In this paper, a two-dimensional photonic crystal biosensor for medical applications based on two waveguides and a nanocavity is presented. The waveguides and nanocavity are created by introducing line and point defects into a photonic crystal, respectively. It could be shown that by injecting an analyte into a sensing hole, and thus changing its refractive index, may shift the resonant wavelength. The proposed structure is designed for the wavelength range of 1.5259–1.6934 μm. Sensitivity, the most important biosensor parameter, was studied and found to be approximately equal to 83.75 nm/refractive index units (RIU). An important specification of this structure is its very small dimensions. Two-dimensional finite-difference time domain and plane-wave expansion methods were used for both to simulate the proposed structure and to obtain the band diagrams.

In this chapter, the definitions of the photonic crystals (PhCs) are presented and the photonic crystal-based sensors are described. The structures of some photonic crystal biosensors that can detect chemical or biochemical molecules are also investigated. Sensing mechanism in the most photonic crystal sensors is based on the refractive index (RI) change mechanism. By binding the chemical or biochemical molecules to active sensing surface, the refractive index will be changed. So, the resonant wavelength or the intensity of the transmission spectrum is changed. This process can be used as a way to measure the concentration of the molecules. To simulate the optical wave behavior in the structure and evaluating the ability of biosensing, the two-dimensional finitedifference time-domain (2D-FDTD) and plane-wave expansion (PWE) methods are used. The sensors that are presented in this chapter are mostly based on photonic crystal resonators. Some structures based on micro-/nano-resonators, structures based on LX resonators, and structures based on ring resonator are investigated. Important parameters on sensing applications for these structures are also calculated.This chapter is organized as follows: an introduction to photonic crystal structures is presented in Section 1. Then, sensing mechanism in chemical sensors based on photonic crystals is described in Section 2. Quality factor, full width at half maximum (FWHM), free spectral range (FSR), detection limit (DL), and sensitivity are the most important parameters in sensing applications which are introduced in Section 3. Finally, the last section describes some structures of photonic crystal chemical/biochemical sensors.