Title : Understanding the Performance of Surface Plasmon Resonance Based Sensors from the First Principle
Starting with the basic laws of electricity and magnetism, we study the optical modes associated with the plasma oscillations of the free electron gas on a metal surface in contact with a dielectric layer. We then obtain
mathematical equations for these optical modes for optical frequencies above the plasma frequency ???????? of these
metals and discuss its characteristics in detail. These optical modes of plasma oscillations can be excited by
resonantly coupling the electromagnetic wave at frequencies above ???????? through the evanescent waves penetrating
from a dielectric in contact with the metal, whose thickness and refractive index are appropriately chosen. These
optical modes are transverse magnetic in nature which satisfy certain boundary conditions and propagate along the boundary of metal and dielectric. These optical modes are very sensitive to any microscopic changes that take
place in the dielectric region adjacent to the boundary and hence are being widely employed as optical sensors.
These optical sensors are based on the principle of surface plasmon resonance (SPR) and hence are named as SPR sensors. The electric field associated with these optical modes propagate in the form of dis-continuous loops along the boundary. Behaviour of the electric field can be described by evaluating the gradient at any point on these dis-continuous electric loops. These plots reveal interesting properties about the electric field associated with these optical plasmon modes at the metal surface, which have pronounced effect on the performance of plasmonic
devices. The gradient of the electric field is shown to have a large value at the metal-dielectric interface. The
gradient reduces as we move away from the interface. A knowledge of the gradient of the electric flux lines near
the metal boundary has proven to be very fruitful in understanding the functioning of optical sensors and can be
utilized in optimizing their performance.
Audience Take Away:
- The proposed results can be used to understand the performance of surface plasmon resonance (SPR) based sensor and evaluate its design parameters for desired sensitivity and figure of merit (FOM).
- Commercial applications: The design methodology of SPR affinity bio-sensors can be prepared from the results discussed herein. The choice of the thicknesses and refractive indices of the various materials employed in the SPR sensors, plays a very important role in its performance when used for the detection of bio-molecules such as E-Coli, pseudomonas bacteria, glucose levels, and triglycerides, etc. in the blood sample.
- Applications in Research/ teaching domain: The proposed technique can be expanded to study other optical devices such as the directional couplers which are extensively being used in integrated optoelectronic devices, and optimize its design parameters.
- The optical TE/TM polarizers can also be studied and their extinction ratio be improved by the proper choice of the refractive indices and thicknesses of the various layers using the proposed technique.