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A cascaded-ring optical sensor with Vernier effect is demonstrated experimentally by using Si nanowires. The Vernier effect helps to obtain ultrahigh sensitivity. In order to enhance the sensitivity further, a suspended Si nanowire is used for the sensing ring by removing the SiO2 insulator underneath with a wet-etching process. The measurement result shows that the sensitivity Δλ/Δna of the demonstrated cascaded-ring optical sensor is up to 4.6×105 nm/RIU experimentally. The detection limit for the index change is about 4.8×10-6.