  To promote the development of two-dimensional (2D) photonic crystals (PCs)
based refractive-index (RI) biosensors, there is an urgent requirement of an
effective approach to improve the RI sensitivity of 2D PCs (usually less than
500 nm/RIU). In this work, the photonic band gap (PBG) feature and the
corresponding RI sensitivity of the air-ring type 2D annular PCs (APCs) have
been studied in detail. Such type of 2D PCs can easily and apparently improve
the RI sensitivity in comparison with conventional air-hole type 2D PCs that
have been widely studied in previous works. This is because the APCs can
naturally exhibit suppressed up edge of PBG that can strongly affect the final
RI sensitivity. In general, an enhanced sensing performance of as high as up to
2-3 times RI sensitivity can be obtained from pure 2D APCs. Such high RI
sensitivity is also available in three typical waveguides developed from pure
2D APCs. Furthermore, a new conception of dual-polarization RI biosensors has
been proposed by defining the RI sensitivity as the difference of crossing
relative sensitivity of up-edge PBG between different polarizations. Based on
pure 2D APCs, a dual-polarization RI biosensor with high RI sensitivity of
around 1190 nm/RIU in infrared range was also presented as an example.
