Real-Time Visualization Method of Surface Plasmon Resonance with Spectroscopic Attenuated Total Reflection

Abstract

We report the real-time visualization method of surface plasmon resonance with the spectroscopic attenuated total reflection. Recently, surface plasmon resonance (SPR) had been studied for plasmonics devices to construct faster processor in the electronic microprocessors. SPR is strong interaction between light and free electron near metal surface, which cause absorption of light due to its resonance. The behavior can be explained with Fresnel’s equation. As the wave number of light with a certain frequency is not matched with that of surface plasmon, a prism or a grating is used in order to compensate this mismatching. In the prism case, the wave number is changed by changing the incident angle to the metal surface inside the prism as ksp=n*k0sinθ, where ksp and k0 is the wave numbers of surface plasmon and incident light, respectively, n is the refractive index of the prism and θ is the incident angle to the metal surface inside the prism. Therefore, the SPR can be observed by absorption of light as functions of the wavelength and the incident angle. This resonance behavior as functions of the wavelength and the incident angle can be observed directly with a two-dimensional detector such as a CCD camera. As the two-dimensional SPR images for 50nm-thick silver films on the prism surface have been observed experimentally, they have good agreement with calculated ones. Kretchmann configuration using a glass prism and an approximately 50-nm-thick silver or gold film was often used in order to evaluate the optical constants of the film. Most of SPR signals had been measured with either angular or spectral dependence with this geometry. In the case of angular dependence, the monochromatic laser, e.g. He-Ne laser at 632.8nm, is often used for the incident light. One can measure reflection loss as a function of an incident angle in the total reflection region. Increase in the resonance angle of SPR is well known when the thin oxide film on the metal film. The two-dimensional image of SPR is called “surface plasmon spectral fingerprint”, because it can inform conditions of metal films whether they are reacted or oxidized. Many fingerprints are expected by changing the thickness of the coating layer on the silver surface. In our method, thin metal film on a prism was excited by focusing beam of white light. SPR was clearly visualized with a spectrometer equipped with a two-dimensional CCD detector in the coordination of the incident angle and the wavelength. Various metal films could be distinguished even in partially oxidized condition. This real-time SPR visualization method would be useful not only for monitoring of surface reaction but for fabricating plasmonic devices.