  We report a refractive index sensor comprising of unique T grating on top of
periodic nano-cavities. The sensor has two resonant modes sensitive to
different regions of the structure with low inter-region interference, hence
allows simultaneous detection of two different analytes or more accurate
detection of a single analyte. The sensor also provides a self-referencing
feature for a broad range of refractive index, from 1.3 to 1.5. Using the FDTD
method, the sensitivities of 801.7 nm/RIU and 1386.8 nm/RIU have been recorded
for the two modes respectively. The versatility of the structure makes the
sensor a prominent candidate for biochemical and other sensing applications.
