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We find strong intensity modulations in surface plasmon polaritons (SPPs) by changing the refractive index and thickness (i.e., the optical length) of the dielectric layer in metal-dielectric nanodevices. We interpret the phenomena by considering the dielectric layer as a Fabry–Pérot cavity. Analysis from finite-difference time-domain numerical studies reveals that the strong periodic variation of the enhancement in SPP fields is due to alternating constructive and destructive interference of optical waves in the dielectric layer. The present results can be applied in designing planar metal-dielectric nanodevices that require optical intensity modulation or high-field enhancement, such as optical switches or thin-film solar cells.