Photonic sensors employ light to convert bio/chemical processes into a detectable signal, i.e., a sensing transduction signal. As compared to other sensing technologies, the strength and versatility of photonic sensors lie in the wide range of optical properties available in both the spectral and time domains that serve to generate the sensing transduction signal. These properties include, but are not limited to, refractive index, optical absorption, fluorescence, polarization, lifetime, and even nonlinear optical processes such as lasing, Raman scattering, and multiphoton absorption and emission. The sensing signals from these optical properties are complementary to each other. When used alone or in combination, they can provide vast amount of information regarding the presence and interaction of bio/chemical molecules. As a result, photonic bio/chemical sensors have broad applications in healthcare, defense, homeland security, the food industry, the biotechnology industry, pharmaceuticals, and environmental monitoring and protection.
For a photonic bio/chemical sensor, three characteristics, among others, are crucial in determining its performance. (1) Light–analyte interaction. This dictates the transduction signal. Stronger light–matter interaction usually results in a higher sensitivity and better (e.g., lower) detection limit. (2) Sensor miniaturization and multiplexing. These are directly related to sample consumption, device portability, detection time, and detection cost. (3) Integration of fluidics with photonic sensing elements. Effective and efficient fluidics not only reduces the sample consumption and hence the cost, but also enhances light–analyte interaction and expedites the detection processes.