Integrated optical chemical sensor using a dispersion-guided photonic crystal structure

There is a growing need for miniature low-cost chemical sensors for use in monitoring environmental conditions. Applications range from environmental pollution monitoring, industrial process control and homeland security threat detection to biomedical diagnostics. Integrated opto-chemical sensors can provide the required functionality by monitoring chemistry induced changes in the refractive, absorptive, or luminescent properties of materials. Mach-Zehnder (MZ) interferometers, using the phase induced from a chemically reactive film, have shown success for such applications but typically are limited to one chemical analysis per sensor. In this paper we present a MZ-like sensor using the dispersion properties of a photonic crystal lattice. Properly engineered dispersion guiding enables the creation of multiple parallel MZ-like sensors monitoring different chemical reactions in a device much smaller than a typical MZ sensor. The phase shift induced in one arm of the photonic crystal structure by the chemical reaction of a special film induces a change in the sensor output. The use of a dispersion guiding photonic crystal structure enables the use of lower refractive index materials because the creation of a bandgap is not necessary. This in turn increases coupling efficiency into the device. Other advantages of this type of structure include the ability to guide both TE and TM modes as well as reduced sensitivity to fabrication tolerances. Two-dimensional FDTD analysis is used to optimize and model the effectiveness of the structure.