Sensor fabrication and characterization. The samples of Pd periodic array were fabricated from a 4-inch fused silica wafer with electron-beam lithography, thermal evaporation, electronbeam evaporation, wet-chemical etching, reactive-ion etching, lift-off, and dicing. The steps involved included: (i) Using a 4-inch fused silica substrate (Siegert Wafer), a lift-off layer of 80 nm MCC NANO Copolymer EL4 (Microlithography Chemicals Corp.) was first spin coated and baked on a contact hotplate for 5 min at 180 o C. Following that, an imaging layer of 70 nm MCC NANO 950k PMMA A2 (Microlithography Chemicals Corp.) was spin coated and baked on a contact hotplate for 5 min at 180 o C. (ii) A 20 nm thick Cr layer was deposited with thermal evaporation (Lesker Nano 36) to enable electrical discharge during electron-beam exposure. (iii)