Localized Refractive index sensing by integrated photonic crystal waveguide with edge-cavity

We have theoretically proposed a highly compact refractive-index sensor consisted of edge-cavity and line-defect waveguide in two-dimensional photonic crystal. The sensing object is completely outside of the single enclosed surface of the sensor. The edge-cavity is designed by engineering the spatial distribution of the cutoff frequency of edge modes. The coupling between the edge-cavity and the waveguide is maximized by optimizing the radius of the rods between them, so that the transmittance spectrum through the waveguide has a sharp anti-peak. As the refractive index of the sensing object changes, the resonant wavelength of the edge-cavity is changed, which in turn changes the wavelength of the anti-peak. The sensitivity of the sensor is up to 40 nm/RIU, and the footprint of the sensor is only 40 $\mu m^{2}$. Because the transmittance spectrum is determined by the overlap between the sensing object and the highly localized resonant mode, the sensor can also perceive spatial distribution of refractive index in the sensing object.