We propose and investigate,both experimentally and theoretically,a novel mechanism for switching and modulating plasmonic signals based on a Fano interference process,which arises from the coupling between a narrow-band optical Fabry-Pérot cavity and a surface plasmon polariton (SPP) source.The SPP wave emitted from the cavity is actively modulated in the vicinity of the cavity resonances by altering the cavity Q-factor and/or resonant frequencies.We experimentally demonstrate dynamic SPP modulation both by mechanical control of the cavity length and all-optically by haessing the ultrafast nonlinearity of the Au mirrors that form the cavity.An electro-optical modulation scheme is also proposed and numerically illustrated.Dynamic operation of the switch via mechanical means yields a modulation in the SPP coupling efficiency of ～ 80％,while the all-optical control provides an ultrafast modulation with an efficiency of 30％ at a rate of ～ 0.6 THz.The experimental observations are supported by both analytical and numerical calculations of the mechanical,all-optical and electro-optical modulation methods.