ABSTRACT
Distributed Bragg reflector (DBR) is a one-dimensional (1D) photoniccrystal (PC) structure having two different materials arranged alternatively. It has the property of refractive-index variation periodically in a single direction. Advancements in modern technology are introducing many new methods for the diagnosis of major life-threatening diseases. An accurate diagnosis of the disease in its early stages will reduce the factor of risk at higher stages and also saves the life of a person. Various methodologies have been researched and implemented successfully in recent years. Optical sensing technology is growing tremendously for the detection of many diseases in their early stages. Major benefits of optical sensors include small in size and weight, very high accuracy, no electromagnetic interference, small sample requirement, high Q factor, good sensitivity, and can be easily integrated with lab-on-chip (LOC) devices.
Refractive index (RI) is one of the key optical parameters which can be used for bio-sensing applications. Using this method, many diseases, 28malignant cells, bacteria, viruses, foreign bodies, etc., can be effectively detected in their initial stages. The medical diagnosis uses initial laboratory tests like enzyme-linked immunosorbent assay (ELISA), reverse transcription–polymerase chain reaction (RT-PCR), micro-immunofluorescence (MIF), etc., for the detection and identification of bacteria, virus, fungi, or any other immigrant cells. These tests regularly use blood, urine, or saliva as a bio-analyte for diagnosis purposes. These bio-analytes have distinguished or unique optical properties like RI, light absorption capability, light scattering, and so on. As the normal and infected cells in the biosample exhibit different RI, it can be easily detected using optical detection techniques. There exists a number of optical detection techniques like surface plasmon resonance (SPR), optical reflectometric interference, bioluminescent, evanescent wave fluorescence, ellipsometry, interferometry, and many more. The optical biosensors can be designed and analyzed using advanced technologies such as micro-electro-mechanical systems (MEMS), micro-electronics, biochemistry, microbiology, molecular biology, nanotechnology, and so on. Fabrication of the optical biosensor is also possible by using advanced micro-machining techniques. Biosensors use different signal transductions like piezoelectric or magnetic or resistive, optical, thermometric, electrochemical, etc., for the detection of target bodies. The optical sensing method has the benefit of label-free sensing, i.e., it does not require any chemical reagents for the detection of target species. Whereas, the normal clinical laboratory tests use chemical stains and reagents for the identification of disease-causing infectants. In an optical sensor, a beam of input light or the optical field interacts with the sample under test and gives the signature at the output. The optical signature can be either the change in amplitude, frequency, wavelength, or intensity of the input light. By comparing the input transmitted signal and the output received signals, with and without bio-analyte, the conclusion can be drawn for the presence or for the absence of diseases in the sample under test. It is also possible to detect the various stages or cycles of infected cells using these optical methods. In the human body, the cells or tissue from the specific organ have unique RI values. The body fluids, viz, blood, saliva, and urine is also exhibiting a different index of refraction in the presence and absence of the malignant cells. Therefore, the RI of a cell or bio-analyte can be effectively used for the identification of various diseases.
In this chapter, a 1D photonic crystal-based DBR is used for the detection of foreign bodies like cancer cells or the infected stages of blood cells 29for quick medical diagnosis. The one-dimensional (1D) Bragg Reflector sensor can also be fabricated by using electron beam lithography or chemical vapor deposition (CVD) techniques.
