مشخصات پژوهش

When a part of an optical wave-front experiences a sharp change in its phase, Fresnel diffraction becomes appreciable. Sharp change in phase occurs as a wave-front reflects from a surface with an abrupt change in reflectivity. We apply this concept to a modified Kretschmann configuration for coupling surface plasmon. In Kretschmann configuration a metal film is placed at the interface of two dielectric media. First medium with higher refractive index is a prism and second medium with lower refractive index can be the air or the solutions of interest. But, in our modified configuration, the metal film is coated only on one half of a specified face of the prism. When a parallel-polarized light travels from the higher refractive index medium to the lower refractive index medium with an angle θ>θc, where θc is the critical angle, the total internal reflection can take place within first medium. Subsequently, enhanced evanescent waves, namely surface plasmons are confined to the metal–dielectric interface. In this case, the total internal reflection amplitude decreases at the half section covering the metal film. Therefore, the Fresnel diffraction fringes are formed due to abrupt change of the reflection amplitude. We measure the visibility of the diffraction fringes versus wavelength of the incident light, for while, we get the maximum visibility. This wavelength is corresponding to the surface plasmon resonance. We present the technique by theory and experiment.