Photonic crystal-based fluid sensors: Toward practical application

We present an uncomplicated and inexpensive system for refractive index measurements of fluids based on the optical properties of photonic crystals (PhCs). The photonic band gap of inverse opals depends on the refractive index contrast between the crystal and the pore fluid. A change of refractive index of the pore fluid causes variations in the photonic band gap which can be observed as change in the wavelength of reflected light of the crystal (color change). Based on this principle a measurement cell for continuous fluid measurements was build (see figure) and used for identification of water/ethanol mixtures. Based on photonic band structure simulations general rules/recommendations for the design of this sensor type are given, for example, periodicity of the inverse opal, choice of material (refractive index), or measurement mode. For the proof of concept tungsten oxide (WO3) inverse opal structure was utilized as sensing layer. However, depending on the application other metal oxides or even polymers can be used. With the presented method a limit of detection (LoD) of 0.001 refractive index unit (RIU) was achieved. The figure shows the cross section of the custom built measurement cell: the reflectivity of the WO3 inverse opal (round image, left) is measured by a spectrometer in dependence of the fluid in contact to the crystal. Different refractive indices of the fluids cause spectral shifts of the reflection peak (right graph).