2. Jiangsu Key Laboratory of Meteorological Observation and Signal Processing, Nanjing, Jiangsu 210044, China
Overview: Photonic crystals have been widely used in sensing, information processing and optical devices since they can manipulate light in the wavelength scale by periodic refractive index distributions, which can also be called optical band gaps. In addition, optical fibers are flexible miniature optical waveguides. Therefore, the combination of photonic crystals and optical fibers could form a miniature sensing platform on fiber, named lab on fiber. In this paper, self-assembly method was applied to fabricated colloidal photonic crystals on optical fiber end facets. Polystyrene opal film, silica inverse opal film and composite opal film are successfully produced on a single optical fiber end facet. Film quality was characterized by SEM and reflection spectra through the other end of optical fiber. Cracks and limited layers of colloidal photonic crystals were observed on the single optical fiber end facet. To increase the photonic crystal film quality, an optimized structure, a capillary ferruled on one end of the fiber and a formed large flat surface, was employed in the fabrication process. As a result, high quality colloidal photonic crystal on the fiber end facet was obtained which is confirmed by optical reflection spectra. Moreover, the produce film stick more firmly on the fiber end facet compared to that on a single optical fiber. Principles of the colloidal photonic crystal film as sensing materials are discussed. Bragg reflection as well as effective refractive index theory was employed to describe the band gap shift of colloidal photonic crystal. A relatively large effective refractive index change or large lattice distance, or both of them will results in a large sensitivity of the colloidal photonic crystal.
Composite photonic crystal on fiber end facet as relative humidity sensor are demonstrated. The sensing mechanism is that silica gel infiltrated in polystyrene spheres can absorb water molecular in high relative humidity, and the water in the silica gel network will evaporate when relative humidity decrease. As a result, the fabricated composite photonic crystal film is sensitive to relative humidity in a range from 12%~86%. A sensitivity of 0.133 nm for reflectance peak at 900 nm wavelength is experimentally demonstrated. When the relative humidity is larger than 86%, reflectance peak of the composite photonic crystal film does not shift obviously due to a saturated absorption of water of the silica gel. As a conclusion, colloidal photonic crystal on optical fiber end facet can be fabricated and could form a platform for optical sensing or analyzing.