Surface plasmon control of excitation and fluorescence in optical biochips

In this paper we report on the development of an optical biochip to control both the excitation and resultant fluorescence using grating coupled surface plasmons. Electron beam lithography is used to fabricate line gratings in thin layers of gold on the surface of 150μm thick coverslips. Laser diodes operating at 630nm are close coupled to the coverslip resulting in the excitation of surface plasmons. In the region of the grating light can radiate into the far-field, and both the angle of emission and beam divergence can be controlled by the grating pitch and the number of lines included in the pattern. A model is presented which treats the grating as an optical antenna array which shows how these characteristics can be explained in terms of the wave vector matching between the surface plasmons and the grating. Fluorescence has also been excited in standard organic dyes on-chip. When placed in close proximity to the surface of the sample strong quenching of the fluorescence is seen in the region of the grating. In contrast an enhancement of the signal is seen when the fluorophores are placed on a 200nm thick spacer layer.