Pmma a4

For the Si substrate sample, a 1 μm of SiO₂ film was grown on a Si substrate through thermal oxidation. The SiO₂/Si substrate was spin-coated with poly(methyl methacrylate) (PMMA A4, MicroChem) and was patterned using e-beam lithography (e-line, Raith). Then, a 3 nm/60 nm of Ti/Au film was deposited on the substrate using e-beam evaporation (Denton Vacuum Explorer 14, Denton Vacuum). Here, the Ti was used to enhance the adhesion between the Au and SiO₂ films. Lastly, the Ti/Au film on the poly(methyl methacrylate) was lifted-off in a solvent stripper (Remover PG, MicroChem). The thickness of the Ti/Au film was measured to be approximately 68 nm using a profilometer (Alpha step 200, Tencor). For the optical fiber sample, a single-mode optical fiber (SMF-28, Corning) was cleaved, so that the fiber end-face was perpendicular to the waveguide direction. The fiber end-face was cleaned by acetone, isopropyl alcohol, and deionized water followed by nitrogen blow. A 3 nm/60 nm of Ti/Au film was deposited on the fiber end-face using e-beam evaporation. The Ti/Au film on the fiber end-face was patterned to the square and rectangular nanopatch array using FIB lithography (GAIA3, Tescan). Note that the Ti/Au films for both optical fiber sample and Si substrate sample were deposited in the same chamber at the same time.