A substrate includes a plurality of protrusions containing a metal. A first metal portion containing a metal including at least one of gold, silver, platinum, copper, and palladium is provided on a first protrusion among the plurality of protrusions. A second metal portion containing a metal including at least one of gold, silver, platinum, copper, and palladium is provided on a second protrusion different from the first protrusion among the plurality of protrusions. A dielectric portion is provided between the first protrusion and the first metal portion, and between the second protrusion and the second metal portion. A gap is provided between the first and second metal portions, and the distance between them is 50 nm or less. The first and second metal portions include functional groups adsorbed or bonded thereto.