  We present a novel differential refractive index sensor based on arrays of
photonic molecules (PM) of dielectric cylinders and two structural defect
cavities. The transmission spectrum of the photonic proposed structure as
sensor shows a wide photonic stop band with two localized states. One of them,
the reference state, is bound to a decagonal ring of cylinders and the other,
the sensing state, to the defect cavities of the lattice. It is shown that
defect mode is very sensitive to the presence of materials with dielectric
permittivity different from that of the surrounding cylinders while the state
in the PM is not affected by their presence. This behavior allows to design a
device for sensing applications. A prototype of the sensor, in the microwave
region, was built using a matrix of 3x2 PM arrays made of soda-lime glass
cylinders (dielectric permittivity of 4.5). The transmission spectra was
measured in the microwave range (8-12 GHz) with samples of different refractive
index inserted in the defect cavities. Simulations with Finite Integration
time-domain Method are in good agreement with experiments. We find that the
response of the sensor is linear. Device measurement range is determined by the
dielectric constant of the cylinders that make up the device. The results here
presented in the microwave region can be extrapolated to the visible range due
to scale invariance of Maxwell equations. This make our prototype a promising
structure as sensor also in the optical range.
