Light-induced Tuning of Microfiber-knot Resonator Overlaid with Azobenzene-doped Nematic Liquid Crystals
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Light-induced Tuning of Microfiber-knot Resonator Overlaid with Azobenzene-doped Nematic Liquid Crystals Vincent K.S. Hsiao1, Zhe Chen*2, Min-Chi Jheng1, Xiaoqing Li2 and Jianhui Yu2. 1 Department of Applied Materials and Optoelectronic Engineering, National Chi Nan University No 1, University Rd. Puli, Nantou Hsien, 54561, Taiwan 2 Key Laboratory of Optoelectronic Information and Sensing Technologies of Guangdong Higher Educational Institutes, Jinan University, Guangzhou, China, 510632 Email: [email protected] ABSTRACT This paper demonstrates light-induced tuning of optical spectrum from optical microfiber knot resonator overlaid with an azobenzene-doped nematic liquid crystal (azo-doped NLC). The high-quality fiber resonator is made by drawing the single mode fiber to the micro-size diameter and self-twisting the microfiber as a knot shape. During the UV light irradiation the azobenzene molecules perform trans-to-cis photoisomerization which disrupts the NLC orientation. The disrupted NLC changes the effective refractive index within the LC overlaid fiber area and shifts the optical spectrum of microfiber knot resonator. The 0.25 nm spectral shifting of resonance wavelength was observed under the irradiation of 50 mW UV light. INTRODUCTION Optical microfiber resonator, a low-loss silica waveguide of subwavelength diameter in a format of loops, knots, or coils, has been demonstrated in the varieties of functionality such as add-drop filter [1], lasing [2-4], nonlinear optics [5], and sensing devices [6,7]. Due to the shrink dimension of microfiber, an evanescent wave leaking from the tapered area is sensitive to the refractive index change of the ambient medium. Liquid crystal, which owns an easily tunable refractive index, has been applied in different photonic structure to achieve actively tunable properties [8-9]. Here, we present a light-induced spectral shifting of a microfiber-knot resonator overlaid with an azobenzene-doped nematic liquid crystal (azo-doped NLC). The refractive index change caused by the light-induced phase transition of the overlaid azo-doped NLC generates a shift of the spectral resonance. The 0.19 nm spectral shifting of resonance wavelength was observed under the irradiation of 50 mW UV light. EXPERIMENT Figure 1A shows the optical image of the fabricated microfiber-knot resonator. The tapering of microfiber was from drawing a standard single-mode fiber using the flame-heading technique [10]. The diameter of the microfiber is around 2.5 μm and the diameter of the loop is around 468 μm. Figure 1B is the setup of optical measurement where a tunable laser (TSL, ANDO AQ4321D, wavelength range 1520nm-1620nm) was used as the light source. The output signal was recorded by the optical spectrum analyzer (OSA, YOKOGAWA QA6317COSA, minimum resolution 0.01 nm). The azo-doped NLC was a homogeneous mixture of 95 wt% nematic LC (MDA 3061, Merck) and 5 wt% azobenzene LC. An UV light source (250 mW,
USHIO SP7-250DB) was used as the light irradiation which could generate the trans-to-cis photoisomer
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