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Introduction of Surface Plasmon Resonance [Part 2]

Updated: Jul 29, 2023

글쓴이: Alex Jang

작성일: 2014년 1월 21일 (수정일: 2020년 5월 30일)

The first part was an introduction, and from now on, we will move on to the main content.

Please remember that we are focusing on the angle-variable SPR device using Kretchmann's method.

<그림 1. Kretchmann configuration>

The Kretchmann method follows the configuration shown in Figure 1. This configuration involves attaching a prism and a gold-coated sensor chip using a one-time adhesive (matching oil or adhesive film). The light is then incident on the inclined side of the prism and reflects after reaching the gold coating. (At this point, the evanescent wave of the incident light and the surface plasmon wave of the gold coating on the sensor chip can produce resonance at a specific angle.)

What is the role of the prism?

A prism (with a refractive index of 1.50 or higher for glass) has a much higher refractive index than air (with a refractive index of 1.00). This increases the refractive index of the medium before reaching the gold layer, allowing the SPR phenomenon to occur. Second, it is important for the light to change angle as it enters. This requires a prism shape like that shown in Figure 1. When light is incident on a flat surface, if the angle is not very high, it is mostly reflected at the lower layer and cannot reach the gold layer.

Total internal reflection is a physical phenomenon that light is reflected entirely at a certain angle of incidence when entering from a higher refractive index material to a lower refractive index material. The SPR phenomenon occurs in the state where total internal reflection is happening, with a metallic film on the interface between a high refractive index material (such as a prism) and a low refractive index material, which can be air, fluid, or a thin film.

If light is incident in this state, total reflection occurs, but at a certain angle, instead of being totally reflected, all the light is absorbed in a very peculiar phenomenon. This is the phenomenon called SPR. The metal film in this case is an object that contains a plasmon, which is a cloud of electrons. When defining SPR in the previous chapter, it was explained that the resonance occurs between the plasmon (metal film) and the external stimulus (energy from light), resulting in the SPR phenomenon.

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