Flexible optical microcavities and their sensing application

Summary form only given. Due to high quality factor and small mode volume, optical whispering gallery microcavities have been employed for fundamental studies of light-matter interaction and significant applications such as low-threshold laser devices, dynamic filters [1], and recently label-free ultra-sensitive biosensors [2]. Here, we demonstrate a novel material composition together with effective methods to fabricate two different high quality microresonators namely microfibers and hemispheres. By incorporating dye molecules in these structures, optically pumped whispering gallery mode (WGM) laser was observed. Moreover, the microlasers can be used as high sensitive refractive index sensors. Microfibers are readily obtained by directly drawing from a mixture of epoxy resin and polymethylmethacrylate (PMMA) as shown in Figure 1(a) [3]. The weight ratio of PMMA and epoxy was about 1: 2. Figure 1(b) shows a scanning electron microscope (SEM) image of dye doped microfiber with a high uniformity and smooth outer surface. Interestingly, by changing the ratio to 1: 6 and placed the fabricated fiber on a hydrophobic distributed Bragg reflector (DBR), hemispheres are self-assembly because the fiber “want” to minimize its surface area as shown in Figure 1(c) [4]. To achieve lasing emission, pulse Nd:YAG laser (wavelength: 532 nm, pulse duration: 1 ns, repetition rate: 60 Hz) was used to excite individual microfiber or hemisphere and the photoluminescence (PL) was recorded by a charge-coupled device (CCD). Figure 1(d) and (e) show lasing spectra from a Rhodamine 6G (R6G) doped fiber and Rhodamine B (RhB) doped hemisphere, respectively. Positions of the lasing modes can be well-explained by WGM theory. In principle, if refractive index of the medium surrounding a cavity changes, spectral shift of resonant modes will be observed and the shifting value is a function of refractive index unit (RIU) [2]. Based on this phenomenon the fiber and hemisphere was used for refractive index sensing of Tetrahydrofuran (THF) solution and acetone vapour with sensitivity around 300 and 130 nm/RIU, respectively [3,4]. In conclusion, we provide a new material composition and an effective technique to fabricate fiber and hemispherical microlasers, and demonstrate their potential application as sensitive optical sensors, which is important for developing of flexible photonic devices.