by means of an oscilloscope. The quality factor (Q-factor) of the microbottle, evaluated by measuring the full width at half maximum of the resonance dip by a Lorentzian fit, is Q~105. In order to estimate the bulk sensitivity of the NOA81 based bottle, the resonance peak shift is measured at different concentrations of water:ethanol mixtures. The measurements are performed by integrating a microfluidic cell with input and output inlets. The experimental results are reported in Figure 2. From the slope of the linear fit of the data we have measured a bulk sensitivity of about 120 nm RIU-1.