The Surface Plasmon (SP) is a propagating electron density wave occurring at the interface between metal and dielectric. Essential for the generation of SPs is the presence of free electrons at the interface of two materials, as a result of the analysis by Maxwell's equations, the materials' (usually a metal and a dielectric) dielectric constants have opposite signs. The field intensity is at its maximum at the interface, and decays exponentially in the normal direction on both sides of the surface. And the field is strongly bound to the interface. The dispersion relation of the SP waves is expressed as:
                              k          sp                =                                                            2                ⁢                π                            λ                        ⁢                                          (                                                                            ɛ                      1                                        ⁢                                          ɛ                      2                                                                                                  ɛ                      1                                        +                                          ɛ                      2                                                                      )                                            1                /                2                                              =                                    ω              c                        ⁢                                          (                                                                            ɛ                      1                                        ⁢                                          ɛ                      2                                                                                                  ɛ                      1                                        +                                          ɛ                      2                                                                      )                                            1                /                2                                                                        (        1        )            where ksp is the wave vector of the SP waves on the metal surface, λ, ω, c are the wavelength, angular frequency of the SP waves and the velocity of the light, respectively, and ∈1 and ∈2 are the dielectric constants of the metal layer and the dielectric layer, respectively.
Surface Plasmon Resonance (SPR) is a phenomenon in physical optics. The SPs can be excited by the evanescent waves generated through the total internal reflection occurs at the prism interface, and the energy can be coupled from the light waves to the surface plasmon waves. When a p-polarized planar light wave is incident on a dielectric/metal interface at a specific angle, where the evanescent wave vector matches with that of the surface plasmon, the energy of the incident light is coupled to the SP waves and the SPR is excited, in which case, the energy of the reflected light is significantly reduced. The phase matching relation of the SPR can be expressed as:kx=ksinθ=ksp  (2)
It can be seen from Equations (1) and (2) that for the same dielectric under detection, the condition for the SPR excitation is a function of both the wavelength and the incident angle of the incident light. Thus, all the interrogation methods currently applied for the SPR detection are based on changing the incident light. These interrogation methods generally involve angular interrogation, wavelength interrogation, intensity interrogation and phase interrogation, among which the first two are mostly used.
1. Angular Interrogation: this is an interrogation method most commonly used in the conventional SPR sensor. In this method, a fixed-wavelength light source is used, and a mechanical device is used to rotate the SPR detection mechanism or the incident light source. So the SPR resonance angle can be found by changing the incident angle. Though the Angular Interrogation is straightforward and simple, and it can achieve the highest precision among all the currently available interrogation methods, the interrogating speed of the scanning system is limited by that of the precise mechanical rotating stage which is very low. Thus, it is difficult to realize fast real-time measurement with high time resolution.
2. Wavelength Interrogation: in this method, the incident angle is fixed, a quasi-planar beam of a broadband light source is used as the incident beam, and the spectrum of the reflected light is measured by using a spectrometer or a monochromator. In that case, the intensity of the response over a range of wavelengths can be obtained, and the corresponding SPR wavelength can be obtained. At an operational band of longer wavelength, the system precision of this method may reach or be even higher than that of the angular interrogation method. However, the speed of the currently available spectrometer is rather slow, which limits the interrogation speed of the system in the method. As a result, the interrogation frequency is very low and the realization of real-time detection for fast-changing signals is not possible.
It can be seen from the above-described SPR interrogation methods that the sensing detection system using the SPR effect has the following technical defects: low interrogation speed, bulky device, and not being able to realize high density multi-channel parallel detections and so on.