SPR waveguide sensor based on transition of modes at abrupt discontinuity

This paper describes a new scheme of enhanced sensitivity based on transition of modes at abrupt discontinuity using combined amplitude and phase sensing. The design consists of a single SPR waveguide structure supporting two modes acting as sensor section. The two modes are made to interfere by an abrupt discontinuity of the sensing waveguide into a single-mode output waveguide. The geometrical parameters of the entire scheme are chosen as to produce almost total cancellation of the field propagating into the output waveguide. This delicate balance is very sensitive to small changes in the waveguide's environment. Calculated values show sensitivities of an order of magnitude higher compared to previously reported waveguide SPR sensors. Additional sensitivity and extended working range can be gained by allowing external control of the differential phase between the propagating modes.