High Coupling Efficiency in 2D Guided-Wave Photonic Band Gap Extended Microcavities for Sensing Applications

Label-free techniques for sensing are of great interest for identification of molecules on a functional surface. Currently, a growing research activity dealing with optical approaches does contribute to the development of new devices. A remarkable advance in the field of bio-detection is given by the use of photonic crystals enabling to design and fabricate high-sensitivity and compact novel optical sensors. Many photonic crystal sensors on several different materials have been proposed in literature. However, further improvement of the sensitivity requires an additional research effort on the optimization of both the design and fabrication process. In this paper we report on the solution of the mode-mismatch occurring at the input and output interfaces of the microcavity. Mode-mismatch is responsible for the increase of the scattering and degradation of transmission characteristics of the photonic crystal. Optimization of the sensor design can be achieved by using an appropriate tapering of some hole rows into reflectors. Numerical results have proved that two physical effects occur simultaneously, i.e. conventional tapering which is dominant at high resonant wavelengths, and resonant conditions of the external cavity, when the radius of the holes in the external row is greater than that of the internal one. This last effect is more clear at low wavelength.