Near field investigation of surface plasmon polaritons

Abstract

The interaction of light with metals contains a resonant phenomenon called the Surface Plasmon Resonance (SPR), at which the free electrons in the metal collectively oscillate. This collective oscillation of the free electrons, called Surface Plasmon Polaritons (SPPs), is highly sensitive to the medium adjacent to the dielectric. The level of sensitivity depends on the sharpness of the resonance. The dynamic range of SPR sensors can be improved by corrugating the metal surface. The SPPs propagating on a corrugated metal surface will scatter back to radiation, which decreases the propagation length and broaden the resonance. Hence the corrugation must be designed in such a way that the dynamic range of the sensor is preserved, without compromising the sensor's sensitivity. The ideas described in the thesis are intended to improve the sensitivity and dynamic range of SPR bio-sensors. An optical excitation of SPPs is only possible by increasing the momentum of light. A sharp tapered optical fiber tip is used to image SPPs with a sub-wavelength optical resolution. An interesting property of SPPs, such as the phase shift at SPR, can be extracted by probing the amplitude and phase of the SPP field on a metal surface using a heterodyne interferometric technique. The extraction of the phase shifts at SPR on two different samples, a flat gold surface and a buried gold grating, is demonstrated. In the former case, the difference between the phase of the evanescent wave on a glass surface and the gold surface is measured for varying incident angles. In the latter case, the resonant phase shift at SPR is extracted using a non-resonant diffracted order as the reference. The radiative scattering of SPPs is investigated for two different types of grating designs: a buried grating and an exposed grating. In a glass-gold-air system, the buried grating has corrugations at the glass-gold interface and the exposed grating has corrugations at the gold-air interface. A significant reduction in the line width of the SPR has been found on the buried grating, which implies an increase in the propagation length of SPPs.